U.S. patent number RE35,498 [Application Number 08/250,782] was granted by the patent office on 1997-04-29 for vehicle location system.
This patent grant is currently assigned to U.S. Philips Corporation. Invention is credited to Michael E. Barnard.
United States Patent |
RE35,498 |
Barnard |
April 29, 1997 |
Vehicle location system
Abstract
Signals from a number of NAVSTAR global positioning system (GPS)
satellites (11,12,13,14) are received by a receiver (16) in a
vehicle (15) and a segment of the signals is stored in a memory
(18) prior to retransmission by a transmitter (19). A base station
(35) receives these transmissions from the mobile unit using a
first receiver (36). The base station also receives signals
directly from the NAVSTAR GPS satellites using a second receiver
(38). A control and calculating apparatus (37) within the base
station can determine the ephemeris (course) information for the
satellites and can measure the transmission times or propagation
delays of signals between the satellites and the vehicle and with
this information the control and calculating apparatus can
calculate the position of the vehicle unit.
Inventors: |
Barnard; Michael E. (Reigate,
GB) |
Assignee: |
U.S. Philips Corporation (New
York, NY)
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Family
ID: |
10671717 |
Appl.
No.: |
08/250,782 |
Filed: |
May 27, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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Reissue of: |
644792 |
Jan 23, 1991 |
05119102 |
Jun 2, 1992 |
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Foreign Application Priority Data
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Feb 28, 1990 [GB] |
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9004433 |
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Current U.S.
Class: |
342/357.46;
342/457 |
Current CPC
Class: |
G01S
5/0036 (20130101); G01S 19/09 (20130101); G01S
19/41 (20130101) |
Current International
Class: |
G01S
5/14 (20060101); G01S 5/00 (20060101); H04B
007/185 (); G01S 005/02 () |
Field of
Search: |
;342/357,457,387
;364/460,449 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0133807 |
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Mar 1985 |
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EP |
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0250211 |
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Dec 1987 |
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EP |
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0545636 |
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Jun 1993 |
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EP |
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8905460 |
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Jun 1989 |
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GB |
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Other References
BD. Nordwall, "GPS Could Improve Sonobuoys, Radiosondes", Aviation
Week & Space Technology, Oct. 18, 1993, pp. 71 and 73. .
McConnell et al, "GPS Translator Tracking System Implementations at
Test Ranges", IEEE 1983 National Telesystems Conference, Nov.
14-16, 1983, San Francisco, CA, pp. 239-245, 239 and 243, Figs. 1,
6 and 7. .
Wells, "Translated GPS Real-Time Tracking", IEEE 1983 National
Telesystems Conference, Nov. 14-16, 1983, San Francisco, CA, pp.
260-264..
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Primary Examiner: Blum; Theodore. M.
Attorney, Agent or Firm: Gathman; Laurie E.
Claims
I claim:
1. A vehicle location system for use in a global positioning system
(GPS), comprising at least one vehicle mounted equipment including
means for receiving signals directly from the GFS, a fixedly sited
base station including first means for receiving signals directly
from the GPS, characterised in that the vehicle mounted equipment
includes means for recording the received GPS signals and means for
retransmitting the recorded GPS signals to the base station, and in
that the fixedly sited base station includes second means for
receiving the recorded GPS signals retransmitted by the vehicle
mounted equipment, and position determining means coupled to the
first and second receiving means for determining the position of
the vehicle at the time when the vehicle mounted equipment received
the GPS signals.
2. A vehicle location system as claimed in claim 1, wherein the
vehicle mounted equipment transmits a time of arrival (TOA) signal
in addition to retransmitting the recorded GPS signals.
3. A vehicle location system as claimed in claim 1 wherein the rate
at which the vehicle mounted equipment retransmits the GPS signals
is lower than that at which the signals were recorded.
4. A vehicle location system as claimed in claim 3, wherein the
vehicle mounted equipment further comprises control means coupled
to the recording means such that signals from the control means
cause the recording means to record the GPS signals at preset
intervals.
5. A vehicle location system as claimed in claim 2, wherein the GPS
is the satellite-based NAVSTAR GPS, and in that the base station
has means for obtaining the GPS ephemeris for the satellites in
use.
6. A vehicle location system as claimed in claim 5, wherein the
means for obtaining the GPS ephemeris in the base station has means
for despreading the NAVSTAR GPS signals without using any locally
generated pseudo random noise codes.
