GPS MULTIPATH MITIGATION

GPS Multipath Equalizer (work performed in 2001 under a contract from NASA Johnson Space Center, NAS9-01040)

This algorithm, which runs inside a GPS receiver, dynamically compensates the PRN code tracking loops for multipath-caused distortions, and the resulting code-phase measurement errors. It does not affect the code tracking operation when multipath is absent. This method is based on certain properties of the discriminator function computed within the code tracking loop.

This multipath mitigation reduces the effects of multipath on the GPS-derived position estimate. It also improves GPS-derived attitude estimate, with some modifications. In general, it improves code tracking performance for any direct-sequence spread-spectrum system (for example, other GNSS, CDMA systems, etc).

Removing effects of multipath is important because multipath affects the GPS receiver carrier and code phase measurements by distorting the carrier and code phase detectors. Multipath affects the code phase to a much higher degree than it affects the carrier phase. In the absence of the carrier phase tracking in a noncoherent receiver, the superimposing direct and delayed/multipath signal components lead to code-phase tracking errors, to errors in pseudo-range measurements, and to errors in receiver’s position measurements.

The two multipath error envelope figures below show the performance of this equalizer for the GPS C/A code, for constructive (0 deg phase) and destructive (180 deg phase) multipath. They correspond to the multipath amplitude of 0.5 (with respect to the direct signal), and the correlator spacing of 0.5 PRN code chip. Multipath delays of up to one PRN code chip are compensated by this equalizer, delays greater than one chip are easily handled using other methods (ex. decorrelation). The uncompensated error envelope is shown in solid line, and the error envelope compensated by this multipath equalizer is shown in dashed line (click on the graph to enlarge it).

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Figure 1. Constructive (0 deg) Multipath Error Envelope

Figure 2. Destructive (180 deg) Multipath Error Envelope

Another performance measure below shows the mean-square error (MSE – in meters square) index values for the two multipath error envelope examples above, for the uncompensated and compensated systems. The code phase tracking error reduction under multipath is substantial as shown by much smaller MSE performance index values for the compensated system (computed for the multipath delays of up to one PRN code chip).

Constructive multipath (0 deg) MSE performance index:

Uncompensated System 926.66 m2 (RMS = 30.44 m), Compensated System 10.51 m2 (RMS = 3.24 m)

Destructive multipath (180 deg) MSE performance index:

Uncompensated System 949.12 m2 (RMS = 30.81 m), Compensated System 74.30 m2 (RMS = 8.62 m)

Anti-Spoofing Use:

In the known absence of multipath, this algorithm may also be used for the detection of the receiver’s spoofing by an outside system. When combined with signal amplitude discrimination, this algorithm may offer some level of protection against spoofing by examining the operation of the receiver’s code tracking loop when spoofing signal is present. This spoofing scenario has not yet been simulated.

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This product is available for licensing, please contact Jack.

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Copyright © 2013-2016 Jack Borninski. All Rights Reserved.

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