U.S. patent application number 09/756118 was filed with the patent office on 2001-06-07 for cellular terminal location using gps signals in the cellular band.
Invention is credited to Steer, David G., Strawezynski, Leo, Watters, J. Michael.
Application Number | 20010002822 09/756118 |
Document ID | / |
Family ID | 22144067 |
Filed Date | 2001-06-07 |
United States Patent
Application |
20010002822 |
Kind Code |
A1 |
Watters, J. Michael ; et
al. |
June 7, 2001 |
Cellular terminal location using GPS signals in the cellular
band
Abstract
Aspects of global positioning system (GPS) technology and
cellular technology are combined in order to provide an effective
and efficient position location system. In a first aspect of the
invention, a cellular network is utilized to collect differential
GPS error correction data, which is forwarded to a mobile terminal
over the cellular network. The mobile terminal receives this data,
along with GPS pseudoranges using a GPS receiver, and calculates
its position using this information. According to a second aspect,
when the requisite number of GPS satellites are not in view of the
mobile terminal, then a GPS pseudosatellite signal, broadcast from
a base station of the cellular network, is received by the mobile
terminal and processed as a substitute for the missing GPS
satellite signal. A third aspect involves calculating position
using GPS when the requisite number of GPS satellites are in view
of a GPS receiver, but when the requisite number of GPS satellites
are not in view of the GPS receiver, then position is calculated
using the cellular network infrastructure. When the requisite
number of GPS satellites come back into view of the GPS receiver,
then position is again calculated using GPS. A fourth aspect
involves using cellular signals already being transmitted from base
stations to terminals in a cellular network to calculate a round
trip delay, from which a distance calculation between the base
station and the terminal can be made. This distance calculation
substitutes for a missing GPS satellite signal.
Inventors: |
Watters, J. Michael;
(Kanata, CA) ; Strawezynski, Leo; (Ottawa, CA)
; Steer, David G.; (Nepean, CA) |
Correspondence
Address: |
Brian J. McNamara
FOLEY & LARDNER
Washington Harbour
3000 K Street, N.W., Suite 500
Washington
DC
20007-5109
US
|
Family ID: |
22144067 |
Appl. No.: |
09/756118 |
Filed: |
January 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
09756118 |
Jan 9, 2001 |
|
|
|
09078444 |
May 14, 1998 |
|
|
|
Current U.S.
Class: |
342/357.64 |
Current CPC
Class: |
G01S 19/35 20130101;
G01S 19/11 20130101; G01S 19/48 20130101; G01S 2205/008 20130101;
G01S 19/071 20190801; G01S 19/46 20130101; G01S 5/14 20130101 |
Class at
Publication: |
342/357.1 |
International
Class: |
H04B 007/185; H04Q
007/20 |
Claims
We claim:
1. A position location system for determining a geographic position
comprising: a plurality of cellular network base stations
positioned at predetermined locations; and a communications network
that communicates with said plurality of cellular network base
stations, wherein at least one of said plurality of cellular
network base stations comprises a generator that generates a global
positioning system pseudosatellite signal independent of receiving
global positioning system satellite signals.
2. The position location system of claim 1, wherein said generator
comprises: a global positioning system processor that generates a
pseudo global positioning system data signal containing data
representing a location of said one of said plurality of cellular
network base stations, a coarse acquisition code portion that
generates a coarse acquisition encoding signal, and a modulator
connected to said global positioning system processor and to said
coarse acquisition code portion, wherein said modulator combines
said pseudo global positioning system data signal and said coarse
acquisition encoding signal to produce said global positioning
system pseudosatellite signal.
3. The position location system of claim 2, wherein said one of
said plurality of cellular network base stations further comprises
a converter connected to said modulator, wherein said converter
converts said global positioning system pseudosatellite signal into
a radio frequency signal having a frequency in a cellular frequency
band.
4. The position location system of claim 3, wherein said one of
said plurality of cellular network base stations further comprises
a level adjuster connected to said converter, wherein said level
adjuster dynamically adjusts an amplitude level of said radio
frequency signal to a predetermined level relative to transmitted
cellular signals.
5. The position location system of claim 4, wherein said one of
said plurality of cellular network base stations further comprises:
a cellular radio that transmits said transmitted cellular signals,
and a combiner connected to said cellular radio and to said level
adjuster, wherein said combiner combines said cellular signals and
said radio frequency signal into a combined cellular signal.
6. The position location system of claim 5, wherein said one of
said plurality of cellular network base stations transmits said
combined cellular signal to a mobile terminal.
7. The position location system of claim 1, further comprising a
mobile terminal positioned in a transmitting vicinity of said one
of said plurality of cellular network base stations, wherein said
one of said plurality of cellular network base stations transmits
said global positioning system pseudosatellite signal to said
mobile terminal.
8. The position location system of claim 7, wherein said mobile
terminal comprises: a cellular antenna that receives a radio
frequency signal having a frequency in a cellular frequency band,
said radio frequency signal containing differential error
correction data; a radio interface unit connected to said cellular
antenna, wherein said radio interface unit converts said received
radio frequency signal into an intermediate frequency signal; a
digital signal processor connected to said radio interface unit,
wherein said digital signal processor converts said intermediate
frequency signal into a data stream; a central processor connected
to said digital signal processor, wherein said central processor
extracts said differential error correction data from said data
stream; and a differential global positioning system processor
connected to said central processor, wherein said central processor
forwards said differential error correction data to said
differential global positioning system processor.
9. A position location system for determining a geographic position
comprising: a cellular antenna that receives a cellular signal
having a frequency in a cellular frequency band, wherein said
cellular signal contains global positioning system pseudosatellite
data; a first converter connected to said cellular antenna, wherein
said first converter converts said received cellular signal into a
first intermediate frequency signal having a first frequency; a
global positioning system receiver antenna that receives global
positioning system satellite signals having frequencies in a
satellite frequency band from a plurality of global positioning
system satellites; a second converter connected to said global
positioning system receiver antenna, wherein said second converter
converts said received global positioning system satellite signals
into a second intermediate frequency signal having a second
frequency, wherein said first frequency of said first intermediate
frequency signal is substantially equal to said second frequency of
said second intermediate frequency signal; a selector connected to
said first converter and to said second converter, wherein said
selector selects one of said first intermediate frequency signal
and said second intermediate frequency signal; and a control
processing unit connected to said selector, said control processing
unit configured to utilize said first intermediate signal and said
second intermediate signal when a requisite number of global
positioning system satellites are not in view of the global
positioning system receiver antenna, to calculate global
positioning system pseudoranges.
10. The position location system of claim 9, further comprising a
mobile terminal that houses said cellular antenna, said first
converter, said global positioning system receiver antenna, said
second converter, said selector, and said control processing
unit.
11. The position location system of claim 10, further comprising: a
plurality of cellular network base stations positioned at
predetermined locations; and a communications network that
communicates with said plurality of cellular network base stations,
wherein at least one of said plurality of cellular network base
stations comprises a generator that generates said cellular signal
containing said global positioning system pseudosatellite data, and
wherein said one of said plurality of cellular network base
stations transmits said cellular signal to said mobile
terminal.
12. The position location system of claim 9, wherein said cellular
signal further contains differential error correction data, and
wherein said control processing unit is further configured to
correct said global positioning system pseudoranges using said
differential error correction data, to obtain corrected global
positioning system ranges.
13. The position location system of claim 9, further comprising an
amplitude controller connected to said first converter, wherein
said amplitude controller adjusts an amplitude of said first
intermediate frequency signal before said first intermediate
frequency signal is forwarded to said selector.
14. A method for determining a geographic position of a mobile
terminal comprising the steps of: receiving global positioning
system satellite signals having frequencies in a satellite
frequency band at the mobile terminal from a plurality of global
positioning system satellites that are in view of the mobile
terminal; converting said global positioning system satellite
signals into a first intermediate frequency signal; receiving a
cellular signal having a frequency in a cellular frequency band at
the mobile terminal, wherein said cellular signal contains global
positioning system pseudosatellite data; converting said cellular
signal into a second intermediate frequency signal; and calculating
the geographic position using both said first intermediate
frequency signal and said second intermediate frequency signal when
a requisite number of global positioning system satellites are not
in view of the mobile terminal.
15. The method of claim 14, further comprising the step of
transmitting said cellular signal from a cellular network base
station to the mobile terminal.
16. The method of claim 15, further comprising the step of
generating said cellular signal in the cellular network base
station, wherein said generating step comprising the steps of
generating a pseudo global positioning system data signal
containing data representing a location of the cellular network
base station, generating a coarse acquisition encoding signal,
combining said pseudo global positioning system data signal and
said coarse acquisition encoding signal into a pseudosatellite
signal, and converting said pseudosatellite signal to produce at
least a part of said cellular signal.
17. A method for determining a geographic position of a mobile
terminal comprising the steps of: receiving global positioning
system satellite signals having frequencies in a satellite
frequency band at the mobile terminal from a plurality of global
positioning system satellites that are in view of the mobile
terminal; receiving a cellular signal having a frequency in a
cellular frequency band at the mobile terminal, calculating a
cellular distance measurement representing a distance between the
mobile terminal and a cellular network base station using said
cellular signal; calculating the position of the mobile terminal
using said received global positioning system signals when a
requisite number of global positioning system satellites are in
view of the mobile terminal; and calculating the position of the
mobile terminal using said received global positioning system
signals and the cellular distance measurement when the requisite
number of global positioning system satellites are not in view of
the mobile terminal.
18. The method of claim 17, wherein said received cellular signal
is a code division multiple access cellular signal.
19. The method of claim 17, wherein said received cellular signal
is a time division multiple access cellular signal.
20. The method of claim 17, wherein said step of calculating the
cellular distance measurement utilizes a round trip delay
measurement.
21. The method of claim 17, wherein said step of calculating the
cellular distance measurement utilizes a plurality of time
difference of arrival measurements.
22. A method for determining a geographic position of a mobile
terminal comprising the steps of: receiving global positioning
system satellite signals having frequencies in a satellite
frequency band at the mobile terminal from a plurality of global
positioning system satellites that are in view of the mobile
terminal; receiving a cellular signal having a frequency in a
cellular frequency band at the mobile terminal, calculating a
cellular distance measurement representing a distance between the
mobile terminal and a cellular network base station using said
cellular signal; correcting said cellular distance measurement
using a cellular correction term, to obtain a corrected cellular
distance measurement; calculating the position of the mobile
terminal using said received global positioning system signals when
a requisite number of global positioning system satellites are in
view of the mobile terminal; and calculating the position of the
mobile terminal using said received global positioning system
signals and said corrected cellular distance measurement when the
requisite number of global positioning system satellites are not in
view of the mobile terminal.
23. The method of claim 22, wherein said cellular correction term
is a round trip delay correction term that is calculated by the
steps comprising: determining an expected round trip delay between
the mobile terminal, whose position is determined using reference
global positioning satellites, and said cellular network base
station, whose position is known; determining an actual round trip
delay between the mobile terminal and said cellular network base
station; calculating a difference between said expected round trip
delay and said actual round trip delay; and storing the calculated
difference as said round trip delay correction term.