7. A vehicle mounted equipment for use .Iadd.in a global
positioning system (GPS) .Iaddend.with .[.the system as claimed in
claim 1.]. .Iadd.a vehicle location system and .Iaddend.including
means for receiving .[.the.]. GPS signals, .[.wherein the equipment
also includes.]. .Iadd.the vehicle location system comprising a
fixedly sited base station including first means for receiving
signals directly from the GPS, second means for receiving recorded
GPS signals transmitted by the vehicle mounted equipment, and
position determining means coupled to the first and second
receiving means for determining the position of the vehicle at the
time when the vehicle mounted equipment received the GPS signals,
said vehicle mounted equipment including .Iaddend.means for
recording the received GPS signals and means for retransmitting the
recorded GPS signals.
8. A vehicle mounted equipment for use .Iadd.in a global
positioning system (GPS) .Iaddend.with .[.the system as claimed in
claim 2.]. .Iadd.a vehicle location system and .Iaddend.including
means for receiving .[.the.]. GPS signals, .[.wherein the equipment
includes.]. .Iadd.the vehicle location system comprising a fixedly
sited base station including first means for receiving signals
directly from the GPS, second means for receiving recorded GPS
signals transmitted by the vehicle mounted equipment, and position
determining means coupled to the first and second receiving means
for determining the position of the vehicle at the time when the
vehicle mounted equipment received the GPS signals, said vehicle
mounted equipment including .Iaddend.means for recording the
received GPS signals and means for retransmitting the recorded GPS
signals and a time of arrival (TOA) signal.
9. A fixedly sited base station for use .Iadd.in a global
positioning system (GPS) .Iaddend.with .[.the system as claimed in
claim 1.]. .Iadd.a vehicle location system comprising at least one
vehicle mounted equipment including means for receiving signals
directly from the GPS, means for recording the received GPS
signals, and means for retransmitting the recorded GPS signals to
the base station, said fixedly sited base station
.Iaddend.including first means for receiving GPS signals directly
from the GPS, .[.wherein the base station also includes.]. second
means for receiving a retransmission of GPS signals from .[.a.].
.Iadd.the .Iaddend.vehicle mounted equipment and means coupled to
said first and second receiving means for determining the position
of the vehicle mounted equipment at the time the GPS signals were
received.
10. A fixedly sited base station as claimed in claim 9, wherein the
means for determining the position of the vehicle mounted equipment
calculates the position of the base station using the GPS and then
calculates the position of the vehicle mounted equipment using a
differential technique.
11. A vehicle location system as claimed in claim 2, wherein the
rate at which the vehicle mounted equipment retransmits the GPS
signals is lower than that at which the signals were recorded.
12. A vehicle location system as claimed in claim 11, wherein the
vehicle mounted equipment further comprises control means coupled
to the recording means such that signals from the control means
cause the recording means to record GPS signals at preset
intervals.
13. A vehicle location system as claimed in claim 2, wherein the
vehicle mounted equipment further comprises control means coupled
to the recording means such that signals from the control means
cause the recording means to record GPS signals at preset
intervals.
14. A vehicle location system as claimed in claim 1, wherein the
vehicle mounted equipment further comprises control means coupled
to the recording means such that signals from the control means
cause the recording means to record GPS signals at preset
intervals.
15. A vehicle location system as claimed in claim 1, wherein the
base station includes a means for obtaining the GPS ephemeris for
satellites of the GPS. .Iadd.
16. A vehicle location system as claimed in claim 1, wherein the
means for determining the position of the vehicle mounted equipment
calculates the position of the base station using the GPS and then
calculates the position of the vehicle mounted equipment using a
differential technique. .Iaddend..Iadd.
17. A mobile equipment location system for use in a global
positioning system (GPS), comprising at least one mobile equipment
including means for receiving signals directly from the GPS, a base
station including first means for receiving signals directly from
the GPS, characterized in that the mobile equipment includes means
for recording the received GPS signals and means for retransmitting
the recorded GPS signals to the base station, and in that the base
station includes second means for receiving the recorded GPS
signals retransmitted by the mobile equipment, and position
determining means coupled to the first and second receiving means
for determining the position of the mobile equipment at the time
when the mobile equipment received the GPS signals.
.Iaddend..Iadd.18. A mobile equipment location system for use in a
global positioning system (GPS), comprising at least one mobile
equipment including means for receiving signals directly from the
GPS, a base station at a known, location, including first means for
receiving signals directly from the GPS, characterised in that the
mobile equipment includes means for recording the received GPS
signals and means for retransmitting the recorded GPS signals to
the base station, and in that the base station includes second
means for receiving the recorded GPS signals retransmitted by the
mobile equipment, and position determining means coupled to the
first and second receiving means for determining the position of
the mobile equipment at the time when the mobile equipment received
the GPS signals.
