Simple Techniques for RFI Situational Awareness and Characterization in GNSS Receivers

Phillip W. Ward

Abstract:	This paper describes simple but effective techniques for radio frequency interference (RFI) situational awareness and characterization in GNSS receiver designs. To be useful, RFI situational awareness in a GNSS receiver must be continuously available anytime that the GNSS receiver is turned on even if the RFI situation is so severe that the receiver cannot acquire or track the GNSS signals in space. This means the RFI situation must be observed prior to and during initial signal acquisition; i.e., without the benefit of tracking the signals in space to make these observations. It also means that the RFI situation must be observed when the receiver has been tracking the GNSS signals in space but the RFI level has increased to the point where the receiver loses lock on every signal in space. Therefore, RFI situational awareness must be performed prior to the correlation process, typically at the intermediate frequency (IF) stage of the GNSS receiver front end. To be simple (and low cost), the analog-to-digital converter (ADC) and automatic gain control designs of the GNSS receiver front end must include a preconceived plan to incorporate RFI situational awareness. This paper describes these simple GNSS receiver RFI situational awareness front end design techniques. The paper also addresses the importance of characterizing the type of RFI that is present and the effect that each type of RFI has on the GNSS receiver at baseband; i.e., after the carrier wipeoff and code correlation processes. Despreading spread spectrum signals plus RFI results in less processing gain for in-band narrowband (NB) RFI than for in-band wideband (WB) RFI assuming both have the same in-band power. It is desirable and possible to remove NB RFI by frequency excision signal processing techniques at IF but these are not simple design techniques. A simple non-uniform ADC design technique to minimize the effects of CW RFI and characterize it is described, but this method cannot be used if NB RFI frequency excision techniques are to be used. These design trade-offs are described. Pulsed RFI is characterized as WB RFI. Because of radars, it is the most common form of civil RFI. Therefore, it is prudent to implement simple non-uniform techniques in the GNSS receiver front end design to mitigate this problem. This paper concludes with RF front end design principles that circumvent the pulsed RFI problem. The nonuniform design techniques described usually render this source of RFI effectively harmless.
Published in:	Proceedings of the 2008 National Technical Meeting of The Institute of Navigation January 28 - 30, 2008 The Catamaran Resort Hotel San Diego, CA
Pages:	154 - 163
Cite this article:	Ward, Phillip W., "Simple Techniques for RFI Situational Awareness and Characterization in GNSS Receivers," Proceedings of the 2008 National Technical Meeting of The Institute of Navigation, San Diego, CA, January 2008, pp. 154-163.
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