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Description  |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an automotive navigation system, and more
particularly to an automotive navigation system having a
satellite-utilizing positioning system which receives radio waves from
artificial satellites and detects the present position of a vehicle.
2. Description of the Prior Art
As disclosed in Japanese Unexamined Patent Publication No. 58(1983)-70117,
there has been known an automotive navigation system in which the present
position of the vehicle is shown on a display device together with a map
of the area around the present position. As the means for detecting the
present position of the vehicle, there has been put into practice a
present position detecting system in which an azimuth sensor such as a
geomagnetism sensor is employed. That is, the traveling distance and the
traveling azimuth of the vehicle from a reference position are detected by
means of a vehicle speed sensor and an azimuth sensor, and the present
position of the vehicle is determined on the basis of the detected
traveling distance and the detected traveling azimuth. However, such a
present position detecting system is disadvantageous in that since the
present position is measured as, say, a position relative to the reference
position, the accuracy of the positioning is deteriorated due to measuring
errors in the traveling distance and/or the traveling azimuth.
Such a problem can be overcome by measuring the present position of the
vehicle as, say, an absolute position by use of radio waves transmitted
from artificial satellites. For example, this can be accomplished by use
of a GPS (Global Positioning System) which is now in the process of
development. By use of a GPS, the present position of a vehicle can be
determined with an accuracy of measurement of about 30 meters (in the case
of C/A code which is to be opened to the public) on the basis of radio
waves transmitted from four artificial satellites (generally called
"NAVSTAR").
In the vehicle provided with a navigation system having a
satellite-utilizing positioning system in which the present position of
the vehicle is detected by use of a GPS, a map of the area where the
vehicle is traveling is displayed, for instance, as a picture on a CRT
carried by the vehicle and the detected present position is shown as a
spot on the picture. The present position must be displayed in response to
movement of the vehicle, and accordingly, the radio waves from the
respective satellites are repeatedly received by the navigation system at
predetermined intervals, and the present position is determined each time
the radio waves are received.
In such a navigation system, while the vehicle is at a stop, the spot
representing the present position must remain stationary on the picture.
However, since the detected present position for a given position of the
vehicle can fluctuate within about 30 meters as described above and the
fluctuation is apt to be increased depending on the positions of the
satellites and/or radio interference, the spot representing the present
position of the vehicle moves, due to the fluctuation in the detected
present position, each time the radio waves from the satellites are
received and the present position of the vehicle is determined on the
basis of the received radio waves, though actually the vehicle remains
stationary.
SUMMARY OF THE INVENTION
In view of the foregoing observations and description, the primary object
of the present invention is to provide an automotive navigation system in
which the spot on the picture representing the present position of the
vehicle is kept stationary while the vehicle is at a stop even if the data
on the present position fluctuate while the vehicle is at a stop.
In accordance with the present invention, there is provided an automotive
navigation system comprising a receiving means for receiving data signals
on present position of the vehicle transmitted from satellites, a present
position detecting means for detecting the present position of the vehicle
on the basis of the data signals, a display device for showing the present
position of the vehicle on the basis of the output of the present position
detecting means, a vehicle speed sensor for detecting the vehicle speed,
and a data change limiting means which limits change of the output of the
present position detecting means to be given to the display device when
the vehicle speed is lower than a predetermined speed.
In a preferred embodiment of the present invention, the outputs of the
present position detecting means during the time the vehicle speed is
lower than the predetermined speed are averaged to obtain the present
position of the vehicle with more accuracy, and when the vehicle speed
reaches the predetermined speed, the averaged output of the present
position detecting means is given to the display device to show the
present position on the basis of the averaged output.