24. The method of claim 22, wherein said received cellular signal
is a code division multiple access cellular signal.
25. The method of claim 22, wherein said received cellular signal
is a time division multiple access cellular signal.
26. The method of claim 22, wherein said cellular correction term
is a time difference of arrival correction term that is calculated
by the steps comprising: determining an expected time difference of
arrival between the mobile terminal, whose position is determined
using reference global positioning satellites, and said cellular
network base station, whose position is known; determining an
actual time difference of arrival between the mobile terminal and
said cellular network base station; calculating a difference
between said expected time difference of arrival and said actual
time difference of arrival; and storing the calculated difference
as said time difference of arrival correction term.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to global positioning
satellite systems and cellular networks, and in particular,
combining aspects of each technology in order to provide an
efficient, reliable, and highly accurate position location
system.
BACKGROUND OF THE INVENTION
[0002] The NAVSTAR (Navigation System with Time and Range) Global
Positioning System (GPS) is a space-based radio-positioning and
time-transfer system. While the system was originally developed
primarily for military purposes, it now also contains a "coarse
acquisition" (C/A) channel that is available for general civilian
use. GPS provides accurate position, velocity, and time (PVT)
information for a given object anywhere on the face of the earth,
such as a moving mobile terminal in a vehicle. The NAVSTAR GPS
includes three major system segments: (i) a space segment, (ii) a
control segment, and (iii) a user segment. Briefly, the space
segment has twenty four NAVSTAR satellites, each of which
broadcasts radio frequency (RF) ranging codes and navigation data
messages. Each navigation data message includes such data as
satellite clock-bias data, ephemeris data (precise orbital data of
the satellite), certain correction data, and satellite almanac data
(coarse orbital data on the 24 satellites). The twenty four
satellites are arranged in six orbital planes with four satellites
in each plane, and the orbital planes are inclined at an angle of
55 degrees relative to the earth's equator. The control segment
primarily consists of a master control station currently at Falcon
Air Force Base in Colorado, along with monitor stations and ground
antennas at various locations around the world. The master control
station monitors and manages satellite constellation. The monitor
stations passively track GPS satellites in view and collect ranging
data for the satellites. This ranging data is transmitted to the
master control system where satellite ephemeris and clock
parameters are estimated and predicted. Furthermore, the master
control system uses the ground antennas to periodically upload the
ephemeris and clock data to each satellite for retransmission in
the navigation data message. Finally, the user segment comprises
GPS receivers, specially designed to receive, decode, and process
the GPS satellite signals.
[0003] Generally, the satellites transmit ranging signals on two
D-band frequencies: Link 1 (L1) at 1575.42 MHz and Link 2 (L2) at
1227.6 MHz. The satellite signals are transmitted using
spread-spectrum techniques, employing ranging codes as spreading
functions, a 1.023 MHz coarse acquisition code (C/A-code) on L1 and
a 10.23 MHz precision code (P-code) on both L1 and L2. The C/A-code
consists of a 1023 bit pseudorandom (PRN) code, and a different PRN
code is assigned to each GPS satellite, as selected from a set of
codes called Gold codes. The Gold codes are designed to minimize
the probability that a receiver will mistake one code for another
(i.e., minimize cross-correlation). The C/A-code is available for
general civilian use, while the P-code is not. In addition, a 50 Hz
navigation data message is superimposed on the C/A-code, and
contains the data noted above.
[0004] In particular, the navigation message has 25 frames of data,
each frame having 1,500 bits. Each frame is divided into five
subframes of 300 bits each. At the 50 Hz transmission rate, it
takes six seconds to receive a subframe, thirty seconds to receive
one data frame, and 12.5 minutes to receive all twenty five frames.
Subframes 1, 2, and 3 have the same data format for all twenty five
frames. This allows the receiver to obtain critical
satellite-specific data within thirty seconds. Subframe 1 contains
the clock correction for the transmitting satellite, as well as
parameters describing the accuracy and health of the broadcast
signal. Subframes 2 and 3 contain ephemeris parameters. Finally,
subframes 4 and 5 contain data common to all satellites and less
critical for a receiver to acquire quickly, namely almanac data and
low-precision clock corrections, along with other data.
[0005] The ranging codes broadcast by the satellites enable the GPS
receiver to measure the transit time of the signals and thereby
determine the range between the satellite and the receiver. It
should be noted, however, that range measurements inherently
contain an error called an offset bias common to all the
measurements created by the unsynchronized operation of the
satellite and the user's clocks. See U.S. Pat. No. 5,467,282 to
Dennis. This user clock error will yield an erroneous range
measurement, making it appear that the user is either closer to or
farther from each of the satellites than is actually the case.
These measurements are therefore more accurately termed
pseudoranges. The navigation data messages enable the receiver to
calculate the position of each satellite at the time the signals
were transmitted.
[0006] In general, four GPS satellites must be in clear view of the
GPS receiver in order for the receiver to accurately determine its
location. The measurements from three GPS satellites allow the GPS
receiver to calculate the three unknown parameters representing its
three-dimensional position, while the fourth GPS satellite allows
the GPS receiver to calculate the user clock error, and therefore
determine a more precise time measurement. The GPS receiver
compiles this information and determines its position using a
series of simultaneous equations.
[0007] In addition, when the GPS receiver is first turned on, it
must calculate its initial position. This initial determination is
known as a "first fix" on location. Typically, the receiver must
first determine which satellites are in clear view for tracking. If
the receiver is able to immediately determine satellite visibility,
the receiver will target a satellite and begin its acquisition
process. If there is no almanac or position information already
stored in the receiver, then the GPS receiver enters a "search the
sky" operation that searches for satellites. Once the satellites
are tracked, the receiver begins receiving the necessary data, as
described above.
[0008] The "time-to-first-fix" (TTFF) represents the time required
for a receiver to acquire the satellite signals and navigation
data, and to calculate its initial position. If the receiver has no
estimate of current time and position and a recent copy of almanac
data, then this process generally takes about 12.5 minutes, which
is the time necessary to receive a complete navigation data message
assuming a 50 Hz transmission rate and receipt of twenty five
frames of data, as described above.
[0009] A common problem with the conventional GPS is not having
four GPS satellites in clear view of the GPS receiver. This
commonly arises, for example, in a city setting such as in an urban
canyon--i.e., in the shadow of a group of tall buildings--which can
block the GPS satellite signals, or indoors in the buildings
themselves. In such situations, the GPS receiver is unable to
accurately determine its location using GPS.
[0010] Therefore, the need arises to find a replacement for the one
or more missing GPS satellite signals. One method for accommodating
this problem is to provide pseudosatellite signals that are
transmitted in the GPS frequency band. They provide much the same
information that the typical GPS satellite does, and are utilized
by the GPS receiver in much the same fashion as the typical GPS
satellite signal. These signals may originate from dedicated
stations that are located on the ground at strategic locations,
such as at airports. However, pseudosatellite signals are stronger
than the GPS satellite signals and therefore, block the GPS
signals. Thus, they generally transmit for only ten percent of the
time. That is, they transmit periodically, known as burst mode,
such as on for ten percent of the time and off for ninety percent
of the time.
[0011] In addition to drowning out actual GPS satellite signals,
the conventional pseudosatellite signal approach has other
disadvantages. For one, there is the need to have specialized
dedicated stations at strategic locations to transmit this
information. This increases the cost of the GPS, and requires the
need for obtaining permission from the landowner to set up and
operate such dedicated stations. In addition, the user must be
located within some specified distance of the station in order to
receive the pseudosatellite signal, which is not always the case.
Therefore, there is a need for a more efficient, less costly, and
reliable alternative for addressing the situation of an inadequate
number of GPS satellites being in clear view of the GPS
receiver.
[0012] In addition, even when four satellites are in view, and the
GPS receiver is readily receiving all of the necessary pseudorange
data for calculating its position, there are further common errors
present that result in erroneous position determinations. These
errors include physical errors such as signal path delays through
the atmosphere, i.e., propagation signal delay, and satellite clock
and ephemeris errors. In addition, for civilian users, the
Government introduces errors for national security reasons,
generally known as selective availability errors (SA). SA primarily
includes ephemeris data error and clock error, and results in an
erroneous position determination of approximately 25 to 100
meters.
[0013] In order to help reduce the effects of these errors, a
differential GPS (DGPS) may be employed. DGPS can achieve
accuracies in the order of ten meters. The typical DGPS
architecture includes one or more reference stations at precisely
known, fixed reference sites, and DGPS receivers. The reference
station includes a reference receiver antenna, a differential
correction processing system, and data link equipment. As an
example, the United States Coast Guard has set up reference
stations that broadcast the differential correction data, which is
typically used by ships.
[0014] There are two primary variations of the differential
measuring techniques. One technique is based on ranging-code
measurements and the other is based on carrier-phase measurements.
In general, the ranging-code differential technique uses the
pseudorange measurements of the reference station to calculate
pseudorange or position corrections for the user receivers. The
reference station calculates the pseudorange corrections for each
visible satellite by subtracting the "true" range, determined by
surveyed position and the known orbit parameters, from the measured
pseudorange. The reference station typically broadcasts the
pseudorange corrections in real-time on a low frequency beacon
channel, which is received in real-time by the DGPS receiver. Of
course, both the DGPS receiver and the reference receivers could
alternatively collect and store the necessary data for later
processing. The DGPS receiver selects the appropriate correction
for each satellite that it is tracking, and subtracts the
correction from the pseudorange that it has measured. For example,
with the reference station set up by the Coast Guard, the station
will broadcast the pseudorange corrections as radio signals. Ships
having DGPS receivers receive this radio signal and process it to
correct the pseudorange data obtained from the GPS satellites.
[0015] The other differential technique is the carrier-phase
differential technique, which is typically used in applications
requiring high accuracy such as in surveying or for an aircraft
landing system. This method measures the difference in phase of the
carrier at the reference and mobile unit. The ambiguity in the
integer number of cycles is determined by either bringing the
antennae of the reference unit and mobile unit close together (less
than one wavelength), or by redundant measurements and complex
search algorithms to determine the correct solutions.
[0016] Furthermore, DGPS may be designed to serve a limited area
from a single reference station, which is generally called a local
area DGPS (LADGPS). In the alternative, the system may use a
network of reference stations and known algorithms to extend the
validity of the DGPS technique over a wide area--known as Wide Area
GPS, or WADGPS.
[0017] The typical DGPS presents certain drawbacks. One drawback is
that the DGPS must use its own frequency band, so as not to
interfere with that of the stand alone GPS. In addition, the DGPS
receiver presents an additional receiver that must operate
independent of the GPS receivers in receiving the differential
correction data. These problems work in direct tension with the
desire to make such systems as small and compact as possible, with
as little additional circuit structure as possible, and still be as
efficient as possible in terms of utilizing limited frequency.
