NBSIR 73-327 (A)

AUTOMATIC VEHICLE LOCATION TECHNIQUES FOR

LAW ENFORCEMENT USE

R. L Fey

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

January 1974

Final Report

Prepared for:

National Institute for Law Enforcement and Criminal Justice

Law Enforcement Assistance Administration

Department of Justice

Washington, D. C. 20537

NBSIR 73-327

AUTOMATIC VEHICLE LOCATION TECHNIQUES FOR

LAW ENFORCEMENT USE

R. L Fey

Electromagnetics Division

Institute for Basic Standards

National Bureau of Standards

Boulder, Colorado 80302

January 1974

Final Report

Prepared for:

National Institute for Law Enforcement and Criminal Justice

Law Enforcement Assistance Administration

Department of Justice

Washington, D.C. 20537

U.S. DEPARTMENT OF COMMERCE, Frederick B. Dent, Secretary

NATIONAL BUREAU OF STANDARDS, Richard W Roberts Director

Contents

Page

Foreword

Abstract

1 . I nt rod uct ion

2. Police Applications of Automatic Vehicle

Location Systems .....

3. Automatic Vehicle Location Techniques

3. 1 3.2

3.3

Dead Reckoning Systems

Proximity Systems . . Radiolocation Systems

Effects

Accuracy

Systems

4. 1

Radio Wave Propagation

1 Very Low Frequency

2 Low Frequency

3 Medium Frequency

4 High Frequency

5 Very High Frequency . . . . 6 Ultra High Frequency . . . . 2 Technical Base for Location

Systems

3 Propagation in Urban Areas . . 4 Types of Radiolocation Systems

4.1 Trilate ration

4.2 Mu 1 1 i I a te ra t i on

4.3 Triangulation

4.4 Hybrid Systems

and Errors in Vehicle Location

Errors

4.1.1 Dead Reckoning

4.1.2 Prox i m i ty

4.1.3 Radiolocation

4.2 Accuracy

4.2.1 AVL Measurement Accuracy ....

4.2.2 Techniques for Improving Accuracy

Location Data Reporting Systems

5.1 Narrowband Communication

5.2 Wideband Communication

5.3 Location Update Rates

5.3.1 Dead Reckoning Rates

5.3.2 Proximity Rates

5.3.3 Radiolocation Rates

I V

I

I

2

3

3

4

4

4

55

6

6

6

6

7

7

9

9

I 3

I 3

I 4

I 5

I 5

I 5

I 6

I 6

1 9

20

2 1 22

22

23

24

24

i i i
6. Information Displays 25

7. Commercial AVL Systems 25

7.1 Dead Reckoning 26

7 . 2 Prox i m i ty 26

7.3 Radiolocation Systems 26

7.3.1 Phase Tr i I aterat I on 26

7.3.2 Pulse Tr i I aterat i on 27

7.3.3 Navigation Systems 27

8. Radio Spectrum Utilization 28

9. System Costs 30

10. Comparisons of AVL System Techniques 30

Appendix A--B i b I I og rap hy 31

iv
FOREWORD

The National Bureau of Standards (NBS) has established

a Law Enforcement Standards Laboratory (LESL) for the National

Institute of Law Enforcement and Criminal Justice (NILECJ) of

the Law Enforcement Assistance Administration, Department

of Justice. LESL's function is to conduct research that will

assist law enforcement and criminal justice agencies in the

selection and procurement of quality equipment.

In response to priorities established by NILECJ, LESL

is (I) subjecting existing equipment to laboratory testing

and evaluation and (2) conducting research leading to the

development of several series of documents, including national

voluntary equipment standards, user guidelines, state-of-theart surveys and other reports.

The Electromagnetics Division of NBS prepared this

report under the direction of Marshall J. Treado, Program

Manager for Communication Systems and Jacob J. Diamond,

Chief of LESL.

This document. Automatic Vehicle Location Techniques

for Law Enforcement Use, describes various techniques used

by law enforcement agencies to automatically monitor and

locate vehicles. It is intended to serve as a background

reference or information guide for users and prospective

users of such equipment.

V

AUTOMATIC VEHICLE LOCATION TECHNIQUES

FOR LAW ENFORCEMENT USE

ABSTRACT

Automatic vehicle location systems employing

communications techniques are presently under development by

several companies. This report discusses such systems with

respect to law enforcement applications. It presents the

principles upon which the various automatic location

techniques are based, summarizes their important

characteristics, and compares them as candidates for law

enforcement use. This report discusses techniques,

available system performance, and the related aspect of

frequency spectrum requirements for these systems. It also

mentions areas where more operational information is needed

before systems for law enforcement operations can be clearly

defined.

1. INTRODUCTION

In order to dispatch patrol cars efficiently, some

knowledge of car location is necessary. The earliest car

location method used was to divide the city into sections,

known as beats, and to assign a patrol car to each beat.

The only assumption made was that the car would be found

somewhere within the beat. Present dispatching methods have

evolved from this beginning. Since it is usually

advantageous to dispatch the car nearest to a scene,

identifying it is important. Frequently a car from another

beat is the nearest one available for dispatching to a given

location. Verbal location by radio is time consuming and Is

poor management of the radio spectrum. An automatic vehicle

location (AVL) system can save time and improve spectrum

usage. By keeping the dispatcher continuously Informed of

patrol fleet deployment, an AVL system can also Improve

coordination of police activities and make possible the

development of improved dispatching strategies.

1 .
2. POLICE APPLICATIONS OF AUTOMATIC VEHICLE LOCATION SYSTEMS

A variety of car location techniques have been suggested

and some have undergone considerable development, although

none have yet undergone large scale testing. Activity in

this field has been stimulated considerably by the report of

the President's Commission on Law Enforcement and

Administration of Justice, February 1967. The Task Force

Report, "Science and Technology," lists four possible types

of vehicle location techniques:

a . A system of patrol car emitters and ca 1 1 box sensors.

b . A modified radar transponder system.

c . A med i um- f requency radio direction-finder system.

d . A car-borne position computation and reporting

system

.

Subsequent developments in the field have made it

appropriate to alter this classification somewhat. The

radar technique as described has not been fully developed.

Parts of the radar transponder scheme, however, have been

incorporated into some of the radio location systems. The

radio direction finder technique may be considered a subclass of a rather extensive class of radiolocation

techniques which has been developed. The first and last

techniques listed above have been pursued and are known

respectively as proximity and dead reckoning systems.

In this report, AVL techniques are classified into three

categor I es

:

1. Dead Reckoning Systems (as in item d above). The

vehicle carries instruments to measure direction

and distance traveled. From these data a computer determines the

estimated vehicle position.

2. Proximity Systems (as in I tern a above). These

systems Identify a vehicle when It passes near certain fixed

locations. These locations may be equipped with either

sensors or emitters for signaling the vehicle, usually by

radio.

3. Radiolocation Systems (as in item c above). These

systems use the unique characteristics of radio fields

themselves to establish vehicle location, as contrasted to

use of radio to report vehicle location. They must employ

more than one fixed radio terminal in order to operate.

Tr 1 I aterat 1 on , t r 1 a ng u I at i on and navigation systems are

Included in this category.

2.
For law enforcement use, an AVL system must provide

location information to a central dispatcher, usually

through radio links. Location information must be presented

to the dispatcher in a form useful to him. Generally this

would be a map display of the city on which is indicated the

vehicle's identification and location as it moves throughout

the service area. A television type display is usually the

primary form of presentation. Where computer-aided

dispatching is included. Interfacing with that system is

necessary. For emergency situations when a patrol officer

requires assistance, AVL systems can be provided with an alarm capability. This enables the officer to signal for

help at his Indicated location.

A variety of AVL systems are now being offered. They

differ In the accuracy achieved, radio spectrum needed, rate

of update of location Information, ability to accommodate

new vehicles In the system, size of vehicle fleet which may

be accommodated, area which may be covered, and tolerance to

problems caused by various urban circumstances. Since AVL

systems are not yet available In commercial production

quantities, and system specifications depend on Individual

Installation circumstances, definite cost figures are

lacking. However, estimates can be made and trends can be

identified.

3. AUTOMATIC VEHICLE LOCATION TECHNIQUES

3.1 Dead Reckoning Systems

These systems obtain necessary location data using

Instrumentation carried entirely within the vehicle and need

no Interaction with externally placed system elements. This

Instrumentation usually consists of an odometer to measure

the distance traveled and a compass to measure the direction

of t rave I

.

