U.S. patent application number 12/638076 was filed with the patent office on 2010-09-09 for method and apparatus for processing a satellite positioning system signal using a cellular acquisition signal.
Invention is credited to Charles Abraham.
Application Number | 20100225537 12/638076 |
Document ID | / |
Family ID | 41430675 |
Filed Date | 2010-09-09 |
United States Patent
Application |
20100225537 |
Kind Code |
A1 |
Abraham; Charles |
September 9, 2010 |
METHOD AND APPARATUS FOR PROCESSING A SATELLITE POSITIONING SYSTEM
SIGNAL USING A CELLULAR ACQUISITION SIGNAL
Abstract
Method and apparatus for processing satellite positioning system
signals is described. In one example, assistance data is received
at a mobile receiver from a first wireless network using a wireless
transceiver. The first wireless network may be a non-synchronized
cellular network. A time synchronization signal is obtained from a
second wireless network at the mobile receiver using a wireless
receiver. A time offset is then determined in response to the time
synchronization signal. Satellite signals are processed at the
mobile receiver using the assistance data and the time offset. The
second wireless network may be a synchronized cellular network or
may be a non-synchronized cellular network that is externally
synchronized to GPS time.
Inventors: |
Abraham; Charles; (Los
Gatos, CA) |
Correspondence
Address: |
MCANDREWS HELD & MALLOY, LTD
500 WEST MADISON STREET, SUITE 3400
CHICAGO
IL
60661
US
|
Family ID: |
41430675 |
Appl. No.: |
12/638076 |
Filed: |
December 15, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10926792 |
Aug 26, 2004 |
7656350 |
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12638076 |
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09993335 |
Nov 6, 2001 |
7053824 |
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10926792 |
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Current U.S.
Class: |
342/357.49 |
Current CPC
Class: |
G01S 19/235 20130101;
G01S 19/258 20130101; G01S 19/256 20130101 |
Class at
Publication: |
342/357.49 |
International
Class: |
G01S 19/12 20100101
G01S019/12 |
Claims
1. A method of processing satellite positioning system signals,
comprising: receiving assistance data at a mobile receiver from a
first wireless network using a wireless transceiver; obtaining a
time synchronization signal from a second wireless network at said
mobile receiver using a wireless receiver; determining a time
offset in response to said time synchronization signal; and
processing satellite signals at said mobile receiver using said
assistance data and said time offset.
2. The method of claim 1, wherein said step of processing
comprises: obtaining expected pseudorange data in response to said
assistance data; determining expected code delay windows using said
expected pseudorange data and a sub-millisecond portion of said
time offset; and correlating said satellite signals in response to
said expected code delay windows to produce correlation
results.
3. The method of claim 2, further comprising: coherently averaging
said correlation results in response to said time offset to
synchronize to navigation data bits.
4. The method of claim 2, further comprising: computing
pseudoranges in response to said correlation results.
5. The method of claim 4, further comprising: sending said
pseudoranges to a server using said first wireless network and said
wireless transceiver; and locating position of said mobile receiver
at said server in response to said pseudoranges.
6. The method of claim 4, further comprising: processing satellite
trajectory data within said mobile receiver using an absolute
portion of said time offset to produce satellite position data; and
locating position of said mobile receiver in response to said
pseudoranges and said satellite position data.
7. The method of claim 1, wherein said first wireless network
comprises a non-synchronized cellular network and said second
wireless network comprises a synchronized cellular network.
8. The method of claim 1, wherein said time synchronization signal
is obtained without a subscription to said wireless network.
9. A mobile receiver, comprising: a wireless transceiver for
communicating with a first cellular network; a cellular acquisition
receiver for receiving a time synchronization signal from a second
wireless network; a satellite signal receiver for receiving
satellite signals from satellite positioning system satellites; a
local clock in communication with said satellite signal receiver;
and a processor for determining a time offset between local time
output by said local clock and satellite time output by said
satellite positioning system satellites in response to said time
synchronization signal.
10. The mobile receiver of claim 9, wherein said processor is
further configured to determine expected code delay windows using a
sub-millisecond portion of said time offset and expected
pseudorange data obtained via said wireless transceiver.
11. The mobile receiver of claim 10, further comprising: correlator
circuitry for correlating said satellite signals in response to
said expected code delay windows to produce correlation
results.
