Innovation: Interference Heads-Up
June 1, 2008 By: Phillip W. Ward GPS WorldReceiver Techniques for Detecting and Characterizing RFI
INNOVATION INSIGHTS with Richard Langley
AS WE ALL KNOW, GPS SIGNALS ARE WEAK. At a receiver's antenna, in the open air, their strength is about –160 dBW or 1 × 10-16 watts. Compare this to a cell-phone signal, which might be –60 dBW or 1 × 10-6 watts — 10 billion times stronger! While code correlation in the receiver lifts the GPS signals above the background noise floor, the signals are still relatively fragile, and building walls and other obstructions can significantly attenuate the received signal power so that they cannot be tracked by a conventional receiver.
 Richard Langley |
It is the ratio of the signal power to the noise power per unit bandwidth that determines the trackability of the signal. Accordingly, if the receiver's noise floor should increase sufficiently, even in an outdoor environment, the signals may also become untrackable. This can happen when the receiver is subjected to intentional or unintentional radio-frequency interference (RFI) by a transmitter operating on or near GPS frequencies. If the interference is strong enough, it can jam the receiver. Although intentional jamming is typically of concern only to military GPS users, unintentional jamming can occur anywhere and anytime and can affect large numbers of users within the range of the jamming transmitter. The jamming incident in San Diego harbor in January 2007, for example, affected all GPS users within a range of about 15 kilometers including a medical services paging network.
Such jamming renders a GPS receiver inoperable. But how do users know that their receivers are being jammed and not suffering some other type of malfunction? Clearly it would be advantageous for users to receive a heads-up when jamming signals are present and, if possible, for the receiver to take corrective action automatically.
In this month's column, we look at some simple techniques, which can be easily incorporated into the design of a GNSS receiver, to detect, characterize, and actually mitigate RFI. Such receiver enhancements will benefit civilian and military users alike.
"Innovation" is a regular column that features discussions about recent advances in GPS technology and its applications as well as the fundamentals of GPS positioning. The column is coordinated by Richard Langley of the Department of Geodesy and Geomatics Engineering at the University of New Brunswick, who welcomes your comments and topic ideas. To contact him, see the "Contributing Editors."
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With the growing proliferation of a large variety of transmitters around the world, there should be little question that the noise floor for GNSS receivers will continue to increase along with the threat of disabling in-band radio-frequency interference (RFI). RFI poses a serious threat to the reliable operation of GNSS receivers when the received RFI power level is high enough to render the GNSS receiver inoperable. RFI that is at or beyond the tolerable jamming capability of the GNSS receiver (which is related to the receiver's background thermal noise level) causes no end of confusion to the user. There are usually no visible external signs of anything being out of order, so the user initially assumes the receiver has experienced an internal failure.
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. A simple jamming-to-noise-power (J/N) monitor can be a low-cost built-in feature of the GNSS receiver if a J/N meter design is preplanned in the original GNSS receiver front-end component design, layout, and implementation. A retrofit to an existing design is usually impractical. An additional justification for including a J/N meter in the initial GNSS receiver design is the significant performance advantages obtained through RFI situational adaptation especially during initial search. Every GNSS receiver user who 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 the military should require it.
 Figure 1 GNSS receiver front end with digital automatic gain control and jamming-to-noise-power meter by-product. |
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