GPS Noise and Interference Test Challenges and Implementation

B. Hessen-Schmidt

Abstract:	GPS is today used as the primary source for critical timing and location applications including: the time-base for CDMA base stations; navigation systems for ships, aircrafts and automobiles; and E-911 services for mobile phones. The recent changes in the political climate, applications and market situation for GPS, requiring a level of immunity to intentional and un-intentional interference, have created the need for dual frequency GPS receivers and the associated testing of such receivers. The challenges of interference testing includes: combining the low level GPS signals with the interference; calibrating the noise level of interference; generating above +20 dBm (100 mW) of pulsed interference with on/off ratios larger than 164.5 dB; and eliminating phase noise of the interferer from skewing the measurement. Testing dual frequency or triple frequency, L1, L2 and L5, GPS receivers additionally entail injection of any combination of above simultaneously at multiple frequencies into the antenna port of the receiver. Since the GPS signals are received and decoded at levels close to the natural noise floor special calibration routines are required to measure and calibrate the in band interference. A flexible programmable noise and interference generator for GPS testing has been developed and will be presented. It consists of a series of arbitrary waveform generators, one for each band, L1, L2 or L5, plus an elaborate switching and combining network. Up to three synthesized 1 to 2 GHz frequency sources are included to generate out of band interference at test levels exceeding +21 dBm. A built-in RF power meter is use for power level calibrations. A calibrated noise source provides the low power reference level enabling the generation of accurate signals over a range larger than 100 dB. The complete noise and interference generator system is implemented in a single standards size instrument case. A fully embedded computer using the Windows XP operating system provides an intuitive and power full graphic user interface.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1478 - 1483
Cite this article:	Hessen-Schmidt, B., "GPS Noise and Interference Test Challenges and Implementation," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1478-1483.
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