System for automatic vehicle location

An automatic vehicle location system wherein the position of a mobile unit is at any time (on a "real-time" basis) defined by a stored predetermined location code communicated to the mobile unit from a remote unit at a predetermined location and dead reckoned location coordinates relative to such stored location. When the mobile unit passes into the vicinity of a different remote unit, a new predetermined location code is communicated to the mobile unit from such remote units, and the dead reckoned coordinates are reset to zero. A novel dead reckoning system for use in conjunction with a specific deployment of the remote units is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS 
Of interest are the following copending applications: Ser. No. 576,604, 
entitled "A Homodyne Communicating System" by G. S. Kaplan and A. D. 
Ritzie, filed May 12, 1975; Ser. No. 625,884, entitled "A Semipassive 
Responder Utilizing a Low Voltage, Low Power Drain Reflective Varactor 
Phase Modulator" by J. Rosen, filed Oct. 28, 1975; and Ser. No. 694,043 
entitled "Heading Sensor for Vehicle Dead Reckoning System", by E. 
Jellinek, filed on June 7, 1976, now U.S. Pat. No. 4,055,750, all of which 
applications being assigned to the same assignee as that of this 
application. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to automatic vehicle location systems, and in 
particular to dead reckoning vehicle location systems. 
2. Description of the Prior Art 
Automtic vehicle location (AVL) systems using dead reckoning for 
determination of vehicle locations are well known. In dead reckoning AVL 
systems, an initial position of the vehicle is determined and transmitted 
to a central station. Such initial position or "origin" determination is 
typically made manually or automatically by a signalling device located at 
a predetermined location which causes a code to be transmitted to the 
central station when the vehicle passes in the vicinity of the signalling 
device. The instantaneous location of the vehicle with respect to the 
initial position, or origin, is thereafter computed by utilizing 
directional changes and the distance traveled by the vehicle occurring 
between the time the vehicle passed the initial position and the time of 
measurement. 
A problem which plagues dead reckoning systems is that distance and heading 
errors are typically cumulative. Thus, in order for a dead reckoning 
system to be accurate, the location information must be frequently 
corrected. Prior art systems have attempted to solve such a problem by 
maintaining in storage accurate location coordinates corresponding to 
known drivable surfaces within a predetermined coverage area and 
approximately correcting the vehicle's location in accordance with a 
comparison of dead reckoned locations to known locations of drivable 
surfaces. One such prior art system is described in U.S. Pat. No. 
3,789,198, to Henson et al. issued Jan. 29, 1974. In the Henson et al. 
system each cooperating vehicle is repetitively interrogated at preset 
intervals, on the order of 5 milliseconds long, and the vehicle responds 
by communicating to a central station its heading, and the distance 
traversed since last interrogated. The calculation of dead reckoned 
position coordinates of the vehicle and the comparison of the dead 
reckoned coordinates to known coordinates are performed in a computer at 
the central station. Another such prior art system, developed by Marconi 
Research Laboratories, is described in U.S. Pat. No. 3,947,807 issued Mar. 
30, 1976 to R. A. Tyler et al. and in "UK vehicle tracker is built around 
microprocessor", Electronics, Mar. 6, 1975, page 48. The Marconi AVM 
system utilizes a data processor and cooperating map storage means self 
contained within the vehicle for performing the comparison and correcting 
function. The Marconi system maintains in the map storage means the 
distances between all intersections of drivable surfaces within the 
coverage area, and all angles associated with each intersection as turned 
through from any possible direction. The dead reckoning system utilizes a 
turn sensor as opposed to a heading sensor, and the dead reckoned vehicle 
location is reset to the associated prestored value automatically at each 
turn. Such prior art systems, are disadvantageous in that large amounts of 
data storage are required and in that the data processor is relatively 
complex and expensive. The Henson et al. system is additionally 
disadvantageous in that it requires a very high rate (density) of 
communication between the vehicle and the central station to prevent the 
position error from becoming so large that it cannot be corrected 
properly. 
