Reseach topics

The Chair of Navigation deals with the development of robust GNSS receivers on the system and algorithm level. External interferences on the position determination are especially jammers, spoofers or multipath signals. The most effective countermeasure against these influences is the spatial processing of incident signals by means of antenna arrays. Research areas of the chair are in particular the design, simulation and experimental validation of algorithms for spatial signal processing. This includes algorithms for the calibration of frontend and array antennas, for the detection and suppression of jammers and spoofers, as well as for the localization of interference sources.

Receiver vulnerability to interference

Due to the large distance to the satellite GNSS signals are received with very low power, which is typically buried below the noise floor of the receiver. Therefore, GNSS receivers are very sensitive to interfering signals. Interferers are any signals received in the GNSS bands but not emitted by a GNSS satellite. On the one hand, this can be unintentional interference, such as harmonics or spurious emissions from terrestrial communication systems that are not sufficiently filtered.

However, there are also intentional jammers, which can basically be divided into two classes. In the case of jamming, the attacker attempts to prevent the reception of authentic GNSS signals at the receiver, e.g., through broadband noise. A fairly recent example is provided by so-called personal privacy devices (PPDs), which have already been used in the past by drivers of delivery trucks in the USA to prevent monitoring by their employers. The use of jammers leads to a degradation of signal quality for affected receivers in their vicinity. The effects can range from an inaccurate estimation of the position to a failure of the entire receiver in the case of stronger jammers.

In addition to jamming, GNSS receivers are also susceptible to so-called spoofing due to a lack of signal authentication. In this case, the interferer transmits signals with authentic GNSS signal structure. Standard receivers cannot distinguish these signals from the real satellite signals, so that in the worst case a false position intended by the interferer is determined at the receiver.

The most effective countermeasure is therefore spatial filtering. The prerequisite for this is the use of an antenna array. So-called uniform rectangular arrays (URA) consisting of four antennas are frequently used. Arrays make it possible to determine the direction of arrival of any incident signal based on the differential phase offset between the individual antennas of the array. Subsequently, a constructive or destructive superposition of the antenna signals can be selected to spatially amplify authentic satellite signals or to suppress the direction of incident interferers.

The image shows a URA and the suppression of an incidence direction

The chair is pursuing two different research foci here: On the one hand, algorithms for compact antenna arrays such as the aforementioned URA are being extended in order to increase their effectiveness (cf. Zorn et. al.). On the other hand, a new conformal antenna array consisting of distributed subarrays for automotive applications is being researched (cf. Brachvogel et. al.) and corresponding algorithms are being designed (cf. Brachvogel et. al.).

The picture shows the division of a URA into two ULAs

Calibration of an antenna-array

Antenna calibration is a mandatory requirement to determine the attitude of a multi-antenna receiver. Moreover, knowing the antenna attitude can vastly improve detection and mitigation of spoofers and / or Radio Frequency Interferences. In todays receivers the time-varying phase offset introduced by the active elements like amplifiers, downconverters as well as various filters and unequal cable lengths are usually calibrated using a pilot signal, which is injected in the antenna signal. This method, however, does not only complicate the manufacturing process, but also increases the complexity of the receiver architecture and power consumptions. New approaches developed recently use GNSS live signals for calibration. These methods are established based on the fact that for each incoming GNSS signal the direction of arrival [abbr.: DOA], is exactly known by means of the ephemeris data, which is transmitted from each satellite. In order to estimate the calibration phases, the DOAs of the received satellite signals are compared with their corresponding expected DOAs, which are determined based on the ephemeris data. The problem, however, lies in the fact, that the expected DOAs are given in the East-North-Up coordinate frame, whereas the measured DOAs are given in the local coordinate frame and are additionally affected by the unknown phase offsets to be calibrated. The Chair of Navigation has developed an algorithm, that estimates the calibration phases and the attitude of the antenna jointly.

Spoofer Detection and Mitigation

Spoof signals have an even worse impact on the position estimation than interference signals. In contrast to interferences a spoofer does not result into a failure of the GNSS navigation but to a wrong position estimation. Whereas a failure can be detected immidiately, a wrong position is only obvious some time later. Due to the increasing number of safety critical applications like autonomous cars, assisted landing maneuver of airplanes, assisted maritime traffic, etc. spoofer detection gains signaficant attention. Spoof signals are GNSS like signals, that are shifted in time. There are some examples listed in the picture above. Because of the enormous amount of different spoofer types is the research topic still not fully covered.

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