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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a system for determining the position of a
vehicle using two or more satellites.
2. Description of the Prior Art
A satellite-based system for determining the position of a vehicle and for
communication service systems for use by moving objects such as vehicles,
airplanes, ships, or the like which uses two or more artificial satellites
and which can also perform message communication is being developed. Such
a system is disclosed in U.S. Pat. No. 4,359,733, in which a transponder
is carried by a moving object (hereinafter, simply referred to as a
vehicle) and message communication between an operation center serving as
a ground station and each vehicle is provided. Each transponder transmits
a responsive signal to an interrogation signal from the operation center
via each satellite. Thus, the operation center can determine the position
of the vehicle on the basis of the time differences between the
propagation times of the respective signals.
Such a satellite-based vehicle position determining and communicating
service system can be used, for example, to manage movement of trucks in a
transport company. For example, when using this system to manage the
movement of trucks, transponders are attached to each of the trucks which
are operated by the transport company. The trucks are distributed in every
district and are moving and their positions constantly change. Message
communication is also performed between the operation center and each
truck. The operation center can monitor the velocity and present position
of each truck. According to this system, the transport company can perform
services such that a customer is informed of the time of arrival of a
parcel, for example. On the other hand, the next schedule of moving a
truck can also be planned, so that the trucks can be efficiently operated.
Such satellite-based vehicle position determining and communicating service
systems include not only a one-way communicating service in which a
message is unidirectionally communicated from a vehicle to the operation
center but also a two-way communicating service in which a message is
bidirectionally communicated between a vehicle and the operation center.
In the two-way communicating service, the position of a vehicle can be
determined and at the same time, message communication can be performed
between the vehicle and the operation center. Therefore, in the two-way
communication service, an instruction for movement can be also given from
the operation center to a vehicle.
FIG. 1 is a conceptional diagram for explaining such a vehicle position
determining system which uses satellites. The system in this diagram can
obtain the position of each vehicle in three dimensions by using three
satellites. A satellite 201 is a geostationary satellite for transmitting
a signal from a vehicle 206 to a ground station 205 and for transmitting a
signal from the ground station 205 to the vehicle 206. Satellites 202 and
203 are geostationary satellites for transmitting the signal from the
vehicle 206 to the ground station 205.
When the position of the vehicle 206 is to be determined, an interrogation
signal from the ground station 205 is sent to the vehicle 206 through the
satellite 201. A transponder mounted in the vehicle 206 receives the
interrogation signal from the ground station 205 and sends an output
response signal in responsive to the interrogation signal. The response
signal is received by the ground station 205 through the satellites 201,
202, and 203. In the ground station 205, the position of the vehicle 206
is calculated from the time differences of the response signals received
from the satellites 201 to 203. The vehicle position is calculated on the
basis of triangulation.
It is assumed that the distances from the ground station 205 to the
satellites 201, 202, and 203 are respectively l.sub.201, l.sub.202, and
l.sub.203 and the distances from the satellites 201, 202, and 203 to the
vehicle 206 are l.sub.210, l.sub.220, and l.sub.230, respectively. An
interrogation signal is transmitted from the ground station 205. It is
assumed that the times when the response signal generated from the
transponder of the vehicle in response to the interrogation signal reaches
the ground station 205 through the satellites 201, 202, and 203 are
t.sub.201, t.sub.202 and t.sub.203, respectively. In this case, the
following equations are satisfied (where, C denotes the velocity of
light).
##EQU1##
Since the distances l.sub.201, l.sub.202, and l.sub.203 are already known,
the distances l.sub.210, l.sub.220, and l.sub.230 can be obtained if the
times t.sub.201, t.sub.202, and t.sub.203 are known. Thus, the position of
the vehicle 206 can be three-dimensionally obtained.
On the other hand, the position of a vehicle can be also measured by using
two satellites, e.g., the satellites 201 and 202. In this case, the
vehicle position can be two-dimensionally measured. The measurement is
performed on the assumption that one of three coordinates of latitude,
longitude, and altitude, e.g., the altitude are known.
In the case for determining the vehicle position by use of, e.g., three
satellites 201 to 203 on the basis of the time differences of the
interrogation signal from the ground station 205 has been output until the
response signals responsive to the interrogation signal are received from
the transponder of the vehicle 206 through the satellites 201 to 203,
respectively, if the times t.sub.201, t.sub.202, and t.sub.203 until the
response signals return to the ground station through the satellites 201,
202 and 203 are not accurately measured in the ground station, a position
measurement error occurs. To prevent this, in the transponder, the
response signal which is output in response to the interrogation signal
from the ground station has to be completely time synchronized with the
interrogation signal. However, a time lag of the response signal to the
interrogation signal actually has a particular value in each transponder.
