J.Padmanabhan
Advanced Micronic Devices Ltd
Bangalore
[email protected]
Of all the applications of GPS, vehicle tracking and navigational systems have brought this technology to the day-to-day life of the common man. Today GPS fitted cars; ambulances, fleets and police vehicles are common sights on the roads of developed countries. Known by many names such as Automatic Vehicle Locating System (AVLS), Vehicle Tracking and Information System (VTIS), Mobile Asset Management System (MAMS), these systems offer an effective tool for improving the operational efficiency and utilization of vehicles.
The switching off of SA has improved the accuracy of GPS to better than 30 meters, which makes it an ideal position sensor for vehicle tracking systems without the overhead of DGPS. Fig. 1 gives the block diagram of a DGPS based VTIS. 
GPS is used in vehicles for both tracking and navigation. Tracking systems enable a base station to keep track of the vehicles without the intervention of the driver where, as navigation system helps the driver to reach the destination. Whether navigation system or tracking system, the architecture is more or less similar. The navigation system will have convenient, usually a graphic, display for the driver which is not needed for a tracking system. Vehicle Tracking Systems combine a number of well-developed technologies. Irrespective of the technology being used, VTS consist of three subsystems: a) In-vehicle unit (IVU), b) Base station and c) Communication link. The IVU includes a suitable position sensor and an intelligent controller together with an appropriate interface to the communication link. Thanks to the US Government announcement of 911E regulation, radio based position technology has witnessed a spurt of developmental activities.
Network Overlay Systems use cell phone infrastructure for locating vehicles. The cell centers with additional hardware and software assess the time of arrival (TOA) and angle of arrival (AOA) of radio signals from vehicles to compute the position of the vehicles. This information is sent to the tracking centre through the cell link or conventional link. Another technique used for locating vehicles computes the time difference for signals from two cell centers to reach the vehicle. This computation is made in the IVU and the position information is sent to the tracking centre through the cell phone link. A more common technique used is direct radio link (DRL). In this system dedicated radio infrastructure is used along with special IVU to compute vehicle location. However all these techniques impose limitation on the operational area. Alternatively, embedded GPS receivers provide absolute position co-ordinates at any point, without any area restrictions. 
Fig.2 shows the block diagram of an IVU. The controller interacts with the GPS receiver, collects co-ordinates at predefined intervals, processes it and sends out to the communication link. Optionally in certain cases a man-machine- interface like a display with key board can be added for message communication between the driver and the base station.
The base station consists of a high-speed system running VTIS application software that will receive the position data from the vehicles and display on a digital map. It too will have the interface to the communication link. Enhanced features include video features, trace mode, history track, vehicle database, network support etc. Fig.3 gives the block diagram of a Base station.
The most costly part of a VTIS is the data link. The data link, together with a suitable communication protocol, has to be selected after a thorough study of various parameters such as the bandwidth requirement, number of vehicles to be tracked, expandability, terrain, area of coverage etc. Sophisticated VTIS are linked to data bases that can support information about the vehicles such as the cargo, the temperature of storage of perishable goods, fuel consumption rate etc. Naturally, such systems demand data link with higher bandwidth. UHF links are suitable for short range without shadow region, as they require line of sight. Cell phone based systems demand minimum infrastructure investment, but is limited in coverage. On the contrary, LEO based systems are expensive and offer largest coverage. The recently introduced WAP and GPRS technologies hold great promises for VTIS. 
When multiple vehicles are being tracked, a suitable communication protocol need to be established to avoid collision of radio signal. The simple technique is TDMA, where each IVU communicates during predefined time slots. This synchronization is easy in a GPS based IVU as the GPS receiver provides very precise time reference signal. However, TDMA based systems have limited expandability, flexibility and are known for under-utilization of bandwidth.
The alternative is polling technique. Here each vehicle is addressed by the control station and in response the IVU sends the information. This arrangement enables variable polling rate for different vehicles, non-polling of specific vehicles and expansion of polling list as new vehicles are added.
The relatively large investment needed for the communication link, makes VTIS an opportunity area for service providers. Fig. 4 shows the global market for GPS based VTIS in the next three years. In US and Europe many vehicle tracking service providers are already in operation. In a large country like India with a very long network of roads and long coastline, this opportunity area is yet to be exploited.