12. The mobile receiver of claim 11, wherein said correlator
circuitry is configured to coherently average said correlation
results in response to said time offset to synchronize to
navigation data bits.
13. The apparatus of claim 11, wherein said processor is further
configured to compute pseudoranges in response to said correlation
results.
14. The apparatus of claim 13, wherein said processor is further
configured to process satellite trajectory data using an absolute
portion of said time offset to produce satellite position data, and
locate position of said mobile receiver in response to said
pseudoranges and said satellite position data.
15. The apparatus of claim 9, wherein said first wireless network
comprises a non-synchronized cellular network and said second
wireless network comprises a synchronized cellular network.
16. The apparatus of claim 9, wherein said cellular acquisition
receiver is configured to obtain said time synchronization signal
without a subscription to said second wireless network.
17. A position location system, comprising: a server for providing
assistance data; a mobile receiver, including: a wireless
transceiver for communicating with said server through a first
wireless network; a cellular acquisition receiver for receiving a
time synchronization signal from a second wireless network; a
satellite signal receiver for receiving satellite signals from
satellite positioning system satellites; a local clock in
communication with said satellite signal receiver; and a processor
for determining a time offset between local time output by said
local clock and satellite time output by said satellite positioning
system satellites in response to said time synchronization
signal.
18. The system of claim 17, wherein said first wireless network
comprises a non synchronized cellular network and said second
wireless network comprises a synchronized cellular network.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of pending U.S. patent
application Ser. No. 10/926,792, filed Aug. 26, 2004, which is a
continuation-in-part of issued U.S. Pat. No. 7,053,824, issued May
30, 2006, which is incorporated by reference herein in its
entirety.
FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
[0002] [Not Applicable]
[MICROFICHE/COPYRIGHT REFERENCE]
[0003] [Not Applicable]
BACKGROUND OF THE INVENTION
[0004] Embodiments of the present invention generally relate to
satellite position location systems and, more particularly, to a
method and apparatus for receiving a global positioning system
signal using a cellular acquisition signal.
[0005] Global Positioning System (GPS) receivers use measurements
from several satellites to compute position. GPS receivers normally
determine their position by computing time delays between
transmission and reception of signals transmitted from satellites
and received by the receiver on or near the surface of the earth.
The time delays multiplied by the speed of light provide the
distance from the receiver to each of the satellites that are in
view of the receiver.
[0006] More specifically, each GPS signal available for commercial
use utilizes a direct sequence spreading signal defined by a unique
pseudo-random noise (PN) code (referred to as the coarse
acquisition (C/A) code) having a 1.023 MHz spread rate. Each PN
code bi-phase modulates a 1575.42 MHz carrier signal (referred to
as the L1 carrier) and uniquely identifies a particular satellite.
The PN code sequence length is 1023 chips, corresponding to a one
millisecond time period. One cycle of 1023 chips is called a PN
frame or epoch.
[0007] GPS receivers determine the time delays between transmission
and reception of the signals by comparing time shifts between the
received PN code signal sequence and internally generated PN signal
sequences. These measured time delays are referred to as
"sub-millisecond pseudoranges", since they are known modulo the 1
millisecond PN frame boundaries. By resolving the integer number of
milliseconds associated with each delay to each satellite, then one
has true, unambiguous, pseudoranges. A set of four pseudoranges
together with a knowledge of absolute times of transmission of the
GPS signals and satellite positions in relation to these absolute
times is sufficient to solve for the position of the GPS receiver.
The absolute times of transmission (or reception) are needed in
order to determine the positions of the GPS satellites at the times
of transmission and hence to compute the position of the GPS
receiver.
[0008] Accordingly, each of the GPS satellites broadcasts a model
of satellite orbit and clock data known as the satellite navigation
message. The satellite navigation message is a 50 bit-per-second
(bps) data stream that is modulo-2 added to the PN code with bit
boundaries aligned with the beginning of a PN frame. There are
exactly 20 PN frames per data bit period (20 milliseconds). The
satellite navigation message includes satellite-positioning data,
known as "ephemeris" data, which identifies the satellites and
their orbits, as well as absolute time information (also referred
to herein as "GPS time" or "time-of-day") associated with the
satellite signal. The absolute time information is in the form of a
second of the week signal, referred to as time-of-week (TOW). This
absolute time signal allows the receiver to unambiguously determine
a time tag for when each received signal was transmitted by each
satellite.
