Digital Wideband Antijam Technology for Global Positioning System (GPS) Protection

The proliferation of inexpensive low-power GPS jammers is an increasing concern. These battery-powered jammers are a menacing problem because they can deployed by the hundreds the air by tethered ballons, for example, or concealed on the ground. Theselow-power jammers operate for several days on a battery pack. Small jammers may be difficult to find and individually are considered low-value targets. Wideband low-power jammers often modulate high-speed CW swept or Chirp interference in order toappear like noise in the frequency spectra. Most spectral analyzers use Fast Fourier Transform (FFT) methods to estimate spectral energy of RF signals. Real time frequency domain filtering using the FFT and inverse FFT has been demonstrated to extractfrequency-stable narrow band CW interference in GPS bands. However, Fourier techniques are not effective against frequency-agile sources of interference, which are rapidly being aggressively marketed. In order to address the frequency-agile interferenceproblem, modern spectral analysis techniques such as wavelet analysis are better suited. The singular advantage of wavelets is the multi-resolution time-frequency signal analysis property.(i)Wavelet sub-band coding and filter banks is a promising newsignal processing algorithm has been demonstrated to be an effective means gaining of 20-30 dB J/S for spread-spectrum receivers. Wavelet sub-band filters was performed by introducing frequency-agile interference (high-speed swept and chirp) on C/A bandof GPS down converted and post-processed. Although the results are promising, the work was not advanced to a wideband (P-code) GPS real-time implementation. (ii) The wavelet sub-band filter technique has been demonstrated to add 20-30 dB of J/S in directsequence spread spectrum receivers against a catalog of frequency agile jammers. In order for this work to yield practical benefits for tactical system, the algorithm needs transition real-time over a 20 MHz band. This technique can be implementedwithout high latency. Spectral interference rejection by digital signal processing does not require a special antenna. In addition, it does not assume constantly radiating interference. These issues affect spatially interference rejection (CRPA)approaches. The inherent processing latency is fixed unlike the null steering antennas, which do not converge upon best performance when platform dyamics and interference transmission cycles on and off. Spectral filtering has been demonstrated to beaffective against multiple jammers simultaneously, while spatial filters performance degrades substantially with each additional null and has a hard limit of (N-1) jammers. As a result, DSP using wavelet filter banks may be used stand alone orcomplementary anti-jam technique within a layered approach.