Another description of the Henson et al. system sometimes known as the 
Boeing "Flair" System may be found in "A Boeing Plot to Find Squad Cars," 
Electronics, Feb. 21, 1974, page 30. A description of various prior art 
systems, including the Henson et al. and Marconi systems, mentioned above, 
may be found in "Decca to enter chancy vehicle location business", by 
Michael Payne, Electronics Weekly, Nov. 12, 1975, page 5. 
Other prior art systems, as noted in the above mentioned U.S. Pat. No. 
3,789,198, have attempted to solve the problem of cumulative errors by 
establishing a plurality of check points within the coverage area which 
cause the vehicle to communicate to the central station a predetermined 
location code as it passes the vicinity of the check point. Such systems 
are disadvantageous for use with a large number of vehicles in that, as a 
practical matter, the time of transmission of such predetermined location 
codes are not controllable and priority conflicts may arise between the 
respective vehicles. Further, a large amount of communication between the 
vehicles and the central station is required. 
SUMMARY OF THE INVENTION 
The present invention provides for an improvement in a system for 
monitoring the position of a vehicle within a predetermined area. Such a 
system comprises a first means within the vehicle to determine the 
instantaneous location coordinates of the vehicle with respect to an 
origin. A plurality of remote units are respectively positioned at 
predetermined locations within the area. Each remote unit communicates to 
the vehicle information indicative of its respective predetermined 
location as the vehicle passes such location. The improvement in the 
system comprises second means, in the vehicle, for receiving the 
communicated predetermined locations information and storing the last 
communicated predetermined location information and wherein further, the 
first means determines the instantaneous location coordinates with respect 
to the stored location.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, there is shown a mobile unit 10 coupled to a 
cooperating vehicle (not shown). Mobile unit 10 includes a suitable 
interrogator 14; suitable storage means 18, a change-in-storage detector 
20; a resettable dead reckoning system 26; and a conventional radio 
transceiver 38. A remote unit 12 is deployed, for example as a signpost, 
at a predetermined roadside location within the coverage area of the 
automatic vehicle location (AVL) system. As will be explained, a plurality 
of such remote units are deployed throughout such coverage area. 
Interrogator 14, directionally transmits a continuous wave signal 15 which 
serves as an interrogation signal to remote unit 12 when mobile unit 10 
comes into the vicinity of remote unit 12. Remote unit 12, in response to 
signal 15, generates a modulated "return" or "reply" signal 17 which is 
radiated back to interrogator 14. Reply signal 17 is modulated in 
accordance with a code word indicative of the predetermined location 
(identity) of remote unit 12. Interrogator 14, in response to reply signal 
17, generates an output signal indicative of the location of remote unit 
12. For a description of suitable interrogators 14 and remote units 12 
reference is made to the aforementioned copending applications Ser. No. 
576,604 filed May 12, 1975 by G. S. Kaplan and A. D. Ritzie entitled "A 
Homodyne Communications Systems", now U.S. Pat. No. 3,984,835. It should 
be appreciated that constantly transmitting remote units such as described 
in U.S. Pat. No. 3,735,335, entitled "Electronic Fence Vehicle Locator 
Transmitter and System Using Same," issued May 22, 1973 to G. S. Kaplan 
and J. Shefer, may also be used. Constantly transmitting signposts would 
be used in conjunction with a receiver in mobile unit 10, such receiver 
replacing interrogator 14. A preferred embodiment of interrogator 14, 
including means to detect the direction of passage of mobile unit 10 with 
respect to remote unit 12, will be hereinafter described in conjunction 
with FIG. 4. 
The output signals of interrogator 14 are applied to storage means 18 
wherein they are temporarily stored. Storage means 18 suitably comprises a 
conventional register. The register may comprise a plurality of fields, to 
facilitate indication of the direction of travel of mobile unit 10 as it 
passes remote unit 12, as well as the location of remote unit 12, as will 
be explained in conjunction with FIG. 4. An alternative suitable storage 
means 18, wherein the storage means comprises a plurality of registers and 
the sequence of the contents thereof operates to indicate the direction of 
travel of mobile unit 10, is described in the aforementioned copending 
application Ser. No. 576,604 by Kaplan and Ritzie. 