For example, assuming that the time lag until the transponder transmits a
response signal after it receives the interrogation signal deviates from a
preset value by 10 nsec, a position measurement error of 3 m will result.
Therefore, in a conventional satellite-based vehicle position determining
and communicating service system, a high speed device such as an ECL
(emitter coupled logic) or the like is used as a circuit device for the
transponder in the vehicle 206 or a time adjusting circuit is interposed,
thereby allowing the time synchronization between the interrogation signal
and the response signal in each transponder. However, the use of such high
speed device causes the circuit size and cost to be increased. In
addition, even if such a high speed device is used, it is difficult to
make the time synchronization perfect between the interrogation signal and
the response signal, so that variations in time lags of the transponders
occur. Further, it is also difficult to perform adjustments so as to make
the time lags of the transponders to coincide.
SUMMARY OF THE INVENTION
It is an object of the present invention to make it possible to
independently set the time lags of the response signals to the
interrogation signals of the transponders mounted in respective vehicles.
Another object of the invention is to enable a vehicle position to be
accurately determined even when variations in time lags of the response
signals to the interrogation signals of the transponders mounted on
respective vehicles occur.
Still another object of the invention is to eliminate the necessity of a
circuit having high response speed performance by permitting variations in
time lags of the response signals to the interrogation signals of
transponders mounted on respective vehicles.
According to the invention, there is provided a system in which a response
signal to an interrogation signal of a transponder which is mounted on a
vehicle is transmitted to a ground station through two or more satellites
and in the ground station, the position of the vehicle is determined on
the basis of the propagation time differences of the signals which are
received from the satellites, wherein when the information of the peculiar
time lag of the response signal to the interrogation signal of the
transponder mounted on each vehicle is stored and the vehicle position is
calculated, by correcting the propagation time on the basis of the
information of the time lag so that the vehicle position can be accurately
decided for each vehicle.
The above and other objects and features of the present invention will
become apparent from the following detailed description and the appended
claims when read with reference to the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are conceptional diagrams for explaining satellite-based
vehicle position determining and communicating systems using three
satellites;
FIGS. 3a-3F are a timing chart of response signals to an interrogation
signal;
FIGS. 4a-4b show signal formats of an interrogation signal and a response
signal;
FIG. 5 is a block constitutional diagram of the main section in a
transponder mounted on each vehicle;
FIG. 6 is a diagram for explaining the measurement of a time lag of a
response signal to an interrogation signal of a transponder mounted on
each vehicle; and
FIG. 7 is a block diagram of a tester which is used in the device of FIG. 6
.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
An embodiment of the present invention is described hereinbelow with
reference to the drawings.
FIG. 2 shows an example of a satellite-based vehicle position determining
and communicating service system to which the invention can be applied.
This system can be used to control the movement of a plurality of
vehicles.
In this system, communication is performed between an operation center 4
which serves as a ground station and each of the vehicles 5A and 5B by
using three geostationary satellites 1, 2 and 3. The position of each of
the vehicles 5A and 5B is determined three-dimensionally by using the
three satellites 1, 2 and 3. The latitude, longitude, and altitude are
obtained and the satellite 1 among three satellites is used to perform
two-way communication between the operation center 4 and each of the
vehicles 5A and 5B. The other satellites 2 and 3 are used to transmit
signals from the vehicles 5A and 5B to the operation center 4.
The operation center 4 is fixed on the ground and receives the transmitted
signals via the satellites from the transponders mounted on the vehicles.
The operation center 4 and a user center 6 are connected via, e.g. a
telephone line 7. The vehicles 5A and 5B are moving objects such as trucks
or other vehicles. A transponder is installed in each vehicle.
Two-way communication can be performed between the operation center 4 and
each of the vehicles 5A and 5B through the satellite 1. The operation
center 4 transmits a signal having a carrier frequency of, e.g., 2.5 GHz
to the satellite 1. This signal is sent to the vehicles 5A and 5B via the
satellite 1. Each of the vehicles 5A and 5B transits a signal having a
carrier frequency of, e.g., 1.6 Ghz and this signal is sent to the
operation center 4 via he satellite 1.
