What’s Going On? RFI Situational Awareness in GNSS Receivers

How familiar is this? You switch on your GNSS receiver. It lights up but won’t acquire satellites and begin tracking signals. Is the problem in the receiver, the signals, or perhaps the operating environment? This article describes a simple but effective receiver-based method for detecting radio frequency interference (RFI) or jamming that can help answer this question. The design does not require the receiver to be tracking for it to reliably provide RFI situational awareness. Instead, the solution uses measurements obtained from the automatic gain control to help determine the jamming-to-noise power ratio, which serves as the key metric for assessing the RF environment.

Phillip W. Ward

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In-band radio frequency interference (RFI) is a serious threat to the reliable operation of GNSS receivers. When the RFI power level is high enough to render the GNSS receiver inoperable, usually no visible external signs appear indicating that anything is out of order; so, the user initially assumes the receiver has experienced an internal failure.

For the purposes of this article, the determination of the presence and seriousness of an RFI problem is called RFI situational awareness. Without the sophistication of built-in RFI situational awareness in the GNSS receiver design, the determination of the presence and seriousness of in-band RFI problems is an extremely inefficient and frustrating process.

Given the growing proliferation of a very large variety of transmitters around the world, the noise floor for GNSS receivers will undoubtedly continue to increase along with the threat of disabling, in-band RFI.

This article describes the design techniques and advantages of built-in RFI situational awareness using a very simple and inexpensive design called a jamming-to-noise power ratio (J/N) meter.

One caveat, however: the J/N meter design is “simple and inexpensive” only if this capability is carefully pre-planned in the design of the original GNSS receiver front-end components, layout, and implementation. Another caveat is that commercial GNSS receiver front-end designs using one-bit analog-to-digital converters (requiring no automatic gain control) do not fit the design prerequisites for this concept.

A retrofit to an existing design is usually impractical. The initial justification for including RFI situational awareness in the design might be based on the significant performance advantages described in this article, but every GNSS receiver user that anticipates or has ever experienced an operational failure due to in-band RFI will greatly value this feature. Certainly, all safety-of-life GNSS applications and military operations should require it.

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Manufacturers

The analog AGC circuit design shown in Figure 2 and characterized in Table 1 is the AD8367 from Analog Devices, Inc., Norwood, Massachusetts USA. The digital AGC amplifier illustrated in Figure 4 and specified in Table 2 is Analog Devices’ AD8325. (To see figures, download the PDF of the complete story.)

Author Profiles

Phil Ward is president of Navward GPS Consulting, which he founded in 1991 in Dallas, Texas. Prior to becoming a consultant, he was senior member of the technical staff in the Defense Systems & Electronics Group of Texas Instruments Inc. Ward has been involved in the field of navigation since 1958 and with GPS receiver design as a systems engineer since 1976. He was the first Institute of Navigation (ION) Congressional Fellow (2001), the chair of the ION Satellite Division (1994-96), and ION President (1992-93). He received the ION Thurlow Award (1989) for developing the first commercial GPS receiver, the TI 4100. Ward received his B.S.E.E. degree from the University of Texas at El Paso and his M.S.E.E. degree from Southern Methodist University in Dallas, Texas. He also took postgraduate courses in Computer Science at Massachusetts Institute of Technology (MIT) while a member of the technical staff on the Apollo Guidance Computer design team at the MIT Instrumentation Lab, now Charles Stark Draper Lab (1967-70).