## 1 Introduction  
GPS-NAVSTAR (Global Positioning System-NAVigation Signal Timing And Ranging), popularly known as the GPS system, has considerably gained in civilian interests, since May 2000. Earlier, the system was practically reserved for military purposes. The positioning accuracy for the civilian sector was _ca._ 100 m due to an intentional error, called Selective Availability. According to report [1], the horizontal positioning error is less than 17 m for 99% of the time in average conditions or 17 m for 90% of the time in worse outdoor conditions. The error depends on many factors, like atmospheric conditions, sun activity, geographical location, terrain type, satellites' constellation, _etc._ In an open space, positioning errors are of _ca._ 2-3 m. However, in dense built-up areas, the location error may reach 100 m [2,3] or even more [4]. The error is introduced due to multipath propagation of signals transmitted by the satellites when there is no line-of-sight. A satellite signal is bounced off the walls of a building before finding its way to a GPS receiver. The propagation time of the signal is delayed and the GPS receiver miscalculates its location with a reference to the satellites. There are many techniques to improve the location accuracy. Along coasts, for marine purposes, special ground DGPS (Differential GPS) reference stations broadcast differential corrections that allow a GPS receiver to eliminate tropospheric, ionospheric, ephemeris and clock errors. The overall error is reduced to 10 m with accuracy decreasing by 1 m with each 150 km increase in distance from the reference station. The corrections are transmitted on a _ca._ 300 kHz carrier frequency. A receiver must be, however, equipped with an additional antenna [5]. A-GPS (Assisted GPS) is a technique that downloads from the Internet the data concerning GPS satellite constellation [6]. Otherwise a GPS receiver may require up to 12.5 minutes to receive data about satellite constellation. By employing the A-GPS, the first fix is provided within few seconds. Additionally, a GSM modem can support a GPS receiver with a rough location which is obtained by measuring the strengths of signals from GSM base stations. Apart from L1 = 1,227 MHz, a second frequency called L2C = 1,575 MHz, has been made available to the civilian sector with the aim of reducing the ionospheric and tropospheric errors which are a well defined function of frequency. Also, the so-called augmentation systems effectively reduce tropospheric and ionospheric errors by sending differential corrections from geostationary satellites directly to GPS receivers. The geostationary satellites imitate also GPS satellites and improve mainly the vertical accuracy. EGNOS (European Geostationary Navigation Overlay Service) works in Europe, WAAS (Wide Area Augmentation System) in the US, MSAS (Multi-functional Satellite Augmentation System) in Japan and GAGAN (GPS aided Geo-Augmented Navigation) in India. Real Time Kinematics technique achieves the accuracy of millimetres in an open space and is designated for cartographic measurements. However, two GPS receiver are necessary. The base receiver is placed in a known position and calculates tropospheric, ionospheric, ephemerids and clock errors of satellites. The corrections are sent via a radio link to a mobile GPS receiver. In the trials reported in [4] RMS error of 2 cm on a mountain high-way was noted whereas trials in urban canyons yielded a 50 m RMS error. All the above mentioned techniques are helpless against multipath propagation errors. Algorithms harnessed in car navigation alleviate these errors by taking advantage of car kinematics and a comparatively sparse network of roads, as described, e.g., in [7] or [8]. An error of 20 m is of no bigger importance to a driver.