[0009] GPS satellites move at approximately 3.9 km/s, and thus the
range of the satellite, observed from the earth, changes at a rate
of at most 800 m/s. Absolute timing errors result in range errors
of up to 0.8 m for each millisecond of timing error. These range
errors produce a similarly sized error in the GPS receiver
position. Hence, absolute time accuracy of 10 ms is sufficient for
position accuracy of approximately 10 m. Absolute timing errors of
much more than 10 ms will result in large position errors, and so
typical GPS receivers have required absolute time to approximately
10 milliseconds accuracy or better.
[0010] Another time parameter closely associated with GPS
positioning is the sub-millisecond offset in the time reference
used to measure the sub-millisecond pseudorange. This offset
affects all the measurements equally, and for this reason it is
known as the "common mode error". The common mode error should not
be confused with the absolute time error. As discussed above, an
absolute time error of 1 millisecond leads to range errors of up to
0.8 meters while an absolute time error of 1 microsecond would
cause an almost unobservable range error of less than 1 millimeter.
A common mode error of 1 microsecond, however, results in a
pseudorange error of 1 microsecond multiplied by the speed of light
(i.e., 300 meters). Common mode errors have a large effect on
pseudorange computations, and it is, in practice, very difficult to
calibrate the common mode error. As such, traditional GPS receivers
treat the common mode error as an unknown that must be solved for,
along with position, once a sufficient number of pseudoranges have
been measured at a particular receiver.
[0011] In some GPS applications, the signal strengths of the
satellite signals are so low that either the received signals
cannot be processed, or the time required to process the signals is
excessive. As such, to improve the signal processing, a GPS
receiver may receive assistance data from a network to assist in
satellite signal acquisition and/or processing. For example, the
GPS receiver may be integrated within a cellular telephone and may
receive the assistance data from a server using a wireless
communication network. This technique of providing assistance data
to a remote receiver has become known as "Assisted-GPS" or
A-GPS.
[0012] In some A-GPS systems, the wireless communication network
that provides the assistance data is not synchronized to GPS time.
Such non-synchronized networks include time division multiple
access (TDMA) networks, such as GSM networks, universal mobile
telecommunications system (UMTS) networks, North American TDMA
networks (e.g., IS-136), and personal digital cellular (PDC)
networks. In such systems, the GPS receiver cannot synchronize to
GPS time without receiving and decoding TOW information from the
satellites signals. In low signal-to-noise ratio environments, TOW
information is difficult, if not impossible, to decode. Without
accurate time-of-day information, the GPS receiver cannot provide
an accurate time-tag for its measurements, thereby deleteriously
affecting the accuracy of the position computed by the network.
[0013] Accordingly, there exists a need in the art for an A-GPS
mobile receiver for non-synchronized communication networks capable
of synchronizing to GPS time.
BRIEF SUMMARY OF THE INVENTION
[0014] Method and apparatus for processing satellite positioning
system signals is described. In one embodiment, assistance data is
received at a mobile receiver from a first wireless network using a
wireless transceiver. The assistance data may comprise acquisition
assistance data (e.g., expected pseudorange data), satellite
trajectory data (e.g., satellite ephemeris), or both. The first
wireless network may be a non-synchronized cellular network. A time
synchronization signal is obtained from a second wireless network
at the mobile receiver using a wireless receiver. A time offset is
then determined in response to the time synchronization signal.
Satellite signals are processed at the mobile receiver using the
assistance data and the time offset. The second wireless network
may be a synchronized cellular network (e.g., a CDMA network) or
may be a non-synchronized cellular network that is externally
synchronized to GPS time (e.g., a GSM network having location
measurement units (LMUs)). The mobile receiver is thus configured
to receive the time synchronization signal without a subscription
to the second wireless network, which eliminates fees for such a
subscription. In addition, the circuitry required for the
receive-only front end is less complex and less costly than that
required for a full transceiver.
BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS
[0015] So that the manner in which the above recited features of
the present invention can be understood in detail, a more
particular description of the invention, briefly summarized above,
may be had by reference to embodiments, some of which are
illustrated in the appended drawings. It is to be noted, however,
that the appended drawings illustrate only typical embodiments of
this invention and are therefore not to be considered limiting of
its scope, for the invention may admit to other equally effective
embodiments.