As the vehicle travels, the data produced by these

Instruments are processed to derive the vehicle's position

with respect to Its starting point. Data processing may be

done within the vehicle, or the data may be relayed to a computer at the central dispatch location for processing.

Since the system operates without referring to external

coordinates, the vehicle's Initial position on the map must

be Inserted In order for Its computed path to correspond to

Its position on the map. Errors In the odometer and errors

In the compass heading cause cumulative errors In the

vehicle's calculated location which eventually cause the

vehicle to become "lost." Such errors are unavoidable In

any system which operates over a period of time without an
external reference. An analogous case is a free running

clock. It must be reset sooner or later if its error In

time is to remain within bounds. Therefore a dead reckoning

AVL system should include a means for correcting errors

before they become too large. One demonstrated method makes

use of the fact that a vehicle is confined to a drivable

surface in a city. Ordinarily this would be the network of

city streets. A street map of the city is stored in

computer memory. The indicated vehicle position is compared

with the map, and path discrepancies are corrected to cause

the vehicle to appear on the nearest appropriate street.

This technique is known as map-matching.

3.2 Proximity Systems

Proximity systems require the vehicle to pass near one

of a set of specially equipped "sign posts" distributed

throughout the city, in order for its location to be

identified. Ordinarily a low-powered microwave transmitter

is mounted on the sign post, and an encoded location address

is transmitted continuously. As the vehicle passes nearby,

it receives the address signal and decodes it. This

location information is stored in the vehicle and sent to

•the central dispatch location by radio when requested.

An inverted proximity system may also be used. In this

form the vehicle sends its identity code continuously by

means of a low power transmitter. When the vehicle passes

near a sign post equipped with a receiver, the identifying

signal is detected and relayed to the central dispatch

location, usually by telephone lines. Police call box

circuits or fire alarm circuits appear to be particularly

suitable for this use since they are readily available in a number of locations throughout most cities.

3.3 Radiolocation Systems

3.3.1 Radio Wave Propagation Effects

Most AVL methods make use of radio propagation both In

the location process and for sending location information to

the dispatcher. A summary of radio propagation effects

f o I 1 ows

.
Radio waves usually travel from the transmitting antenna

to the receiving antenna by several different routes. In

free space they propagate directly from the antenna,

decreasing in amplitude with distance. This is known as

free space or space wave propagation. If the antennas are

both near the ground, this provides a means for another mode

of propagation: a wave which travels along the ground and

is called a ground wave. The groundwave attenuation is

lowest for the lowest frequencies. A third type of wave is

possible because the earth is surrounded by free electrons

In a region above the atmosphere known as the ionosphere.

The ionosphere electrons tend to be concentrated in several

layers. Radio waves are reflected from these layers under

certain conditions and are returned to the earth. Such

waves are called ionospheric waves or sky waves. One or

more of the three types of wave is involved in any radio

communication system. However, propagation effects depend

strongly on the frequency used. For convenience, the radio

frequency spectrum has been divided into bands which have

been given descriptive names. The most important

characteristics of these bands follow.

3.3.1.1 Very Low Frequency, VLF, 3-30 kHz

This is the lowest frequency band of interest in

automatic vehicle location. It has the advantage of stable

and reliable propagation over very long distances, up to

worldwide. Both ground waves and ionospheric waves are

present for distances up to several thousand miles; after

that, only ionospheric waves are important. The ionospheric

waves are those reflected from the lowest, most stable layer

of the ionosphere. This band can be used for communication,

but only at a very low information rate. The long range

navigation system called Omega operates in this band. It is

useful for ocean navigation, but reducing its uncertainty to

less than a mile is difficult.

3.3.1.2 Low Frequency, LF, 30-300 kHz

This band, similar to VLF, is also characterized by

stable long range propagation, but much less than worldwide.

Higher communication rates are possible. The Loran C

navigation system (see bibliography) operates in this band

with considerably better accuracy than Omega but over a sma I I er a rea

.
3.3.1.3 Medium Frequency, MF, 300-3000 KHz

The ground wave coverage in this band continues to

decrease until it is only a few tens of miles at the high

end of the band. Lonospheric waves reflected from the upper

layers of the ionosphere become important for long distance

transmissions. However, these layers are generally unstable

and change greatly from day to night, over the yearly

seasons, and with sun spot cycles. The AM broadcast band

occupies the lower end of this band.

3.3.1.4 High Frequency, HF, 3-30 MHz

Ven/ long distance transmissions are sometimes possible

in this band through ionospheric reflections, subject to the

same kinds of uncertainties as in the MF band.

3.3.1.5 Very High Frequency, VHF, 30-300 MHz

As the frequency increases, free-space type waves which

travel over I i ne-o f - s i g h t paths become predominant. At the

lower end of this band ionospheric reflections sometimes

account for very long distance transmission. In the middle

of this band the free space waves are bent by the

atmosphere, sometimes resulting in very long distance

transmissions. At the higher end of the band the free space

waves are sometimes trapped by the multi layered ionosphere

and reflected, or are reflected by the troposphere , again

resulting in very long distance transmissions. These bending

phenomena are affected by varying weather conditions such

as atmospheric temperature and water vapor content. This

dependence increases as the frequency increases. In urban

areas, wave reflections occur from buildings and other objects.

Signals arriving by means of these reflections interfere with

the direct line-of-sight waves, sometimes causing reinforcement

and sometimes nearly complete cancellation due to destructive

interference. These effects are known as multipath fading.

3.3.1.6 Ultra High Frequency, UHF, 300-3000 MHz

The direct waves are most important here. Multipath

phenomena also remain important in urban areas. If a receiving antenna is moving, multipath fading occurs more

often at the higher frequencies.
3.3.2 Technical Base for Location Systems

Radio signals have two important properties which make

them useful in position location or navigation systems.

First, the velocity of propagation of radio waves is

essentially constant at the speed of light. This allows a measurement of the time it takes for a radio wave to travel

from transmitter to receiver to be converted to a measurement of distance. Methods which employ this

principle are known as tr i I aterat i on methods (described in

paragraph 3.3.4.1). The term is also used to describe

vehicle location systems which employ this principle.

Second, radio waves travel essentially in straight lines

from a transmitting antenna. This allows a measurement of

the wave direction to be made at the receiver. When plotted

on a map, this becomes the direction to the transmitter from

the receiver. Methods which employ these measurements are

known as t r i a ng u I a t i on methods (described in paragraph

3.3.4.3). As before, the same term is used to describe

vehicle location systems which employ this principle.

3.3.3 Propagation in Urban Areas

The presence of buildings and other man-made structures

in cities significantly affects propagation characteristics.

Furthermore, man-made electrical noise and interference are

concentrated in the cities. These effects Influence the

operation of radiolocation AVL devices In urban areas,

generally causing degradation of accuracy and reliability.

The effects apply in two different ways, first through

reflection of radio waves from man-made structures and

second by attenuation of the waves in passing through such

structures. These effects are a function of the length of

the propagated wave relative to the dimensions of the manmade structures involved,

3.3.3.1 Multipath

In the VLF and LF bands, the wavelengths used vary from

about 20 miles to a mile or so. Only long metallic

structures such as power lines, metallic bridges, and so

forth have dimensions comparable to an appreclabl'e fraction

of a wavelength and can affect propagation appreciably.

When waves are reflected or reradiated, the phase of the

signal Is altered. In addition, if a receiving antenna is

located near or under large metallic structures, the VLF or

LF signal received may be attenuated significantly.
In the VHF and UHF bands, where the wavelengths are a few meters or less, wave reflections are more common. These

reflections occur mostly from buildings but also from the

ground or pavement. The result at a receiving antenna is

the reception of the same signal over numerous paths. The

direct path transmission, being the shortest distance,

occurs with the least time delay; the others occur with

increased delays up to 10 microseconds or more,

corresponding to reflections from several miles away. For

radio reception in urban environments this causes a problem.

The additional multipath signals alter the phase of the

received signal and also interfere with the direct signal,

causing it to fade. Severe fading of the signal often

occurs as the receiving antenna is moved through a distance

comparable to a wavelength. On the average, the multipath

effect causes a VHF or UHF radio wave to be attenuated more

than if it were propagating in free space. This attenuation

increases as frequency increases.