.Iaddend..Iadd.19. A mobile equipment location system as claimed in
claim 17, wherein the mobile equipment transmits a time of arrival
(TOA) signal in addition to retransmitting the recorded GPS
signals. .Iaddend..Iadd.20. A mobile equipment location system as
claimed in claim 17, wherein the rate at which the mobile equipment
retransmits the GPS signals is lower than that at which the signals
were recorded. .Iaddend..Iadd.21. A mobile equipment location
system as claimed in claim 20, wherein the mobile equipment further
comprises control means coupled to the recording means such that
signals from the control means cause the recording means to record
the GPS signals at preset intervals. .Iaddend..Iadd.22. A mobile
equipment location system as claimed in claim 19, wherein the GPS
is the satellite-based NAVSTAR GPS, and in that the base station
has means for obtaining the GPS ephemeris for the satellites in
use. .Iaddend..Iadd.23. A mobile equipment location system as
claimed in claim 22, wherein the means for obtaining the GPS
ephemeris in the base station has means for despreading the NAVSTAR
GPS signals without using any locally generated pseudo random noise
codes. .Iaddend..Iadd.24. A mobile equipment for use in a global
positioning system (GPS) with a mobile equipment location system
and including means for receiving GPS signals, the mobile equipment
location system comprising a base station including first means for
receiving signals directly from the GPS, second means for receiving
recorded GPS signals transmitted by the mobile equipment, and
position determining means coupled to the first and second
receiving means for determining the position of the mobile
equipment at the time when the mobile equipment received the GPS
signals, said mobile equipment including means for recording the
received GPS signals and means for retransmitting the recorded GPS
signals. .Iaddend..Iadd.25. A mobile equipment for use in a global
positioning system (GPS) with a mobile equipment location system
and including means for receiving GPS signals, the mobile equipment
location system comprising a base station including first means for
receiving signals directly from the GPS, second means for receiving
recorded GPS signals transmitted by the mobile equipment, and
position determining means coupled to the first and second
receiving means for determining the position of the mobile
equipment at the time when the mobile equipment received the GPS
signals, said mobile equipment including means for recording the
received GPS signals and means for retransmitting the recorded GPS
signals and a time of arrival (TOA) signal. .Iaddend..Iadd.26. A
base station for use in a global positioning system (GPS) with a
mobile equipment location system comprising at least one mobile
equipment including means for receiving signals directly from the
GPS, means for recording the received GPS signals, and means for
retransmitting the recorded GPS signals to the base station, said
base station including first means for receiving GPS signals
directly from the GPS, second means for receiving a retransmission
of GPS signals from the mobile equipment and means coupled to said
first and second receiving means for determining the position of
the mobile equipment at the time the GPS signals were received.
.Iaddend..Iadd.27. A base station as claimed in claim 26, wherein
the means for determining the position of the mobile equipment
calculates the position of the base station using the GPS and then
calculates the position of the mobile equipment using a
differential technique. .Iaddend..Iadd.28. A mobile equipment
location system as claimed in claim 19, wherein the rate at which
the mobile equipment retransmits the GPS signals is lower than that
at which the signals are recorded. .Iaddend..Iadd.29. A mobile
equipment location system as claimed in claim 28, wherein the
mobile equipment further comprises control means coupled to the
recording means such that signals from the control means cause the
recording means to record GPS signals at preset intervals.
.Iaddend..Iadd.30. A mobile equipment location system as claimed in
claim 19, wherein the mobile equipment further comprises control
means coupled to the recording means such that signals from the
control means cause the recording means to record GPS signals at
preset intervals. .Iaddend..Iadd.31. A mobile equipment location
system as claimed in claim 17, wherein the mobile equipment further
comprises control means coupled to the recording means such that
signals from the control means cause the recording means to record
GPS signals at preset intervals.
.Iaddend..Iadd. 2. A mobile equipment location system as claimed in
claim 17, wherein the base station includes a means for obtaining
the GPS ephemeris for satellites of the GPS. .Iaddend..Iadd.33. A
mobile equipment location system as claimed in claim 18, wherein
the mobile equipment transmits a time of arrival (TOA) signal in
addition to retransmitting the recorded GPS signals.
