Advanced Interference Detection and Mitigation in Septentrio’s High Precision Receivers

Wim De Wilde, Jean-Marie Sleewaegen, Bruno Bougard, Jan Van Hees

Abstract:	Today’s high precision receivers exploit the full range of navigation signals in order to optimize accuracy and reliability of the positioning solution. These signals originate from multiple satellite constellations and each of these constellations transmits wide band modulations on at least three carrier frequencies. In addition many receivers support precise point positioning corrections broadcasted by geostationary satellites. In order to receive all those signals, the receiver needs to process over 200 MHz of frequency spectrum. The combination of this wide frequency range and the weak signal power associated to space-earth signals makes interference a critical issue. Some GNSS signals share the spectrum with other systems. The L2 and E6 bands overlap with the 23-cm radio amateur band and the L5 signals reside in an aeronautical band which is also used for Distance Measurement Equipment (DME). The L1 GLONASS band can be affected by powerful earth-space transmissions from satellite phones operating in an adjacent frequency band. Interference can also originate from unintended cross-talk from local sources. A typical example is a 1.6 GHz CPU operating near the GNSS antenna. Sometimes the receiver could be affected by intentional interference in the GPS L1 band. Inexpensive GPS jammers are sold on the Internet. They are frequently used to impair operation of GPS tracking devices in road vehicles. High precision receivers are often used as critical elements in high operating cost applications such as offshore operations, excavator control, aerial survey, etc. Interference related outages can block operations and have important financial implications. Therefore it is important to identify interference issues as fast as possible, ruling out other potential causes for failure of a complex RTK system. For this purpose, Septentrio receivers have a built-in tool to monitor the RF spectrum. This provides insight in the time and frequency domain structure of the signal received by the antenna. The signal can be monitored in real time by the user on the graphical user interface and the data can also be recorded for further analysis. This allows support engineers to identify the precise nature of an interfering signal and the likely interference source. This information is then used to trigger the right countermeasure, for example changing the antenna set-up or disabling the interference source. The RF signal analysis tool was introduced in Septentrio products as a support tool in 2006 and exposed as a standard feature in the graphical user interface in 2010. Over the years, it has proved very successful in resolving interference issues at customers, while allowing us to build an extensive data-base of field interference cases. This paper discusses some of the most representative cases, including various types of radio amateur interference, illegal video interference as well as local cross-talk. As the receiver collects signals in the time domain the data allows distinguishing harmless pulsed interference from harmful continuous interference, as will be shown in the paper. Besides, a recording of a chirp jammer will be presented. The recorded chirp jammer completely wipes out the GPS L1 band and appears in the spectrum as a wide-band signal. The corresponding time domain plot reveals the swept frequency nature of this signal. The process of analyzing interferences manually and mitigating them by switching off the source takes time and required skilled personnel. In many cases, it is simply not possible to remove the cause of the interference. In those cases, the ability to continue operating entirely depends on the interference mitigation capabilities built in the receiver itself. Septentrio’s AsteRx4 multi frequency receiver modules feature advanced in-band interference mitigation circuitry, residing in the mixed-signal GReCo4 baseband SoC. This chip has a high-resolution high-speed signal processing section, which recovers all GNSS bands after cleaning them from interference. Afterwards the cleaned signals are supplied to a massive amount of tracking and acquisition channels without risk for saturation, offering ultimate GNSS availability and accuracy to the user. The interference mitigation hardware comprises two types of circuits. The first type is a set of Adaptive Notch Filters (ANF). The ANF addresses continuous or slowly pulsed signals with bandwidths up to 1 MHz. It is fully adaptive and only removes the part of the spectrum which is affected, preserving useful spectral content of GNSS signals. As some interferers tend to drift in center frequency, it is able to track this drift. The circuit can mitigate up to three interferers at a time. The field data shows this is sufficient to cover any realistic interference scenario. The ANF includes a rapid interference detector, which almost instantaneously detects a harmful interferer anywhere in the GNSS spectrum. Upon detection the interference rejection is enabled, without needing user intervention and with minimal impact on continuity. The second circuit is the Wide-band Interference Mitigation Unit (WIMU). The WIMU is designed to cope with interferers which exceed 1 MHz of bandwidth. As clear from field data, the L2 frequency range is sometimes affected by video interferers. Video signals have a wide bandwidth, often close to 10 MHz. They rarely fully overlap with a GNSS band, but they often saturate correlator hardware resulting in a loss of signal tracking. In case such an interferer shows up, the WIMU will automatically discard the spectral region associated with this signal and avoid saturation. Even though it typically rejects part of a GNSS signal as well, a sufficient amount of signal power will be preserved to maintain signal tracking. The spectrum of some interferer types completely overlaps with a GNSS signal band. One example is DME interference in the L5 band as perceived by aircraft at cruising altitudes. Chirp jamming signals are another example. Nonetheless the WIMU can reject these signals as it also exploits time domain information. Finally, the paper will demonstrate the operation of the interference mitigation circuits by comparing performance of the AsteRx4 receiver module to receivers with limited or no interference mitigation, when exposed to realistic interference scenarios as presented earlier in the paper.
Published in:	Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015) September 14 - 18, 2015 Tampa Convention Center Tampa, Florida
Pages:	1656 - 1683
Cite this article:	De Wilde, Wim, Sleewaegen, Jean-Marie, Bougard, Bruno, Van Hees, Jan, "Advanced Interference Detection and Mitigation in Septentrio’s High Precision Receivers," Proceedings of the 28th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2015), Tampa, Florida, September 2015, pp. 1656-1683.
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