[0018] Another area of interest for the present invention is
cellular technology. FIGS. 1 and 2 show a typical cellular network,
and its main components. See U.S. Pat. No. 5,546,445 to Dennison et
al. The typical cellular network 100 covers a contiguous area that
is generally broken down into a series of cells 110. Each cell has
a base station 210 that maintains communication with the mobile
terminal 220 (e.g., a cellular phone). The base station 210
includes a transmitter and receiver (or transceiver), and an
antenna that transmits a wireless signal over a given area. The
transmit power of the base station is directly related to the size
of the cell, where the greater the transmit power of the base
station, the larger the size of the cell.
[0019] The overall management of the cellular system is handled by
a mobile telecommunications switching office (MTSO) 120. The MTSO
provides numerous functions for the cellular system, such as
assigning calls to a cell based on availability and signal
strength, call statistics, and billing for the cellular network.
The MTSO also functions as the interface between the cells and the
Public Telephone Switching Network (PTSN) 140 for connection to the
local telephone company 230 and long distance toll centers.
[0020] In configuring the cellular network, the desired size of the
cell depends on the geographic nature of the coverage area and the
amount of traffic expected in that area. Each cell uses a group of
assigned frequencies or channels. In addition, where traffic
becomes too heavy in a given area, the cell may be split into
smaller cells by a process known in the art as "cell splitting."
This concept is generally illustrated in FIG. 1.
[0021] In many instances, a cellular user also wishes to determine
their location. The cellular user may carry around a GPS receiver
for determining location. An alternative is to have the GPS
receiver incorporated into the cellular mobile terminal. See, for
example, U.S. Pat. Nos. 5,043,736 to Darnell et al and 5,625,668 to
Loomis et al. Methods also exist for determining location in a
cellular system independent of GPS in order to determine location,
such as using the cellular network infrastructure. Two examples for
calculating position (though not the only methods) are (i) using
Time Of Arrival (TOA) measurements when the time of transmission of
the signal from the base stations is known, or (ii) using Time
Difference of Arrival (TDOA) measurements when the actual time of
transmission is not known, but periodic signals are available, as
explained below.
[0022] Referring generally to FIG. 3, a typical urban street
pattern 300 is shown to illustrate the first method of using TOA
measurements. When the time of transmission of the signal from a
base station 310 is known, a mobile terminal 320 simply determines
when that transmitted signal is received. The difference in time
from transmission to receipt, also known as the propagation delay,
multiplied by the speed of light, provides a radial distance
measurement R between that base station and the mobile terminal.
Calculating the distance between the mobile terminal and three
different base stations provides an accurate location fix for the
mobile terminal, as the intersection of three spheres.
[0023] In the second method of utilizing TDOA measurements, while
this approach can also be used when the actual time of transmission
of the signal from the base stations is available, it may also be
used when such time of transmission is not available, but periodic
signals are. This may occur with some cellular systems. Some CDMA
(Code Division Multiple Access) systems, such as those conforming
to the IS-95 standard, do provide transmissions at well defined
times.
[0024] The periodic signal entails each of the base stations
transmitting periodic signals that are synchronized with one
another. In that regard, all of the base stations may transmit
their periodic signals at the same exact time, or with some
specified timing offset between base stations. In this method, the
mobile terminal measures the difference in time between the arrival
of a signal from one base station with respect to another. This
time difference of arrival (TDOA), together with the known
locations of the two base stations and the speed of radio signal
transmission, defines a hyperbolic surface with the base stations
at the foci. The mobile terminal's location is somewhere on this
surface. Thus, a single TDOA measurement does not uniquely define
the location of the mobile terminal. However, a similar measurement
for signals from other pairs of base stations defines additional
surfaces. By measuring the TDOA of the signals from three base
stations, three surfaces can be determined, the common intersection
of which establishes the location of the mobile terminal.
[0025] Further information and systems regarding conventional TDOA
location systems and methods may be found in Krizman et al.,
"Wireless Position Location Fundamentals, Implementation
Strategies, and Sources of Error," presented at the IEEE Conference
on Vehicular Technology, Phoenix, Ariz., May 5-7, 1997 and in the
issue of the IEEE Communications Magazine, Apr. 1998, Vol. 36, No.
4, pages 30-59. The entirety of this reference is hereby
incorporated into the present disclosure for its teachings
regarding conventional TDOA location methods and systems.
[0026] However, problems exist with using these two methods for
determining location. One significant problem results from
multi-path errors. Such errors result from changes in the
transmission path of the signal that the mobile terminal receives
from the base station. For example, when the user of the mobile
terminal goes around a corner, the mobile terminal may receive a
new signal from the base station that has followed a completely
different transmission path compared to the old signal that the
mobile terminal was previously receiving before the user turned the
corner. Therefore, the distance traveled by the signal will likely
differ. This causes a change in time measurement by the mobile
terminal that does not accurately represent the actual distance
change of the mobile terminal from the base station, thereby
rendering an inaccurate location determination by the mobile
terminal.
[0027] Another problem encountered is that the typical clock in a
cellular mobile terminal does not measure time precisely, and may
have a tendency to drift, generally known as clock drift.
Therefore, the time measurements made by the terminal are not
extremely accurate, which results in an erroneous time--and
therefore location--determination. The error due to the drift grows
larger the longer the mobile terminal clock is used.
[0028] In sum, as shown above, a need exists for a more efficient
and less costly structure compared to the conventional DGPS
receiver. In addition, a need exists for more efficient, reliable,
and effective solutions to address the problem of receiving an
inadequate number of satellite signals from the GPS satellites.
SUMMARY OF THE PRESENT INVENTION
[0029] It is an object of the present invention to provide a system
that combines GPS and cellular technology in order to overcome
deficiencies associated with the use of either technology alone, in
order to provide a more efficient, reliable, and effective position
determination for a given object such as a mobile terminal.
[0030] It is a further object of the present invention to provide a
position location system that utilizes the cellular network to
forward DGPS error correction information to a mobile terminal.
[0031] It is yet another object of the present invention to provide
a position location system that efficiently utilizes the cellular
frequency band available in a cellular network for forwarding DGPS
error correction information to a mobile terminal.
[0032] It is a further object of the present invention to provide a
position location system that provides a cellular network with the
capability of receiving and forwarding DGPS error correction
information that is utilized by the mobile terminal for accurately
determining its position.
[0033] It is another object of the present invention to provide a
position location system that provides a cellular network with the
capability of receiving and forwarding DGPS error correction
information to a DGPS processor, and also forwarding GPS
pseudoranges from the mobile terminal to the DGPS processor,
wherein the GPS pseudoranges are corrected.
[0034] It is yet another object of the present invention to provide
a position location system that compensates for the inability of a
mobile terminal containing a GPS receiver to view the requisite
number of GPS satellites to obtain an accurate fix on its
location.
[0035] It is yet a further object of the present invention to
provide a position location system that utilizes the base station
of a cellular network to transmit GPS pseudosatellite signals such
that if the requisite number of GPS satellites are not in clear
view of the mobile terminal containing the GPS receiver, the mobile
terminal can accurately determine its position using both
cellular-based pseudosatellite signals and available GPS
signals.
[0036] It is another object of the present invention to provide a
position location system that includes a base station in a cellular
network that is capable of generating and transmitting GPS
pseudosatellite signals independent of receiving GPS signals.
[0037] It is a further object of the present invention to provide a
position location system that includes a mobile terminal capable of
receiving a GPS pseudosatellite signal from a base station of a
cellular network, and processing that signal as a substitute for a
missing GPS satellite signal, and in combination with available GPS
satellite signals, to determine position.
[0038] It is another object of the present invention to provide a
position location system that efficiently makes use of both a
position location scheme using the cellular network infrastructure
and a GPS location system when the requisite number of GPS
satellites are not in clear view of the mobile terminal containing
the GPS receiver.
[0039] It is a further objective of the present invention to
provide a position location system that makes use of information
from both the GPS and the cellular network infrastructure to
provide improved accuracy and reliability than could be achieved by
either system working alone.
[0040] It is yet a further object of the present invention to
provide a position location system that makes use of a position
location scheme using the cellular network infrastructure and a GPS
location system as appropriate to minimize power consumption in the
terminal.
[0041] It is a further object of the present invention to provide a
position location system that efficiently switches to a position
location scheme based on a combination of using the cellular
network infrastructure and the available GPS satellite signals when
the requisite number of GPS satellites are not in clear view of the
mobile terminal containing the GPS receiver.
[0042] It is yet another object of the present invention to provide
a position location system that converts from a position location
scheme using the cellular network infrastructure to GPS when the
requisite number of GPS satellites are in clear view of the mobile
terminal containing the GPS receiver.
[0043] It is yet a further object of the present invention to
provide a position location system that utilizes GPS technology to
reduce the problems associated with the position location scheme
that uses the cellular network infrastructure.
[0044] It is another object of the present invention to provide a
position location system that reduces the effects of radio
multi-path propagation and clock drift associated with position
location schemes using cellular network infrastructure by utilizing
GPS technology.
[0045] It is yet a further object of the present invention to
provide a position location system that efficiently utilizes
cellular signals of a CDMA or TDMA system to augment the position
determination.
[0046] In order to achieve these and other objects, the present
invention provides a position location system that incorporates
particular aspects of the cellular network with GPS. For one, in
the position location system of the present invention, the cellular
network is utilized to collect DGPS error correction information,
and forward it to the mobile terminal over the established cellular
network in the cellular band. The mobile terminal includes a DGPS
processor that processes the information, along with pseudoranges
received from a GPS receiver, in order to calculate a more precise
position than that obtained from GPS standing alone. In the
alternative, the DGPS processor is connected to a communications
network which is also connected to the base station, and receives
DGPS error correction data, along with the pseudoranges from the
GPS receiver, over the cellular network (from the mobile). The DGPS
processor uses this information to correct the pseudoranges to
obtain more accurate ranges.
[0047] Second, in the position location system of the present
invention, when the requisite number of GPS satellites are not in
view of the GPS receiver, the system utilizes a GPS pseudosatellite
signal that is generated by one or more base stations of the
cellular network independent of the GPS, that is, independent of
having to receive GPS signals at the base station. The base
stations are modified to generate and broadcast such
pseudosatellite signals, and the pseudosatellite signal is received
and processed by the mobile terminal as a substitute for an actual
GPS satellite signal. Processing this information along with the
satellite signal information from the GPS satellites that are in
clear view, the mobile terminal is able to determine its position
more accurately.
[0048] Third, in the position location system of the present
invention, when the requisite number of GPS satellites are not in
clear view of the GPS receiver, the system switches from relying on
the GPS portion of the system to utilizing cellular network
infrastructure to determine location. This can be done, for
example, by using either the TOA or TDOA methods for determining
location in a cellular network portion of the system. Furthermore,
when the mobile terminal is moved to a location where the requisite
number of satellites are again in clear view of the GPS receiver,
the system efficiently switches back to using the GPS portion of
the system to determine location. An alternative is to use a
combination of GPS satellite signals and cellular signals from base
stations to calculate position.