Storage means 18 is coupled to change-in-storage detector 20. In accordance 
with the present invention, change-in-storage detector 20 generates a 
signal to reset dead reckoning system 26 to zero, as will be explained, 
each time the contents of storage means 18 change. The contents of storage 
means 18 change, for example, when any of the following events occur: (1) 
when mobile unit 10 first passes a remote unit; (2) when mobile unit 10 
passes a remote unit different from the last passed remote unit or (3) 
when mobile unit 10 passes the same remote unit from the opposite 
direction. Change-in-storage detector 20 suitably comprises a conventional 
storage register 22 and comparator 24. The instantaneous contents of 
storage means 18 are applied to both register 22 and comparator 24 (input 
A). Register 22 is not loaded, however, with the contents of storage means 
18 except on command from comparator 24 as will be explained. Compartor 24 
has applied thereto, in addition to the contents of storage means 18, the 
instantaneous contents of register 22, and operates to compare such 
contents. When the contents of storage means 18 change from a previous 
value, register 22 does not reflect such change but, rather, maintains the 
previous value. Thus, the contents of storage means 18 and register 22 
become different and comparator 24 generates an output signal indicative 
of the occurrance of a change in the contents of storage means 18. Such 
change in storage signal is utilized as the load command signal to 
register 22, to ready register 22 for detection of the next "change in 
storage". The change in storage signal is also utilized to reset dead 
reckoning system 26 to zero, as will be explained. 
Dead reckoning system 26 comprises: a conventional distance increment 
sensor 28, such as an odometer; a suitable heading sensor 30; suitable 
switching logic 32; and conventional bi-directional (up-down) counters 34 
and 36 (hereinafter referred to as X-counter 34 and Y-counter 36). Heading 
sensor 30 generates a signal indicative of the angular heading of mobile 
unit 10 with respect to a reference direction by the direction of travel 
of mobile unit 10 when it passed remote unit 12. Heading sensor 30 may be 
of any known type, such as compass, inertial or two-odometer type heading 
sensors. It is preferable that heading sensor 30 produce a digital signal 
and, in addition, be resettable to facilitate changing of the reference 
direction. For a description of such a heading sensor, reference is made 
to aforementioned copending application Ser. No. 694,043 filed on June 
7,1976. In addition, a preferred embodiment 26a of dead reckoning system 
26 including a preferred form of heading sensor 30, for use in conjunction 
with a specific deployment of remote units is depicted in FIG. 2 and will 
be hereinafter described. 
As will hereinafter be explained in more detail, switching logic 32 
operates to route proportionally, pulses from distance increment sensor 28 
to X and Y counters 34 and 36 respectively, in proportion to and in 
accordance with the cosine and sine of the heading angle represented by 
the output signal of heading sensor 30. Thus switching logic 32 converts 
the polar coordinates defined by the output of distance sensor 28 and 
heading sensor 30 into cartesian coordinates accumulated in counters 34 
and 36. Switch logic 32 may comprise digital multipliers and look-up 
tables, such as ROMs, in a manner well known in the art. In accordance 
with one aspect of the present invention, heading sensor 30, and X and Y 
counters 34 and 36, are all reset to zero by the output signal of 
change-in-storage detector 20. Zero contents in both X and Y counters 34 
and 36 thereafter corresponds to the location of mobile unit 10 when the 
counters are reset. Similarly, a "zero" output signal from heading sensor 
30 thereafter corresponds to mobile unit 10 heading in the same direction 
that it was travelling when heading sensor 30 was reset to zero. Thus, the 
contents of X counter 34 and Y counter 36 are respectively representative 
of the X and Y cartesian coordinates of the instantaneous location of the 
mobile unit 10 on a coordinate system defined by the location and 
direction of travel of mobile unit 10 at the instant dead reckoning system 
26 is reset to zero by change-in-storage detector 20. 