When the present positions of the vehicles 5A and 5B is to be calculated, a
response signal is output from each transponder which is mounted on the
vehicles 5A and 5B in response to an interrogation signal which is sent
from the operation center 4 through the satellite 1. This response signal
is sent to the operation center 4 through the satellite 1 and is also sent
to the operation center 4 through the satellites 2 and 3. Due to the
differences in length of the propagation paths, time differences occur in
the arrival times of the response signals transmitted through the
satellites 1, 2 and 3 to the operation center 4. In the operation center
4, the positions of the vehicles 5A and 5B can be obtained on the basis of
these time differences.
For example, assuming that an accident occurred in the vehicle 5A, a
message indicative of this situation is transmitted from the vehicle 5A to
the operation center 4 through the satellite 1.
On the other hand, the present position of the vehicle 5A can be calculated
since the response signal which is output from the transponder mounted on
the vehicle 5A in response to the interrogation signal transmitted from
the operation center is transmitted to the operation center 4 through the
satellites 1, 2 and 3.
The situation of and the present position of the vehicle 5A are sent to the
user center 6 via the telephone line 7. Therefore, in the user center 6,
the velocity and the present position of each vehicle can be known and
movement management can be efficiently performed. Assuming that an
accident had occurred in the vehicle 5A and that the vehicle 5B is to be
moved to the location of the accident so as to rescue the vehicle 5A as
mentioned above, the instruction for this condition is sent from the user
center 6 to the operation center 4 and the instruction is given from the
operation center 4 to the vehicle 5B through the satellite 1. At this
time, the situation of and the present position of the vehicle 5A are
momently supplied from the operation center 4 to the vehicle 5B through
the satellite. Thus, the vehicle 5B can promptly move to the location of
the vehicle 5A which has had an accident.
The present invention is applied to the foregoing satellite-based vehicle
position determining and communicating service system. As mentioned above,
in this system, when the vehicle position has been determined, the
response signals are output from the transponders mounted in the vehicles
in response to the interrogation signal from the operation center 4. The
response signals are sent to the operation center 4 through the satellites
1, 2 and 3. The position of each vehicle is calculated using the time
differences among the arrival times of the response signals transmitted
through the satellites 1, 2 and 3 to the operation center 4, respectively.
FIGS. 3A-3F are timing charts of respective signals which exist until the
interrogation signal generated from the operation center 4 are received by
the transponders mounted in the vehicles and the response signals in
response to the interrogation signal are then received by the operation
center through the satellites, respectively.
When the position of a vehicle is to be determined, an interrogation signal
S.sub.1 consisting of frames F.sub.1, F.sub.2, F.sub.3, . . . is output
from the operation center 4 as shown in FIG. 3A. The signal S.sub.1 is
received by the transponder of each vehicle through the satellite 1 as
shown in FIG. 3B. The frame signal corresponding to each vehicle is
included in each frame of the interrogation signal S.sub.1 from the
operation center 4. The transponder of each vehicle receives the
interrogation signal S.sub.1 and discriminates whether the frame number
obtained by decoding the received signal coincides or not with its own
frame number assigned to the transponder. If they coincide, the vehicle
outputs a response signal S.sub.2 including the own frame number assigned
as shown in FIG. 3C. Therefore, the response signal S.sub.2 is output
from the transponder at a timing which almost coincides with the leading
edge of a predetermined frame in the signal (FIG. 3B) which corresponds to
the interrogation signal S.sub.1 sent from the operation center 4 and
which is received by the transponder of each vehicle. The response signal
S.sub.2 is sent to the operation center 4 through the satellites 1, 2 and
3, respectively.
As shown in FIGS. 3D to 3F, the times until the response signal S.sub.2
reaches the operation center 4 through the satellites 1, 2 and 3 have time
differences which correspond to the distances from the operation center 4
to the satellites 1, 2 and 3 and to the distances from the satellites 1, 2
and 3 to the vehicles, respectively. In the operation center 4, the times
until the response signal S.sub.2 which are responsive to the
interrogation signal reaches the operation center 4 through the satellites
1, 2 and 3 are measured by using the transmission time of each frame of
the interrogation signal S.sub.1 (FIG. 3A) as a reference, respectively.
It is now assumed that the time until the response signal S.sub.2 arrives
through the satellite 1 is t.sub.1, the time until the response signal
S.sub.2 arrives through the satellite 2 is t.sub.2, and the time until the
response signal S.sub.2 arrives through the satellite 3 is t.sub.3. The
position, i.e., the latitude, longitude, and altitude of each vehicle are
calculated using the times t.sub.1, t.sub. 2, and t.sub.3.