Since the detected present positions can be considered to be substantially
normally distributed about the true present position, a more accurate
present position can be obtained by averaging outputs of the present
position detecting means during the time the vehicle speed is
substantially zero.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a partly cutaway perspective view showing a vehicle provided with
an automotive navigation system in accordance with the present invention,
FIG. 2 is a schematic view showing an arrangement of an automotive
navigation system in accordance with an embodiment of the present
invention,
FIG. 2A is a schematic view for illustrating a modification of the control
unit,
FIG. 3 is a schematic perspective view for illustrating a GPS,
FIG. 4 is a view for illustrating the principle of positioning using a GPS,
FIG. 5 is a block diagram showing a transmitting circuit on the satellite,
FIG. 6 is a block diagram showing a satellite-utilizing positioning system,
FIG. 7 is a flow chart for illustrating the operation of the navigation
system of the embodiment,
FIG. 8 is a flow chart for illustrating the controlling operation of the
control keys,
FIG. 9 is a front view showing a navigation unit to be mounted on the
dashboard of the vehicle in the navigation system of this embodiment,
FIG. 10A is a flow chart for illustrating the control of the map to be
shown on a CRT,
FIG. 10B is a flow chart for illustrating the controlling of the spot that
shows the present position,
FIG. 11 is a flow chart for illustrating the operation of the navigation
system of another embodiment, and
FIG. 12 is a flow chart of the process for effecting correction of error in
measurement of the traveling azimuth by the geomagnetism sensor by use of
the GPS data.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
As sown in FIGS. 1 and 2, an automotive navigation system in accordance
with an embodiment of the present invention comprises a GPS receiver 2 for
receiving radio waves from satellites, a vehicle speed sensor 4, a present
position detecting means 3 for detecting the present position of the
vehicle on the basis of the radio waves received by the GPS receiver 2,
and a control unit 10 which receives the output signal of the present
position detecting system 3 and controls various signals as will become
apparent later. A memory 5 in which map information and the like are
stored and which may be, for instance, a compact disk or a ROM is
connected to the control unit 10 by way of a decoder 6, and an operating
device 7 having various control keys is connected to the control unit 10
by way of an encoder 8. Further, a display device (e.g., a cathode-ray
tube) 16 and a video RAM 15 are connected to the control unit 10 by way of
a display control system 14.
In the memory 5, there are stored maps and the like bearing thereon various
information for guiding the driver. The operating device 7 comprises
various control keys (not shown) for manually changing the contents to be
displayed and the like.
The control unit 10 comprises a microcomputer having an operational circuit
12 and ROM 11a and RAM 11b connected to the operational circuit 12. The
operational circuit 12 is connected to the present position detecting
system 3 and the like by way of an interface 13. In the control unit 10,
the present position of the vehicle is calculated on the basis of a signal
from the present position detecting system 3, and a map of the area around
the present position of the vehicle is retrieved from the memory 5. The
map thus retrieved from the memory 5 is displayed on the screen of the
display device 16 or stored in the video RAM 15.
The GPS receiver 2 and the present position detecting system 3 together
form a satellite-utilizing positioning system which forms a user side
portion of the GPS. As shown in FIG. 3, in the GPS, eight to twenty-one
artificial satellites are controlled by a main control station 1a on the
ground by way of four ground antennas 1b, and the satellite-utilizing
positioning system receives radio waves emitted from four satellites S1 to
S4 in the field of view, and determines the present position of the
vehicle on the basis of the received radio waves. The accuracy of
positioning by the satellite-utilizing positioning system can be
deteriorated depending on the position of the satellites, perturbation of
the motion of the satellites, the condition of the ionosphere and the
like, and sometimes positioning by the satellite-utilizing positioning
system becomes locally infeasible, though for a short time. Further, where
the radio waves from the satellites cannot be received or are difficult to
receive, e.g., the inside of a tunnel or a ground location blocked by an
obstacle, positioning by the satellite-utilizing positioning system
becomes infeasible or difficult.