[0016] FIG. 1 is a block diagram depicting an exemplary embodiment
of a position location system;
[0017] FIG. 2 is a block diagram depicting an exemplary embodiment
of a mobile receiver constructed in accordance with the
invention;
[0018] FIG. 3 is a flow diagram depicting an exemplary embodiment
of a method for processing satellite positioning system signals in
accordance with the invention; and
[0019] FIG. 4 is a flow diagram depicting another exemplary
embodiment of a method for processing satellite signals in a mobile
receiver in accordance with the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0020] A method and apparatus for processing satellite positioning
system signals is described. Those skilled in the art will
appreciate that the invention may be used with various types of
mobile or wireless devices that are "location-enabled," such as
cellular telephones, pagers, laptop computers, personal digital
assistants (PDAs), and like type wireless devices known in the art.
Generally, a location-enabled mobile device is facilitated by
including in the device the capability of processing satellite
positioning system (SPS) satellite signals, such as Global
Positioning System (GPS) signals.
[0021] FIG. 1 is a block diagram depicting an exemplary embodiment
of a position location system 100. The system 100 comprises a
mobile receiver 102 in communication with a server 104 via a
wireless communication network 106 (e.g., a cellular telephone
network). The server 104 may be disposed in a serving mobile
location center (SMLC) of the wireless communication network 106.
The mobile receiver 102 obtains satellite measurement data with
respect to a plurality of satellites 110 (e.g., pseudoranges,
Doppler measurements). The server 104 obtains satellite navigation
data for at least the satellites 110 (e.g., orbit trajectory
information, such as ephemeris). Position information for the
mobile receiver 102 is computed using the satellite measurement
data and the satellite navigation data.
[0022] Satellite navigation data, such as ephemeris for at least
the satellites 110, may be collected by a network of tracking
stations ("reference network 120"). The reference network 120 may
include several tracking stations that collect satellite navigation
data from all the satellites in the constellation, or a few
tracking stations, or a single tracking station that only collects
satellite navigation data for a particular region of the world. An
exemplary system for collecting and distributing ephemeris is
described in commonly-assigned U.S. Pat. No. 6,411,892, issued Jun.
25, 2002, which is incorporated by reference herein in its
entirety. The reference network 120 may provide the collected
satellite navigation data to the server 104.
[0023] The mobile receiver 102 is configured to receive assistance
data from the server 104. In one embodiment, the assistance data
comprises acquisition assistance data. For example, the mobile
receiver 102 may request and receive acquisition assistance data
from the server 104 and send satellite measurement data to the
server 104 along with a time-tag. The server 104 then locates
position of the mobile receiver 102 (referred to as the mobile
station assisted or "MS-assisted" configuration). Acquisition
assistance data may be computed by the server 104 using satellite
trajectory data (e.g., ephemeris or other satellite trajectory
model) and an approximate position of the mobile receiver 102. An
approximate position of the mobile receiver 102 may be obtained
using various position estimation techniques known in the art,
including use of transitions between base stations of the wireless
communication network 106, use of a last known location of the
mobile receiver 102, use of a location of a base station of the
wireless communication network 106 in communication with the mobile
receiver 102, use of a location of the wireless communication
network 106 as identified by a network ID, or use of a location of
a cell site of the wireless communication network 106 in which the
mobile receiver 102 is operating as identified by a cell ID.
[0024] The acquisition assistance data includes expected
pseudorange data. In one embodiment of the invention, the
acquisition assistance data includes expected pseudoranges from the
satellites 110 to an assumed position of the mobile receiver 102
(approximate position) at an assumed time-of-day. The expected
pseudoranges may be computed using the satellite trajectory data.
The details of such computations are well known in the art and, for
purposes of clarity, are not repeated herein. In one embodiment,
the expected pseudoranges are derived from a model that is valid
over specified period of time ("pseudorange model"). The mobile
receiver 102 may apply a time-of-day to the pseudorange model to
extract appropriate expected pseudorange parameters. Exemplary
processes for forming pseudorange models as acquisition assistance
data are described in commonly-assigned U.S. Pat. No. 6,453,237,
issued Sep. 17, 2002, which is incorporated by reference herein in
its entirety. The expected pseudoranges or a pseudorange model may
be computed by the server 104 and transmitted to the mobile
receiver 102 upon request. Alternatively, if the mobile receiver
102 has obtained satellite trajectory data and an approximate
position, the mobile receiver 102 may compute the expected
pseudoranges or pseudorange model. That is, the mobile receiver 102
may compute expected pseudoranges or a pseudorange model using the
same computation as that performed by the server 104.