For example, it is now assumed that the position of the vehicle 5A is to be
obtained. In FIG. 2, assuming that the distance between the operation
center 4 and the satellite 1 is l.sub.1 and the distance between the
satellite 1 and the vehicle 5A is l.sub.10.
2(l.sub.1 +l.sub.10)=C.multidot.t.sub.1 (1)
where C denotes the velocity of light. Similarly, assuming that the
distance between the operation center 4 and the satellite 2 is l.sub.2 and
the distance between the satellite 2 and the vehicle 5A is l.sub.20,
(l.sub.1 +l.sub.10)+(l.sub.20 +l.sub.2)=C.multidot.t.sub.2 (2)
Assuming that the distance between the operation center 4 and the satellite
3 is l.sub.3 and the distance between the satellite 3 and the vehicle 5A
is l.sub.30,
(l.sub.1 +l.sub.10)+(l.sub.30 +l.sub.3)=C.multidot.t.sub.3 (3)
The satellites 1, 2 and 3 are geostationary satellites. The operation
center 4 is the ground station and is fixed on the ground. Therefore, the
distances l.sub.1, l.sub.2, and l.sub.3 are already known. Therefore, if
the times t.sub.1, t.sub.2, and t.sub.3 are obtained, the distances
l.sub.10 l.sub.20, and l.sub.30 can be obtained by using the equations (1)
to (3). Thus, the position of the vehicle 5A can be calculated by applying
the principle of triangulation using the distances l.sub.10, l.sub.20, and
l.sub.30.
When the position of each vehicle is determined in this manner, in order to
decide the accurate position of each vehicle, in the transponder mounted
on each vehicle, the time of reception of the leading edge of, e.g., a
frame of the interrogation signal S.sub.1 which is sent from the operation
center 4 must be accurately made to coincide with the output time of the
response signal S.sub.2 which is responsive to the interrogation signal
S.sub.1.
However, since the circuit device constituting the transponder in each
vehicle has its own operation delay time, it is difficult to accurately
make the output timing of the response signal S.sub.2 coincide with the
reception time of the leading edge of a frame of the interrogation signal
S.sub.1 sent from the operation center 4.
Therefore, according to the present invention, the time until the
transponder mounted on each vehicle receives the interrogation signal and
then transmits a responsive signal in response to the interrogation signal
is measured using every transponder. When the vehicle position is
determined, the position measurement difference is eliminated by using
this response time information.
FIGS. 4A and 4B show examples of signal transmission formats of each frame
of the interrogation signal which is output from the operation center and
of the response signal output from a vehicle. The response time
information which is unique for each of the transponders is transmitted
from each transponder to the operation center through the satellite.
Each frame of the interrogation signal S.sub.1 shown in FIG. 4A which is
sent from the operation center 4 includes a sync signal portion 11, a
frame number portion 12, and a message portion 13.
The response signal S.sub.2 which is transmitted from each transponder
mounted on the vehicles has a sync signal portion 21, a frame number
portion 22, and a message portion 23 as shown in FIG. 4B. A portion 24 for
transmitting the response time information is provided in the message
portion 23. The frame number which locates each frame of the interrogation
signal S.sub.1 is sent from the operation center 4 and is detected. When a
special frame number coincides with the frame number assigned to the
vehicle, the frame number assigned which corresponds to this special frame
number is set in the frame number portion 22 in the response signal.
The response time information TE is provided so as to instruct a time lag
until each vehicle receives the interrogation signal and then transmits a
response signal in response to this interrogation signal. For example, a
standard response time of the transponder is used as a reference and the
response time difference is transmitted as the response time information
to the operation center 4. As another example, a method whereby the
response time itself is transmitted is also considered. When the response
time difference is transmitted as information, for example, the data area
of one byte to transmit the response time difference .DELTA.t from -128
nsec to +127 nsec is held. The response signal S.sub.2 is sent to the
operation center 4 through the satellites 1, 2 and 3. In the operation
center 4, the frame in the signal S.sub.1 to which the response signal
S.sub.2 corresponds is determined on the basis of the number in the frame
number portion 22 in the response signal S.sub.2. The position of each
vehicle is decided on the basis of the time differences among the times
until the response signals S.sub.2 transmitted through the satellites 1, 2
and 3 reach the operation center 4. At this time, the reception times of
the signals transmitted via the different propagation paths and received
by the vehicles are corrected by the response time difference information
TE included in the response signal S.sub.2.