The rate of deterioration of the accuracy of positioning by the
satellite-utilizing positioning system depends upon the deterioration
coefficient and the field intensity. That is, the deterioration
coefficient is a value determined according to the geometric relation
between the satellites and the vehicle upon positioning, and as the
deterioration coefficient increases, positioning error increases,
deteriorating the accuracy of positioning. Other factors which can
deteriorate the accuracy of positioning appear as deterioration of the
field intensity. When the deterioration coefficient increases and/or the
field intensity is deteriorated, positioning error is increased. The
deterioration coefficient can be calculated on the basis of data on the
positions of the satellites utilized for positioning which are sent from
the respective satellites on the basis of the result of tracking the
satellites by the ground antennas 1b and data reception at monitor
stations 1c on the ground. The field intensity can be detected on the
basis of intensity of received radio waves.
The principle of positioning by the GPS is as follows.
Assuming that the transmitting point and the receiving point are
respectively provided with clocks which are in perfect synchronization
with each other and the transmitting signal is controlled by the clocks,
the propagation delay time between the transmitting point and the
receiving point can be detected by clocking the time the transmitting
signal is received at the receiving point, and the distance between the
transmitting point and the receiving point can be obtained by multiplying
the propagation delay time by the velocity of light. When it is assumed
that there are three satellites S1, S2 and S3 in the field of view of the
user as shown in FIG. 4 and the satellites S1 to S3 transmit distance
measuring signals under the control of clocks which are in synchronization
with each other, the distances between the receiving point P and the
satellites S1 to S3 can be determined by clocking the times the respective
measuring signals are received, whereby the receiving point P can be
determined as the intersection of sphericities having the respective
centers on the satellites S1 to S3. (FIG. 4 shows this two-dimensionally
for the purpose of simplicity.) However, it is technically very difficult
and disadvantageous from the viewpoint of the manufacturing cost of the
receiver to synchronize the clock at the receiving point with the clocks
at the transmitting point, i.e., on the satellites. This problem can be
overcome by increasing the number of the satellites from which the
receiver receives radio waves. If the clock at the receiving point is
.DELTA.tu behind the clocks on the satellites, the detected distances
between the receiving point P and the satellites, i.e., the radii of said
three sphericities, become larger than the actual values by .DELTA.tu.c (c
representing the velocity of light) and the three circles which should
intersect with each other at a point cannot do so as shown by the solid
lines in FIG. 4. By correcting the value of .DELTA.tu.c, the position of
the receiving point P and the value of .DELTA.tu can be simultaneously
determined. In the GPS, a measured value of the distance between a
receiving point and a satellite i which differs from the true value Ri of
the same by .DELTA.tu.c is called a false distance. The false distance Ri'
between the receiving point P and the satellite i is represented by a
formula
Ri'=Ri+c.DELTA.tai+c(.DELTA.tu-.DELTA.tsvi)
wherein .DELTA.tai represents the delay time of a radio wave in the
ionosphere and the troposphere, and .DELTA.tsvi represents the time offset
of the clock on the satellite i. Instead of synchronizing the atomic
clocks on the respective satellites, the time offset of the atomic clock
on each satellite is measured and predicted to convert it into a form
which enables calculation of .DELTA.tsvi, and the time offset in the
converted form is transmitted. In order to effect three-dimensional
positioning, false distances are detected for four satellites and four
unknowns, i.e., three coordinates and .DELTA.tu, are determined on the
basis of the false distances. Similarly, the three-dimensional velocity of
the user can be measured on the basis of detected values of Doppler
frequencies of the signals from the respective satellites, i.e., detected
values of the rates of change in the false distances between user and
satellites.
When position of the user is to be determined on the basis of the positions
of the satellites, data on positions of the satellites which are
constantly changing and the conditions of the clocks on the respective
satellites must be given to the user. These data are transmitted from the
satellites in a manner described later.
