Interference Mitigation for Highly Dynamic GPS Receivers Using Intelligent Tracking Loops

A.M. Kamel, D. Borio, J. Nielsen, G. Lachapelle

Abstract:	In recent years, most autonomous navigation systems (ANS) used onboard missiles or fighters depend on Global Navigation Satellite Systems (GNSS) for positioning and navigation. Global Positioning System (GPS), as an example of GNSS, uses satellites that broadcast very weak signals. The performance of a GPS receiver usually depends on the Phase Lock Loops (PLL) used to down-covert these incoming weak signals and track their carrier phase and frequency. Only reliable signal tracking allows a GPS receiver processor to calculate pseudoranges and extract the navigation data. A PLL can properly work only if its bandwidth is wide enough to track the signal dynamics that can be significantly high if the ANS is onboard a missile or fighter. On the other hand, wide loop bandwidths allow larger portions of noise and interference to enter the tracking loops and disturb the signal tracking process. Excessive noise and interference can lead to loss of lock. Using aiding from a Frequency Lock Loop (FLL) is a possible solution that allows the reduction of the PLL bandwidth. This solution, however, cannot prevent frequent loss of lock and can be strongly affected by interference. The tradeoff between bandwidth requirements motivates the design of alternative tracking systems replacing conventional FLL-assisted-PLLs. In this paper, an innovative FLL-assisted-PLL is designed and tested using the concept of fuzzy systems. More specifically, a fuzzy controller is used to replace standard loop filters whose output is used to drive the Numerically Controlled Oscillator (NCO). The proposed Fuzzy Frequency Phase Lock Loop (FFPLL) uses both frequency and phase discriminators outputs to generate the required frequency changes to tune the NCO, which in turn generates the local carrier used for the signal down-conversion. The proposed tracking scheme is an extension of the work presented by Kamel (2010) where the FFPLL was proposed for tracking signals under high dynamic conditions. In (Kamel 2010), the interference problem was only considered under a constant signal power level and the system was designed to operate under nominal conditions where the interference level is known. The main core of any fuzzy system is its fuzzy sets or Membership Functions (MF) that maps input/output parameters into defined linguistic variables describing the input/output states. Loop discriminator outputs mainly depend on the incoming signal Carrier-to-Noise power density ratio (C/No) and have a Probability Density Function (PDF) that, under lock conditions, can be accurately approximated by a Gaussian distribution. Although the mean of this Gaussian distribution is zero under normal tracking conditions, it can be affected by sudden changes in the presence of dynamics that can cause cycle slips and other phase errors. The standard deviation of this distribution is also dependent on the signal quality and hence on the interference level. For these reasons, the discriminator output values have been clustered into several overlapped Gaussian MFs that can linguistically describe their state. The variance of the Gaussian MFs assigned to the phase and frequency discriminator outputs are adaptively tuned according to the incoming signal quality. So any change in the interference power level leads to variations in the Gaussian MF variance to ensure accurate linguistic description of the discriminator output signal. The fuzzy rules are selected to tune the NCO and ensure accurate and robust signal tracking. In this paper, the performance of the previously described fuzzy tracking system is assessed in the presence of different power levels of interference. To generate GPS signals corrupted by Radio Frequency (RF) interference, a hardware GPS signal simulator combined with two external signal generators are used. Different interference levels combined with missile harsh dynamics are applied for testing the proposed system. Results show that the use of the fuzzy tracking system leads to a significant improvement in system robustness and accuracy such that it is able to track very high dynamics with reduced tracking jitter. The proposed system is shown to be resilient against strong interference up to a certain extent where increasing jamming levels are compensated by the online adaptation of the MF distribution on the basis of a small amount of data or C/No information. The proposed system performs favorably against standard tracking loops that cannot sustain the same level of dynamics and interference. The proposed adaptive FFPLL can sustain interference power levels up to 40 dB higher than the GPS signal power. Even when the proposed algorithm loses lock, a fast and reliable reacquisition is obtained when the interference power is reduced.
Published in:	Proceedings of the 2011 International Technical Meeting of The Institute of Navigation January 24 - 26, 2011 Catamaran Resort Hotel San Diego, CA
Pages:	374 - 383
Cite this article:	Kamel, A.M., Borio, D., Nielsen, J., Lachapelle, G., "Interference Mitigation for Highly Dynamic GPS Receivers Using Intelligent Tracking Loops," Proceedings of the 2011 International Technical Meeting of The Institute of Navigation, San Diego, CA, January 2011, pp. 374-383.
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