[0049] Fourth, the cellular signals already transmitted, for
example, in a CDMA or TDMA (Time Division Multiple Access) system
may be used as a replacement of a missing GPS signal or to augment
and improve GPS measurements. Using either the CDMA or TDMA system,
a round trip delay is calculated with respect to a base station,
from which the radius of the terminal from the base station is
calculated. Further calibration can be achieved by calculating a
timing offset correction, to achieve a more accurate radius
measurement.
[0050] Other objects, features, and advantages of the present
invention will become apparent from reading the following Detailed
Description in conjunction with the accompanying drawing
figures.
BRIEF DESCRIPTION OF THE DRAWING FIGURES
[0051] FIG. 1 illustrates a conventional cellular network area
divided into a plurality of cells.
[0052] FIG. 2 illustrates the major components of a conventional
cellular network scheme.
[0053] FIG. 3 illustrates the concept of determining position of a
mobile terminal based on Time of Arrival measurements in a cellular
network system.
[0054] FIG. 4 is a block diagram of a mobile telecommunications
switching office (MTSO) and base station for implementing a first
aspect of the position location system of the present
invention.
[0055] FIG. 4A is a block diagram of a communications network and
base station for implementing a first aspect of the position
location system of the present invention.
[0056] FIG. 5 is a block diagram of a mobile terminal for carrying
out the first aspect of the position location system of the present
invention.
[0057] FIG. 6 is a block diagram of an alternative embodiment of
the MTSO and base station for implementing the first aspect of the
position location system of the present invention.
[0058] FIG. 6A is a block diagram of an alternative embodiment of a
communications network and base station for implementing a first
aspect of the position location system of the present
invention.
[0059] FIG. 7 is a block diagram of an alternative embodiment of
the mobile terminal for carrying out the first aspect of the
position location system of the present invention.
[0060] FIG. 8 is a block diagram of a base station for implementing
a second aspect of the position location system of the present
invention.
[0061] FIG. 9 is a block diagram of a mobile terminal for carrying
out the second aspect of the position location system of the
present invention.
[0062] FIG. 10 is a block diagram of a mobile terminal and base
stations according to a third aspect of the position location
system of the present invention.
[0063] FIG. 11 is a flowchart depicting operation of the position
location system according to the third aspect of the present
invention.
[0064] FIG. 12 is a flowchart depicting operation of the position
location system according to an alternative of the third aspect of
the present invention.
[0065] FIG. 13 is a flowchart depicting operation of the position
location system according to a further alternative of the third
aspect of the present invention.
[0066] FIG. 14 is a block diagram of a base station for
implementing a fourth aspect of the position location system of the
present invention.
[0067] FIG. 15 is a flowchart depicting carrying out a correction
calculation according to the fourth aspect of the position location
system of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0068] The various aspects of a position location system according
to the present invention are described below.
[0069] First Aspect
[0070] Referring to FIGS. 4 and 5, a first aspect of a position
location system of the present invention will be described.
Broadly, FIG. 4 shows a source for DGPS error correction data 400,
a cellular mobile telecommunications switching unit 410, and a base
station 440. FIG. 5 shows a mobile terminal 500, which is typically
at a remote location relative to the cellular mobile
telecommunications switching unit and in the transmitting vicinity
of the base station 440. The transmitting vicinity is the area over
which the base station broadcasts its signals. In general, this
aspect of the invention involves the use of the cellular network to
transmit DGPS error correction data to the mobile terminal, where
it is used to perform corrections on pseudorange data also received
at the mobile terminal.
[0071] First, in FIG. 4, the source 400 is responsible for
providing DGPS error correction data (i.e., differential error
correction data). Numerous alternatives for source 400 exist to
provide such information, including using Government sources,
commercial operators, or the cellular operator. For example, the
source 400 may be a Government source, such as the Coast Guard,
which broadcasts DGPS error correction data as radio signals from
reference stations that it has established. Alternatively, a
commercial supplier may be used to supply the DGPS error correction
data. Two examples of such commercial suppliers are Differential
Correction, Inc. (DCI) of California, and Omnistar, Inc. of Texas.
In particular, DCI currently uses FM radio stations to broadcast
the correction information while Omnistar uses a geostationary
satellite to broadcast the correction information.
[0072] A third alternative is that the cellular provider set up its
own reference stations that calculate the pseudorange corrections
for each visible satellite and broadcast them over the cellular
network. In that case, the reference stations may be part of the
base stations in the cellular network system.
[0073] For any particular application, depending on which
alternative is selected for use as the source 400, the source will
contain an appropriate receiver, such as a satellite receiver, an
FM receiver, a beacon receiver, etc. For example, if Omnistar were
used as the source, then a satellite receiver would be necessary.
It should be noted that the signal received will be in the
satellite frequency band. The typical overall satellite frequency
band includes approximately 1200-1600 MHz and 3500-4300 MHz. The
Omnistar system, for example, uses the 1551.489, 1554.497 and
1556.825 MHz frequencies for its coverage of the United States.
[0074] Furthermore, the present invention contemplates a system
that has two or more of the above-noted sources available, and
obtaining the information from one or more of those sources as
desired. For example, the system may have the ability to receive
DGPS error correction information from both a commercial supplier
and reference stations set up by the cellular provider. In that
circumstance, the source 400 would include circuitry that would
decide which source to utilize, based on, for example, availability
of each source.
[0075] In addition to receiving the differential error correction
data, the source 400 will generally also convert the data into a
standard DGPS signal, such as, for example, as defined by RTCM
SC-104 (Radio Technical Commission for Maritime Services, Special
Committee-104), which has developed international standards for
digital messaging. The DGPS signal is forwarded over a data link
405 to the cellular mobile telecommunications switching unit 410,
which is here a modified version of a mobile telecommunications
switching office (MTSO) known in the art. Data link 405 may be any
type known in the art and compatible for use with the MTSO 410.
[0076] Furthermore, relevant to this aspect of the invention, the
MTSO 410 includes a processing unit 415, a central unit 420 that is
capable of receiving data and messages from other sources, a
multiplexer 425, and a switching unit 430. Processing unit 415 is
responsible for converting the DGPS signal into a proper format for
further transmission over the cellular network. For example,
processing unit 415 may convert the received signal into a short
message by using a short message service (SMS) as defined in the
Global System for Mobile Communication (GSM) standard. GSM
represents a mobile cellular system as defined by a set of
operating standards, as introduced by the European body ETSI. For
the purposes of understanding the present invention, a short
message is essentially a data packet containing the DGPS
signal.
[0077] Central unit 420 contains other data and message sources
that are supplying information that must be forwarded by the MTSO
410. Such information would include other short messages intended
for transmission to the same base station, voice data, data traffic
for users having modems at the mobile terminal, and the like. This
other information in the central unit 420 is combined with the
short message containing the differential error correction data
using multiplexer 425 to create a combined signal.
[0078] Thereafter, the combined signal is transmitted to a
switching unit 430 of the MTSO 410 which is responsible for
switching the data to an appropriate data link 435 for forwarding
to one of numerous cellular network base stations in the cellular
system. Here, the intended base station for receiving the combined
signal is represented by reference numeral 440. Base station 440
includes a base station modulator 445 that modulates the signal,
and then transmits it through a radio interface 450 and a base
station antenna 455. It should be noted that the transmission of
the signal by the base station will be in the cellular frequency
band. The typical overall cellular frequency band includes
approximately 800-900 MHz and 1850-1990 MHz.
[0079] An analogous FIG. 4A shows the preferred arrangement. In
this arrangement the source of DGPS data 400 is connected to a
communications network 460 by a data link 406. In this view the
source of DGPS data may be, for example, a workstation or server
attached to the Internet and providing DGPS data for many base
stations in one or more mobile networks. Although this server
provides a logically separate function, it may be combined with, or
physically located in conjunction with, a part of the
communications network (for example such as an MTSO). It should be
noted that the communications network may simply be an MTSO (as
shown in FIG. 4), include an MTSO or a plurality of MTSOs with
other components, or may itself not include any MTSOs. The server
may also be operated by a third party, separate from the mobile
network operator, and located remotely from the communications
network components. The base station 440 is also connected to the
communications network through data link 436. The communications
network interconnects the source of DGPS data and the base station
and provides the similar functions to the MTSO shown in FIG. 4 of
receiving messages from the DGPS source and combining these
together with other data and messages destined for the base station
for transmission to the mobile terminals served by the base
station. Such a communications network, for example, is provided by
the Internet and the associated Internet protocols (IP) for
addressing, formatting, sending and receiving messages to devices
attached to the network.
[0080] The signal containing the short message with the DGPS error
correction data is transmitted as a radio frequency signal in the
cellular band to the mobile terminal 500, which is shown in FIG. 5.
Mobile terminal 500 generally includes a cellular antenna 505, a
radio interface 510, a digital signal processor (DSP) 512, a
central processor 515, a DGPS processor 520, a control unit 525, a
speaker 530, and a GPS receiver 550 having a GPS antenna 555.
Mobile terminal 500 receives the radio frequency signal via the
cellular antenna 505, and forwards it to the radio interface 510.
Radio interface 510 converts that signal into an intermediate
frequency (IF) signal that is compatible with the DSP 512, and
forwards the signal to the DSP 512. The DSP 512 demodulates the
signal into a data stream and forwards that data stream to the
central processor 515. Central processor 515 is responsible for
determining the contents of the data stream and forwarding the
appropriate portions of the data stream to their intended
destinations. For example, a typical data stream may contain short
messages, one of which is the DGPS error correction data, control
data, voice data, and other information.
[0081] Regarding the short message carrying DGPS error correction
data, the central processor 515 utilizes the protocol of the short
message, which use of protocol is known in the art, to extract the
error correction data out of the signal and forward it to the DGPS
processor 520. In addition, for illustrative purposes, the central
processor 515 forwards control data to the control unit 525, while
voice data may be processed and forwarded to the speaker 530. FIG.
5 does not depict all of the various destinations for all types of
data encountered by the central processor 515.
[0082] In addition, the GPS receiver 550 receives satellite signals
by the GPS antenna 555 from the GPS satellites that are in view,
and calculates the pseudoranges between the mobile terminal and
each of the GPS satellites. GPS receiver 550 forwards these
pseudoranges to the DGPS processor 520.
[0083] DGPS processor 520 utilizes the DGPS error correction data
in a conventional manner to correct the calculated pseudoranges.
The corrected ranges are forwarded back to the GPS receiver 550,
which then utilizes the information to calculate a more accurate
position of the mobile terminal 500 than could be achieved by using
GPS standing alone.
[0084] It should be noted that the cellular antenna 505 and the GPS
antenna 555 may be formed as a single antenna that is capable of
receiving both types of signals, and may use, for example, an
amplifier that responds to both cellular and satellite bands.