.Iaddend..Iadd.34. A mobile equipment location system as claimed in
claim 18, wherein the rate at which the mobile equipment
retransmits the GPS signals is lower than that at which the signals
were recorded. .Iaddend..Iadd.35. A mobile equipment location
system as claimed in claim 34, wherein the mobile equipment further
comprises control means coupled to the recording means such that
signals from the control means cause the recording means to record
the GPS signals at preset intervals. .Iaddend..Iadd.36. A mobile
equipment location system as claimed in claim 33, wherein the GPS
is the satellite-based NAVSTAR GPS, and in that the base station
has means for obtaining the GPS ephemeris for the satellites in
use. .Iaddend..Iadd.37. A mobile equipment location system as
claimed in claim 36, wherein the means for obtaining the GPS
ephemeris in the base station has means for despreading the NAVSTAR
GPS signals without using any locally generated pseudo random noise
codes. .Iaddend..Iadd.38. A mobile equipment for use in a global
positioning system (GPS) with a mobile equipment location system
and including means for receiving GPS signals, the mobile equipment
location system comprising a base station at a known location
including first means for receiving signals directly from the GPS,
second means for receiving recorded GPS signals transmitted by the
mobile equipment, and position determining means coupled to the
first and second receiving means for determining the position of
the mobile equipment at the time when the mobile equipment received
the GPS signals, said mobile equipment including means for
recording the received GPS signals and means for retransmitting the
recorded GPS signals. .Iaddend..Iadd.39. A mobile equipment for use
in a global positioning system (GPS) with a mobile equipment
location system and including means for receiving GPS signals, the
mobile equipment location system comprising a base station at a
known location including first means for receiving signals directly
from the GPS, second means for receiving recorded GPS signals
transmitted by the mobile equipment, and position determining means
coupled to the first and second receiving means for determining the
position of the mobile equipment at the time when the mobile
equipment received the GPS signals, said mobile equipment including
means for recording the received GPS signals and means for
retransmitting the recorded GPS signals and a time of arrival (TOA)
signal. .Iaddend..Iadd.40. A base station at a known location for
use in a qlobal positioning system (GPS) with a mobile equipment
location system comprising at least one mobile equipment including
means for receiving signals directly from the GPS, means for
recording the received GPS signals, and means for retransmitting
the recorded GPS signals to the base station, said base station
including first means for receiving GPS signals directly from the
GPS, second means for receiving a retransmission of GPS signals
from the mobile equipment and means coupled to said first and
second receiving means for determining the position of the mobile
equipment at the time the GPS signals were received.
.Iaddend..Iadd.41. A base station as claimed in claim 40, wherein
the means for determining the position of the mobile equipment
calculates the position of the base station using the GPS and then
calculates the position of the mobile equipment using a
differential technique. .Iaddend..Iadd.42. A mobile equipment
location system as claimed in claim 33, wherein the rate at which
the mobile equipment retransmits the GPS signals is lower than that
at which the signals are recorded. .Iaddend..Iadd.43. A mobile
equipment location system as claimed in claim 42, wherein the
mobile equipment further comprises control means coupled to the
recording means such that signals from the control means cause the
recording means to record GPS signals at preset intervals.
.Iaddend..Iadd.44. A mobile equipment location system as claimed in
claim 33, wherein the mobile equipment further comprises control
means coupled to the recording means such that signals from the
control means cause the recording means to record GPS signals at
preset intervals. .Iaddend..Iadd.45. A mobile equipment location
system as claimed in claim 18, wherein the mobile equipment further
comprises control means coupled to the recording means such that
signals from the control means cause the recording means to record
GPS signals at preset intervals. .Iaddend..Iadd.46. A mobile
equipment location system as claimed in claim 18, wherein the base
station includes a means for obtaining the GPS ephemeris for
satellites of the GPS. .Iaddend..Iadd.47. A mobile equipment
location system as claimed in claim 17, wherein the means for
determining the position of the mobile equipment calculates the
position of the base station using the GPS and then calculates the
position of the mobile equipment using a differential technique.
.Iaddend..Iadd.48. A mobile equipment location system as claimed in
claim 18, wherein the means for determining the position of the
mobile equipment calculates the position of the base station using
the GPS and then calculates the position of the mobile equipment
using a differential technique. .Iaddend.
Description
This invention relates to a .[.vehicle.]. .Iadd.mobile equipment
.Iaddend.location system which makes use of a satellite-based
global positioning service (GPS) of the NAVSTAR type and which has
particular but not exclusive application to an automatic vehicle
location (AVL) system for use with a fleet of vehicles, each of
which is in radio contact with a base station.