In the operation of an AVL system in accordance with the present invention, 
when mobile unit 10 comes into the vicinity of remote unit 12, the 
following occur. The location code of remote unit 12 is communicated to 
mobile unit 10 and loaded into storage means 18. Storage means 18 also 
contains information indicative of the direction of travel of mobile unit 
10 as it passes remote unit 12 (hereinafter "the original direction of 
passage"). The derivation of such original direction of passage 
information will hereinafter be described in conjunction with FIG. 4. The 
contents of storage means 18 are thus indicative of the location code 
(identity) of remote unit 12 and the original direction of passage of 
remote unit 10 with respect to such remote unit. If remote unit 12 is 
different from the last previously passed remote unit or if remote unit 12 
is the last previously passed remote unit, but was previously passed from 
the opposite direction, change in storage detector 20 operates to reset 
dead reckoning system 26 to zero. The contents of storage means 18 thus 
identify the position and direction of travel of mobile unit 10 at the 
instant dead reckoning system 26 is reset. The contents of storage means 
18 therefore define a rectangular (cartesian) coordinate system having an 
origin at the position of remote unit 12 and a positive Y axis in the 
original direction of passage of mobile unit 10 with respect to remote 
unit 12. The contents of counters 34 and 36 having each been reset to zero 
by remote unit 12 are thereafter indicative of the X and Y coordinates of 
the instantaneous location of mobile unit 10 on that coordinate system. 
When mobile unit 10 comes into the vicinity of a remote unit different 
from remote unit 12, or passes remote unit 12 from the opposite direction, 
the location code of the most recently passed remote unit and original 
direction of passage of mobile unit 10 with respect to such remote unit 
are loaded into storage means 18, and dead reckoning system 26 is again 
reset to zero. Thus, a new coordinate system is established, defined by 
the location of the most recently passed remote unit and the original 
direction of passage with respect to that remote unit. The instantaneous 
position of mobile unit 10 is therefore identified by the contents of 
counters 34 and 36 in conjunction with the contents of storage means 18. 
The contents of storage means 18 identify the present coordinate system, 
while the contents of X and Y counters 34 and 36 are indicative of the X 
and Y coordinates of the instantaneous location of mobile unit 10 on such 
coordinate system. 
To monitor automatically the position of mobile unit 10, the contents of 
storage means 18, X counter 34, and Y counter 36 are transmitted by radio 
transceiver 38, upon command or interrogation, to a central station 40. 
Central station 40, decodes the transmitted information and files it as 
the location of the particular mobile unit at the time of transmission. 
The filed information can be used for various purposes including that of 
displaying vehicle locations. 
It should be noted, that by deploying the remote units on streets 
throughout the AVL coverage area separated by predetermined distances, 
dead reckoning system 26 can be made to reset with known probability prior 
to mobile unit 10 traveling such a distance that the accumulated error in 
the dead reckoned position becomes unacceptable. 
A preferred form of an AVL system utilizing a simplified dead reckoning 
system 26a and a specific deployment of remote units will be described. As 
noted above, heading sensor 30 generates a signal indicative of the angle 
between the instantaneous heading of mobile unit 10 and the original 
direction of passage with respect to the last passed remote unit. 
Switching logic 32 proportionately routes the distance increment pulses 
from sensor 28 to X counter 34 and Y counter 36 respectively in accordance 
with the cosine and sine of the heading angle. In general, heading sensor 
30 must be capable of relatively fine resolution between heading angles. 
Switching logic 32, accordingly must also be capable of such resolution, 
typically involving dividing the distance increment pulses between the 
respective X and Y counters (34, 36) in accordance with sines and cosines 
having values other than unity. Such heading sensors and switching logic 
are relatively complex and costly. 
In urban areas, however, street intersections are typically perpendicular, 
and fine angular resolution is thus only occasionally required. 
