For effective range safety, global positioning system (GPS) metric tracking must be robust to interference with, and jamming of, GPS signals. The conventional approach to mitigating interference and jamming is to use a Controlled Reception Pattern Antenna (CRPA). These few-element phased arrays are used to steer nulls in the directions of interference sources, and/or to point beams in the directions of GPS satellites. The use of CRPAs is limited by their cost and size, as well as the difficulties of integrating the array into a platform. The problems are compounded for a launch vehicle, which must acquire and track GPS signals at high speed and acceleration, and undergo vibration and temperature conditions not common to CRPA use.

The invention enables distributed small and simple GPS fixed-reception pattern antennas (FRPAs) that do not need to be precisely arranged, to suppress wideband interference and/or jamming, and to provide sufficiently accurate and timely position and velocity measurements from the C/A code for launch vehicle range safety, antenna pointing, and attitude determination.

The new technology is compatible with existing launch-capable GPS antennas and receiver hardware, and requires the addition of cabling and a common processor (and can accommodate channel mismatch in the receivers or added hardware). The digital IF signals from multiple receivers are transferred to a common processor and jamming/interference is suppressed. The post-suppression signals are processed individually to measure the positions and velocities of receivers, and together to measure the vehicle orientation.

The suppression is based on the fact that the signal that any antenna receives from a given transmitter, be it a GPS satellite or a jammer, consists of a set of scaled, delayed, and Doppler-shifted replicas of the transmitted signal (multiple replicas are due to multipath). By determining how the parameters of the replicas differ from receiver to receiver, then taking an appropriate combination of the received signals, the invention suppresses the jammers. GPS signals are not suppressed because the suppression depends sensitively on the direction to the transmitter. Instead, the post-suppression signals contain multiple copies of the signal from each satellite. The replicas have insignificant effects on receiver position and velocity measurements, but affect measurements of orientation.

The suppression of a jammer has a processing load comparable to that for a single receiver tracking three GPS satellite signals, so that rapid tracking updates are possible.

This work was done by Stephen Hershkowitz of Mark Resources Inc. for Kennedy Space Center. KSC-13392

Test & Measurement Tech Briefs Magazine

This article first appeared in the February, 2015 issue of Test & Measurement Tech Briefs Magazine.

Read more articles from the archives here.