3.3.3.2 Noise

All radio systems are limited in their performance by

background noise. In order to be usable, a received signal

must be distinguishable from this noise. Figure 1 shows

typical values of various types of noise. In quiet rural

areas, background noise is due to thunderstorm activity,

which is known as atmospheric noise, and to galactic noise,

which comes from regions outside the earth. At frequencies

below about 30 MHz atmospheric noise predominates. Its

intensity fluctuates greatly, depending on the distance of

the receiving antenna from significant thunderstorm regions.

This causes atmospheric noise levels to undergo seasonal and

diurnal changes and to vary depending on location on the

earth. In urban areas, man-made noise predominates. All

three types of noise increase at the lower radio

frequencies. On the average, man-made noise in populated

rural areas is about 15 dB higher than noise in unpopulated

rural areas, about 5 dB less than noise in residential

areas, and about 9 dB less than noise in business areas.

Man-made noise is significant at frequencies below 100 MHz.

In specific locations, man-made noise above 20 Ml-iz is

correlated with traffic density, while noise below 20 MHz is

correlated with power consumption. Atmospheric noise

increases more rapidly, by 50 to 60 dB as frequency Is

decreased by a factor of 10, and becomes noticeable below 30

MHz .

8.
3.3.4 Types of Radiolocation Systems

3.3.4.1 Tr i I aterat I on

Of the various approaches taken by manufacturers In

attempting to develop AVL systems, t r i I ate rat I on has been

the most common. Some variation In detail is possible but

all such systems require radio transmissions between the

vehicle and three fixed locations. The installations at the

three fixed locations may consist of radio transmitters, in

which case the vehicle receives transmissions from each of

the three locations; or the vehicle may transmit and the

three fixed locations receive. In the case of VLF or LF

systems, the vehicle must receive, because at such long

wavelengths a suitable transmitting antenna would be

impractical for a vehicle. Several VLF and LF systems are

already in operation for ship and aircraft navigation and

are known as Navigation Systems. At VHF and UHF frequencies

either scheme may be used, but most commercial systems use

the vehicle to transmit. This eases the data handling and

equipment problem since, if the vehicle receives, the

received Information must be retransmitted to the dispatch

location, with or without processing on board the vehicle;

while if the vehicle transmits, receivers are only necessary

at the three fixed sites, and all data processing may be

done at a central location. In principle, the frequency

bands between LF and VHF could be used for vehicle location

purposes, but the present crowding of these bands and the

possibility of Interference over very long distance paths

makes them unattractive for such use.

Whether a vehicle transmits or receives, t r i I aterat i on

vehicle location systems employ the same geometry and are

based on the fact that radio signals travel with a constant

velocity. Thus a knowledge of the time taken for a radio

signal to propagate over a given path allows the length of

that path to be calculated. This becomes the basis for the

most straightforward, although rather expensive, vehicle

location system, often called the circular or range-range

system

.

3.3.4.1.1 Range-Range System

This system requires each fixed site and the vehicle to

have access to accurately synchronized clocks. Since radio

waves take about 5 microseconds to travel one mile, clock

synchronization accuracy must be to within a small fraction

of 5 microseconds to attain a vehicle location accuracy of a small fraction of a mile. Then, merely by noting the time

of emission of a radio signal and its time of reception, its

travel time over a given path and the path length can be

determined. To insure that both time of transmission and

time of reception are known, this information can be

9.
sent by radio link, or the transmitter can emit on a predetermined schedule. Once the distance is known, the

vehicle location must be on a circle with its center at the

fixed site and the known distance as its radius (see fig.

2). The same is true of another circle about a second fixed

site. Now the vehicle must be at an intersection of the two

circles in order to lie on both circles. Since, in general,

two circles intersect at two points, a third fixed point

with its circle must be used to resolve this ambiguity and

determine the actual vehicle location. Although the rangerange system has some attractive features, high quality

clocks are very expensive, and another system exists which

does not require a vehicle clock. It is known as the

hyperbolic, or range difference system.

3.3.4.1.2 Hyperbolic System

In this system, single path propagation times cannot be

measured. Instead, the difference in propagation time for

signals traveling over two different paths is measured.

This can be done if there are synchronized clocks at the

three fixed sites, whether the vehicle transmits or

receives. A given time difference, corresponding to a given

path distance difference between the radio transmissions

over two paths, means that the vehicle must be somewhere on

a curve which has that constant difference in distance from

the two fixed sites. This curve is a hyperbola rather than

a circle. Using the third fixed site plus one of the

original two fixed sites, another time difference and thus

path difference may be determined. This results in another

hyperbolic curve on which the vehicle must lie. The

intersection of the two hyperbolas corresponds to the actual

vehicle I oca t ion.

3.3.4.1.3 Navigation Systems--Loran , Omega, Decca

Two extensive long-distance navigation systems are being

operated by the U.S. Coast Guard. Loran C is a pulse

ranging system operating in the 100 kHz region. A Loran C

chain consists of three or four transmitters. Presently

eight chains exist, giving coverage over important portions

of the Pacific, the Atlantic, and the Mediterranean.

However, good coverage exists only over the eastern third of

the continental U.S. The Omega system is a phase ranging

system operating in the 10-14 kHz band and presently employs

four transmitters. These transmitters cover the continental

U.S. Four more transmitters are planned to provide

worldwide coverage. Shorter baseline systems using either

Loran or Omega methods could be set up to cover smaller

regions for vehicle location systems. An LF phase ranging

system called Decca, employing techniques somewhat similar

to Omega, has been developed in England and provides

extensive European coverage. All these navigation systems

use the hyperbolic mode.

10.
3.3.4.1.4 Tr i I aterat I on Signal Formats

In t r i I aterat i on AVL systems, two types of modulation

are being used. These are either pulse modulation, known as

pulse t r i I a te rat i on ; or sine wave modulation, known as phase

t r i I aterat i on

.

3.3.4. 1 .4. 1 Pulse

The use of f ast-r i set i me pulses at a suitable rate is

the most straightforward modulation method. It has an advantage over phase t r i I a te ra t i on in its ability to

discriminate against errors caused by multipath propagation.

This is because the essential information contained in the

transmission is the time of occurrence of the leading edge

of the pulse. It is necessary for the pulse to arrive at

the receiver by the direct free space route in order for the

distance between transmitter and receiver to be directly

proportional to pulse propagation time, which is the

measured quantity. If the pulse arrives after being

reflected from a building or other object, that is, by

multipath, it will have traveled a longer distance and taken

a longer time than by the direct route, thus indicating a greater distance than is correct. However, in urban areas,

usually enough signal arrives by the direct route to enable

a time measurement to be made on the leading edge of the

direct-path pulse. Then multipath only has the effect of

lengthening the pulse as received or, if the indirect path

is long enough, to cause one or more pulses to appear

following the initial pulse. These spurious effects can be

largely ignored and errors greatly reduced by the use of

proper measuring circuitry. The chief errors occur when the

direct-path signal amplitude is so low that the earliest

multipath pulse is the-one on which the time measurement is

made. This always causes an over-estimation of path

propagation time. The chief disadvantage of this method is

the wide bandwidth, approximately 10 MHz, needed to transmit

a pulse with suitably short rise time, approximately 0.1

m i croseconds

.

3.3.4.1.4.2 Phase

In contrast to the pulse method, phase t r i I a te ra t i on

uses sine wave modulation and requires a relatively low

bandwidth. However, instead of the time of occurrence of a fast rise pulse, the essential information is now contained

in the time of occurrence of zero-crossings of a low

frequency sine wave. This information is obtained at the

receiver by measuring the phase of the sine wave. The

effect of multipath is to add reflected sine waves to the

directpath sine wave at the receiver. The phase of the

resultant is now the only information available, and no

method exists to separate the direct wave from the reflected

waves. The errors are considerably larger on the average

than with pulse t r i I ate ra t i on , and can cause underestimation

of path propagation times as well as ove res t i ma t i on

.

1 1 .
3.3.4.1.5 Data Exchange

The three fixed tr i I ate rat I on system sites generally

must be provided with means for data exchange among them.

In addition, if transmitters are located at these sites, it

Is preferable, but not essential, that they operate from

synchronized clocks. Details vary according to the system

type, but generally two types of communication channels may

be employed. One is a data channel; the other is a clock

channel which must have a time delay stability suitable to its use

for clock synchronization or comparison.