Accordingly, a preferred form of the present invention for use in such 
urban areas, utilizes simplified dead reckoning system 26a, which 
quantizes angular heading into 90.degree. increments, with remote units 
specifically deployed at each non-perpendicular intersection within the 
AVL coverage area. Thus, dead reckoning system 26a accurately dead reckons 
the position of mobile unit 10 along a route in which all turns are at 
perpendicular intersections, and is reset and thereby shifted to a new 
coordinate system when a turn is made by mobile unit 10 at a 
non-perpendicular intersection. Thus, in accordance with one aspect of the 
present invention, the requirement for a relatively costly fine resolution 
heading sensor is avoided. 
A preferred form of such a simplified dead reckoning system 26a is shown in 
FIG. 2. In such simplified dead reckoning system 26a, heading sensor 30 
comprises: a turn sensor 50 generating output signals indicative of right 
and left turns, such as doppler radar sensors or a mechanical switch 
coupled to the steering column of the vehicle cooperating with mobile unit 
10; and a two bit bidirectional counter 52, hereinafter sometimes referred 
to as heading register 52. Distance increment sensor 28, and X and Y 
counters 34 and 36 are of the same type as those same-numbered elements 
described in conjunction with FIG. 1. Heading register 52 and counters 34 
and 36 are all reset to zero by the output signal of change-in-storage 
detector 20 as described above. The reference direction of heading sensor 
30 is thus equal to the original direction of passage of mobile unit 10 
with respect to the last passed remote unit. 
Turn sensor 50 operates to increment heading register 52 in response to a 
turn by mobile unit 10 in, for example, a right hand direction and to 
decrement heading register 52 in response to left hand turns by mobile 
unit 10. The contents of heading register 52 thus represent the relative 
heading of mobile unit 10 with respect to the original direction of 
passage as follows: 
TABLE 1 
______________________________________ 
Relative Heading Contents of Heading 
(degrees) Register 52 
______________________________________ 
0 00 
90 01 
180 10 
270 11 
______________________________________ 
It should be noted that when a right hand turn is made, from a prior 
relative heading of 270.degree. (heading register 52 contents 11), two bit 
register 52 will overflow, causing the contents to be equal to 00, 
reflecting thereby a return of the heading of mobile unit 10 to the 
original direction of passage. 
Switching logic 32 is receptive of signals indicative of increments of 
distance traveled by mobile unit 10, and operates to direct such distance 
increment signals to increment or decrement the respective counters 34 and 
36, in accordance with the contents of heading register 52. It should be 
noted that confining the possible heading angles to 0.degree., 90.degree., 
180.degree. and 270.degree. restricts the sine and cosine of the heading 
angle to values of zero, -1, or +1. Thus, the respective values of the 
sine and cosine of a given angle are either equal to zero and plus or 
minus one, or plus or minus one and zero. Accordingly, switching logic 32 
causes the distance signals to increment or decrement counters 34 or 36 in 
accordance with Table 2. 
TABLE 2 
______________________________________ 
Contents of Heading 
Register 52 Switching Logic State 
______________________________________ 
00 increment counter 36 
01 increment counter 34 
10 decrement counter 36 
11 decrement counter 34 
______________________________________ 
Thus, the contents of X counter 34 are incremented or decremented in 
accordance with the distance traveled by mobile unit 10 in a direction 
parallel to the X coordinate, and the contents of Y counter 36 are 
incremented or decremented in accordance with the distance traveled 
parallel to the Y coordinate. The instantaneous contents of counters 34 
and 36 therefore indicate the instantaneous position of mobile unit 10 on 
the coordinate system defined by the contents of storage means 18. 
As noted above, dead reckoning system 26a provides accurate dead reckoning 
of the location of mobile unit 10 so long as mobile unit 10 makes turns 
only at perpendicular intersections. However, dead reckoning system 26a is 
not capable, in itself, of discriminating between turns at perpendicular 
and nonperpendicular intersections. Accordingly, dead reckoning system 26a 
is used in conjunction with remote units specifically deployed at each 
non-perpendicular intersection within the AVL coverage area. Such 
deployment of remote units is illustrated in FIG. 3. 