Fleets of vehicles such as messengers and taxis have traditionally
kept a base station informed of their location by using speech
messages from the vehicle over a radio link to an operator or
controller at the base station. This technique has significant
disadvantages which include errors due to mis-heard messages,
distraction of the vehicle driver and the large amount of time that
the operator has to spend in simply updating a map or schedule. One
solution to this problem involves using an automated vehicle
locating system based on the NAVSTAR satellite-based global
position system (GPS).
The NAVSTAR GPS is described in "Global Positioning by Satellite"
by Philip G. Mattos, Electronics and Wireless World. February 1989
but the salient points of the system are repeated here. The NAVSTAR
GPS consists of a number of satellites in approximately 12 hour,
inclined orbits of the earth, each satellite transmitting
continuous positional information. Two positioning services are
provided by NAVSTAR, the precise positioning service (PPS) which is
reserved for military use and the standard positioning service
(SPS) which is available for general use, The following description
is confined to the SPS although some features are common to both
systems. By measuring the propagation time of these transmissions
and hence the distance from three satellites to himself, a user can
make an accurate calculation of his position in three dimensions.
To make a valid positional fix, the user needs to measure the
propagation times to an accuracy of better than 100 ns and to
facilitate this the satellite signals each have timing marks at
approximately 1 .mu.s intervals. However, each satellite's signals
are synchronised to an atomic clock and the normal user of the
system will not maintain such an accurate clock. As a result the
user's clock is said to be in error (in other words, different from
satellite time) by a clock bias C.sub.B. By measuring the apparent
satellite signal propagation times from four satellites rather than
three, the redundancy can be used to solve for C.sub.B and the
three accurate propagation times required can be calculated. The
signal propagation times correspond to ranges of the user from the
satellites related by the speed of light c. Prior to correction for
the user's clock bias C.sub.B, the apparent ranges of the
satellites are all in error by a fixed amount and are called
pseudoranges.
FIG. 1 of the accompanying drawings shows a radio receiver 16 in a
user's vehicle 15 receiving signals from four GFS satellites 11,
12, 13 and 14. The four pseudoranges of the satellite signals are
denoted R1, R2, R3 and R4. The positions of the satellites and the
vehicle are shown as three-dimensional coordinates whose origin is
the centre of the earth. FIG. 2 of the accompanying drawings shows
the equations used by a GPS receiver to calculate the dimensional
coordinates and the clock bias from a knowledge of four satellite
positions and their respective pseudoranges. While it is not
essential, these equations are usually solved using numerical
techniques to hasten the calculations. It is important to note that
the clock bias C.sub.B has the dimension metres in order to agree
with the remainder of the equation. C.sub.B has the dimension
metres in order to agree with the remainder of the equation.
C.sub.B can be converted to a time .Iadd.by .Iaddend.division by
the speed of light c.
The data transmitted by each satellite consists broadly of three
sets of information, the ephemeris, the almanac and the clock
correction parameters. The ephemeris consists of detailed
information about the satellite's own course over the next two
hours, the almanac consists of less detailed information about the
complete satellite constellation for a longer period and the clock
correction parameters allow the user to correct for the GPS
satellite's own clock errors. The satellite transmissions consist
of a direct sequence spread spectrum (DSSS) signal containing the
ephemeris, almanac, and the clock correction information at a rate
of 50 bits per second (bps). In the case of the SPS a pseudo random
noise (PRN) signal which has a chip rate of 1.023 MHz and which is
unique to each satellite is used to spread the spectrum of the
information, which is then transmitted on a centre frequency of
1575.42 MHz. The PRN signal is known as a coarse/acquisition (C/A)
code since it provides the timing marks required for fast
acquisition of GPS signals and coarse navigation. The signals
received at a user's receiver have a bandwidth of approximately 2
MHz and a signal to noise ratio (S/N) of approximately -20 dB. In
addition, since the satellites are each moving at a speed in excess
of 3 km/s, the GPS signals are received with a Doppler frequency
offset from the GPS centre frequency. As a result, a stationary GPS
receiver has to be capable of receiving signals with frequencies of
up to .+-.4 KHz from the GPS centre frequency, and a mobile
receiver (as is usually the case) has to be able to receive signals
over an even greater frequency range. To recover the data and
measure the propagation time of the satellite signals, the GPS
receiver must cancel or allow for the Doppler frequency offset and
generate the C/A code relevant to each satellite. Initially, at
least, this can be very time consuming since to despread the DSSS
signals, the incoming and locally generated PRN codes must be
exactly at synchronism. To find the PRN code delay the receiver
must compare the locally generated code and the incoming code at a
number of different positions until the point of synchronism or
correlation is found. With a code length of 1023 chips this
comparison can be a lengthy procedure. However, once the frequency
offset and the PRN code delay for each satellite are known, tracing
them is relatively easy.