Each of the satellites is provided with a receiving circuit (not shown) for
receiving a radio wave transmitted from the main control station 1a by way
of the antenna 1b and a transmitting circuit 20 shown in FIG. 5. The
transmitting circuit 20 comprises a reference frequency oscillating
circuit 21 which outputs a reference frequency signal at, for instance,
10.23 MHz, a first multiplier 22 which multiplies the frequency of the
reference frequency signal output from the reference frequency oscillating
circuit 21 by 154 to form an L1 carrier (1575.42 MHZ) as a first carrier,
and a second multiplier 23 which multiplies the frequency of the reference
frequency signal by 154 to form an L2 carrier (1227.6 MHZ) as a second
carrier. The transmitting circuit 20 further comprises a clock formation
circuit 24 which forms a clock signal having a predetermined period on the
basis of the reference frequency signal, a code generating circuit 25
which forms two kinds of code signals called P code and C/A code on the
basis of the reference frequency signal and the clock signal, and a
computer 26 which outputs data on the position of the satellite which is
changing from moment to moment and on the condition of tee clock on the
satellite under the timing-control of the clock signal. The P code is a
secret code having a high accuracy and is available only to the military
and other limited users. The P code is superposed on data output from the
computer 26 and is then transmitted by orthogonal modulation of both the
L1 and the L2 carriers. The repetition frequency of the P code is 10.23
Mbit/s and the duration of the P code is one week. The C/A code is used
for rough positioning (normal positioning) and acquisition of the P code,
and is available to the public. The C/A code is superposed on data output
from the computer 26 and is then transmitted by modulation of both the L1
and the L2 carriers. The repetition frequency of the C/A code is 1.023
Mbit/s and the duration of the C/A code is 1023 bits. That is, the C/A
code is repeated every 1 ms. The C/A code is generated, for instance, by a
Gold code generating circuit comprising a pair of ten-stage shift
registers. Data to be output from the computer 26 are stored in a memory
circuit (not shown) on the satellite on the basis of measurement and
prediction at a control section on the ground, and are successively read
out. These data are transmitted at a predetermined timing at a
transmission speed of, for instance, 50 bit/s. These data include a
telemeter language, an ionosphere correction parameter, a delay correction
for a single-wave receiver, a date of data on a clock correction, a
reference time for clock correction, a GPS system time, a date of orbit
prediction, a reference time for orbital elements, a mean anomaly at a
reference time for orbital elements, eccentricity, a square root of the
longer diameter, a right ascension of an ascending node, inclining angle
of the orbit, a perigee argument, a perturbation of an ascending node, a
mean motion correction, a parameter for an inclining angle correction, a
correction term for an orbital disturbance, an identification number of
the satellite, a reference time for a data subframe, condition of the
satellite and the like. Further, almanac data of the other satellites in
the system is included in the said data to enable prediction of the period
for which the receiver of the user can receive the signals from the
satellites, selection of combination of the satellites in the field of
view providing the best positioning accuracy, presetting of the receiving
circuit in order to acquire the signals from the satellites at the
earliest possible moment, and the like.
Said control section comprises the main control station 1a, the ground
antennas 1b disposed at fixed points (expected to be at least four in
number) on the ground, and the monitor stations 1c disposed at fixed
points (expected to be at least four in number) on the ground. The main
control station 1a is a manned facility having a large computer and a
series of operation control tables which tracks the satellites by way of
the ground antennas 1b, predicts the clocks on the satellites and orbits
of the satellites on the basis of the result of the tracking, and
transmits data to store the predicted clocks and orbits in the memories on
the satellites in order to broadcast them. Further, the main control
station 1a receives telemetry data for controlling the satellites and
commands. The monitor station 1c is an unmanned facility provided with a
receiver for receiving the signals from the satellites, an atomic clock,
and meteorological instrumentation for calculating the tropospheric delay.
As shown in FIG. 2 and as described above, the satellite-utilizing
positioning system comprises the GPS receiver 2 for receiving the signals
from the satellites and the present position detecting system 3 which
determines the present position on the basis of the received signals and
outputs a position signal representing the present position. As shown in
FIG. 6, the satellite-utilizing positioning system includes a quartz
oscillator 38 which outputs said reference frequency signal as an overall
timing control signal, and a clock oscillator 39 which forms, on the basis
of the reference frequency signal, a clock signal for controlling the
timing of operation of a signal processing means 37. An antenna 31, a
pre-amplifier 32 and a band filter 33 are connected to the front stage of
the GPS receiver 2.