3.3.4.1.6 Fixed Site Used for Transmitting

If the three fixed sites employ transmitters, system

operation is simplified if the transmitter clocks are

synchronized. This may be accomplished in several ways. A

separate communication channel can be used to send

synchronizing pulses from one transmitter site to another.

Such channels must have sufficient bandwidth and known

signal delay characteristics. Alternatively, each broadcast

itself can be monitored at the other transmitter locations,

allowing time adjustments to be made to keep all clocks in

synchronism. This procedure is simplified if one station

clock is designated as the master and the others are

adjusted to agree with it. The system can also operate if

the transmitter clocks are unsy nch ron i zed . In that case a monitor is necessary to determine the disagreement, or

differences, among the clocks at the various transmitters.

These differences must then be incorporated into each

computation of vehicle location, and this information must

be suppi led to the faci I i ty where these computations are

made. A data communication channel, in contrast to a stable

time synchronization channel, is all that is needed for this

purpose.

3.3.4.1.6.1 Loran and Omega

The only radio location systems presently under

discussion employing fixed sites used for transmitting are

the VLF and LP Omega and Loran systems. The Omega

transmitters are kept in synchronism by monitoring al I the

broadcasts to determine what individual station adjustments

are necessary. In the case of Loran, each chain consists of

a "master" transmitter and two or three "slaves." Their

broadcasts are monitored, and the slaves are adjusted

individually to agree with the master.

1 2 .
3.3.4.1.6.2 Differential Loran and Omega

For improving Loran or Omega system accuracy over

limited areas, monitoring stations may be employed in a different mode. Instead of being used to determine

transmitter adjustments, they can be used to measure local

variations in received signals due to propagation changes.

This information can be disseminated and used to refine the

position estimation made by a navigation receiver or a vehicle location system. Systems employing this technique

are known as differential Omega or differential Loran and

are sti I I under development.

3.3.4.1.7 Fixed Site Used for Receiving

In this case two modes of operation exist. The field

sites may contain clocks which are synchronized through

suitable clock channels. The times of reception of the

vehicle signal may then be determined at each site. This

information is sent by data channel to the facility where

the vehicle location computation is made. Alternatively,

the transmissions from the vehicle may be received at each

site and relayed, or transmitted, by clock channel to the

vehicle location computation facility.

3.3.4.2 Multi lateration

The minimum number of fixed sites required of radio

location systems which operate by measuring signal

propagation time is three. Additional stations may be used

both to provide a greater area of coverage and to improve

system accuracy through increased data acquisition. Such

systems are frequently called mu 1 1 i I aterat i on systems.

3.3.4.3 T r i a ng u I a t i on

AVL systems based on measurement of the direction of

arrival of radio signals are known as t r i a ng u I a 1 1 on systems

A signal may be transmitted from a vehicle and received at

fixed site where its direction of arrival is determined.

This determines a position line on which the vehicle must b

located. A similar measurement at a second fixed site

determines another position line. The vehicle is located

where the two lines intersect. This procedure is not very

accurate when the vehicle is located on a line between the

two fixed sites, so a third fixed site is necessary. The

same technique may be used if the vehicle receives

transmissions from the fixed sites. The technique for

determining the direction of signal arrival employs

directional antenna systems similar to those that have been

13.
extensively used in radio direction-finding systems.

Multipath reflections from objects close to the transmitter

do not greatly change the direction of arrival of the signal

and therefore do not cause significant errors. However, the

opposite is true of reflections close to the receiver.

Therefore, it is advantageous to place the transmitter in

the vehicle and locate the receiving antenna away from

nearby reflection points.

3.3.4.4 Hybrid Systems

For some applications it may be advantageous to combine

elements of two systems to improve overall system

performance or to overcome specific problems.

Because the dead reckoning system depends on having a correct initial reference point, a temporary system failure

produces more drastic restart problems than it would with

the other systems. One suggestion to reduce this problem

might be to provide a limited number of proximity sign posts

which would then provide reca I i b ra t i on points at intervals

throughout the service area.

Alternatively, a proximity system could be provided with

a simple odometer dead reckoning arrangement to Interpolate

position between sign posts in order to improve system

accuracy. If this is done on a fixed route vehicle, such as

a bus, it could provide accurate position information

continuously. Its utility for random route police patrol is

doubtful

.

Another method which has received attention is to add a signal amplitude measuring capability to a proximity system.

This would permit making an estimate of a vehicle's distance

from the sign post based on received signal amplitude. The

average attenuation rate of signal levels vs. distance in an urban environment is known. Although high accuracy is not

possible, the knowledge that the vehicle is nearer or

farther than a certain range would be of considerable value,

and perhaps not too difficult to obtain.

Another way of combining systems could be to use one

type in a certain area of a city and another type in a different area. For instance, the downtown high-rise area

of a city is a very difficult multipath and interference

environment for radio t r i I aterat i on systems. It is also the

area where patrol beats are smallest. To provide the same

advantage, an AVL system must have higher accuracy in a sma I I beat than In a large beat. Thus, where the highest

accuracy is needed, t r i I ate rat I on systems have the most

difficulty providing it. On the other hand, performance of

14.
proximity systems is unaffected by high-rise environments.

They can be placed as close together as needed to obtain

accuracy, and lowering transmitter power to avoid

interference. Since the downtown high-rise area is a small

part of a city, the number of sign posts required may not

increase the cost excessively. A t r i I ate rat i on system could

then provide coverage for the rest of the city. Some

elements of the two systems could be shared, such as their

communication facilities.

4. ACCURACY AND EFFORS IN VEHICLE LOCATION SYSTEMS

Accuracy is a measure of how correctly a vehicle's

location is indicated by the vehicle location system. In

order to evaluate system performance and compare different

systems, a quantitative specification of accuracy is

necessary. To be meaningful, this specification must be

based on a realistic model of system behavior with respect

to errors. Such models will differ depending on which of

the three types of systems is being considered: dead

reckoning, proximity, or radiolocation. The main

characteristics of error performance of these systems are

discussed in the following paragraphs.

4 . 1 Errors

4.1.1 Dead Reckoning

The dead reckoning system which has been demonstrated

depends on the use of map matching to correct errors;

otherwise, errors would increase indefinitely as the system

operated. Generally, as the system operates, errors

gradually accumulate until they exceed a certain

predetermined amount. Then they are corrected in one step,

indicating that the vehicle is back on a street on the map.

Lateral errors, where the vehicle path appears to gradually

depart from a street on an angle, are caused by compass

heading errors and are easily recognized. Distance errors,

where the vehicle appears to go faster or slower than Its

actual speed, are due to odometer errors and cannot be

recognized as long as the vehicle travels in a straight

line. Their only manifestation would be a velocity error.

However, the map matching system does detect these errors if

the vehicle appears to make a turn where no cross street is

indicated on the map. In such a case, if the distance error

is less than half a block, the error would be properly

corrected by the system. If the error is greater than half

a block, the system would change the indicated vehicle

position to be on the nearest, but incorrect, street. By

employing more sophisticated computer software, even these

errors can be later corrected when further path

discrepancies are noted. The most serious problem would

occur if the computer failed and lost all memory of the

15.
vehicle position. This problem could be eventually

corrected if the city nap remained in memory and if vehicle

position data could be reinserted into memory.

4.1.2 P rox i m i ty

In this system the city can be considered to be divided

into a set of continuous cells with a sign post In each. In

the ideal system, the cell boundaries would be located such

that as a vehicle crossed one, it would begin to communicate

with the sign post within the new eel I and fai I to

communicate with the sign post in the old cell. If sign

posts are placed too close together, the vehicle would

sometimes be indicated in two cells simultaneously. If the

sign posts are too far apart, there would be an area between

posts when the vehicle would not be recognized at all,

although its last indicated position would remain in memory.

In this case the dispatcher would continue to assign a vehicle according to its last position. System accuracy for

proximity systems is evidently a function of cell dimension,

being some fraction of cell radius.

4.1.3 Radiolocation

Radiolocation systems use a continuous grid or

coordinate system which overlays the service area. Usually,

this consists of a set of hyperbolic curves, or lines of

position, each with constant propagation time or phase

difference between signals propagating between the vehicle

and a pair of fixed sites. A vehicle position determination

consists of measuring at least two of these differences,

locating the appropriate curves and finding their

intersection. In an AVL system this operation is carried

out by a computer. Location resolution depends on how

accurately these time differences for pulse ranging systems,

or phase differences for phase ranging systems, can be

determined. Their accuracy depends on several factors

including the effects of noise and interference, multipath

signals, skywave or groundwave propagation effects, and

instrumentation errors.