[0085] An alternative structure for carrying out the correction of
the pseudoranges is shown by FIGS. 6, 6A and 7. Much of the
structure remains similar as in the embodiment shown in FIGS. 4, 4A
and 5, for which analogous reference numerals have been used.
However, in this embodiment, the mobile terminal 700 has no DGPS
processor. Rather, a DGPS processor 675 is connected to a source of
DGPS data 600 by a data link 605, and is connected by another data
link 612 to a processing unit 615 (which may be a multitude of
processors) in a MTSO 610.
[0086] The remaining elements are a central unit 620, a multiplexer
625, and a switching unit 630 in the MTSO 610, a data link 635, and
a base station 640 that includes a base station modulator 645, a
radio interface 650 and a base station antenna 655.
[0087] An analogous FIG. 6A shows the preferred arrangement. In
this arrangement the source of DGPS data 600 is connected to a
communications network 660 by a data link 606. In this view the
source of DGPS data may be, for example, a workstation or server
attached to the Internet and providing DGPS data to one or more
base stations in one or more mobile networks. Similarly the DGPS
processor 675 is also attached to the communications network
through data link 614. In this view the DGPS processor may also be,
for example, a server attached to the Internet and providing DGPS
processing services for one or more base stations and their
associated mobile terminals in one or more mobile networks. The
base station 640 is also connected to the communications network
through data link 636. The communications network interconnects the
source of DGPS data, the DGPS processor and the base station and
provides the similar functions to the MTSO shown in FIG. 6 of
receiving messages from the DGPS source and combining these
together with other data and messages destined for the base station
for transmission to the mobile terminals served by the base
station. Similarly the communications network provides the function
of transporting messages reporting GPS pseudoranges received by the
base station from the mobile terminals to the DGPS processor. Such
a communications network, for example, is provided by the Internet
and the associated Internet protocols (IP) for addressing,
formatting, sending and receiving messages to devices attached to
the network.
[0088] Furthermore, the mobile terminal 700 contains a cellular
antenna 705, a radio interface 710, a digital signal processor
(DSP) 712, a central processor 715, a control unit 725, a speaker
730, and a GPS receiver 750 having a GPS antenna 755.
[0089] In this embodiment, the GPS receiver 750 receives satellite
signals by the GPS antenna 755 from the GPS satellites that are in
its view and calculates the pseudoranges between the mobile
terminal 700 and each of the GPS satellites. The results of this
determination, i.e., the calculated pseudoranges, are then
forwarded through the system to the DGPS processor 675 as described
below. It should be noted that this may be done automatically in a
continuous fashion, i.e., each time the GPS receiver 750 calculates
pseudoranges it also forwards the pseudoranges to the DGPS
processor 675. Alternatively, it may be done upon a specific
request to the GPS receiver 750.
[0090] Forwarding of the calculated pseudoranges to the DGPS
processor 675 may be carried out by forwarding the calculated
pseudoranges to the central processor 715, which multiplexes the
calculated pseudoranges into a data stream that is forwarded to the
DSP 712. The DSP 712 modulates the data stream into an IF signal
and forwards the signal to the radio interface 710. The radio
interface 710 converts the IF signal into a radio frequency signal
having a frequency that is in the cellular band. The cellular
signal is transmitted to be received at the base station 640. Base
station 640 demodulates the signal, and forwards the demodulated
signal message through the data links to the DGPS processor 675. If
the signal message passes through data link 635 to the MTSO 610,
the MTSO extracts the calculated pseudorange messages, for example,
by using the processing unit 615, and forwards them over the data
link 612 to the DGPS processor 675. If the signal message passes
through data link 636 to a communications network, it will be
received by the DGPS processor through data link 614 where the
relevant pseudorange messages may be extracted.
[0091] In addition, the source 600 provides a signal containing the
DGPS error correction data to the DGPS processor 675. Therefore,
the DGPS processor 675 will have the pseudoranges as calculated by
the GPS receiver 750, along with the necessary error correction
data from the source 600. DGPS processor 675 performs the required
corrections on the calculated pseudoranges and transmits the
corrected ranges back through the system to the GPS receiver 750.
GPS receiver 750 then calculates a more accurate position of the
mobile terminal 700 using these corrected pseudoranges.
[0092] In the alternative, DGPS processor 675 could calculate the
corrected position of the mobile and forward the position to the
mobile for display, or to the network service or other service
requesting the position. The requesting service may be a process of
the cellular network itself, or it may be a server operated by a
third party connected to the communications network.
[0093] It should be noted that in either of the embodiments shown
in FIGS. 4 (4A), 5 and FIGS. 6 (6A), 7, one or more of the elements
may be integrally formed with other elements, i.e., be formed as
part of a single unit. For example, the DGPS processor may be
integral with the central processor or the GPS receiver of the
mobile terminal. In addition, while not described in detail, the
DGPS processor may alternatively be located at the base station or
other locations and connected to the base station through the
communications network. In addition, the first aspect of the
position location system of the present invention may be used with
either a local area DGPS (LADGPS) system or a wide area DGPS
(WADGPS) system.
[0094] This aspect of the invention provides numerous advantages
over the conventional systems. For one, there is no need for a
separate beacon receiver at each mobile terminal for receiving the
DGPS error correction information from the source. That is, in the
conventional DGPS approach, each individual mobile terminal
containing a DGPS receiver/processor needs a beacon receiver for
receiving the DGPS signal. By way of the present invention, the
need for a beacon receiver at each mobile terminal is eliminated.
Rather, the mobile terminal need only have a DGPS processor to
process the DGPS signal and correct the pseudoranges calculated by
the GPS receiver. This means a reduction in the amount of structure
present in the mobile terminal, thereby allowing for a smaller size
and lower cost.
[0095] Furthermore, in the embodiment where the DGPS processor is
connected to the base station through the communications network,
there is also no need for a separate DGPS processor at each mobile
terminal. This reduces the number of DGPS processors necessary,
since there is no need to have a separate processor in each mobile
terminal. This allows for a further reduction in size and cost of
the mobile terminal without sacrificing the accuracy of position
determinations made possible using DGPS. This configuration also
reduces the message traffic between the network and the terminal by
allowing the position location determined by the DGPS processor to
be sent directly to the network service requesting the
information.
[0096] In addition, the present invention utilizes the frequency
band already dedicated for cellular use for performing additional
functions, here the transmission of DGPS error correction data.
Therefore, more efficient use of the cellular band is made with
this invention. Moreover, the cellular network provides an
efficient means for having the DGPS error correction information
transmitted to the mobile terminal. That is, in certain instances,
the mobile terminal may be located in an area where reception of
the DGPS signal from a specified reference station is not possible.
For example, the mobile terminal may not be within the proper range
of the reference station. The cellular network is generally
available in such circumstances, and it can provide the necessary
DGPS error correction information. Therefore, the user will not
lose the accuracy attributable to DGPS.
[0097] Second Aspect
[0098] Referring now to FIGS. 8 and 9, a second aspect of the
position location system of the present invention will be
described. This aspect addresses the problem described above of a
GPS receiver not having the requisite number of GPS satellites in
clear view. For example, in a typical situation, four GPS
satellites must be in clear view of the GPS receiver in order for
the GPS receiver to gain an accurate fix on its location. Of
course, the requisite number of GPS satellites that must be in view
may be less than four when the GPS receiver has one or more
components of its location already known. It is assumed for the
purposes of illustration of this embodiment that four GPS
satellites are needed, but only three of those satellites are in
clear view. This may occur, for example, when the GPS receiver is
in a city setting, such as in an urban canyon, i.e., in the shadow
of a group of tall buildings, or indoors. Of course, there are
numerous other reasons why the GPS receiver may not be receiving
signals from the requisite number of GPS satellites, which are not
listed here.
[0099] Broadly, in the present invention, a base station of the
cellular network provides a pseudosatellite signal that may be used
by the GPS receiver as a replacement for a GPS satellite that is
not in clear view. Furthermore, the base station provides the
pseudosatellite signals within the cellular band, i.e., over the
cellular network. In order to carry out this aspect of the
invention, the base station needs to be modified, along with the
mobile terminal, as described below.
[0100] Referring to FIG. 8, a base station 800 according to this
aspect of the present invention is shown. Base station 800 includes
a time standard 805 and a GPS processing unit 810. The time
standard may be an independent reference unit such as a
commercially available cesium based reference clock. Alternatively,
the cellular network infrastructure, used to synchronize base
station transmissions, may also be used to provide a time
reference. GPS processing unit 810 is programmed with location code
and is responsible for calculating pseudo GPS data representing the
base station's location and other related information. In addition,
a C/A code unit 820 provides the encoding scheme for signals in the
civilian band. It should be noted that the base station 800 does
not act as a conduit for forwarding GPS satellite information in
this aspect of the invention. Rather, the base station 800 produces
a pseudosatellite signal that is based on the programmed code in
the GPS processing unit 810 and the encoding scheme of the C/A code
unit 820. The calculated pseudo GPS data from the GPS processing
unit 810 and the C/A code generated by the C/A code unit 820 are
combined by a modulator 825 to produce a pseudosatellite signal. Of
course, other generators may replace the shown GPS processing unit
810, C/A code unit 820 and modulator 825, as long as they are able
to generate a GPS pseudosatellite signal. The pseudosatellite
signal produced has the same characteristics as a normal GPS
satellite signal.
[0101] Next, in order to place the signal into proper form for
transmission over the cellular band, a converter converts the
pseudosatellite signal to an appropriate radio frequency (RF)
signal for transmission over the cellular band. One example of a
converter is illustrated as an up converter 830, which is supplied
with a local carrier frequency, and a filter 835. In this
embodiment, the filter 835 has a 2 MHz bandwidth, with a center
frequency equal to the local carrier frequency.
[0102] In addition, the base station 800 may also be transmitting
other signals created by a cellular radio 845. These represent the
typical signals that are transmitted to the mobile terminals to
provide the typical speech and signaling services associated with
mobile phones. If both signals need to be transmitted together, it
is desirable to set the pseudosatellite signal at a level such that
it does not interfere with the remaining cellular signals that are
to be transmitted. One possible working parameter is to have the
pseudosatellite signal broadcast at a level which is at least
twenty decibels (dB) lower than the remaining cellular signals.
[0103] To carry out this level adjustment, the RF signal is passed
through a level adjuster 840 which adjusts the amplitude level of
the RF signal such that it is at some predetermined level below
that of the remaining cellular signals transmitted through the
cellular radio 845. The level of the cellular band signals from the
cellular radio 845 may vary over time depending on the cellular
system traffic or any transmit power control process utilized by
the cellular system. The level adjuster 840 may thus change the
level of the pseudosatellite signal over time (dynamically) in
response to changes in cellular radio transmissions in order to
keep the pseudosatellite signal at the maximum level possible, yet
not interfere with the cellular transmissions.