Some considerable effort has been directed towards making more
accurate location systems using the GPS. One technique for
obtaining improved accuracy is to use a differential system which
makes propagation time measurements for a mobile receiver and for a
fixed receiver at a known location. Patent specification WO
87/06713 describes such a differential system which additionally
smooths the values of propagation time over a number of
measurements to obtain improved accuracy. There are numerous
applications of the GPS, however, which do not require pinpoint
accuracy; the operator of a fleet of vehicles, for example, will
probably be satisfied with locations having an accuracy of only
several hundred metres.
As can be appreciated, a receiver for use with the GPS is rather
complex and hence expensive and it is the aim of the present
invention to provide a considerably simplified system, based on the
GPS, for locating a distant vehicle or vehicles from a fixed
point.
According to a first aspect of the present invention there is
provided a .[.vehicle.]. .Iadd.mobile equipment .Iaddend.location
system for use in a global positioning system (GPS), comprising at
least one .[.vehicle mounted.]. .Iadd.mobile .Iaddend.equipment
including means for receiving signals directly from the GPS, a
.[.fixedly sited.]. base station including first means for
receiving signals directly from the GPS, characterised in that the
or each .[.vehicle mounted.]. .Iadd.mobile .Iaddend.equipment
includes means for recording the received GPS signals and means for
retransmitting the recorded GPS signals to the base station, and in
that the .[.fixedly sited.]. base station includes second means for
receiving the recorded GPS signals retransmitted by the .[.vehicle
mounted.]. .Iadd.mobile .Iaddend.equipment, and position
determining means coupled to the first and second means, for
determining the position of the or each .[.vehicle.]. .Iadd.mobile
equipment .Iaddend.at the time when the 171 vehicle mounted.].
.Iadd.mobile .Iaddend.equipment received the GPS signals.
The maximum rate at which the retransmission of the GPS signals
takes place will be determined by the capacity of the radio channel
between the mobile unit(s) and the base station(s). This
retransmission rate will generally be somewhat lower than the
original rate of the GPS signals and at a different carrier
frequency.
It is envisaged that a vehicle location system in accordance with
the present invention will make use of a vehicle mounted
communications transmitter that is already a part of the vehicle's
equipment and also serves one or more other purposes although this
is by no means essential.
The vehicle mounted equipment can make the necessary recordings of
GPS dates on receipt of a request signal from a base station, at
predetermined intervals, or continuously, using a first in, first
out (FIFO) type of storage means. The data can be retransmitted on
receipt of a request signal from a base station, upon the lapse of
a given amount of time from the beginning of the recording of the
data or at predetermined time intervals. To make a position fix the
transition time of the stellite signals has to be known accurately
and the redundancy available due to reception of four satellite
signals will only resolve errors of up to a few milliseconds. A
coarser measure of the signal arrival time, that is nonetheless
accurate to within a few milliseconds will thus be required by the
base station. One solution to this problem would be for the vehicle
mounted equipment to transmit a time of arrival (TOA) signal with
the recording of the satellite data. Another solution would be for
the vehicle mounted equipment to record the satellite data at
certain, known intervals and to retransmit the data before the
commencement of the next interval. In most cases the vehicle
mounted equipment will also transmit an identifying signal with the
recorded satellite signals so that the base station has a knowledge
of the origin of any particular signal. Where a specific mobile
unit has been requested to retransmit its recorded data, this
identification signal may be superflous, but its inclusion does
provide a degree of extra protection in the event of receipt of
corrupted request signals from the base station.
According to a second aspect of the present invention there is
provided a vehicle mounted equipment for use with a vehicle
location system in accordance with the first aspect of the present
invention, including means for receiving GPS signals, characterised
in that the equipment also includes means for recording the
received GPS signals and means for retransmitting the recorded GPS
signals.
According to a third aspect of the present invention there is
provided a fixedly sited base station for use with the system in
accordance with the first aspect of the present invention,
including first means for receiving GPS signals directly from the
GPS, characterised in that the base station also includes second
means for receiving a retransmission of GPS signals from a vehicle
mounted equipment and means coupled to said first and second means
for determining the position of the vehicle mounted equipment at
the time that the GPS signals were received.