The GPS receiver 2 comprises a frequency synthesizer 61 which produces a
signal having the same pattern as data on the carriers transmitted from
the transmitting circuit 20 on each satellite, position of the satellite
and condition of the clock on the satellite on the basis of the reference
frequency signal generated by the quartz oscillator 38, a code generating
circuit 62 which receives a clock signal output from the clock oscillator
39 and forms a code signal having the same pattern as the distance
measuring signal, a data-and-carrier detector 63 which correlatively
detects the data on the clock on the satellite and the orbit of the
satellite and carriers under the control of the output signals of the
frequency synthesizer 61 and the code generating circuit 62, and a
code-locked detector 64 which correlatively detects the distance measuring
signals under the control of the code signal output from the code
generating circuit 62. The signal processing means 37 is time-controlled
by the clock signal output from the clock oscillator 39.
Though the GPS receiver 2 shown in FIG. 6 has a single receiving channel,
the GPS receiver may be provided with two receiving channels so that one
of the channels can be exclusively used for switching reception of the
signals from the satellites in the field of view, and the other channel is
used for receiving the data broadcast from the respective satellites and
for preliminary acquisition of the signal from the satellite which is due
to be received next. With this arrangement, interruption of positioning
during reception of the data from the satellites can be avoided. When a
receiver having five channels is used, switching of satellites can be
effected in an instant by continuously tracking four satellites with four
of the channels while the satellite due to be used next is preliminarily
acquired with the other channel.
In the GPS, errors in measuring distances are all converted into distances
which are referred to as "UERE" (user equivalent range error). The causes
of the UERE and the nominal values of the UERE in the case of the P code
for the respective causes are shown in Table 1. In the case of the C/A
code, the UERE caused by the ionosphere and the receiver is expected to be
several times as much as in the case of the P code.
The positioning error (positioning accuracy) in the GPS depends solely upon
the product of the UERE and the deterioration coefficient GDOP. In the
case of the C/A code, the nominal value of the positioning accuracy is 40
m (50%) in the term of the probable radial error.
TABLE 1
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Kinds and values of UERE (P code)
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Space part Stability of clock on satellite
4.5 m
Vibration of satellite
3.0
Other 0.5
Control part
Orbit prediction 2.5
Other 0.5
User part Ionospheric propagation delay
2.3
Tropospheric propagation delay
2.0
Receiver noise 1.5
Multipath 1.2
Other 0.5
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With reference to the flow chart in FIG. 7, present position detection in
the navigation system of this embodiment will be described, hereinbelow.
In FIG. 7, radio waves from the satellites are received by the GPS
receiver 2 in step S1, and in step S2 it is determined whether the
deterioration coefficients GDOP are smaller than a predetermined value.
When it is determined in the step S2 that the deterioration coefficient is
not smaller than the predetermined value, present position detection is
transferred to a different present position detecting system as will
become apparent later. Otherwise, the present position of the vehicle,
co-ordinates (x, y), is calculated on the basis of the data received in
step S3. Then in steps S4 and S5 it is determined whether the vehicle
speed V is higher than a predetermined speed k. When it is determined that
the vehicle speed V is higher than the predetermined speed k, the present
position calculated in the step S3 is transmitted to the control unit 10
and to the display device 16 by way of the display control system 14 to be
displayed on the CRT of the display device 16 (step S6). In this case, the
present position which changes with the movement of the vehicle is shown
as a spot which moves on the CRT since the present position detection is
repeated at predetermined intervals. When it is determined that the
vehicle speed V is not higher than the predetermined speed k, that is,
that the vehicle is at a stop, it is determined in step S7 whether data on
the present position (X, Y) have been stored. When it is determined that
data on the present position (X, Y) have not been stored, the present
position as calculated in the step S3 is transmitted to the control unit
10. Otherwise, the present position of the vehicle is not determined. That
is, when the vehicle is at a stop, the data of the present position are
not refreshed until the vehicle is started again, and on the other hand,
when the navigation system begins to be operated while the vehicle is at a
stop, the data on the present position are fixed at those obtained in the
step S3. Accordingly, in accordance with this embodiment, the spot on the
CRT representing the present position cannot be moved until the vehicle is
started again even if the data on the present position received change
during the stop.