FIG. 3 is a schematic street map of a portion 60 of an AVL coverage area, 
including north-south running streets 62-68, east-west running streets 
70-78 and skewed street 80. Remote units are deployed at each 
non-perpendicular intersection (82, 84, 86) within coverage area portion 
60 such that dead reckoning system 26a (FIG. 2) is reset to zero whenever 
a mobile unit 10 makes a turn of other than 90.degree.; 88, 90 and 92 at 
intersection 82; 94, 96, 98 and 100 at intersection 84; and 102, 104, 106 
and 108 at intersection 86. Such a deployment of remote units allows 
utilization of simplified dead reckoning system 26a without introducing 
additional error into the AVL. Other remote units, such as remote unit 
110, are positioned on the various streets in the AVL coverage at 
distances calculated to reset dead reckoning systems 26a prior to an 
unacceptable accumulation of error in dead reckoning system 26a, as 
mentioned above. 
With reference to FIGS. 1, 2 and 3, the operation of an AVL system in 
accordance with the present invention utilizing simplified dead reckoning 
system 26a will be described, as mobile unit 10 travels along the route 
indicated by dotted line 136 on FIG. 3. The route indicated by line 136 
is: north on street 62 to street 70, east on street 70 to street 64, north 
on street 64 to street 80, north-east on street 80 to street 66, then 
north on street 66. 
As mobile unit 10 comes into the vicinity of remote unit 110, located on 
the western side of street 62, the identity (location) of remote unit 110 
is communicated to interrogator 14 (FIG. 1). Storage means 18 is loaded 
accordingly. As noted above, the contents of storage means 18 identify 
remote unit 110 and the original direction of passage of mobile unit 10 
with respect to remote 110. Assuming the new contents of register 18 to be 
different from the prior contents, change-in-storage detector 20 resets 
heading register 52 (FIG. 2) and X and Y counters 34 and 36 of dead 
reckoning system 26a. The reference direction and origin for dead 
reckoning system 26a are thus respectively established as north and the 
location of remote unit 110. As mobile unit 10 travels north on street 62, 
the contents of heading register 52 are thus 00, and Y counter 36 is 
incremented in accordance with the distance traveled. 
When mobile unit 10 turns at street 70, turn sensor 50 generates a signal 
indicative of a right hand turn to increment heading register 52. The 
contents of register 52 are thus 01 (indicative of 90.degree. relative 
heading) and as mobile unit 10 travels east on street 70, X counter 34 is 
incremented in accordance with the distance traveled. When mobile unit 
turns to street 64, turn sensor 50 generates a signal indicative of a left 
hand turn, to decrement thereby heading register 52. The contents of 
register 52 are thus again 00, indicative of mobile unit 10 again 
traveling in the original direction of passage (i.e.-north or 0.degree. 
relative heading). Accordingly, Y counter 36 is incremented in accordance 
with the distance traveled as mobile unit 10 travels north on street 64. 
The intersection 84 of streets 64 and 80 is non-perpendicular and 
accordingly, remote units 94-100 are positioned at the intersection. In 
turning onto street 80 from street, mobile unit 10 passes the vicinity of 
and communicates with remote unit 94, storage means 18 is loaded 
accordingly, and heading register 52, X counter 34 and Y counter 36 are 
reset by change in storage detector 20. However, mobile unit 10 almost 
immediately thereafter comes into the vicinity of remote unit 100. Mobile 
unit 10 then communicates with remote unit 100, storage means 18 is loaded 
accordingly and dead reckoning system 26a is again reset to zero. 
Thereafter, the position of mobile unit 10 is defined on a coordinate 
system having an origin established by remote unit 100 and a positive Y 
axis in a north-easterly direction along street 80. The contents of 
heading register 52 are 00 while mobile unit 10 proceeds northeast on 
street 80, and Y counter 36 is incremented in accordance with the distance 
traveled. 
In turning at the non-perpendicular intersection 86, mobile unit 10 passes 
and communicates with remote unit 104 and subsequently with remote unit 
106. Communication between mobile unit 10 and remote unit 106 thereby 
establishes a new coordinate system originating at intersection 86 with a 
positive Y axis running north on street 66. 