Assuming that the time difference value transmitted as the response time
difference information TE is .DELTA.t, the equations (1) to (3) are
corrected by this value .DELTA.t. Thus, the distances l.sub.10, l.sub.20,
and l.sub.30 are obtained as follows:
##EQU2##
Therefore, if the response time which is peculiar to the transponder
mounted on each vehicle is previously known, the reception time can be
corrected for every transponder when the vehicle position is calculated.
Even if the output timing of the response signal S.sub.2 is not perfectly
coincident with the leading edge of a frame of the interrogation signal
S.sub.1 and a time lag peculiar to the transponder attached to each
vehicle exists, the position of the vehicle can be accurately determined.
FIG. 5 shows an example of the construction of a main section in the
transponder which is installed in each vehicle to which the invention is
applied. In FIG. 5, a response time difference data memory 31 serves as a
time lag information storing means for storing the time lag information.
The data corresponding to the response time difference .DELTA.t is
measured by a tester 51 in a manner which will be explained hereinafter,
and is previously stored in the memory 31. In this example, a temperature
sensor 32 and a table 33 of data for temperature correction are provided.
An output of the memory 31 and an output of the data table 33 are supplied
to a response time difference data generating circuit 34 which serves as
the time lag correcting means.
Each circuit device constituting the transponder of each vehicle can be
influenced by temperature changes, so that the response time lag changes.
The temperature sensor 32 is provided so as to measure the ambient
temperature of the circuit device constituting the transponder and to
correct for changes in response time lag. The data table 33 for
temperature correction is used with an output signal of the temperature
sensor 32. The response time lag which is stored in the memory 31 is
corrected by the output signal of the data table 33.
The data indicative of the response time lag which is output from the
generating circuit 34 is included as the response time difference
information TE to the response signal S.sub.2 and is transmitted.
The interrogation signal transmitted from the operation center 4 is
spectrum diffused by a PN code (high speed pseudo random code train) and
is transmitted by using a carrier which has a carrier frequency of e.g.,
2.5 GHz. By transmitting a signal after the spectrum has been diffused as
explained above, the resistance to interference and the speech security
property are improved. The accuracy of determined position is also
improved. The signal from the operation center 4 is received by an antenna
59 on each vehicle through the satellite 1. The signal received by the
antenna 59 is supplied from an antenna terminal 35 to a receiving circuit
36.
The signal received by the receiving circuit 36 is amplifier and converted
into a center frequency having a predetermined frequency and is further
converted into a base band signal. An output of the receiving circuit 36
is supplied to a decoder 37.
A PN signal generator for generating the same PN series as the PN series of
the spectrum diffused interrogation signal which is sent from the
operation center 4 through the satellite 1 is provided in the decoder 37.
The PN code of the input signal is compared with the code which is
generated by the PN code generator and the signals are synchronized and
locked at a position of the maximum correlation value. In this manner, the
spectrum inverse diffusion is performed. The phase synchronization is held
by a costas loop and the data is decoded.
The decoded data is supplied to a terminal 38. The terminal 38 may be an
input/output device such as a key switch, a display, or the like. The
terminal 38 is made operative on the basis of the data sent from the
decoder 37. A command from the operation center 4 is displayed on the
display. The information which is to be transmitted to each vehicle is
input by a keyboard or other means.
So as to determine the vehicle's position, a packet of the response signal
S.sub.2 is formed from the terminal 38 as shown in FIG. 3B. The response
time difference information TE which is generated from the generating
circuit 34 is added to the response signal S.sub.2 by an adder 39. The
response signal S.sub.2 which includes the information TE is supplied to
an encoder 40. A message is also included in the response signal S.sub.2.
The encoder 40 spectrum diffuses the response signal S.sub.2 by using the
PN code and transmits the diffused signal using a carrier with a carrier
frequency of, 1.6 GHz, for example. The response signal S.sub.2 is sent
from the encoder 40 to a transmitter 41 with a timing which coincides with
the leading edge of a predetermined frame of the interrogation signal
S.sub.1 which was received as an input at the antenna terminal 35. In
order to synchronize the leading edge of the frame of the interrogation
signal S.sub.1 with the output timing of the response signal S.sub.2, a
clock signal CK and a timing signal TS are supplied from a frame sync
circuit 42 in the decoder 37 to a transmission timing generator 43 in the
encoder 40. The timing signal TS is synchronized with the leading edge of
the frame of the interrogation signal S.sub.1 which is received. When the
timing signal TS is supplied to the encoder 40, the spectrum diffused
response signal S.sub.2 is supplied from the encoder 40 to the transmitter
41 and then to the antenna 60 and is transmitted using a carrier having a
carrier frequency of 1.6 GHz.
