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"Everyday Engineering"

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Tue, 18 Oct 94

A Brief Survey of Automatic Vehicle Location Systems

The long-haul trucking companies and metropolitan area fleets are making increasing use of automatic vehicle location (AVL) systems for fleet management. Some AVL systems in use today rely on the Global Positioning System (GPS) for location, whereas others use communication satellites or land-based networks to compute location. The definition of AVL will become clearer with the following descriptions of some principal AVL systems, that are currently operational or will be available shortly:

OmniTRACS mobile satellite communications system is the largest AVL worldwide. It provides real-time, two-way communications between the carriers and their trucks in transit. OmniTRACS supplies precise vehicle location reports and trailer monitoring information automatically. >From the keyboard unit in the cab, the drivers can contact their home offices from any place and at any time. The carriers (headquarters) may inform their drivers of any urgent instructions. This two-way communications allows greater flexibility to divert shipments for pick-ups or drop-offs along the way. Since 1988, some 250 fleet operators have installed OmniTRACS on a total of 65,000 long-haul trucks. OmniTRACS computes position by measuring the round trip delay of synchronized transmissions from two geostationary satellites separated by 12-24 degrees. The network management at the OmniTRACS hub computes the range of each satellite and derives the third measurement needed for position from a topographic model of the earth. Accuracy is typically one-quarter mile, which is more than adequate for long-haul trucking requirement. OmniTRACS terminals sell for $3,500-$4,000, and monthly subscription fees average about $70 per vehicle.

American Mobile Satellite Corporation (AMSC) AVL system uses GPS for location and INMARSAT-C service for data communications. AMSC customer vehicles are equipped with Trimble Galaxy Standard-C/GPS terminals, which cost less than $2,000. Typical monthly usage charge for fleet customers is $50-$70 per vehicle. When AMSC launches its own satellite (planning in late 1994), which will support both voice and data communications, the terminals will be redesigned to support interface to cellular network also.

In the 1970s, FCC established interim rules under which licenses to operate in the 902-928 MHz band, sometimes referred to as the AVM (automatic vehicle monitoring) band, would be granted to companies for the operation of vehicle- tracking systems in metropolitan areas. PacTel Teletrac is the largest AVL system using this technology. It provides vehicle location and messaging services to 800 metropolitan area fleets operating some 25,000 vehicles. Teletrac is a network of transmitting and receiving antennas installed around the metropolitan area. Teletrac uses time of arrival measurements on received signals from four or more antennas to compute a vehicle's location with an accuracy of about 100 feet. Teletrac transceivers are priced around $400, and the monthly subscription charge averages about $20 per vehicle.

Several companies are promoting the use of the cellular network for computing location. KSI Inc. has developed a technique that measures the angle of arrival at multiple cell sites of cellular phone control channel transmissions. A phone's (vehicle's) location is computed as the intersection of the observed angles, with typical accuracy of 150 feet or better. KSI's system requires no modification to the cellular phone but does require an overlay to the existing cellular network. Another company, TrackMobile, has developed a technique for computing location by measuring the signal strength of transmissions from multiple cell sites and using these measurements to derive the ranges to the sites. Accuracy is claimed to be 100-1000 feet.

Another interesting system is the Terrapin Corporation's PINS (Position Information Navigation Subsystem). PINS computes a vehicle's location by tracking the pilot tone from existing commercial FM radio transmitters. PINS measures the phase of the signal and derives the range to the transmitter, the location of which is known. Four or more FM stations are tracked by the PINS receivers, which use multilateration to compute the vehicle's location with an accuracy of 10-20 meters. To get and maintain this accuracy, Terrapin installs a fixed observer in each metropolitan area, which operates in much the same way as a differential GPS reference station. The fixed observer computes the frequency drift of the FM signals and transmits the data to the receivers. The FM subcarrier that transmits the fixed observer data can also transmits traffic information, road conditions, and other broadcast data.

There are many more AVL systems than are described here. It is sufficient to say that the use of vehicle location by commercial fleets is expected to grow significant over the next several years. GPS will be widely used as a source of accurate location data, but other systems will also be used as well. Many of these systems and technologies will be adopted for IVHS-related (Intelligent Vehicle Highway Systems) consumer services. Then, we will be looking at a market of many order of magnitude bigger.

Viet-Dung Hoang, Ph.D.

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