As shown in FIGS. 1 and 9, the operating device 7 (FIG. 2) is incorporated
with the display device (CRT) 16 into a navigation unit which is mounted
on the dashboard 9 of the vehicle, and is provided with a plurality of
control keys including an on-off key 91 for turning on and off the
navigation system, a list key 92 for changing the contents to be displayed
on the CRT 16, e.g., selectively showing gas stands, parking lots and the
like in the map displayed on the CRT 16, a cancel key 93 for cancelling
the content to be displayed, a reset key 94 for resetting the content to
be displayed, a scale-up-and-down key 95 for scaling up and down the
picture on the CRT 16 and a scroll key 96. These keys may be of a type
pictorially displayed on the CRT 16.
The control keys are generally operated by the driver. However, during
travel this is not preferred from the viewpoint of safety. Especially
during high speed travel, it is very dangerous for the driver to select
one of the keys and operate it. Accordingly, the navigation system of this
embodiment is controlled to reject non-essential operation of keys while
the vehicle is traveling at high speed. This control is done according to
the flow chart shown in FIG. 8. In FIG. 8, when one of the keys is
operated, the code of the key is read in step S1, and in step S2 it is
determined whether the present vehicle speed Vc is higher than a
predetermined speed Vm. When it is determined that the present vehicle
speed Vc is higher than the predetermined speed Vm, this flow is
immediately ended, and accordingly the operation of the key is ignored. On
the other hand, when the present vehicle speed Vc is not lower than the
predetermined speed Vm, the control corresponding to the operated key is
performed in step S3.
Though, in the flow chart shown in FIG. 8, operation of any key is ignored
when the vehicle speed Vc is higher than the predetermined speed Vm, the
navigation system may be arranged to accept operation of some of the keys
even if the present vehicle speed Vc is higher than the predetermined
speed Vm.
FIG. 10A shows a flow chart according to which the map to be displayed on
the CRT 16 is determined, and FIG. 10B shows a flow chart according to
which the present position of the vehicle is displayed on the CRT 16.
In FIG. 10A, a map area including the present position detected by the
present position detecting system 3 is set in step S1, and in step S2 the
file reference number under which the map area is stored in the memory 5
is determined. Then the map including the present position is loaded from
the memory (CD-ROM) 5 in step S3.
In FIG. 10B, the present position (X , Y) in an absolute coordinate system
as detected by the present position detecting system 3 is input into the
control unit 10 in step S1. Then in step S2, the position (Xo , Yo) on the
CRT 16 representing the present position (X , Y) is calculated, and in
step S3, the present position is shown as a spot on the CRT 16.
Though, in the embodiment described above, the present position of the
vehicle is not determined when the vehicle speed is not higher than the
predetermined speed, the present position may be calculated irrespective
of the vehicle speed so long as the calculated data are not transmitted to
the display device. In the modification shown in FIG. 2A, the vehicle
speed signal is input into a write limiting section 100, and data on the
present position of the vehicle calculated by the present position
detecting means are stored b in the RAM 11b under the control of a write
command section 101 which is operated every time the data on the present
position are input. The RAM 11b reads out data stored therein under the
control of a read command section 102 which is operated at predetermined
intervals. The write limiting section 100 prevents the write command
section 101 from permitting the data to be stored in the RAM 11b when the
vehicle speed is lower than the predetermined speed.