The output of the transmitter 41 is supplied from an antenna terminal 44 to
the antenna 60 and is sent to the operation center 4 through the
satellites 1, 2 and 3.
In the foregoing embodiment, the transmission timing of the response signal
S.sub.2 is made to be coincident with the leading edge of the frame of the
interrogation signal S.sub.1. However, the transmission timing of the
response signal S.sub.2 is not limited to the leading edge of the frame of
the interrogation signal S.sub.1 but can be also be made to be coincident
with other portions of the signal.
As mentioned above, the response time lag data corresponding to the
response time difference .DELTA.t is stored in the memory 31. This
response time difference .DELTA.t is preliminarily measured by the tester
51 as shown in FIG. 6. The response time difference .DELTA.t measured by
the tester 51 is stored in the memory 31.
As shown in FIG. 7, the tester 51 comprises: a transmitting unit 52 so as
to generate a signal having a carrier of a predetermined carrier frequency
which is obtained by spectrum diffusing the interrogation signal S.sub.1
which is similar to the interrogation signal which is output from the
operation center 4. A receiving unit 53 receives a response signal from a
transponder 61 which has been measured. The tester 51 also includes a time
difference detecting circuit 54 so as to measure the response time
difference .DELTA.t. An output terminal 55 of the tester 51 is connected
to the antenna terminal 35 of the transponder 61. An input terminal 56 of
the tester 51 is connected to the antenna terminal 44 of the transponder
61.
The interrogation signal from the transmitting unit 52 of the tester 51 is
output from the output terminal 55 and is supplied to the transponder 61.
A timing signal TS.sub.1 is output from a transmission timing generator 57
in response to the leading edge of each frame of the interrogation signal
which is output from the transmitting unit 52.
When the interrogation signal is supplied from the transmitting unit 52 of
the tester 51 to the transponder 61, the response signal S.sub.2 is formed
in the transponder 61 and the response signal S.sub.2 with a packet format
which has a carrier with a predetermined carrier frequency and which has
been spectrum diffused is output from the antenna terminal 44 of the
transponder 61. This output signal is supplied from the antenna terminal
44 to the input terminal 56 as an input to the receiving unit 3 of the
tester 51. When the response signal S.sub.2 having the packet format from
the transponder 61 is detected by the receiving unit 53, a timing signal
TS.sub.2 is output from a packet sync circuit 58 in the receiving unit 53.
The timing signal tS.sub.1 from the transmission timing generator 57 in the
transmitting unit 52 and the timing signal TS.sub.2 from the packet sync
circuit 58 is the receiving unit 53 are supplied to the time difference
detecting circuit 54. The time difference between the timing signals
TS.sub.1 and TS.sub.2 is measured by the detecting circuit 54. By
subtracting a preset standard response time for each transponder from this
measured time difference, the data indicative of the response time
difference .DELTA.t is formed. This response time difference data is
stored in the memory 31 of the transponder 61.
Therefore, according to the invention, the response time difference of the
output timing of the response signal S.sub.2 is previously measured on the
basis of the leading edge of the frame of the interrogation signal S.sub.1
as a reference. The response time difference data which is unique for each
transponder and which has been stored in the transponder is included in
the response signal S.sub.2 and this signal is transmitted to the
operation center 4. In the operation center 4, the times of receival of
the response signal S.sub.2 through the satellites 1, 2 and 3 are
corrected by using the response time difference data. Thus, even if the
output timing of the response signal S.sub.2 is not precisely coincident
with the leading edge of the frame of the interrogation signal S.sub.1,
the position measurement difference due to disturbance of the output
timing is eliminated. In the invention, there is not need to perform
adjustments to precisely make the output timing of the response signal
S.sub.2 coincide with the timing of, the leading edge of the frame of the
interrogation signal S.sub.1. Also, there is no need to use a high speed
device such as an ECL or the like as a circuit device as the transponder
on a vehicle. Therefore, the circuit size and cost can be reduced as
compared to prior art systems.
The present invention is not limited to the foregoing embodiments but many
modifications and variations are possible which are within the spirit and
scope of the appended claims of the invention.
* * * * *
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