A Blind Anti-Jammer Pre-Processor for GPS Receiver

1. Introduction Global positioning system (GPS) employing satellite-based location techniques has been widely used in providing precision navigational capabilities for aircraft, ships, and ground traffic. Due to a large processing gain with spread spectrum techniques employed, GPS receivers can be effective in extracting the navigational information signals transmitted from satellites at a power level below that of thermal noise [1]-[2]. Nevertheless, performance will significantly degrade if any strong interfering source coexists. Typically, jamming power level less than 40 dB with respect to the signal power level, i.e. jammer-to-signal ratio (JSR) of 40 dB, can be tolerated. This however is so impossible, since, in practice, the existence of strong unintentional radio frequency interference and intentional jammers, which are easily generated due to the weakness of received GPS satellite signal about –160 dBm at earth, will disable the receiver. Consequently, a powerful technique for jammer suppression has gained much attention to guarantee the reliable GPS operations [3]-[4]. In this paper, a novel GPS receiver is proposed for alleviating the performance degradation due to the strong jammer. Specifically, an anti-jammer scheme prior to the correlators is first proposed for assuring the reasonable level of JSR in the inputs of correlators. From the fact that the desired signals are well below the possible jammers, the jammer terms associated with the received data will significantly enlarge the eigenvalue spread of the received data correlation matrix. Based on the received data correlation matrix, a detection algorithm is developed to determine the existence of the strong jammers. An optimal combining is then constructed so as to produce nulls for suppressing the strong jammers as the resultant of the jammer detector is true. Contrarily, a mainbeam is formed for maximum reception of the Signals-of-Interest (SOIs). Moreover, the outputs of the optimal combiner are sent to regular GPS correlators to despread the received data (chip level) into the bit stream. In spite of the success in suppressing the strong jammer, optimal combining performed on the SOIs will introduce phase rotation, which will induce significant degradation in performance. As a remedy, a phase compensator is developed based on the predetermined 8-bit preambles in the first two words (telemetry and TLM) associated with each subframe of GPS data to estimate the phase rotation introduced by the combining scheme. Final, a hard decision detector is used to recover the GPS navigation data bits. Since the strong jammers have been removed preliminarily, the …