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Introduction
By augmentation of the Global Positioning System (GPS) with other satellite or land-based navigation monitoring methods, we can acquire precise positioning of any point where a receiver or an antenna can be planted. This precise positioning data can be made meaningful and can be put to various uses through integration with Geographic Information Systems (GIS), which have various forms of data organized as layers of information. The GIS can integrate the various data layers and present the same from different perspectives, which are pertinent to the problem at hand in a manner that best appeals to human perception. Fleet management addresses the problem of managing fleets of trailers, containers, boxcars, taxi-cabs, locomotives, business jets and other modes of public transport. Such operations of management of a fleet of vehicles require solutions to various problems like dynamic assignment, trip allocation, dynamic routing, responding to real-time customer demands and dispatch instructions, automatic vehicle location (AVL), trip and freight reporting and monitoring driver and vehicle characteristics to attain efficient and optimized performance with available resources. Owing to the tough competition among transportation companies, fleet management problems with their implications of logistics and optimization are vital issues to be attended to for achieving the best possible performance and maximum profits. GPS-GIS integrated systems provide vital data in a graphic form that is easily comprehendible by the drivers, customers, fleet operators and fleet owners. The data that is provided by these systems is then fed to the logistics and optimization software that various fleet operators use to manage their operations.

GPS Augmentations
Various methods have been explored as augmentations to GPS signals to derive continuous precise positioning on GIS-based maps, which is vital for fleet management applications. A few have been discussed below.

(1) Differential GPS: This method is used to minimize errors in positioning by mutual comparison of current data and previously known data for a base station and applying the corresponding correction to the rover station. This correction can be applied in the following 2 ways:

• The 'Block Shift Technique' uses the computed coordinates at any time and compares them with the known coordinates of the base station. The error, which is in terms of a correction in coordinates, is applied to the rover. This method requires essentially that the two positions be found using the same set of satellites.
• The 'Range Correction Technique' uses the instantaneous and known base station coordinates to compute the error in all pseudo-ranges. Since, this correction is computed for all pseudo-ranges, this method gives the flexibility to the rover receiver to use whichever satellites it wants to compute the corrected coordinates.

(2) Beacons and Antennae: These are used along routes where GPS signals are hard to receive or are faced with multi-path problems due to tall buildings, canopy or terrain conditions. These instruments detect the presence of the vehicle along the road and relay information to the control station to help track the vehicle even during loss of GPS signals. They can also be placed inside long tunnels where tracking becomes very difficult and loss of signals for a long time cannot be afforded.

(3) GLONASS and Galileo Integration: The GLONASS is a system similar to the GPS and was established by the USSR (now the Russian Federation). The Galileo is a similar system being developed by the European Union, to be functional by 2008 (Rizos, 2000). The integration of GPS with these systems is favourable because they complement each other and can provide good accuracy on integration. Other benefits are multiple frequency allocations, better geometry and large number of available satellites and hence greater reliability of data (Han, 1999).

(4) Pseudolites: These are ground-based pseudo-satellites that transmit GPS-like signals and can be used as substitutes for space-borne satellites when their signals are hard to track. They are normally located on high buildings or hills from where they can track vehicles just like a normal satellite would do in absence of obstructions and are also free of ionospheric and tropospheric errors that are faced by GPS signals. The pseudolites then transmit signals to the control station to allow continuous tracking of vehicles even in difficult terrain like deep open-cut pits and mines and downtown urban canyons. Inertial Navigation Systems (INS) like gyroscopes and accelerometers have also been used in integration with GPS signals and perform to a better level of accuracy when coupled with pseudolites. A problem faced by pseudolites is the multi-path error due to their location close to the ground. To resolve this problem, High Altitude Platform Systems (HAPS) have been amongst the research projects taken up by many countries. Japan has been investigating the use of an airship system that will function as a stratospheric platform (at a height of about 20 km) above the ground for applications in communications, broadcasting and environment monitoring. The possibility of these airships being equipped with pseudolites to provide centimeter level accuracy is being explored in Japan (Tsujii et al., 2002).