High Temperature Superconducting (HTS) Filters for GPS Interference Mitigation

Anna Leese de Escobar and Harper J. Whitehouse

Abstract:	Out-of-band interference can be a significant problem for GPS receivers that operate in close proximity to high power transmitters. To provide protection from this type of interference and to mitigate against unintentional jamming, high performance radio frequency (RF) filters can be used between the antenna and the GPS receiver. These RF filters need to be low loss, have high out-of-band rejection and have nearly constant group delay. Well-designed superconducting filters can provide this level of performance. High temperature superconducting (HTS) films can be used to implement these filters which when used in conjunction with closed cycle mechanical refrigerators can provide a high degree of out-of-band signal rejection. Superconductor Technologies, Incorporated have constructed, under contract, a pair of constant amplitude infinite impulse response (IIR) L1 and L2 filters as well as low noise amplifiers (LNAs) in a rack mounted dewar for experimental evaluation. These HTS filters have 3dB bandwidths of 20 MHz and out-of-band rejection of greater than 50dB at frequencies greater than 20 MHz away from their respective center frequencies. Experiments conducted at China Lake in California using these experimental filters have shown a 20dB improvement in interference rejection when the filters are used in conjunction with a GPS MAGR. In addition, link margins are improved by 6dB for both an RF MAGR and the combination of an AE-4 and an intermediate frequency (IF) MAGR. Performance improvements can be made through a redesign of the HTS filters. For applications such as adaptive antennas, that require matched group delay, a constant group delay design is preferable to a constant amplitude design. For the new military signal Lm a filter without impulse response ripples will help in the tracking of the code correlation function peak. Although these filter modifications can be achieved using HTS IIR filters, finite impulse response (FIR) filters such as HTS surface acoustic wave (SAW) filters are an attractive alternative. The significance of this work is that high temperature superconducting (HTS) radio frequency (RF) filters are a practical GPS modernization option. In addition, new filter designs coupled with next generation small size coolers should permit superconducting filters in airplane and missile applications.
Published in:	Proceedings of the 2001 National Technical Meeting of The Institute of Navigation January 22 - 24, 2001 Westin Long Beach Hotel Long Beach, CA
Pages:	364 - 368
Cite this article:	de Escobar, Anna Leese, Whitehouse, Harper J., "High Temperature Superconducting (HTS) Filters for GPS Interference Mitigation," Proceedings of the 2001 National Technical Meeting of The Institute of Navigation, Long Beach, CA, January 2001, pp. 364-368.
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