[0025] In one embodiment, the acquisition assistance data may be
formatted as described in ETSI TS 101 527 (3GPP TS 4.31), which is
shown below in Table 1. Notably, the acquisition assistance data
defined in 3GPP TS 4.31 may include a satellite vehicle identifier
(SVID), zeroth and first order Doppler terms, a Doppler
uncertainty, an expected code phase (e.g., sub-millisecond
pseudorange), an integer code phase, a code phase search window,
and expected azimuth and elevation data. The range of possible
values and associated resolutions are shown for each of the
parameters.
TABLE-US-00001 TABLE 1 Parameter Range Resolution SVID/PRNID 1-64
(0-63) n/a Doppler (0.sup.th order term) -5,120 Hz to 5,117.5 Hz
2.5 Hz Doppler (1.sup.st order term) -1-0.5 n/a Doppler Uncertainty
12.5 Hz-200 Hz n/a [2.sup.-n(200) Hz, n = 0-4] Code Phase 0-1022
chips 1 chip Integer Code Phase 0-19 1 C/A period GPS Bit number
0-3 n/a Code Phase Search 1-192 chips n/a Window Azimuth 0-348.75
deg 11.25 deg Elevation 0-78.75 deg 11.25 deg
[0026] In another embodiment, the assistance data comprises
satellite trajectory data (e.g., ephemeris, Almanac, or some other
orbit model). Upon request, the server 104 may transmit satellite
trajectory data to the mobile receiver 102 via the wireless
communication network 106. Alternatively, the mobile receiver 102
may receive satellite trajectory data via a communications network
122 (e.g., a computer network, such as the Internet). Notably, the
satellite trajectory data may comprise a long term satellite
trajectory model, as described in commonly-assigned U.S. Pat. No.
6,560,534, issued May 6, 2003, which is incorporated by reference
herein in its entirety. Having received the satellite trajectory
data, the mobile receiver 102 may locate its own position using the
satellite measurement data (referred to as the "MS-Based"
configuration). In addition, the mobile receiver 102 may compute
its own acquisition assistance data (described above) using the
satellite trajectory data.
[0027] The server 104 illustratively comprises an input/output
(I/O) interface 112, a central processing unit (CPU) 114, support
circuits 116, and a memory 118. The CPU 114 is coupled to the
memory 118 and the support circuits 116. The memory 118 may be
random access memory, read only memory, removable storage, hard
disc storage, or any combination of such memory devices. The
support circuits 116 include conventional cache, power supplies,
clock circuits, data registers, I/O interfaces, and the like to
facilitate operation of the server 104. The I/O interface 112 is
configured to receive satellite navigation data from the reference
network 120. The I/O interface 112 is also configured for
communication with the wireless communication network 106. Various
processes and methods described herein may be implemented using
software stored in the memory 118 for execution by the CPU 114.
Alternatively, the server 104 may implement such processes and
methods in hardware or a combination of software and hardware,
including any number of processors independently executing various
programs and dedicated hardware, such as application specific
integrated circuits (ASICs), field programmable gate arrays
(FPGAs), and the like.
[0028] The mobile receiver 102 is also configured to receive one or
more cellular broadcast signals 123 as a form of one-way
communication from a cellular base station 108 of a cellular
network 122. Notably, the base station 108 is configured to
broadcast a time synchronization signal to enable the mobile
receiver 102 to synchronize to the base station 108 as a first step
in establishing communication with the cellular network 122. While
the mobile receiver 102 is not configured to establish
communication with the cellular network 122 (e.g., the mobile
receiver 102 may not have a subscription to the cellular network
122), the broadcast time synchronization signal may be used as a
source of time. For purposes of clarity by example, the network 122
is described as being a cellular network. It is to be understood,
however, that the network 122 may comprise other types of wireless
networks that broadcast acquisition signals including a time
synchronization signal.
