«Anti-Jamming locate, communicate, accelerate techniques in u-blox GPS receivers GPS receivers differ considerably in their anti-jamming effectiveness ...»
locate, communicate, accelerate
GPS receivers differ considerably in
their anti-jamming effectiveness
white paper by: Andreas Thiel,
Founder and Executive VP, u-blox
VP Embedded Software Development, u-blox
Table of contents Executive Summary 3 Anti-Jamming 4 Capturing extremely weak signals 4 Mitigation measures 5 Field comparisons 7 Conclusion 7 About the Authors 8 About u-blox 9 Anti-Jamming techniques in u-blox GPS receivers / White Paper Published by u-blox AG | 2 Executive Summary A critical factor when selecting components for a GPS system is the receiver’s immunity to external noise, or “jamming”. The ability to lock onto typically faint GPS signals in the presence of noise generated from other electronic devices has a large influence on the system’s ability to provide correct location data.
The use of an advanced, proprietary adaptive digital filtering technology allows the u-blox 5 and u-blox 6 GPS positioning engines to overcome jamming signals up to 25 dB stronger than conventional GPS receivers can withstand. The result is the most sensitive and reliable GPS receiver technology available.
Anti-Jamming techniques in u-bloxGPS receivers / White Paper Published by u-blox AG | 3 Anti-Jamming While underway, whether walking or driving a car, you’ll notice how the signal strengths from satellites as read by a GPS receiver vary quite a bit, and in some areas for some unexplained reason you can completely lose satellite lock. While such reductions in signal strength can be due to attenuation and signal blockage while in urban canyons, inside building or even underneath heavy tree cover, it can also be due to unintentional jamming – signals from everyday electrical and electronic devices that create so much noise that they mask the satellite signals. These jamming signals can even come from ordinary battery-operated consumer devices such as portable music players or mobile phones as well as automobile electronics.
One reason that jamming is an issue with any GPS receiver is that the signals GPS Satellites transmit with a power output of roughly 30 W coming from the satellites are so weak. Satellites transmit with a power outand are 20,000 km above us. put of roughly 30 W and are 20,000 km above us. Just imagine: how much strength would the signal with the strength of a weak light bulb have after traveling that distance? In fact, a typical signal, when acquired outdoors with open sky, is in the range of –120 dBm (1 x 10–15 W), and moving inside a normal residential building can add 20 or 30 dBm of attenuation. With such a weak signal, other signals in the same GPS frequency band don’t need to be very strong at all in order to override the satellite signals.
Figure 1: This simplified block diagram of a GPS receiver based on the u-blox 5 / 6 technology shows the SAW filter at the signal input to block out-of-band interference plus the softwarecontrolled digital filter bank that blanks out specific interference sources.
Figure 2: Measured directly at the GPS carrier frequency, an interference signal of only about –110 dBm is sufficient to degrade the signal by 3 dB for receivers using conventional approaches (blue trace), whereas u-blox 5 / 6 can handle interference that is 25 dBm stronger before it reaches its 3-dB desensitization point (red trace).
Figure 3: Field tests using conventional technology (left) and u-blox 5 / 6 technology (right) at various levels of a singlefrequency, in-band interference signal. The upper part of each screen shot shows the signal strength of the GPS satellites, and dots in the lower part track several calculated positions. As the jamming signal increases, the number of satellites each technology can track and their signal strengths decreases.