The original direction of passage of mobile unit 10 with respect to a 
remote unit can be determined several ways. The original direction of 
passage can be established by storing a sequence of location codes in 
storage means 18, as described in the aforementioned copending application 
Ser. No. 576,604 by Kaplan and Ritzie. Alternatively, the original 
direction of passage can be determined by utilizing remote units which 
communicate different code words to mobile units approaching from opposite 
directions. Such remote units are described in the aforementioned U.S. 
Pat. No. 3,735,335 to Kaplan and Shefer. When a heading sensor 30 is a 
resettable compass type or inertial heading sensor, the original direction 
of passage may be determined by storing a digital number indicative of the 
instantaneous absolute heading of mobile unit 10 as it passes the remote 
unit. In addition, a preferred form of interrogator 14 including 
provisions for determining the original direction of passage of mobile 
unit 10 with respect to a remote unit will be described. In accordance 
with one aspect of the present invention, the remote units are positioned, 
at their respective predetermined location, on predetermined sides of the 
street. Information, specifying the street and side of the street where 
each remote unit is located is prestored in central station 40 (FIG. 1). 
Mobile unit 10 traveling along a given street can pass a remote unit only 
from one of two directions on such street. When mobile unit 10 passes a 
remote unit from a given direction, one side of mobile unit 10 always 
faces the remote unit. When passing the remote unit from the opposite 
direction the opposite side of mobile unit 10 faces the remote unit. Since 
the direction of the street and the side of such street where the remote 
unit is positioned are known, the direction of passage can be determined 
by detecting which side of mobile unit 10 faces the remote unit as mobile 
unit 10 passes. 
A preferred embodiment 14a of interrogator 14, including provisions to 
detect which side of mobile unit 10 faces a remote unit as mobile unit 10 
passes is depicted in FIG. 4. Interrogator 14a includes a suitable radio 
frequency (RF) source 118, coupled through conventional circulators 120 
and 122 to first and second antenna networks 124 and 126. Antenna networks 
124 and 126 preferably comprise printed circuit corporate feed antennas 
such as described in U.S. Pat. No. 3,587,110, issued June 22, 1971 to O. 
M. Woodward. Antenna networks 120 and 122 have directional antenna 
patterns disposed at 180.degree. with respect to each other and are 
directed to irradiate remote units positioned on the respective lateral 
sides of the vehicle cooperating with mobile unit 10. Circulators 120 and 
122 respectively route signals received by antennas 124 and 126 to 
conventional mixers 128 and 130. Mixers 128 and 130 are also receptive of 
samples of interrogation signal 15, suitably via directional couplers 132 
and 134. The output terminals of mixers 128 and 130 are coupled to 
suitable signal detectors 136 and 138 and to the input terminals of a 
suitable logic gate 140. Detectors 136 and 138 are respectively coupled to 
the set (S) and reset (R) terminals of a conventional RS flip flop 142, 
the Q output terminal of which is coupled to the control input of gate 
140. The output terminal of gate 140 is coupled to the input terminal of a 
suitable processor 144. For a description of a suitable processor 144 
reference is again made to copending application Ser. No. 576,604 by 
Kaplan and Ritzie. 
In the operation of interrogator 14a, RF source 118 generates interrogation 
signal 15, which signal 15 is radiated by antennas 124 and 126. When 
mobile unit 10 comes into the vicinity of a remote unit 110, interrogation 
signal 15 illuminates the remote unit, causing the remote unit to generate 
reply signal 17. It should be noted that only the antenna facing the 
remote unit receives reply signal 17. The output signal of the mixer 
corresponding to the receiving antenna is indicative of the location 
(identity) code of the remote unit. The output signal of the mixer 
corresponding to the non-receiving antenna is typically zero. Detectors 
136 and 138 detect which of antennas 124 or 126 receives reply signal 17 
and sets or resets flip flop 142. Accordingly, the Q output signal of flip 
flop 142 is indicative of the antenna receiving reply signal 17; logic 1 
is indicative of antenna 124, logic 0 of antenna 126. Gate 140, in 
accordance with the Q output signal of flip flop 142, routes the 
appropriate mixer output signal to processor 144. 