[0029] In one embodiment, the time synchronization signal includes
a timing message that is related to GPS time. The timing message
may be related absolutely to GPS time (e.g., the timing message may
be a system time message) or may be related to a sub-millisecond
portion of GPS time (e.g., the timing message may be a frame number
and information relating the frame number to GPS time). In either
case, information from the time synchronization signal may be used
to establish a timing reference for the mobile receiver 102.
[0030] For example, the base station 108 may employ a separate
synchronization channel for broadcasting a time message that
contains the system time relative to timing markers in the
synchronization channel. The system time may be equivalent to GPS
time or may have some known relationship to GPS time. The mobile
receiver 102 may derive the system time from the time
synchronization signal and determine a time offset GPS time and
time provided by a local clock. The time offset may be used to
calibrate the local clock circuits within the mobile receiver 102,
or may be used to compensate for local clock error while
processing. In one embodiment, the computed time offset is further
compensated to account for the distance of the mobile receiver 102
from the base station 108. This compensation makes use of a
measurement of the round trip signal delay between base station 108
and the mobile receiver 102. In this manner, the mobile receiver
102 may be synchronized to GPS time.
[0031] An exemplary cellular communication network that employs
such a timing synchronization signal is the North American CDMA
(code division multiple access) standard (IS-95). The IS-95 system
employs a separate 26.67 millisecond synchronization channel that
is spread using a PN sequence of 215 chips. Additionally, the
synchronization channel is modulated with a particular Walsh code,
allowing it to be separated from paging and traffic channels using
different Walsh codes. The synchronization channel carries a
message containing a time of day relative to the frame boundaries
of the synchronization channel ("CDMA system time"). The CDMA
system time is precisely related to GPS time. In one embodiment, to
accurately determine GPS time from the CDMA system time, the CDMA
time obtained from the synchronization channel is adjusted to
remove an offset that is added by the delay in the transmission of
the CDMA system time from the base station 108 to the mobile
receiver 102. This adjustment is made by measuring the round-trip
delay for a signal being transmitted from the mobile receiver 102
to the base station 108 and back. The synchronization channel
structure for the IS-95 CDMA system is well known in the art. For
purposes of clarity by example, aspects of the invention are
described with respect to an IS-95 CDMA system. It is to be
understood, however, that the invention may be used with other
types of synchronized cellular communication networks that provide
time synchronization signals, such as CDMA-2000, W-CDMA, and the
like.
[0032] The present invention may also be used with non-synchronized
cellular communication systems that include a mechanism for
relating a non-synchronized system time to GPS time, such as global
system for mobile communication (GSM), universal mobile
telecommunications system (UMTS), North American time division
multiple access (TDMA) (e.g., IS-136), and personal digital
cellular (PDC) networks. That is, the cellular network 122 may be a
non-synchronized cellular network. For example, in a GSM system,
the time synchronization signal comprises a synchronization burst
periodically transmitted by the base station 108 and a timing
message that provides a GSM time stamp associated with the
synchronization burst. In some GSM networks, GSM time is not
synchronized to GPS time. However, such networks may include
location measurement units (LMUs). As is well known in the art, an
LMU includes a GPS receiver, which is used to receive and decode
time information (TOW) from the satellites in view of one or more
base stations. The LMU then computes an offset value between GPS
time and the time as known by the base station(s) that are near the
LMU ("air-interface timing"). The offset is provided to the base
station(s) for use in relating the air-interface timing to GPS
time. Notably, the base station 108 may transmit an offset between
its air-interface timing and GPS time to the mobile receiver 102.
For example, the offset may be supplied to the mobile receiver 102
as part of an acquisition assistance data exchange as defined in
3GPP TS 4.31.
[0033] FIG. 2 is a block diagram depicting an exemplary embodiment
of a mobile receiver 102 constructed in accordance with the
invention. The mobile receiver 102 comprises a GPS receiver 203, a
cellular acquisition receiver 205 (also referred to as a wireless
receiver), a wireless transceiver 210, a processor 218, a memory
220, and a local time keeping counter 222 (also referred to as a
local clock). The GPS receiver 203 comprises a GPS front end 208
and a GPS baseband processor 210. The GPS front end 208 filters and
downconverts satellite signals received by an antenna 202 to
produce a near baseband (e.g., intermediate frequency) or baseband
signal. The GPS baseband processor 203 processes output from the
GPS front end 208 to produce measurement data. The GPS baseband
processor 203 uses a time reference generated by the local time
keeping counter 222. Notably, the GPS baseband processor 203
includes correlator circuitry 226 for correlating satellite signals
with corresponding reference codes to produce correlation results.