The present invention will now be described, by way of example,
with reference to FIGS. 3, 4 and 5 of the accompanying drawings,
wherein:
FIG. 1 shows a radio receiver in a vehicle receiving signals from
four GPS satellites,
FIG. 2 shows equations used by a GPS receiver for certain
calculations,
FIG. 3 shows signals from four NAVSTAR satellites being received by
a mobile unit and retransmitted to a base station,
FIG. 4 is a block schematic diagram of a receiver, data store and
transmitter for a mobile unit, and
FIG. 5 is a block schematic diagram of a GPS receiver, a
transceiver and a data store in a base station.
In the drawings corresponding features have been identified using
the same reference numerals.
FIG. 3 shows the vehicle location system operating with just one
vehicle 15 and one base station 35. Transmissions from NAVSTAR GPS
satellites 11, 12, 13 and 14 are received by both a vehicle mounted
receiver 16 and a base station receiver 38. The GPS signals
received by mobile receiver 16 are fed to a storing and control
means 18 which records a short section of the satellite signals.
The control means 18 might record GPS signals at preset intervals,
upon receipt of a request signal (not shown) from the base station,
or continuously where the stored signals at any instant will be
those most recently received. The recorded signals are then
retransmitted, at a lower data rate and on a different carrier
frequency to that used by the satellites, by a transmipl 19 to a
receiver 36 in the base station. The signals could be retransmitted
at a predetermined interval after the commencement of the recording
or upon receipt of a request signal from the base station. In the
former case it may be necessary for the retransmitted signal to
include some kind of identifier so that the base station knows from
which vehicle the signals have originated. The rate at which the
signals are retransmitted will depend upon the capacity of the
radio channel between the mobile unit and the base station but will
be approximately 1,000 times less than the rate at which they were
received and sampled if a voice channel is used. The received and
recorded satellite signals have a S/N ratio of approximately -20 dB
and retransmission will probably not cause significant further
deterioration of the S/N. As a result, it is not usually necessary
to include any error detection or correction codes with the
retransmitted data. The base station also includes means for
receiving the GPS signals directly from the satellites using
receiver 38. Signals from receiver 38 are passed to a processing
means 37 which maintains a copy of the GPS ephemeris and clock
correction data for those satellites currently in use by the
system. The processing means 37 can, with the data received by
receivers 36 and 38, calculate the position of the vehicle 15 by
removing the Doppler offset frequency successively from each of the
recorded satellite signals, correlating the relevant C/A code with
each of the signals and calculating the satellite pseudoranges. The
processing means 37 may also maintain a copy of the GPS almanac so
that the vehicle location system can use the signals from the most
favourable satellites and find newly visible satellites more
quickly.
Two main problems can arise from this offline, remote processing of
the satellite signals. Firstly, if the mobile unit is at a great
distance from the base station it is possible that the base station
will not be able to receive signals from a satellite that is
visible to the vehicle and which is essential to the positional
fix. The base station will thus be deprived of up-to-date ephemeris
information for that satellite. To reduce the likelihood of this
problem, the antenna for the base station GPS receiver should be
omnidirectional and mounted in an area clear of obstructions. Where
a very large area is to be covered, the use of a number of
physically separated base stations with means for
intercommunication might be the best solution. For the base station
to obtain the ephemeris data from the retransmission by the mobile
unit is not a practical proposition since the recording and
retransmission of sufficient spread spectrum data to provide a
complete satellite ephemeris would take several hours. Secondly,
there is a range ambiguity problem that, while present in a
conventional vehicle mounted GPS receiver system, may be more
difficult to solve in this case. The PRN codes used by the
satellites repeat every millisecond and as a result the circular
correlation of the received and locally generated PRN codes only
allows a GPS receiver to calculate the submillisecond part of the
satellite signal transit time. The integer number of milliseconds
in the signal transit times can usually be calculated from the
approximate position of the vehicle. Since a 1 millisecond
difference in transit time corresponds to a difference in the
satellite pseudorange of 300 km. a knowledge of the vehicle
position to within approximately 100 km will allow the calculation
of the integer number of milliseconds in the signal transit times.
