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Expert Advice: Low-End Jam Resilience May Not Be Desirable

May 29, 2015  - By
Jan Wendel

Jan Wendel

By Jan Wendel

At the European Navigation Conference held in Bordeaux, France, April 7–10, a keynote session and ensuing panel discussion addressed the issue of “GNSS Resilience for Terrestrial and Naval Applications.” During the discussion, two questions from the floor drew these responses from panelist Jan Wendel of Airbus Defence & Space GmbH, a leading European aerospace company.

Do you believe that receiver manufacturers will be able to deliver resilient receivers in the future?

JW: In order to achieve resilience, regulatory measures can only provide a mid- to long-term solution. Therefore, resilience needs to be addressed at the receiver level as well.

Considering spoofing, I am not aware of any confirmed spoofing incident. Iran has been claiming to have spoofed a CIA drone, which became for me at least theoretically feasible when I heard the rumor that this drone was equipped with a GPS C/A code receiver. Also, there has been a wrongly configured repeater at the Hannover airport. Nevertheless, spoofing to me does not seem to be a current threat.

However, jamming is clearly a reality nowadays. In my opinion, we should first decide which level of resilience we actually want to achieve for which type of user receiver. If the simple receivers like in smartphones become more and more robust against jamming, the simple jammers available on the Internet will react with an increasing jamming power. This will leave less margin for the receivers used in more critical applications, which we really would like to see functioning permanently.

Therefore, resilience for low-end receivers might not be a good idea; maybe it would be better to see them fail in some scenarios.

Another aspect in the discussion we have had so far is the spreading-code encryption for authentication purposes. Actually, I see spreading-code encryption more as a means to restrict the access of a GNSS signal to authorized users and as an anti-spoofing measure, but not primarily as a means for authentication. Here, we must be aware that the access is not necessarily as restricted as we would like to think.

With directive antennas, blind demodulation techniques and a communication link, it is possible with a slight delay to achieve a position, velocity and time solution at a rover, without being an authorized user of the respective service.

We must understand resilience also in a more global sense, that such a possibility must not be detrimental to the applications assuming a restricted access to specific GNSS services.

Do standards help?

JW: In general, standards are a good thing, as they help in the construction of complex systems by assuring interface compatibility and also minimum performances. However, care needs to be taken when the standards are defined. For example, in the NMEA 0183 protocol, essential information is missing that is required for integration of a GNSS receiver with an inertial navigation system, for example, vertical velocity, full variance-covariance matrices of the receiver’s position and velocity, or raw data like pseudorange, delta ranges and ephemeris to name a few. Clearly, the NMEA protocol was not designed for GNSS/INS integration, and for its intended use the NMEA protocol fits perfectly.

However, for many applications, it is not usable. Being a de-facto standard offered by most receivers, I think it would be beneficial if this protocol would follow more a general-purpose spirit, like most of the proprietary protocols of the different receiver manufacturers do. So with the NMEA protocol lacking relevant information, we are in a situation where for many applications either the receiver manufacturers’ proprietary protocols have to be used — given these protocols offer the required information — or the receiver cannot be used at all. For me, this is an example where a standard is not of great help, also because the process of developing such a standard towards an extended scope takes considerable time, if possible at all.

Jan Wendel is a system engineer at Airbus DS GmbH in Munich, Germany, where he is involved in activities related to satellite navigation, including tracking, integrity and sensor integration algorithms. He received the Dr.-Ing. degree from the University of Karlsruhe, where he is also a private lecturer.

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