Operation of the GPS front end 208, the GPS baseband processor 203,
and the correlator circuitry 226 is well known in the art. For a
detailed understanding of the GPS receiver 203, the reader is
referred to commonly assigned U.S. Pat. No. 6,453,237, cited
above.
[0034] The cellular acquisition receiver 205 comprises a cellular
acquisition front end 212 and a cellular acquisition baseband
processor 214. The cellular acquisition front end 212 receives
cellular acquisition signals (e.g., time synchronization signal)
via an antenna 204. The cellular acquisition baseband processor 214
locks and decodes the cellular acquisition signals using, for
example, conventional digital processing techniques that are well
known in the design of cellular telephones. The cellular
acquisition receiver 205 is configured to only receive broadcast
cellular acquisition signals.
[0035] Notably, in an IS-95 CDMA compatible environment, the
cellular acquisition receiver 205 detects a pilot channel of a
nearby base station (e.g., the base station 108 of FIG. 1) and then
proceeds to decode a synchronization channel broadcast by the base
station. The cellular acquisition receiver 205 achieves
synchronization to the framing of the synchronization channel and
receives a time message containing a time of day relative to the
frame boundaries. Since the time of day derived from the
synchronization channel is related to GPS time used by the GPS
satellites, the processor 218 may derive a time offset between GPS
time and time provided by the local time keeping counter 222. The
time offset may be further compensated for the round-trip delay of
a signal communicated between the cellular acquisition receiver 205
and the base station 108. The processor 218 may calibrate the local
time keeping counter 222 using the time offset. Alternatively, the
processor 218 may provide the time offset to the GPS baseband
processor 210 so that the GPS baseband processor 210 can compensate
for clock error in the local time keeping counter 222.
[0036] In a GSM compatible environment, the mobile receiver 102
receives a time signal from the base station 108 that relates the
air-interface timing of the base station 108 to GPS time. The
cellular acquisition receiver 205 achieves synchronization to the
framing of the GSM signal and receives a GSM time message
containing a time of day relative to the frame boundaries. The
processor 218 derives a time offset between GPS time and time
provided by the local time keeping counter 222 using the time
offset between the air-interface timing and GPS time. The processor
218 may calibrate the local time keeping counter 222 using the time
offset. Alternatively, the processor 218 may provide the time
offset to the GPS baseband processor 210 so that the GPS baseband
processor 210 can compensate for clock error in the local time
keeping counter 222. In either the CDMA or GSM environments, the
mobile device 102 may use a cellular acquisition signal broadcast
by the base station 108 to precisely track GPS time, typically to
within a few microseconds.
[0037] The wireless transceiver 204 processes cellular signals
received by an antenna 206. The wireless transceiver 204 is
configured for two-way communication with a cellular network.
Notably, the wireless transceiver 204 may be used to request and
receive assistance data from the server 104 through the cellular
network 106. The mobile receiver 102 may include a modem 224 or
other type of communications transceiver for receiving data (e.g.,
satellite trajectory data) from a separate communications link,
such as the Internet. The processor 218 may comprise a
microprocessor, instruction-set processor (e.g., a
microcontroller), or like type processing element known in the art.
The processor 218 is coupled to the memory 220. The memory 220 may
be random access memory, read only memory, removable storage, hard
disc storage, or any combination of such memory devices. Various
processes and methods described herein may be implemented using
software stored in the memory 220 for execution by the processor
218. Alternatively, the mobile receiver 102 may implement such
processes and methods in hardware or a combination of software and
hardware, including any number of processors independently
executing various programs and dedicated hardware, such as ASICs,
FPGAs, and the like.
[0038] FIG. 3 is a flow diagram depicting an exemplary embodiment
of a method 300 for processing satellite positioning system signals
in accordance with the invention. Aspects of the method 300 may be
understood with simultaneous reference to FIGS. 1-3. The method 300
begins at step 302, where assistance data is received at the mobile
receiver 102. At step 304, a time synchronization signal is
obtained at the mobile receiver 102. The time synchronization
signal is obtained using the receive-only cellular acquisition
receiver 205. Thus, the mobile receiver 102 does not require a
subscription to the cellular network 122 and may employ less
complex and costly circuitry as compared to a full communication
transceiver.