Noise and interference effects can be reduced by

increasing transmitter power and narrowing receiver

bandwidth. If the frequency of coherent interfering signals

is close enough to the desired signal to be within receiver

bandwidth, the signals will not be rejected and are likely

to cause errors. However, narrowing the receiver bandwidth

Increases system response time, and thus the measuring time,

and so cannot be carried too far.

16.
Multipath signals affect pulse ranging systems less than

phase ranging systems. Less Information Is available about

direction finding systems In urban environments, but they

may provide some discrimination against multipath signals.

Skywave effects are unlikely except with VLF systems

such as Omega where they are Inherent in system operation,

and their fluctuations account for much of the system error.

Skywave effects occur also with the LF Loran C system,

appearing as a delayed replica of the transmitted pulse

being received after the desired groundwave pulse. As In

any pulse ranging system, such undesired delayed signals can

be ignored by using circuits which measure only the early portion

of the pulse.

Ground wave effects are also possible with the VLF and

LF Omega or Loran C type systems. Variations in ground

conductivity cause small changes In propagation velocity.

Irregular terrain also causes variations. Very little

investigation of such phenomena has been carried out for

Omega, but experiments have shown appreciable effects for

Loran C.

Instrumentation errors depend on the particular system

concept and design. Generally, time and phase delays in the

receiver front end must be stable, or a means provided to

calibrate for variations. It is difficult to maintain

constant time or phase delays when signal amplitude varies.

In pulse systems the transmitted pulse shape must be

maintained constant.

Each line of position can be determined within some

value of error, depending on how accurately the

corresponding time difference can be measured. Then the

error in a position determination, made by finding the point

of intersection of the two lines of position, is dependent

on the crossing angle of these lines. If they cross at

right angles, the least uncertainty results. If they are

nearly parallel, crossing at an acute angle, a large

uncertainty results. This is a geometric factor which

depends on the relationship of the vehicle position to the

fixed site locations. The effect Is referred to as geometric dilution of precision.

Because the coordinate system is self-contained In a radio location system such as Loran C or Omega, a vehicle

location determination can be made without referring to

stored information concerning the vehicle's previous

location. Thus, each determination at a new location becomes

a separate measurement of vehicle location. Computer

failure will Interupt system operation; but when the

computer Is restored, the positioning operation can resume

immediately with no need for external Information to

reestablish the vehicle's previous position.

17.
4 . 2 Accuracy

The concept of accuracy includes the effects of

unpredictable variations in system performance and is

necessarily statistical in nature. For vehicle location

systems the error is usually expressed as a certain distance

interval from the vehicle's true location, and a confidence

level is given. For example, a performance specification

used by the Department of Transportation (DoT) for mass

transit purposes is that "System accuracy shall be such that

the vehicles' location, as estimated by the system, shall be

within 500 feet of true location with 95 percent

confidence." The confidence level is a statistical quantity

determined from past system performance. It is obtained by

making a series of checks of an operating system to

determine vehicle location errors at specific points

throughout the system service area. The result of such a test will consist of a set of errors distributed over a range of values. The distance for which a specified

percentage of these errors, for instance 95 percent, is less

than or equal to a certain value can be determined. If the

check points have been chosen at random, and if system

performance or service area conditions do not change, the

percentage of points with values no greater than a given

error will be a quantity which can be repeated in subsequent

tests. The agreement from test to test wi I I depend on the

number of data points used, becoming better with more

points.

4.2.1 AVL Measurement Accuracy

A curve similar to the Gaussian, or familiar bell shaped

curve, is obtained in testing tr i I aterat i on vehicle location

systems. Instead of making a large number of measurements

at one location, the measurements are made at random points

throughout a city and distance measurement errors determined

in each case by reference to a map. This set of errors,

when plotted as before, results in an error curve resembling

the Gaussian, with average value zero. It is more nearly

Gaussian for the phase t r i I ate rat I on system, since

deviations from the true location may be either positive or

negative. For pulse t r i I a te ra t i on , negative errors do not

occur except as a result of instrument malfunction. Thus,

the error curve would be asymmetrical with most of the

negative error points missing.

18.
The discussion thus far has referred to measurements in

one dimension. If the process takes place in two

dimensions, as with vehicle location in a city, the Gaussian

curve is modified. Instead of errors in the x coordinate or

the y coordinate, errors may occur in both simultaneously.

To specify vehicle location system measurement accuracy the

radial error should be used. The statistical curve required

to illustrate simultaneous errors in x and y coordinates is

known as the Rayleigh distribution. In this case, the 95

percent confidence interval, or radius, equals 2.45 times

the standard deviation along one axis. The Rayleigh

distribution also turns out to be the one which describes

radio signal level distribution over channels which involve

multlpath reflection. The resultant signal fading is known

as Rayleigh fading.

If errors are known to be random, this fact may be used

to improve measurement accuracy through averaging

techniques. If errors are Independent, or uncorrelated with

each other, the average value of a number of measurement

points Is subject to less fluctuation and has greater

accuracy than the individual points. This Improvement is

proportional to the square root of the number of independent

points used. The disadvantage of the use of this procedure

for AVL is that the Increased time needed to measure the

added points causes a delay in obtaining vehicle location

information. This delay Is likely to be more of a problem

for law enforcement than for public transportation use.

4.2.2 Techniques for Improving Accuracy

The technique of map matching can be made an Integral

part of the dead reckoning system. This method works

because errors are not allowed to become too large to enable

the system to distinguish between vehicle locations a block

apart. In principle this technique could be used for the

other systems as well.

Another method related to map matching Is recognizing

bias errors and removing them. If a system, through testing

In a city, is found to have an error bias, or offset, from

an average error value of zero, this error may be applied as

a correction to Improve accuracy. This bias may vary

throughout the city so that a map of bias as a function of

city location can be constructed. This Information can be

stored In the computer and used to Improve vehicle position

measurements. This method Is applicable to radiolocation

systems.

19.
Additional data processing methods applicable to

radiolocation systems are based on filtering and smoothing

techniques to improve accuracy. This cannot be carried too

far without creating unacceptable time delays. One form of

filtering consists of rejecting data points which disagree

too much with the average. In order to obtain improvement

by averaging, only independent data points may be used.

Because the major errors are due to multipath, and are not

independent of each other until the locations are separated

by a number of wavelengths, only data obtained from widely

separated locations are useful in averaging. Additional

data points taken while the vehicle is stationary are of no

value. Therefore, a means of indicating vehicle movement

between data points used in averaging will improve averaging

performance. An elementary odometer arrangement can provide

such data.

5. LOCATION DATA REPORTING SYSTEMS

To be useful, police AVL systems must include a method

of supplying location information to the dispatcher.

Digital data techniques are often employed to transmit this

information. In addition to location data, the capability

of sending other data from the vehicle enhances the system

usefulness, e.g., sending an emergency "silent" alarm call

for help from the vehicle to the dispatcher. Digital

messages may be sent over either a radio channel or

telephone channel. The speed with which the message may be

sent, or message rate, depends on the channel bandwidth, a faster rate requiring a larger bandwidth.

All message channels include noise which tends to cause

errors. Errors may be reduced by methods which involve

slowing down the message, or by increasing the signal level.

One of the advantages of using digital methods is that

message channel error rates may be accurately predicted, on

the average, if the nature of the competing background noise

is known. Message rates and signal levels can then be

adjusted to keep error rates below some acceptable value.

For any message to be sent on a radio channel, it must be

impressed, or modulated, on the radio frequency carrier

using one or a combination of several techniques. Amplitude

modulation (AM) and frequency modulation (FM) are two

commonly used techniques. For digital messages two types of

FM are often used: Frequency Shift Keying (FSK), where the

binary coding is accomplished by shifting between two

specified frequencies, and Phase Shift Keying (PSK), where

the phase of a single modulation frequency is shifted

between two specified values. The system designer thus has

a wide range of choices open to him if he is free to specify

the entire digital communication system.

20.
5.1 Narrowband Communication

In the Interest of minimizing costs It Is appealing to

use existing police mobile channels and equipment. In the

450 MHz band, existing channels are separated by 25 kHz with

frequency deviation restricted to ±5 kHz, by FCC regulation.

