Antenna Diagram Shaping for Pseudolite Transmitter Antennas – A Solution to the Near-Far Problem

Sven Martin

Abstract:	Since GPS has become operational the idea of augmenting the system has been addressed. One idea is the usage of pseudolites, which are ground-based transmitters. Pseudo-lites broadcast a GPS-like signal and can be used to en-hance the accuracy, integrity and availability of GPS. The major disadvantages are the „Near-Far Problem“ which limits the coverage area and transmitter generated mul-tipath due to ground reflections and nearby reflectors. Dif-ferent approaches have been made to overcome these problems, e.g. pulsed or unpulsed systems, signals trans-mitted with a frequency offset from L1 or the usage of al-ternative codes [8], [6], [10], [11]. The Institute of Flight Guidance and Control has devel-oped such a pseudolite system since 1995. The system has been successfully demonstrated in flight and driving trials. During these experiments it has become obvious that the transmitter antenna is a very important part which strongly affects the system’s performance. Therefore a research project has been launched to find an optimal antenna dia-gram shaping for pseudolite transmitter antennas. The goal was to find a beam shape which minimizes the Near-Far problem (i.e. extend the coverage area) and which excludes possible ground reflectors (e.g. hangars, buildings etc.) to prevent multipath. Research activities started with modelling different an-tenna types like patch antennas, helix antennas and phased array antennas using a special software package. Thereby parameters have been identified to shape the antenna pat-tern. Based on these results different antennas have been built to validate the theoretical models. The developed an-tennas are usable with pulsed and unpulsed systems. This paper describes results from antenna diagram simula-tions as well as measurements from field trials. Different antenna types will be presented with their advantages and disadvantages. It will be shown that using the best fit an-tenna diagram, the Near-Far problem will be minimized and the usable area can be extended.
Published in:	Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999) September 14 - 17, 1999 Nashville, TN
Pages:	1473 - 1482
Cite this article:	Martin, Sven, "Antenna Diagram Shaping for Pseudolite Transmitter Antennas – A Solution to the Near-Far Problem," Proceedings of the 12th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1999), Nashville, TN, September 1999, pp. 1473-1482.
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