[0039] At step 306, a time offset is determined using the time
synchronization signal. At step 308, expected pseudorange data is
obtained or computed from the assistance data received at step 302.
In one embodiment, expected pseudorange data may be extracted from
acquisition assistance data (e.g., a 3GPP TS 4.31 an acquisition
assistance message). In another embodiment, expected pseudorange
data may be computed within the mobile receiver 102 using satellite
trajectory data and an approximate location of the mobile receiver
102.
[0040] At step 310, expected code delay windows are determined
using the expected pseudorange data and the time offset. Notably,
the expected pseudoranges are used to provide a code delay window
within which satellite signal acquisition is expected. If the local
time keeping counter 222 is not calibrated to GPS time, the timing
of the locally generated C/A code within the GPS baseband processor
210 is arbitrary relative to the satellite signals. In other words,
there is an uncertainty component in the expected delay windows
computed from the expected pseudorange data caused by the local
clock error (common mode error). The time offset may be used to
solve for this uncertainty component. In one embodiment, the time
offset may be used in conjunction with the local time keeping
counter 222 to program the starting point of locally generated
reference codes relative to GPS time in order to solve for the
uncertainty component. In another embodiment, the time offset may
be used to calibrate the local time keeping counter 222
directly.
[0041] At step 312, satellite signals are correlated within the
expected code delay windows. The correlation process is well known
in the art. Optionally, the time offset determined at step 306 may
be used by the mobile receiver 102 to improve a coherent averaging
process performed by the correlator circuitry 226 of the GPS
baseband processor 210. As is well known in the art, coherent
averaging improves signal-to-noise ratio by averaging correlation
results over a particular interval. The effectiveness of the
coherently averaging process may be limited due to the navigation
data bits that modulate the PN codes of the satellite signals.
Specifically, due to the navigation data bits, a GPS signal
undergoes a potential 180 degree phase transition every 20 C/A code
cycles. The coherent averaging process should be synchronized to
the navigation data bit timing, otherwise changing data bits may
partially defeat such an averaging process. Thus, in one
embodiment, the time offset computed at step 306 may be used in
conjunction with the local time keeping counter 222 to control the
start and stop times of coherent averaging to make the coherent
averaging intervals coincident with incoming navigation data
bits.
[0042] FIG. 4 is a flow diagram depicting another exemplary
embodiment of a method 400 for processing satellite signals in a
mobile receiver in accordance with the invention. Aspects of the
method 400 may be understood with simultaneous reference to FIGS.
1, 2, and 4. The method 400 begins at step 402, where a time
synchronization signal is received at the mobile receiver 102. The
time synchronization signal is obtained using the receive-only
cellular acquisition receiver 205. Thus, the mobile receiver 102
does not require a subscription to the cellular network 122 and may
employ less complex and costly circuitry as compared to a full
communication transceiver.
[0043] At step 404, a time offset is determined. At step 406,
satellite trajectory data is obtained at the mobile receiver 102.
For example, the satellite trajectory data may be obtained from the
server 104 via the cellular network 106 or the communication
network 122. At step 408, a time of day is determined using an
absolute component of the time offset determined at step 404.
Notably, the absolute component of the time offset may be used in
conjunction with the local time keeping counter 222 to provide time
of day. At step 410, the satellite trajectory data is processed
using the time of day to produce satellite position information. At
step 412, pseudoranges are obtained by the mobile receiver 102. At
step 414, position of the mobile receiver 102 is located using the
pseudoranges and the satellite position information.
[0044] In the preceding discussion, the invention has been
described with reference to application upon the United States
Global Positioning System (GPS). It should be evident, however,
that these methods are equally applicable to similar satellite
systems, and in particular, the Russian GLONASS system, the
European GALILEO system, combinations of these systems with one
another, and combinations of these systems and other satellites
providing similar signals, such as the wide area augmentation
system (WAAS) and SBAS that provide GPS-like signals. The term
"GPS" used herein includes such alternative satellite positioning
systems, including the Russian GLONASS system, the European GALILEO
system, the WAAS system, and the SBAS system, as well as
combinations thereof.
[0045] While the foregoing is directed to embodiments of the
present invention, other and further embodiments of the invention
may be devised without departing from the basic scope thereof, and
the scope thereof is determined by the claims that follow.
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