The Development of an Anechoic GPS Test Facility

Neil J. Boasman and Peter Briggs

Abstract:	GPS provides position, velocity and time information measurements with unparalleled accuracy to an ever increasing number of world-wide users in many civil and military applications. However, due to the low powered intensity of GPS signals, it is extremely vulnerable to interference or jamming. Controlled Radiation Pattern Antenna (CRPA) technologies have been under development for a number of years. These multi-element phased antenna arrays allow the gain pattern of the user’s receive antenna to be electronically manipulated such that interference source(s) are placed within a ‘null’, whilst at the same time allowing the wanted GPS signals into the receiver – increasing the operational capabilities of the GPS user when functioning in such a hostile electronic environment. The testing of such an antenna system could be carried out in real-world trials involving, for example, a number of static interference sources directed towards a dynamic airborne target CRPA system. However, these tests are not highly repeatable. The GPS constellation is constantly changing and flight paths would undoubtedly differ from sortie to sortie. Also, in a world where GPS is used in such a wide range of activities, from the automatic location of emergency vehicles to mountaineering, trials involving GPS jamming have to be carefully planned and RF power constrained such that the minimum amount of disruption is caused. Interference sources from outside the trials control can also cause concern over the accuracy of the results obtained. The anechoic test facility under development by QinetiQ Telematic Solutions provides a repeatable and accurate setting in which the performance of GPS receiver and antenna technologies can be evaluated. The spherical shaped chamber, certified up to 18GHz, is based within a hardened aircraft shelter at Boscombe Down, Wiltshire, UK and provides a high integrity, low noise environment with exceptionally low signal reflectivity performance, beyond that of a conventional anechoic chamber. The test facility’s noteworthy features include: • A GPS signal simulator, comprising 12 discrete antenna outputs which is used to feed a constellation of source antennae, placed around the periphery of the chamber to replicate a real GPS constellation. • Jamming source antennae, which are moveable anywhere within the chamber. • A variety of RF interference sources (Broad / narrow band, pseudo random noise, frequency swept etc.) • A dynamic platform to facilitate real time attitude control of the receive antenna in sympathy with the simulated host vehicle trajectory. • A STANAG 4572 compliant interface is available which injects real time inertial aiding data into the unit under test to enable testing of integrated GPS and Inertial navigation systems. It would be true to say that this facility is stretching the limits of anechoic chamber capabilities. This paper explores the difficulties faced when designing such a test facility, from choosing the correct antenna types to ensuring that the reflectivity at GPS frequencies is low enough to avoid reflections being seen by the unit under test as another jamming source. Also discussed are some of the initial results obtained to date and suggestions of other uses for the chamber.
Published in:	Proceedings of the 58th Annual Meeting of The Institute of Navigation and CIGTF 21st Guidance Test Symposium (2002) June 24 - 26, 2002 Hyatt Regency Hotel Albuquerque, NM
Pages:	483 - 494
Cite this article:	Boasman, Neil J., Briggs, Peter, "The Development of an Anechoic GPS Test Facility," Proceedings of the 58th Annual Meeting of The Institute of Navigation and CIGTF 21st Guidance Test Symposium (2002), Albuquerque, NM, June 2002, pp. 483-494.
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