[0104] Thereafter, the signals from the cellular radio 845 and the
pseudosatellite signal (i.e., the RF signal) are combined by a
combiner 850. The combined signal is adjusted by a power amplifier
855 and then transmitted in the cellular band of the cellular
network.
[0105] It should be understood that, at this point, the base
station 800 has created the pseudosatellite signal such that it can
be broadcast, together with other cellular signals, in the cellular
band of the cellular network to the mobile terminal. The
pseudosatellite signal may be decoded by the mobile terminal to
determine the range to the base station. The mobile terminal is
described in connection with FIG. 9 below.
[0106] Turning to FIG. 9, a mobile terminal 900 according to the
second aspect of the present invention is described in detail.
Mobile terminal 900 generally contains a cellular mobile portion
905 and a GPS receiver portion 910, and a control processing unit
940. Cellular mobile portion 905 generally includes a cellular
antenna 915, a low-noise amplifier 920, a down converter 925, a
filter 930, an IF section and base band processor 935, and audio
outputs 936. The GPS receiver portion 910 generally includes a GPS
receiver antenna 945, a low-noise amplifier 950, a first down
converter 955, a first filter 960, a second down converter 965, a
second filter 970, an analog-to-digital (A/D) converter 975, and a
digital signal processor (DSP) 980. In addition, the mobile
terminal 900 contains an automatic gain controller or amplitude
controller 990 and a switch 995. As readily understood by one of
ordinary skill in the art, the particular components shown for the
cellular mobile portion and the GPS receiver portion are not
necessarily exhaustive of the components contained therein, which
any other components are generally known in the art. Furthermore,
the present invention contemplates substitutes of the components
shown that are capable of carrying out substantially the same
functions.
[0107] It should be noted that the cellular antenna 915 and the GPS
antenna 945 may be formed as a single antenna that is capable of
receiving both types of signals. This alternative has the advantage
of creating a much more compact mobile terminal. If the single
antenna is used, then the low noise amplifiers 920 and 950 may also
be combined into a single unit, with response designed for both
cellular and GPS bands, and with outputs to connect to down
converters 925 and 955.
[0108] The cellular signal broadcast from the base station 800,
which contains both the pseudosatellite signal and other cellular
signals, is received at the mobile terminal 900 by the cellular
radio antenna 915, and is then processed through the low-noise
amplifier 920, the down converter 925, and the filter 930. These
last three elements work to convert the received signal, which has
a radio frequency in the cellular band, into a signal having a
predetermined intermediate frequency (IF). Of course, any suitable
converter may replace the shown low-noise amplifier 920, down
converter 925, and filter 930. The IF signal is forwarded to the IF
section and base band processor 935, and depending on its contents,
is forwarded to the necessary destination in the mobile terminal
900. For example, voice data would be forwarded to audio outputs
936, and control data is forwarded to the control processing unit
940. In addition, the IF signal is also forwarded to the automatic
gain controller 990 and thereafter to the switch 995, at input A.
The automatic gain controller 990 adjusts the amplitude of the
pseudosatellite signal to correspond to the GPS satellite signals,
which are described below.
[0109] It should be noted that the filter 930 must be constructed
to be capable of handling pseudosatellite signals, which are
typically wider than conventional cellular signals. Based on the
present scheme of satellite signals transmitting at frequencies
described above--e.g., C/A code with 1.023 MHz rate--the filter 930
should have the requisite width. For example, a width of at least
approximately 2 MHz may be used. In that regard, it is most
practical if the cellular system have a bandwidth that is similar
to the pseudosatellite signal bandwidth. The typical bandwidth of
IS-95 CDMA cellular signals is approximately 1.25 MHz. Therefore, a
GPS pseudosatellite signal that is approximately 2 MHz wide could
underlay the center of one IS-95 channel, which would mean that it
would also overlap two adjacent channels. Other options for
transmission over the IS-95 channels could be used. The IF section
of 935 will typically include a channel selection filter to
separate the desired cellular channel from the other, overlapped,
cellular channels.
[0110] Moreover, an alternative to the filter 930 would be to have
two filters. In such a scenario, one filter handles the
pseudosatellite signals and forwards them to automatic gain
controller 990. The other filter handles the cellular signals and
forwards them to the IF section and base band processor 935. With
this dual filter arrangement, the channel selection is performed
together by the cellular section of filter 930 and the channel
selection filter section of 935.
[0111] Referring now to the GPS receiver portion 910, it receives
satellite signals by the GPS antenna 945 in the satellite frequency
band from the GPS satellites (not shown) that are in clear view.
The GPS receiver portion 910 converts those signals to correspond
to the IF signal representing the pseudosatellite signal; that is,
having the same frequency and substantially the same amplitude.
That conversion may be carried out by routing the GPS satellite
signals through the low-noise amplifier 950, the first down
converter 955, and the first filter 960. The IF signal of the GPS
satellite signals is sent to the switch 995, at input B.
[0112] The control processing unit 940 is generally responsible for
controlling the operation of the switch 995. The control processing
unit 940 can be programmed such that it normally has the switch set
to input B, but when it determines that it is not receiving the
requisite number of GPS signals, it toggles the switch to input A
to accept the pseudosatellite signal. Thus, the control processing
unit 940 utilizes as many GPS satellites that are in view, and
toggles to input A to also receive a pseudosatellite signal when
the requisite number of GPS satellites are not in view of the
mobile terminal. Those of ordinary skill in the art will recognize
that there are numerous alternatives to the structure shown for
switching between the pseudosatellite signals and the GPS satellite
signals.
[0113] Therefore, the switch 995 chooses the signal at either input
A or input B. The selected signal is then processed by another
converter here, the second down converter 965 and the second filter
970--in order to convert the signal into a baseband signal.
Thereafter, the signal is converted from an analog signal to a
digital signal by the A/D converter 975. Next, the DSP 980
processes the signal to produce a data stream. The control
processing unit 940 receives the data stream from the DSP 980 and
derives the position of the mobile terminal 900 in a manner known
in the art, including calculating pseudoranges that are used to
determine the position. It should be noted that the function of
calculating position may be carried out by a separate processing
unit that is separate from the control processing unit 940.
[0114] Thus, the control processing unit 940 is configured to
utilize the GPS signals from GPS satellites that are in view, and
when the requisite number of GPS satellites are not in view, to
also utilize one (or more) pseudosatellite signals from base
stations to substitute for the missing GPS satellite signal. That
is, a combination of GPS satellite signals and the pseudosatellite
signals are utilized to calculate position of the terminal. This
approach has the advantage of utilizing GPS, which provides the
most reliable position data, to the fullest extent possible, and
only rely on the pseudosatellite signals as necessary when GPS
alone does not provide the required information.
[0115] In the above-described embodiment, the base station may be
controlled to broadcast the pseudosatellite signal as a continuous
wave signal, i.e., in a continuous manner at all times, or may be
controlled to broadcast the signal in a burst mode--i.e.,
broadcast, for example, twenty percent of the time. If the
pseudosatellite signals are broadcast in bursts, the mobile portion
905 must synchronize its reception with the bursts which it may do
through knowledge of the burst timing. In any event, the mobile
terminal 900 receives this pseudosatellite information whenever it
is broadcast by base station 800. Therefore, it can be seen that
the broadcast of the pseudosatellite signal by the base station
800, and the receipt of the pseudosatellite signal by the cellular
mobile portion 905 in the mobile terminal 900, is carried out
without regard to whether GPS receiver portion 910 actually has
four GPS satellites in view.
[0116] However, when the GPS receiver portion 910 has four GPS
satellites in view, the pseudosatellite signal is not needed. If
nothing else is done, this signal is disregarded due to the switch
995. However, if desired, the control processing unit 940 may be
programmed to utilize the pseudosatellite signal even when the
requisite number of GPS signals are available. In such a case, the
processor 985 will have available to it five or more pseudoranges
(four or more from the GPS satellites and one or more from the base
stations). This exceeds the number of signals necessary to carry
out a position calculation, but does not negatively affect the
position calculation made, and indeed, can improve the accuracy of
the position calculation in such an over-determined system.
[0117] It should further be noted that in this embodiment of the
invention, the base station 800 need not have any GPS satellite
signal receiving capabilities itself. That is, the base station
produces a pseudosatellite signal independent of receiving and
utilizing GPS signals, and therefore it need not have the structure
necessary for receiving actual GPS satellite signals. Rather, the
base station 800 simply produces a pseudosatellite signal based on
a code programmed therein and a particular time reference, which
need not be derived from GPS.
[0118] This aspect of the invention provides an efficient scheme
for producing a pseudosatellite signal, and forwarding that signal
to the mobile terminal. The present invention accomplishes this by
using an established network--the cellular network--to carry out
these functions. By adding very little additional structure to
already existing base stations, the need to build dedicated
reference stations at strategic locations is obviated. Furthermore,
the present invention provides for more efficient use of the
cellular band by utilizing it to send additional types of
information.
[0119] Moreover, the coverage provided by the cellular network
generally includes the strategic locations where dedicated
reference stations would otherwise be placed, including at airports
and in city settings. Furthermore, the cellular network has the
advantage of providing coverage where the available number of GPS
satellite signals is likely to be insufficient, including in city
settings such as in urban canyons or indoors.
[0120] In addition, the present invention overcomes the problem
that with dedicated reference stations broadcasting pseudosatellite
signals, the pseudosatellite signal is stronger than actual GPS
satellite signals and drowns out the GPS satellite signals. In the
present invention, the pseudosatellite signal produced and
broadcast by the base station will not interfere in any way with
the actual GPS satellite signals, and therefore will not drown out
the GPS satellite signals. In fact, the two are broadcast in
completely different frequency bands. That is, the GPS satellite
signals are broadcast in the satellite band, while the base station
broadcasts the pseudosatellite signal in the cellular band.
[0121] Furthermore, when the alternative employing the switch 995
is used, then receipt of the pseudosatellite signal by the control
processing unit 940 may be completely prevented when four GPS
satellites are in view of the GPS receiver portion. Again, this is
an improvement over the conventional system as it enables the local
control to select between the pseudosatellite signals and the GPS
satellite signals that are available.
[0122] As referred to above, the second aspect of the invention may
also be expanded to where more than one base station transmits
pseudosatellite signals, and therefore, the position of the mobile
terminal 900 may be determined even when two (or more) GPS
satellites are not in view. For example, two base stations may
transmit pseudosatellite signals, such that the GPS receiver
portion 910 may accurately determine its location when only two GPS
satellites are in its view. Indeed, any combination of base
stations and satellites could be used. When no GPS satellites are
in view, then four base stations could be used to calculate
position. Of course, it may also be desirable to use as many GPS
satellites that are in view, along with all the available
pseudosatellite signals, to obtain an over-determined result that
may be more accurate.