This degree of accuracy of the vehicle position may be available
from a knowledge of which .[.call.]. .Iadd.cell .Iaddend.of a
cellular radio system is being used to retransmit the signals to
the base station. If the vehicle position is not known to this
degree of accuracy (100 km may be less than one hour's motoring)
the data bit edges on the satellite signals can act as timing marks
with a spacing of 20 ms. Since the modulation of the satellite
signal by the data is synchronised to an atomic clock, the position
of the data bit edges in the received, despread signals gives a
coarse measure of transit time which is nonetheless accurate to
within one millisecond. To use this measurement technique, at least
20 ms of satellite signals will need to be recorded to ensure that
the recording contains a data bit edge from each satellite. A third
alternative is to use the Doppler shift on the received GPS signals
to calculate an approximate user position. However, this method
still requires at least 20 ms of satellite data and is
mathematically more complex, especially if the user's vehicle is in
motion.
FIG. 4 is a block schematic diagram of a mobile receiver and
transmitter suitable for use in a vehicle locating system in
accordance with the present invention. Satellite signals are
received at an antenna 20 which feeds an rf amplifier 22. The input
stage of the rf amplifier 22 will usually include a bandpass
filter. The output of the amplifier 22 is mixed with the output of
local oscillator 24 in a mixer 23 and the output of the mixer is
filtered by a bandpass filter 26. Although only one down-conversion
stage is shown, the front end of the receiver could include two or
more such stages. The nominal intermediate frequency to which the
satellite signals are mixed down could be anything from zero to
several MHz. In the case of a zero IF receiver, the filter 26 would
be a low pass type. The output of filter 26 is digitised in an
analogue to digital converter 27 whose sampling rate is determined
by the Nyquist sampling criterion.
The output of the analogue to digital converter 27 is stored in a
random access memory (RAM) 28 which is addressed by a counter 31,
the counter itself being under the control of a receiver controller
30. The size of this RAM will be determined by the rate of sampling
and the length of time that the incoming satellite signals are to
be recorded for. For example, sampling at 2.046 MHz (to satisfy the
Nyquist criterion) for 8 ms will require just under 16 kbits of
memory. The contents of the RAM 28 are transmitted serially by
transmitter 32 via antenna 33. In a practical system the
transmitter 32 may be part of an existing transceiver within the
mobile unit.
These signals are received and processed by the base station, an
embodiment of which is shown in block schematic form in FIG. 5. The
retransmitted signals from the mobile unit are received by antenna
43 and fed to a transceiver 44. Again, the transceiver 44 could be
part of an existing communications link. A base station controller
42 is connected to the transceiver and in addition to receiving the
signals from the mobiles and calculating their positions it
maintains an up to date copy of the ephemeris data for all the
satellites currently in view. The GPS signals are received by a GPS
receiver 38 via an antenna 40. The purpose of this receiver is to
decode satellite ephemeris and clock correction data and it will
probably also decode almanac data to facilitate satellite signal
acquisition. Since positional information is not required for the
base station it does not need to determine the propagation delays
of the GPS signals. It is thus possible to use a signal despreading
technique based on non-coherent demodulation which does not use any
logically generated C/A codes. In all other respects the satellite
data is received as described previously for a conventional system
and stored in a RAM 41 for use by the base station controller 42 in
calculating the satellite pseudoranges in respect of the or each
vehicle. One advantage of using a complete GPS receiver at the base
station rather than one employing a non-coherent demodulation
technique is that it permits location fixes to be made by a
differential technique. The base station uses the GPS to determine
its own position and, since this is already known accurately, can
calculate an up to date error term for the GPS. When the mobile
unit(s) position is calculated, this error can be removed from the
mobile unit's pseudoranges which gives an improvement in the
accuracy of the positional fix. The transceiver 44 enables request
signals to be passed from the base station to the mobile units for
commencement of data logging and/or data transmission. It can also,
if required, relay vehicle position or directions back to the
driver of the vehicle.
From reading the present disclosure other modifications will be
apparent to persons skilled in the art. Such modifications may
involve other features which are already known in the design,
manufacture and use of GPS systems and component parts thereof and
which may be used instead of or in addition to features already
described herein. Athough claims have been formulated in this
application to particular combinations of features, it should be
understood that the scope of the disclosure of the present
application also includes any novel feature or any novel
combination of features disclosed herein either explicitly or
implicitly or any generalisation thereof, whether or not it relates
to the same invention as presently claimed in any claim and whether
or not it mitigates any or all of the same technical problems as
does the present invention. The applicants hereby give notice that
new claims may be formulated to such features and/or combinations
of such features during the prosecution of the present application
or of any further application derived therefrom.
* * * * *