Storage means 18 for use with integrator 14a comprises a single register, 
including a first field (hereinafter the "identity" field) receptive of 
the location code of the remote unit from processor 144 and a second 1-bit 
field (hereinafter the direction field) receptive of the Q output signal 
of flip flop 142. When the contents of storage means 18 are communicated 
to central station 40, the location code of the remote unit will identify 
the street, the position on the street and the side of the street where 
the remote unit is located. Thus, the original direction of passage is 
established by the contents of the bit representative of the antenna 
receiving reply signal 17. For example, assume that the identity field 
precedes the direction field in register 18 and the location (identity) 
code of remote unit 110 (FIG. 3) is binary 1000. Remote unit 110 is known 
to be located on north-south running street 62 (FIG. 3). When mobile unit 
10 passes remote unit 110 in a northern direction, antenna 124 receives 
reply signal 17, flip flop 142 is accordingly set by detector 136 and gate 
140 passes the output signals of mixer 128 to processor 144. The identity 
and direction fields of storage means 18 are thus respectively loaded with 
the code indicative of remote unit 110 and a logic one. Contents of 10001 
in register 18 is therefore indicative of a coordinate system originating 
at the location of remote unit 110 with a positive Y axis in a northern 
direction. If mobile unit 10 passes remote unit 110 moving in a southern 
direction, antenna 126 will receive reply signal 17, flip flop 30 will be 
reset by detector 138, gate 140 will accordingly pass the output signal of 
mixer 130 to processor 144, and register 18 will be loaded with binary 
10000. Such register 18 content (10000) is accordingly decoded as 
indicating a coordinate system originating at the location of remote unit 
110 but with the positive Y axis in a southern direction. 
Interrogator 14a as depicted in FIG. 4 is designed to cooperate with remote 
units such as described in the aforementioned copending application Ser. 
No. 576,604 by Kaplan and Ritzie. It should be appreciated, however, that 
interrogator 14a can be adapted to cooperate with continuously 
transmitting remote units. In such an arrangement, antennas 124 and 126 
would be connected directly to mixers 128 and 130; RF source and 
circulators 120 and 122 would be omitted, and a local oscillator would be 
coupled to mixers 128 and 130. 
It should be noted that the various interconnections between the elements 
of mobile unit 10 are shown in the drawing as single connectors solely for 
ease of illustration, and may, in fact, be plural connections. 
It should be appreciated that the contents of storage means 18, and 
counters 34 and 36 may be displayed in mobile unit 10 by conventional 
means, for use as a navigational aid to a driver, in addition to or 
instead of being communicated to central station 40. 
It should be apparent from the foregoing disclosure that the present 
invention provides a particularly advantageous AVL system. A minimum of 
digital data storage space is required. In addition, a relatively simple 
dead reckoning system utilizing a simple perpendicular turn sensor is 
provided which does not require computation of the X and Y coordinates by 
determination of trigonometric functions of the heading angle and 
multiplication by the distance signals by fractional numbers. Moreover, a 
minimum of communication is required between the central station and 
mobile units. 
It will be further appreciated and understood that the embodiment of the 
invention described above utilizes a resettable heading sensor which 
provides a signal indicative of changes in heading from a varying 
reference heading, specifically described in the specification as the 
"original direction of passage." Such a heading sensor may be termed a 
relative heading sensor. A heading sensor which indicates a heading in an 
absolute sense, for example, with reference to the compass rose, without 
resetting capabilities and as such having a reference direction which is 
constant, may be used to practice the invention. In the practice of the 
invention utilizing absolute heading sensors, the computation of the 
location of the mobile unit by the computer in the central station 40 will 
be based on a plurality of coordinate systems, fixed with regard to the 
direction of the axes, in which, for example, the x and y axes of each of 
the coordinate systems will be fixed relative to the conventional compass 
points. Accordingly, in such case, the original direction of travel is not 
determined and stored in storage means 18. The origin of each of such 
coordinate system will be defined by the respective locations of the 
remote units.