With reference to the flow chart in FIG. 11, present position detection in
a navigation system in accordance with another embodiment of the present
invention will be described, hereinbelow. In FIG. 11, radio waves from the
satellites are received by the GPS receiver 2 in step S1 and in step S2 it
is determined whether the deterioration coefficients GDOP are smaller than
a predetermined value. When it is determined in the step S2 that the
deterioration coefficient is not smaller than the predetermined value,
present position detection is transferred to a different present position
detecting system as will become apparent later. Otherwise, the present
position of the vehicle, coordinates (x, y), is calculated on the basis of
data received in step S3. In steps S4 and S5 it is then determined whether
the vehicle speed V is higher than a predetermined speed k. When it is
determined that the vehicle speed V is higher than the predetermined speed
k, a counter N is reset to 0 in step S6 and the sums of X and Y are reset
to 0 in step S7. Then, the present position calculated in the step S3 is
transmitted to the control unit 10 and to the display device 16 by way of
the display control system 14 to be displayed on the CRT of the display
device 16 (step S8). In this case, the present position which changes with
the movement of the vehicle is shown as a spot which moves on the CRT
since the present position detection is repeated at predetermined
intervals. When it is determined that the vehicle speed V is not higher
than the predetermined speed k, that is, that the vehicle is at a stop,
the counter N is incremented by one in step S11. Further, in step S10, the
values x and y representing the present position detected in the step S3
are added to the sums of X and Y, and the sum of X and x and the sum of Y
and y are respectively divided by the value of the counter N, thereby
obtaining average values x' and y' which are stored as the values
representing the present position. These steps are repeated so long as the
vehicle is at a stop, thereby refreshing the average values x' and y'
every time positioning is effected When the vehicle is started again, the
present position of the vehicle is displayed on the CRT 16 on the basis of
the last average values x' and y'.
As shown in the flow charts in FIGS. 7 and 11, it is preferred that an
azimuth sensor such as a geomagnetism sensor be employed instead of the
present position detecting system using the GPS when the deterioration
coefficient GDOP becomes larger than a predetermined value such as when
the vehicle is in a tunnel. In the geomagnetism sensor, positioning
accuracy is adversely affected mainly by error in measurement of the
traveling azimuth, and accordingly, if the error in measurement of the
traveling azimuth can be corrected with a high accuracy, the positioning
accuracy by the geomagnetism sensor can be substantially improved.
Correction of the error in measurement of the traveling azimuth by the
geomagnetism sensor can be effected by use of averaged GPS data on the
position of two places at which the vehicle stops. That is, by use of the
averaged GPS data on the position of two places at which the vehicle
stops, the traveling azimuth can be accurately detected and by correcting
the traveling azimuth detected by the geomagnetism sensor, the positioning
accuracy by the geomagnetism sensor can be substantially improved. For
example, this can be done according to the flow chart shown in FIG. 12.
In FIG. 12, flag i is set to 1 in step S1, and flag F is set to 0 in step
S2. In step S3, it is determined whether the vehicle is at a stop. Step S3
is repeated until it is determined that the vehicle is at a stop. When it
is determined in step S3 that the vehicle is at a stop, GPS data on the
present position of the vehicle during the stop are averaged to obtain
averaged values x.sub.i and y.sub.i of X and Y for a given value of i.
That is, when it is a first stop, first averaged values x.sub.1 and
y.sub.1 are obtained. (steps S4 and S5) Then in step S6, it is determined
whether the flag F is 0. When it is determined that the flag F is 0 in
step S6, flag F is set to 1 in step S7, flag i is set to i+1 in step S8,
and then the steps S3 to S5 are repeated to obtain second averaged values
x.sub.2 and y.sub.2. After the second averaged values x.sub.2 and y.sub.2
are obtained, the flag F has been set to 1, and accordingly the flow
process proceeds to step S10. In step S10, the traveling azimuth .theta.
of the vehicle is calculated on the basis of formula .theta.=tan.sup.-1
(y.sub.2 -y.sub.1)/(x.sub.2 -x.sub.1). Then in step S11, the traveling
azimuth .theta.1 detected by the geomagnetism sensor is corrected by the
traveling azimuth .theta. thus obtained on the basis of the GPS data.
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