[0123] Third Aspect
[0124] Referring now to FIGS. 10-13, a third aspect of the present
invention will be described. As with the second aspect of the
invention, this aspect also addresses the situation where the
requisite number of GPS satellites are not in view of a GPS
receiver. Broadly, in this aspect of the invention, the mobile
terminal has the capabilities of calculating its position using
GPS, and also using the cellular network infrastructure, such as by
the TOA or TDOA methods of determining location. The mobile
terminal will generally use the GPS structure to calculate
position. However, when the requisite number of GPS satellites are
not in view, then it will switch to calculating position using
either only the cellular network infrastructure, or a combination
of the GPS satellite signals available and the cellular network
infrastructure. When the requisite number of GPS satellites return
into view, then the mobile terminal switches back to relying
exclusively on GPS.
[0125] First, in FIG. 10, a mobile terminal 1000 includes a GPS
receiver portion 1005, a mobile cellular portion 1010, and a
central processor 1015. Of course, the central processor 1015 may
alternatively be formed integral with either the GPS receiver
portion 1005 or the mobile cellular portion 1010, or a single
processor can be used that performs the functions of all three
components. GPS receiver portion 1005 includes a GPS processor 1020
for calculating position, while the mobile cellular portion 1010
contains a cellular position processor 1025 that computes position
using the cellular network infrastructure. Finally, as shown in
FIG. 10, base stations 1030, 1035, and 1040 are part of the
cellular network, and for purposes of explanation, they represent
the base stations whose transmitting vicinity includes the location
of the mobile terminal 1000.
[0126] Operation of the mobile terminal 1000 for determining
position is described in connection with the flowchart in FIG. 11.
At block 1100, GPS receiver portion 1010 obtains a first fix on the
location of the mobile terminal, if necessary. Next, at block 1105,
the first fix location and the locations of three (or more) nearby
base stations are transmitted to cellular position processor 1025.
In this case, the nearby base stations are base stations 1030,
1035, and 1040. Cellular position processor 1025 utilizes this
information, along with the location of mobile terminal 1000 as
determined in step 1100, to determine the expected time difference
of arrival of periodic signals from base stations 1030, 1035, and
1040. For the purposes of this explanation, it is assumed that
periodic signals are available in the cellular network, and the
mobile terminal uses the TDOA process, described earlier, to
determine its position from the cellular network signals. However,
the cellular position processor could alternatively use a TOA
technique.
[0127] In particular, in order to calculate the TDOA of the
periodic signals from each base station 1030, 1035, 1040, the
cellular position processor 1025 calculates the distance between
the mobile terminal 1000 and each of the base stations 1030, 1035,
and 1040, using the location measurement for each item. Using the
known distances and the speed of propagation of the radio signals,
the cellular position processor 1025 calculates the expected time
difference of arrival of the signals from each base station pair.
In a completely synchronized system, the time of transmission of
the periodic signals from each base station is the same, or at some
specified time offset (which can be subtracted out). By comparing
the expected TDOA with the measured TDOA, the cellular position
processor 1025 can determine the time offset of the signals from
each base station, and use these to correct later TDOA measurements
for these base stations. In addition, the range, or distance to the
base station, may be calculated from the amount of time that it
took for the periodic signal to travel from the base station to the
mobile terminal, i.e., the propagation delay of the signal.
[0128] Therefore, knowledge of the position of mobile terminal
1000--as calculated by the GPS receiver portion 1005--helps to
determine the time offset of transmission of the periodic signals
from the base stations, which may be unknown in cellular systems
(except, for example, some CDMA systems implementing the IS-95
standard). This time offset of transmission for each base station
pair is stored in cellular position processor 1025. Alternatively,
the time offset of transmission may be stored in central processor
1015.
[0129] Thereafter, as long as the requisite number of GPS
satellites are in view of GPS receiver portion 1005, the GPS
receiver portion continues to calculate position in this manner.
Thus, in block 1110, the GPS receiver portion 1005 determines
whether the requisite number of GPS satellites are in view. If the
requisite number of GPS satellites are in view (i.e., the answer to
the inquiry is "yes"), then at block 1115, the GPS receiver portion
1005 again calculates (or simply updates) the position of the
mobile terminal using GPS. This process is generally repeated every
second or few seconds. Of course, the time between successive
calculations can be set to any desired amount. In addition, at
block 1115, the time offset of transmission of the periodic signals
from each base station may be recalculated and updated, and stored
in the cellular position processor 1025 or the central processor
1015. Then, the next step will be at block 1110, where the same
inquiry as to the requisite number of GPS satellites being in view
is made.
[0130] However, if the answer to the inquiry at block 1110 is "no,"
that is, the number of visible GPS satellites has dropped below the
requisite level (usually four), then the position determination for
the mobile terminal 1000 is switched over to the cellular position
processor 1025. This is shown at block 1120. The switching over may
be carried out by the central processor 1015. In addition, the
information of the last known position of the mobile terminal 1000
and each of the base stations 1030, 1035, and 1040, along with the
time offset of transmission of the periodic signal from the base
stations, is made available to cellular position processor 1025 if
it does not already have the information. The cellular position
processor 1025 calculates position using the periodic signals from
the base stations, as described below.
[0131] Turning to operation of the cellular position processor
1025, indicated at block 1125, the processor 1025 will measure the
TDOA for the signals from the base stations. However, because the
mobile terminal has likely since moved, the measured TDOA will
differ from the previous values after correction for the
transmission time offsets. The processor 1025 may then calculate
the new position using the new, corrected TDOA measurements as the
intersection of the hyperbolic surfaces as discussed earlier.
[0132] In essence, at block 1125, the cellular position processor
1025 uses the cellular network infrastructure to determine its
location, for example, by using either the TOA or TDOA methods for
determining location, as discussed previously. Alternatively, any
other method of determining location based on the cellular network
infrastructure can be used. In general, the point is that the
cellular position processor 1025 calculates position based on using
cellular position signals, such as the periodic signals, rather
than using GPS satellite signals. That is, the cellular position
signals do not contain GPS information and rather are independent
of GPS. Thus, this aspect is unlike the second aspect where the
pseudosatellite signals generated and forwarded by the base station
are like GPS signals.
[0133] An alternative to switching over to using a method for
determining position of the mobile terminal based exclusively on
the cellular network infrastructure, such as the TOA or TDOA
method, is to use a combination of the GPS satellite signals and
the base station periodic signals to determine location. This is
generally indicated by dotted block 1126 and the associated dotted
lines. For example, for purposes of explanation, it may be assumed
that only three GPS satellites are in view of the GPS receiver
portion. The pseudorange determined for two of these satellites
provides a distance measurement between each GPS satellite and the
mobile terminal. The third satellite signal provides a time
reference used to calculate the range to the other two satellites.
That is, the mobile terminal 1000 lies somewhere on a sphere around
the GPS satellite, having a radius equal to the distance
therebetween. In addition, calculating the distance between the
mobile terminal and one base station, the method for which is
described above, provides a third distance measurement. Therefore,
the three GPS satellites, along with the one base station, provides
three spheres whose intersection represents the location of the
mobile terminal. Of course, more than one base station may be used
to ensure that an accurate location determination is made.
[0134] In either case--using the method according to block 1125 or
block 1126--once the location determination is made, the next
inquiry at block 1130 will be the same as the inquiry made at block
1110, i.e., whether the requisite number of GPS satellites are in
view of the GPS receiver portion. If the requisite number of GPS
satellites are not in view, then the same process--in either block
1125 or block 1126--is performed to determine position. This loop
between blocks 1125 or 1126 and block 1130 continues until the
requisite number of GPS satellites come back into view.
[0135] When enough GPS satellites are again in view of the GPS
receiver portion, then the answer to the inquiry at block 1130
becomes "yes," and the system proceeds to block 1135. At block
1135, determination of position again becomes the responsibility of
GPS receiver portion 1005. This switching back to using the GPS
receiver portion for calculating position may be carried out by the
central processor 1015. In addition, at this block, determinations
of the location of the nearest base stations, and their locations
and time offsets of transmission of their periodic signals, are all
updated. In sum, the system is recalibrated just as if it started
out using GPS. Thereafter, the process is returned to block 1105,
and is repeated.
[0136] In addition, it should be noted that at block 1100, if an
initial location fix cannot be made using the GPS receiver portion
1005 because the requisite number of GPS satellites are not in
view, then the central processor 1015 can request that the cellular
position processor 1025 provide an initial location of the mobile
terminal using the conventional TDOA method of determining location
using the cellular network infrastructure, as described previously
herein. However, once the requisite number of GPS satellites come
into view of the GPS receiver portion, then the central processor
1015 will cause the system to switch over to determining location
of the mobile terminal using the GPS receiver portion.
[0137] The GPS provides good accuracy of position when its signals
are available to the mobile terminal. This may be more accurate
than methods using exclusively signals provided by the cellular
network infrastructure. Thus, it can be preferable to make use of
the GPS for determining position, and to make use of a combination
of the GPS and the cellular infrastructure when the GPS is
unavailable, or partly unavailable, to the mobile terminal. It is,
of course, preferable to make use of as many signals from both
systems as are available to develop a more accurate result than
could be obtained by working with either system exclusively.
[0138] However, a competing interest is reducing power consumption.
As shown in FIG. 10, two receivers are present in the terminal. To
save power consumption (i.e., save battery power), it may be
desirable to rely on the cellular network infrastructure to
calculate position. During this time, the GPS receiver would not be
turned on. Only when the cellular network infrastructure does not
provide the necessary signals to determine position, or if a GPS
recalibration was needed, would the GPS receiver be turned on and
utilized to determine position. This alternative approach conserves
battery power.
[0139] The alternative approach is shown in FIG. 12. At block 1200,
a first fix is calculated. Then, at block 1210, the GPS receiver is
turned off, and at block 1220, position is calculated using the
cellular network infrastructure. At block 1230, a determination is
made whether the requisite number of signals in the cellular
network infrastructure are available for calculating position
(e.g., three signals in the TDOA approach). If so, then block 1220
is repeated. If not, then at block 1240, the GPS receiver is turned
on and the position is calculated using GPS. Thereafter, the
process is repeated starting at block 1210. Alternative inquiries
at block 1230 include determining how long (in time) it has been
since the last GPS update and/or how far (in space) the mobile
terminal has moved since the last GPS update. If a predetermined
amount of time has passed, such as two minutes, or the mobile
terminal has moved a predetermined distance, such as 100 meters,
then a GPS update would be called for and the process proceeds to
block 1240. Of course, the rate at which GPS updates are needed
will depend on the conditions under which the mobile is operating,
with less frequent updates needed if the mobile terminal is in an
area where the cellular network infrastructure signals are known to
be of good accuracy.