These parameters were assigned for voice communication

purposes. They may be adapted to digital data communication

purposes and are presently being used that way under

experimental authorization from the FCC. Modulation and

encoding techniques vary among AVL systems, but they all

operate under similar urban noise conditions using similar

average power levels. This means that message rates also

must be similar. A representative value of AVL message rate

for these land mobile channels Is 1200 bits per second,

although Individual systems might vary from this by a factor

of 2. The number of bits required for each message depends

on the AVL system. The system should also be capable of

sending a certain amount of additional Information from the

vehicle. A silent alarm message is the minimum suggested.

Other routine message capabilities may be desirable,

although this Is usually considered to constitute a separate

mobile digital communications system. Some systems send

vehicle identification and status Information as part of

their AVL function. A total of perhaps 10 to 30 bits of

data for each update message Is a reasonable number to

assume. At 1200 bits per second, this would require 25

milliseconds per message. Allowing additional time for a location signal in t r i I aterat I on systems, plus some more

time to insure message separation, a total of 50

milliseconds might be required per transmission. Nontr I I aterat i on systems using shorter messages might require

only half this time. Allowing some leeway for functions

such as calibration, a single voice channel could

accommodate as many as 2000 vehicles per minute. If the

vehicle has to acquire Its own location data, and then

transmit It to the dispatcher, as with dead reckoning,

proximity, and the LF and VLF navigation networks, a 64 bit

message would not be an unreasonable estimate of the

required message length.

21 .
5.2 Wideband Communication

Since pulse ranging t r i I a tera 1 1 on systems require a wide

bandwidth In order to transmit a f ast- r i se-t i me pulse, this

wide bandwidth becomes available for data communication

purposes as well. A nominal pulse bandwidth of 10 MHz Is

necessary. With voice channel spacing restricted to 25 kHz,

one pulse channel has a communication capacity of

approximately 400 voice channels. (To date the use of such

a wide bandwidth channel for AVL has only been authorized

experimentally by the FCC). This very large communication

capacity for pulse systems compared to phase systems may be

used In several ways for AVL systems: to increase vehicle

position update rate, to Increase the number of vehicles the

system may accommodate, to Increase the amount of

Information which may be sent In each message, to improve

the reliability of message reception, or to Improve system

flexibility. Probably any pulse type AVL system which is

developed would incorporate all of these advantages to some

degree. Methods for obtaining these results include use of

faster bit rates, redundant message encoding and spread

spectrum techniques, message repetition, and more detailed

control over the vehicle broadcast format from the dispatch

station transmitter.

5.3 Location Update Rates

The Department of Transportation has given baseline

requirements for AVL systems for public transportation use.

The position sampling rate has been specified as 80 percent

of the vehicles once per minute and 20 percent every fifteen

seconds, with a fleet of 1000 vehicles In a service area.

Law enforcement requirements would usually require more

frequent sampling than this, especially in critical

situations, but It would be difficult to determine these

rates without further study. AVL systems also have their

own update requirements for proper functioning; these depend

on system details. Systems may be designed so that the

update rate Is selectable by the dispatcher, although this

Increases system complexity.

22.
Updating is generally accomplished by either of two

techniques, time sequence reporting or polling. In time

sequence reporting, each vehicle sends its radio

transmission during its own specified time Interval, or

slot, within a total fixed time period. At the end of this

period the sequence is repeated. Thus each vehicle reports

in turn, one at a time. This technique Is adapted to public

transportation use where reporting rates are not likely to

require adjustment for Individual vehicles. Proper timing

is maintained by having a clock in each vehicle preset to

activate the transmitter during Its specified slot. Clocks

in all the vehicles are maintained in synchronism with each

other by being reset upon reception of a synchronizing

signal from the central transmitter. This synchronizing

signal must be sent often enough to keep each vehicle's time

slot from drifting with respect to other vehicle time slots,

causing overlapping transmissions which would interfere with

each other. Clock drift Is caused by the fact that clock

crystal oscillators all unavoidably operate at slightly

different frequencies. A reasonable accuracy for the

frequency of commercial oscillators is 1 part in 10^. If two

oscillators differ in frequency by 1 part in 10^, clocks

driven by them will disagree by 60 microseconds in one

minute. After 10 minutes two clocks drifting in opposite

directions at this rate would depart from each other by 1.2

milliseconds. If, then, a time of a few milliseconds is

provided to allow for this clock drift and for differences

in propagation time over different distances,

resynchron 1 zat I on is needed every 5 to 10 minutes.

In reporting by polling, a specific vehicle is addressed

by sending a properly encoded signal from the dispatch

transmitter. The specific car to which it is addressed then

replies with its appropriate transmission. With this

technique, the update rate for specific vehicles is more

easily adjustable, and vehicles may be more readily added or

removed from the system as they go In or out of service.

However, more message time from the dispatch transmitter Is

necessary; and, therefore, more radio spectrum is needed

than with time sequence reporting.

5.3.1 Dead Reckoning Rates

Dead reckoning systems do not measure actual vehicle

position, but calculate it from distance and heading

information. The central computer continually corrects and

calculates positions by referring to a map of the city.

Data must be updated and calculated positions corrected

before accumulated errors reach about one half block;

otherwise, the system is likely to mislocate the vehicle by

a b I ock

.

23.
A likely source of location error occurs when a turn Is

made at some unknown time between updates. This uncertainty

in time causes a location error the same as an error in the

heading or distance traveled sensor, and places a maximum on

the time between updates before excessive errors occur. One

manufacturer estimates 4 seconds as this maximum time. This

time could probably be extended to a number of minutes by

providing some simple position computation capability on

board the vehicle. The position information would be stored

in the vehicle between updates, and could be transmitted at

whatever rate is needed for police operations, rather than

by the faster rate needed by dead reckoning system

req u i reme n ts

.

5.3.2 Proximity Rates

Proximity systems acquire new location information only

when the vehicle passes near a sign post transmitter. This

is not necessarily the time when they would be reporting

their data to dispatch headquarters under either the polling

or time sequence reporting methods. Therefore, the vehicle

should have the capability to store data for retransmission

between the times when the data are acquired from the

signposts. These data indicate the specific sign post

region in which the vehicle is located at a specific time.

This region changes suddenly as the vehicle reports data

from a new sign post. Reporting rates are not limited by

system requirements, but depend on police operations needs.

5.3.3 Radiolocation Rates

Since each location measurement provides an estimate of

a vehicle position, reporting rate may be determined by

police operations requirements. However, the use of various

types of averaging and data treatment techniques to improve

location accuracy creates the possibility that the optimum

data rate may depend on the type of data processing in use.

24.
6. INFORMATION DISPLAYS

Requirements for presenting vehicle location information

at police headquarters are for a real time display of

vehicle location that is easy to read. A television type

display is often used. Methods have been developed for

providing a street map of the city appearing as an overlay

on the vehicle path shown on the television screen. The map

can be stored in computer memory and appropriate sections

reproduced. Also a conventional map can be scanned by a television camera, and the image superimposed on the same

screen where the vehicle location is shown. Identification

numbers and various status numbers or symbols can be

provided along with a moving spot to represent each vehicle.

Controls may be provided to change the scale and to display

various parts of the city. Displays are available which

provide a permanent record of a vehicle path. An x-y

plotter is usable for tracing the vehicle path on a city

map. Magnetic tape storage can also be used for retaining

vehicle trip information if desired. Vehicle movements can

also be replayed, either at actual vehicle rates or speeded

up for later analysis.

7. COMMERCIAL AVL SYSTEMS

Commercial AVL systems are becoming available in all

three categories: dead reckoning, proximity, and

radiolocation. Radiolocation methods have received the most

attention, although some manufacturers who were involved in

early work on phase ranging systems have now withdrawn from

the field. At least one system each, employing pulse

t r i I aterat i on , Loran C, and dead reckoning, and several

systems employing phase tr 1 I aterat i on and proximity methods

are advertised. These systems are intended for both public

transportation and police use. Detailed specifications and

applications will depend on the particular use. Up to now,

company specifications are usually derived from the results

of in-house testing, often in cities where they were

developed or in nearby cities, making direct comparison of

specifications difficult. A series of tests were sponsored

by DoT in the city of Philadelphia in 1972. The objective

of these tests was to obtain technical data to assess the

performance of several types of AVL systems being considered

for mass transportation use. Tests were completed on one

proximity system, one navigation system employing Loran C,

and two phase tr i I aterat i on systems. A dead reckoning

system was not available at the time of these tests, and a pulse t r i I ate rat i on system was not tested.

