ION GNSS 2012
Session D2: GPS & GLONASS Modernization

Title: High Fidelity Chip Shape Analysis of GNSS Signals using a Wideband Software Receiver
Author(s): S. Gunawardena and F. van Graas, Ohio University
Date/Time: Wednesday, September 19, 2012, 4:23 p.m.
Room: Grand Ballroom West (Renaissance)

The GPS SVN49 signal anomaly sparked a flurry of research into novel GNSS signal processing techniques that are focused on detailed analysis of a GNSS signal´s subtle (and often unique) modulation characteristics [1]. In particular, this type of processing attempts to recover the time-domain shape of the chips modulated onto the RF carrier. More often than not, peculiarities observed in the range domain such as pseudorange natural biases, multipath-like errors, and the elevation dependence of these effects has a one-to-one correspondence with the signal´s chip shape distortions as observed in the time domain. These distortions include the frequency, magnitude and damping response of the ringing in the rising and falling edges of chip transitions, non-symmetries of the chips, and hitherto unreported types of distortions that will be described in this paper.
The observation of a below-noise-floor GNSS signal directly in the time domain can only be accomplished with a high-gain dish antenna on a per-satellite basis. However, the fact that the GNSS signal structure repeats combined with the assumption that any deformations must also be repeating in the short term allows the application of special transformations to the local carrier and code replicas followed by long coherent integration to the extent that the signal´s chips become readily observable in the time domain complete with all distortions.

For this type of processing to be used as a meaningful tool for the detailed analysis of GNSS signal quality, the following requirements must be satisfied: 1) the GNSS receiver antenna and RF front-end must have adequately-high bandwidth - preferably greater than or equal to the GNSS signal´s bandwidth, 2) as much as possible, the observation environment must be free of multipath, and 3) the receiver antenna and front-end electronics subsystem´s time-domain response must be measured and calibrated out. The latter two requirements stem from the fact that the observed chip shape characteristic is the composite of satellite borne effects, multipath, and the receiver´s own transfer function. In fact, related processing techniques are used for advanced multipath estimation and mitigation [2], [3].

Chip shape processing applied to sampled IF data collected using Ohio University´s wideband Transform-Domain Instrumentation GNSS Receiver (TRIGR) [4] has revealed hitherto unreported details of GNSS signal deformations including modulation phase imbalances that are on the order of one degree (approx. 0.5 mm). This paper will report on these observations for the current GPS constellation including their dependence on satellite elevation angle. A rudimentary model that attempts to describe these deformations will also be outlined. This model is based on known public information about a typical GPS satellite´s electronics payload.

References: [1] Thoelert, S., et. al, "A Multi-Technique Approach for Characterizing the SVN49 Signal Anomaly, Part 2: Chip Shape Analysis," GPS Solutions, January 2011

[2] Fenton, P. C. and Jones, J., "The Theory and Performance of NovAtel Inc.´s Vision Correlator," proc. ION GNSS 2005, Long Beach, CA, September 2005.

[3] York, J., Little, J., Munton, D., Barrientos, K., "A Fast Number-theoretic Transform Approach to a GPS Receiver", NAVIGATION, Vol. 57, No. 4, Winter 2010-2011.

[4] Gunawardena, S., van Graas, F., "Multi-Channel Wideband GPS Anomalous Event Monitor," proc. ION GNSS 2011, Portland, OR, September 2011.

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