[0140] Another approach with the third aspect for determining
position is that shown in FIG. 13. At block 1300, a first fix is
obtained. At block 1310, all the available GPS satellite signals
are received by the GPS receiver portion. Then, at block 1320, all
the available cellular positioning signals are received. At block
1330, position is calculated using all of these signals. This is an
overdetermined system, and the results may be combined. This may be
done by weighting each location measurement in the average by the
confidence, or expected error, in the measurements. Such a
technique is known as weighted average. This technique may be used
to combine together location measurements based on available GPS
signals with measurements based on cellular infrastructure signals
with appropriate regard to the accuracy of each measurement. The
weighted average may be calculated according to the following
general expression for an averaged position coordinate x
x=1/w.SIGMA.x.sub.n/.sigma..sub.n.sup.2
[0141] where x.sub.n are the measurements, .sigma..sub.n are the
expected errors, or variances, in the measurements used to weight
the average, and w=.SIGMA.1.sigma..sub.n.sup.2 is the sum of the
variances used to normalize the estimate. The summations are
performed over the total number of measurements N. This choice of
weighting factors minimizes the variance of the estimate of x.
[0142] The third aspect of the invention has numerous advantages.
For one, as with the first and second aspects of the invention,
this aspect efficiently utilizes the cellular network, and
positioning determination methods available therewith, to compute
the position of a mobile terminal when the GPS receiver does not
have the required number of GPS satellites in view. Furthermore,
the system is efficient because it utilizes the more accurate means
for determining location--GPS--whenever the requisite number of GPS
satellites are available.
[0143] In addition, there is another advantage to switching back to
GPS after the mobile terminal has been using the cellular network
infrastructure for some period of time to determine location. By
switching back to GPS, and recalibrating the system, the negative
effect of multi-path problems associated with using the TOA and
TDOA methods for determining location is reduced. That is, after
some period of time of using the TOA or TODA methods to determine
location, the mobile may have moved out of the region for which the
last calibration is appropriate. Recalibrating with the GPS enables
any errors due to the use of the cellular infrastructure signals to
be again determined and corrected.
[0144] By switching back to using GPS as soon as it becomes
feasible, the TOA or TDOA methods will be used for the absolute
minimum amount of time, which limits the effect of errors due to
multi-path problems. Using both the GPS and cellular signals, an
estimate may be made of the multi-path propagation errors in the
cellular signals.
[0145] Furthermore, another problem associated with calculating
position based on cellular network infrastructure is clock drift of
the clock in the mobile terminal. That clock drift can result in
erroneous time--and therefore location--measurements. By switching
back to GPS as soon as it becomes available, the time in which the
TOA or TDOA methods are used is relatively short. Thus, the amount
of error due to clock drift, which increases the longer the
cellular network infrastructure methods are used, can be minimized.
In addition, by measuring the rate of clock drift in the mobile
terminal using the GPS time information while the GPS signals are
available, a software routine may be implemented, for example, in
the central processor, to compensate for this clock drift when GPS
is not available at a subsequent point in time.
[0146] Fourth Aspect
[0147] A fourth aspect of the invention may be described in
connection with the structure shown in FIG. 10, and FIGS. 14-15. In
a standard IS-95 CDMA system, the pilot signal component of the
CDMA cellular base station signal can be used to augment and
improve the accuracy and availability of position location using
GPS. The CDMA cellular signal can provide the functional equivalent
of the GPS pseudorange. The advantage is that this approach
requires very little adaptation of the base station, for example,
compared to the second aspect described above. The CDMA pilot
signal component, such as used in the IS-95 standard, has the
advantage of being transmitted continuously from each base station
at a constant power level that enables it to typically be received
by a mobile terminal from more than one base station. The pilot
signal includes periodic signals that are transmitted at specified
times with specified offsets. Thus, the time difference of arrival
of two pilot signals from two base stations may be readily measured
by the mobile terminal. The IS-95 standard signalling and control
process also includes methods whereby the terminal may be
instructed to measure the TDOA of the pilot signals it can receive
and to report these measurements to the CDMA control process. Other
facets of the IS-95 pilot signals and the standard transmissions
enable a round trip delay ("RTD") measurement to be made which may
be used to determine the mobile's range, or distance, from the
serving base station. Either the RTD measurement or a plurality of
TDOA measurements may be used to obtain a cellular distance
measurement, which represents the distance between the mobile and
the serving base station.
[0148] In this approach, a base station, such as base station 1035,
transmits timing signals to the terminal. In fact, this is already
being done as part of the regular cellular system transmissions
such as according to IS-95 or GSM standards. The terminal will
receive the signals at some later time due to the delay in
transmission. The terminal will extract the timing signals from the
base station, though those signals will be offset in time due to
the propagation delay. Further, the terminal must transmit back to
the base station at defined times with respect to its receipt of
the timing signals from the base station. For example, the terminal
may be required to respond within plus or minus one microsecond
(.+-.1 .mu.s) of its specified transmission timing. The base
station then receives the return signals from the terminal at some
later time due to delay in return transmission. The total delay
measured at the base station is the RTD. The radius of the
sphere--representing the distance between the base station and the
terminal--is one half the RTD times the speed of radio signals in
air. This measurement may be generally referred to as a cellular
distance measurement. The cellular distance measurement, like the
cellular position signal of the third aspect, does not contain GPS
information and rather is independent of GPS.
[0149] Therefore, this distance measurement can be used as a
substitute for a GPS signal when the requisite number of GPS
satellite signals are not in view, or to supplement the information
available from GPS so that both GPS and cellular signals are used
together in order that a more accurate position determination may
be made than by either system operating alone. The advantage of
this approach is that the only change to the base station is the
addition of the RTD determination, which can be done by a
processor. FIG. 14 shows a typical base station 1400 with a
cellular transceiver 1410 for performing conventional functions and
a RTD processor 1420 that determines RTD.
[0150] However, errors are present in the above position estimate,
largely due to the terminal's internal delays and multi-path
propagation. While the uncertainty is generally less than 1
microsecond, this translates into an uncertainty in distance of
about 150 meters.
[0151] To compensate for this error, GPS can be used. This approach
is depicted in FIG. 15. When a requisite number of reference GPS
satellites are in view, the position of the terminal can be
determined. The reference GPS satellites need not be the same GPS
satellites that the mobile makes use of at some later point in time
when the requisite number of GPS satellites are not in view. This
determination is shown in block 1500. The position may be sent to a
position processor which may be located either in the mobile
terminal (such as 1025 in FIG. 10) or may be a server in the
network which operates to calculate positions for terminals using
the cellular and communications networks. This is shown at block
1510. In addition, it is noted that the locations of the base
stations are known. At block 1520 the position processor determines
the expected signal timing using information on the position of the
base station and the position of the mobile as determined using
GPS. For example, in the case of a system using the RTD measurement
process described earlier, the expected RTD would be calculated.
Alternatively, for a system using the TDOA measurement process
described earlier, the expected TDOA would be calculated. Then at
block 1530 a measurement is made of the actual signal timing, for
example the RTD or the TDOA. At block 1540, the difference between
the expected and the measured values is determined. The difference
is stored, at block 1550, as a cellular correction term (i.e. a RTD
or TDOA correction term) for later use by the position processor.
If the position processor is located in the mobile terminal, then
the RTD measurements, which are made in the base station, may be
sent to the mobile terminal using the standard message signalling
facilities of the cellular system. Similarly, if the position
processor is connected to the communications network, the TDOA
measurements made in the terminal may be sent to the position
processor using the standard message signalling facilities of the
cellular system. Of course, to determine the position to the best
accuracy, the position processor may make use of both TDOA and RTD
measurements in its calculations and need not be restricted to a
single measurement type.
[0152] Thereafter, when the requisite number of GPS satellite
signals are not in view at some later point in time, and position
is calculated based in part using the RTD measured at the base
station, or the TDOA measured at the terminal, the correction term
can be used to reduce the error due to the unknown delay in the
terminal as seen at the serving base station. It is contemplated
that the position location calculation be done by the network
server, or in a suitable processor in the terminal (such as central
processor 1015 or position processor 1025 in FIG. 10). In the
alternative, the cellular position processor 1025 in the mobile
terminal could measure TDOA of the cellular pilot signals and use
these to supplement the GPS position information.
[0153] It should be noted that other systems (in addition to the
IS-95 CDMA technique discussed) exist that are capable of measuring
the RTD, such as the GSM (European standard) that utilizes TDMA
(Time Division Multiple Access) cellular techniques. The same
concept of utilizing cellular distance measurements as substitutes
for missing GPS satellite signals may be readily applied in such
systems. Typically GSM systems are operated with unknown time
offsets in the transmissions from each base station. In this case
the correction terms, determined through the use of the GPS
calibrations, also compensate for the unknown time offsets of the
GSM base station transmissions.
[0154] It should be understood that while the four aspects of the
position location system of the present invention have each been
described, and may be used, separately, two or more aspects may be
combined in a single position location system. For example, the
DGPS error correction information may be provided to the mobile
terminal through the cellular network according to the first aspect
of the invention, and in that same system, a base station of the
cellular network may also provide a pseudosatellite signal. In such
a system, the control processing unit 940 would utilize the error
correction data and correct the GPS pseudoranges to obtain
corrected GPS ranges. Thus, the DGPS processor would essentially be
part of the control processing unit. Of course, a separate DGPS
processor could be used instead.
[0155] Alternatively, the first and fourth aspects could be
combined. DGPS error correction data would be provided to the
mobile terminal and the base station could provide the CDMA
cellular signals that provide the functional equivalent of the GPS
satellite signal. Another alternative is that the DGPS error
correction information be provided through the cellular network
infrastructure in a system that also switches between using GPS and
the cellular network infrastructure to determine location.
[0156] Furthermore, the position location system may have the
second, third, and fourth aspects of the invention available, and
the system merely determines which aspect to utilize when the
requisite number of GPS satellites are not in view of the GPS
receiver. The selection of the desired approach could be carried
out, for example, by a processor such as the central processor 1015
of the mobile terminal, shown in FIG. 10. Finally, all four aspects
may be combined in a single position system as well. Therefore, the
present invention is not limited to using only one aspect of the
invention in a single position location system. Rather, two or more
aspects of the invention can be used in the same position location
system.
[0157] In addition, changes to the structures as presented above do
not depart from the scope of the present invention. For example, a
mobile terminal may contain both the GPS and cellular portions in
the same mobile unit, or each component can be housed separately
with a relevant means for communication between the two. In that
regard, the term "mobile terminal" may refer to a terminal
containing either a GPS receiver or a cellular mobile terminal, or
both. Furthermore, many of the individual components may be
combined into a single unit with other components. For example, the
central processor in the third aspect of the invention may be
contained in either the GPS receiver portion or the mobile cellular
portion.
[0158] Furthermore, substitute components that provide
substantially the same functions as those disclosed also do not
depart from the scope of the invention. For example, alternate
structures known in the art for converting a received signal into
an intermediate frequency (IF) may be used rather than the
structure disclosed herein. Finally, the general components shown
for the mobile terminals, base stations, and MTSO do not
necessarily indicate that this is the only structure present in
these items. Rather, the illustration of certain components is
intended for an easier understanding of the present invention.
Finally, while the connections in the Figures, for example between
the DGPS processor and the GPS receiver in FIG. 5, are shown as
electrical connections, it should be understood that other
connections are possible, such as optical connections.
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