25.
The tests were carried out in three different urban

environments: a high-rise area, a low-rise area, and one

with special propagation problems. The results were that

the proximity system exceeded the DoT accuracy requirement

of 500 feet with 95 percent confidence while the other

systems did not meet the requirement. The Loran C system

suffered from severe signal degradation in certain locations

and from man-made interference, leaving its ultimate

capabilities in doubt. The best results from the phase

t r i I aterat i on tests compared reasonably well with the

performance as estimated from theoretical models of urban

propagation using computer simulation.

Based on data now available, including the DoT tests in

Philadelphia, various company tests, and certain analytical

studies, some general conclusions can be made concerning the

accuracies of AVL systems.

7 . 1 Dead Reckon i ng

This method has achieved the highest accuracy

demonstrated so far. One company reports 50-foot accuracy,

but the analysis has not yet been carried out on a basis

such that it can be directly compared with the Philadelphia

test results. The effects of momentary or short-term

malfunctions are more serious with this system and can cause

the vehicle to be "lost" to the dispatcher. Such

circumstances would degrade any overall accuracy figure

greatly. These occurrences can be minimized by improving

system reliability so they do not represent a practical

I imitation.

7.2 P rox i m i ty

These systems could probably achieve 100-foot accuracy

using sufficiently close sign-post spacing and individual

adjustment of transmitter powers. Accuracy was demonstrated

to be 450 feet with 95 percent confidence in Philadelphia,

but this was largely a matter of sign-post spacing.

7.3 Radiolocation Systems

7.3.1 Phase T r i I a te ra t i on

The Philadelphia test results indicate that at the 95

percent confidence level, accuracies approaching one

thousand feet are achievable in real time. This agrees

quite well with analytical results obtained by Turin et

al., based on computer simulation. Better accuracies were

obtained by filtering the data, but the time lag involved

would seriously limit system usefulness for some police

app I i cat ions.

26.
7.3.2 Pulse Tr I I aterat i on

A test of such a system has been conducted under company

sponsorship in a high-rise area of New York City. Accuracy

was reported to be 300 feet at 95 percent confidence. This

represents reasonable agreement with computer simulation

results obtained by the Turin group. Again, this accuracy

figure cannot be compared directly with the DoT administered

tests in Philadelphia.

7.3.3 Navigation Systems

7.3.3.1 Loran C

The Philadelphia test results indicate the accuracy was

slightly over a thousand feet for the low-rise area to

slightly over two thousand feet for the high-rise area, with

95 percent confidence.

7 . 3 . 3 . 2 Decca

This system has been demonstrated for the Montclair

Police Department in the Los Angeles area, but the

transmitters have since been removed, and no Decca

installation now exists In the U.S. The tests and analysis

were limited, compared to the Philadelphia tests. Results

were reported to be 500 foot accuracy at 95 percent

confidence. Again this result cannot be compared with those

obtained by DoT

.

7.3.3.3 Omega

Omega has received very little testing for AVL use. One

inherent limitation is that the broadcast format repeats

only once every 10 seconds, making it impractical at present

for position information to be updated more frequently than

that.

7.3.3.4 Differential Navigation Systems

Some ongoing work oriented toward aircraft or shipboard

use Indicates that the accuracy of either Loran C or Omega

can be improved significantly using this technique.

27.
8. RADIO SPECTRUM UTILIZATION

Since the radio spectrum Is a scarce resource, and it

must serve a number of different uses. Its efficient

allocation among those uses is an important consideration.

All AVL systems make some use of radio transmissions; all

AVL-equipped vehicles either emit radiolocation signals or

send location information to dispatch headquarters by radio.

All AVL information reporting Is done by digital techniques,

which make very efficient use of the spectrum, especially

when the information to be sent is selected from a limited

number of p ref ormu I ated short messages such as the police 10

codes. Nearly all AVL systems now being offered include the

capability of sending at least 10 bits of general purpose

telemetry data with each position report. This was one of

the DoT system requirements. A variety of routine

information has also been suggested for regular reporting to

the dispatcher, including vehicle condition parameters, such

as fuel level.

Police departments are starting to use digital

techniques in their day-to-day communications. Coordination

of AVL systems with other digital systems should result in a more efficient use of the spectrum.

A given amount of information sent digitally requires a certain radio spectrum bandwidth for a certain length of

time. To shorten the transmission time requires a wider

bandwidth. For optimum utilization of the radio spectrum,

the required message rate should be matched to the assigned

channel bandwidth. If the bandwidth is too large, there

will be wasted idle time between messages. If the bandwidth

is too small, some messages will be sent too late, or the

error rate will be unacceptable. The data available for AVL

systems are not sufficient to determine the optimum location

data rate. However, once the rate is determined for a given

system, the location data rate is essentially predictable,

and an optimum channel bandwidth could be assigned provided

the system contained a fixed number of vehicles. This is

not the case for patrol vehicles because the workload varies

as a function of time of day and other external

requirements. If the system is to accommodate the peak

number of vehicles, the spectrum will be u n de r- u t i I i zed the

rest of the time. One way of improving spectrum utilization

is to share an AVL system among a number of users, choosing

users whose peak loads occur at different times. This has

the added advantage of sharing fixed costs among a number of

users. However, such shared systems will experience added

problems of coordination, and the systems will require

g rea te r f I ex i b i I i ty

.

28.
The inclusion of messages other than those essential to

the vehicle location function compounds the problem of

efficient spectrum use. Although average message rates can

be determined, they would have to include some overestimation because of the short term fluctuations in message

length. This is wasteful from the utilization standpoint if

the system is designed to accommodate the highest expected

rates.

Wideband and narrowband are essentially the only two

types of radio channels presently proposed for AVL systems.

AVL system demonstrations have been conducted using both

types of channels under experimental authorization by the

FCC. The proposed spacing for wideband channels is 10 MHz.

A petition submitted by a pulse AVL system manufacturer

asking for this type of allocation has been before the FCC

since late 1970. The petition asks for the allocation of

two such channels in the 900 MHz band with a 6 MHz channel

between them. This band is presently allocated to ISM

( I nd u s t r i a I - Sc i e n t i f i c-Me d i ca I ) usage. The wideband channel

has been occupied simultaneously with ISM usage, but AVL

must not interfere with that use. At present, AVL is

relatively immune from Interference due to ISM activity.

Narrowband voice channels which already exist in the 150 and

450 MHz bands are those which have been used for AVL

system demonstrations.

A! I present AVL systems use narrowband channels except

the pulse systems. Due to variations in system design and

message content, the number of vehicles accommodated on one

narrowband channel has a considerable range, from about 10

to 100 per second. The wideband system has also been

operated at different rates, varying from 250 to 1000

vehicles per second, while having the capability of

supplying more Information per message. For law enforcement

use, a reasonable assumption for required location and

message update time is 5 to 15 seconds. Thus, with present

systems, one narrowband channel might accommodate from 50 to

1500 vehicles while the wideband channel might accommodate

from 1250 to 15,000 vehicles.

29.
9. SYSTEM COSTS

It Is difficult to make meaningful comparisons of AVL

systems based on present cost information. For a 100 car

system, vehicle equipment cost estimates range from $500 to

$3000 per vehicle for the various systems. Fixed station

equipment costs and installation estimates range from onehalf to several million dollars. These variations reflect

pre-production uncertainties and different company pricing

policies and strategies as well as foreseeable equipment

costs

.

Until several systems have undergone at least the pilot

demonstration project stage, reliable price comparisons are

unlikely to be available. If a full-scale manufacturing

stage is reached, a number of economies become possible

which may drastically lower the unit price.

10. COMPARISONS OF AVL SYSTEM TECHNIQUES

Some of the pertinent information concerning parameters

which vary among AVL systems is summarized in the following

table. It is based on the most recent information availabi

concerning technical characteristics. However, no police

department operational experience with AVL systems in this

country is yet available. For this reason it is too early

to clearly define a system to meet particular requirements.

Information obtained from pilot law enforcement studies is

needed in order to make meaningful system comparisons.

30.
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3 I
Appendix A

Annotated Bibliography/

The Challenge of Crime in a Free Society. A Report by

the President's Commission on Law Enforcement and

Administration of Justice, U.S. Government Printing Office,

Feb. 1967.

Task Force Report: Science and Technology. A Report to

the President's Commission on Law Enforcement and

Administration of Justice, U.S. Government Printing Office,

1967.

The first report above contains a general discussion of the

crime problem In the U.S., including a chapter summarizing

the conclusions of the second report. The second report

includes in Appendix E a discussion of car location systems and

mobile digital communications with considerations relevant to

standardization, entitled "Electronics

Equipment Associated with the Police Car," by Raymond

Kn I c ke I .

Propaqa 1 1 on

Young, Jr., W. R., "Comparison of Mobile Radio

Transmission at 150, 450, 900, and 3700 Mc," The Bell System

Technical Journal, (Nov. 1952) pp. 1068-1085.

Okumura, Y., Ohmorl, E., Kauano, T., and Fukuda, K.,

"Field Strength and Its Variability in VHF and UHF LandMobile Radio Service," Review of the Electrical

Communication Laboratory, Vol. 16, No. 9-10, (Sept. -Oct.

1968) pp. 825-873.

Black, D. M.,and Rendlnk, D. 0., "Some Characteristics

of Mobile Radio Propagation at 836 MHz in the Philadelphia

Area," IEEE Transactions on Vehicular Technology, Vol. VT21, No. 2, (May 1972) pp. 45-51.

Cox, D. C, "Delay Doppler Characteristics of Multipath

Propagation at 910 MHz in a Suburban Mobile Radio

Environment," IEEE Transactions on Antennas and Propagation,

Vol. AP-20, No. 5, (Sept. 1972) pp. 625-635.

Cox, D. C, "Time and Frequency-Domain Characteristics

of Multipath Propagation at 910 MHz in a Suburban MobileRadio Environment," Radio Science, Vol. 7, No. 12, pp. 1069-

1077.

32
Hefley, G., "The Development of Loran-C Navigation and

Timing," Nat. Bur. Stand. (U.S.), Monogr. 129 (Oct. 1972).

These papers present results of radio propagation

experiments in urban environments.

Urban Propagation Models Applied to Vehicle Location

Techn i ques

Engel, J. S., "Effects of Multipath Transmission on the

Measured Propagation Delay on an FM Signal," IEEE

Transactions on Vehicular Technology, Vol. VT-18, No. 1,

(May 1969) pp. 44-52.

Turin, G. L., Clapp, F. D., Johnston, T. L., Fine, S.

B., and Lavry, D., "A Statistical Model of Urban Multipath

Propagation," IEEE Transactions on Vehicular Technology,

Vol. VT-21, No. 1, (Feb. 1972) pp. 1-9.

Turin, G. L., Jewell, W. S., and Johnston, T. L.,

"Simulation of Urban Vehicle-Monitoring Systems," IEEE

Transactions on Vehicular Tech no I ogy , Vo I . VT-21, No. 1,

(Feb. 1972) pp. 9-16.

Staras, H.,and Honickman, S. N., "The Accuracy of

Vehicle Location by Tr i I aterat I on in a Dense Urban

Environment," IEEE Transactions on Vehicular Technology,

Vol. VT-21, No. 1, (Feb. 1972) pp. 38-43.

These papers use measured urban propagation data to develop

propagation models from which vehicle location accuracy

performance predictions are derived.

Radio Noise

"World Distribution and Characteristics of Atmospheric

Radio Noise," CC I R Report 322, International

Telecommunications Union, Geneva (1964).

Disney, R. T., "Electromagnetic Interference from ManMade Radio Noise Estimates of Business," Residential and

Rural Area Characteristics.

White, D. R. J., "Handbook on Electrical Noise and

Electromagnetic Interference Specifications" (1971).

Details are given of mathematical descriptions of noise and

methods for obtaining noise predictions.

33
Information Handling in Mobile Systems

Cole, R. E., and Hanks, J. D., "Efficient Dispatching of

Mobile Forces," IEEE Transactions on Vehicular Technology,

Vol . VT-1 8, No. 1 (May 1969) . Braun, W. V.,and Walker, D. L., "Vehicular Location and

Information Systems," IEEE Transactions on Vehicular

Technology, Vol. VT-19, No. 1 (Feb. 1970).

These papers discuss the kinds of information transfer that

are needed with AVL and AVM systems and the application of

digital techniques to information handling.

Vehicle Location System Technical Reports

An Analytic and Experimental Evaluation of Alternative

Methods for Automatic Vehicle Mon i tor i ng , s u bm i tte d to the

Chicago Transit Authority and the U.S. Department of

Transportation by the Institute of Public Administration and

Teknekron, Incorporated, July 1968. This report contains a detailed treatment of the three types of vehicle location

methods with system analysis applicable to public

transportation requirements.

Vehicle Location Study Phase 0 Report, Task 119, Jet

Propulsion Laboratory, California Institute of Technology,

Pasadena, California, March 1970. This report contains a detailed discussion and history of direction finding and

navigation systems and techniques, as well as briefer

treatment of the other methods.

Automatic Vehicle Monitoring Technology Review, S. H.

Roth. Prepared for the Urban Mass Transportation

Adm i n i st rat i on , Department of Transportation, MTR 6059,

August 1971. This report gives an extensive discussion of

the three vehicle location methods including Department of

Transportation requirements and background information for

the Philadelphia AVL tests.

Vehicle Location and Status Reporting System (LOCATES),

submitted to the California Council on Criminal Justice,

Final Report, project number 182, March 1972. This report

gives an extensive discussion of AVL systems, operations and

demonstrations oriented toward police use in a small city,

for the city of Montclair, California.

34
Urban Vehicle Monitoring: Technology Economics, and

Public Policy, in three volumes. Prepared by Technekron,

Inc., for the Office of Research and Technology, the U.S.

Department of Housing and Urban Development. Vol. II,

Technical Analysis and Appendices, contains an extensive

technical discussion of AVL systems with results of

experiments and computer simulation studies. Including some

cost estimates and description of possible display systems.

Vol. Ill, Economic and Institutional Analysis and

Appendices, discusses police operations with AVM

applications and benefits, as well as other dispatched

service discussions. It also considers markets, pricing

structures, financing, and regulation and control Issues.

Vol. I, Summary Report, is a summary of the other two

reports Including recommendations concerning government

involvement in demonstration projects. Two major

conclusions are that the benefits of AVM to police users are

larger than for any other groups; and that joint user

systems are necessary to reduce costs, economize on radio

spectrum and Insure system compatibility.

Urban Systems

Larson, R. C, "Urban Police Patrol Analysis" (the MIT

Press, Cambridge, Massachusetts; and London, England, 1972).

This book contains an extensive discussion of police patrol

and dispatching, with applications of mathematical modeling

techniques to these operations in order to make quantitative

comparisons of various methods. It includes a chapter on

evaluating the use of automatic car location systems.

(

35
20 I I

I I mil—till I nil—I I I i mil—i ii 1 1 iiii—i i ii ii

lOOkHz I MHz 10 MHz 100 MHz I GHz 10 GHz

FREQUENCY

Figure 1. Typical values of various types of noise.

36
Three fixed receiver sites are required to dete

mine the vehicle location when using the rangerange system.

37
NBS-1 14A (REV. 7-73)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR 73-3272. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

AUTOMATIC VEHICLE LOCATION TECHNIQUES FOR LAW

ENFORCEMENT USE

5. Publication Date January 1974

6. Performing Organization Code

/. A U 1 HUKto)

R. L. Fey

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

Boulder, Colorado 80302

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

National Bureau of Standards

M. J. Treado, Div. 400.09

Law Enforcement Standards Laboratory

Washington, D. C. 20234

13. Type of Report & Period

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary at most signilicar\t information. If document includes a significant

bibliography or literature survey, mention it here.)

Automatic vehicle location systems employing communications techniques

are presently under development by several companies. This report discusses

such systems with respect to law enforcement applications. It presents the

principles upon which the various automatic location techniques are based, summarizes

their important characteristics, and compares them as candidates for law enforcement

use. This report discusses techniques, available system performance, and the

related aspect of frequency spectrum reqtiirements for these systems. It also

mentions areas where more operational information is needed before systems for

law enforcement operations can be clearly defined.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Automatic vehicle locator; car locators; law enforcement.

18. AVAILABILITY Unlimited

For Official Distribution. Do Not Release to NTIS

1

' Order From Sup. of Doc, U.S. Government Printing Office Washington. D.C. 20402. SD Cat. No. C13

1 ! Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

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