Session B4: Spectrum: Protection and Optimization

GNSS Interference Detection and Geolocation from LEO Satellites – Satellite Formation and Payload Design Specific Considerations and Their Impact on the Detection Sensitivity and Geolocation Accuracy
Nikolas Dütsch, Hepzibah Ernest, Thomas Pany, University Bundeswehr Munich; Alberto Prada Campello, Dustin Borheck, Jan Speidel, Hacer Sunay, OHB
Date/Time: Thursday, Sep. 19, 2:12 p.m.

The evolution in space technology has aided significant improvements in Global Navigation Satellite Systems (GNSS) and its compatible receiver technology. Today, satellite-based positioning techniques have become one essential tool to navigate anywhere on Earth. GNSS satellites are orbiting the earth in the Medium Earth Orbit (MEO) and emit ranging signals in the L-Band. Recent GNSS radio occultation experiments from the International Space Station (ISS) had observed several interesting cases of GNSS jamming and spoofing activity on earth. These instruments were primarily optimized for studying the composition of the earth’s atmosphere. However, with the rising threat of deliberately operated GNSS emitters on earth, the performance and the integrity of the Position Velocity and Time (PVT) information of GNSS receivers in the vicinity of interference sources are degraded and compromised. Therefore, many GNSS users in the public safety, security and military domain would profit from globally monitoring the activities of deliberately emitted GNSS interferences from LEO satellites without the need for installation and maintaining a large ground-based network of monitoring stations.
Two German research projects were initiated focusing on the detection and geo-localization of GNSS interferences from Low-Earth-Orbiting (LEO) satellites. The first project is SeRANIS (Seamless Radio Access Networks for Internet of Space) that is supported by Zentrum für Digitalisierungs- und Technologieforschung der Bundeswehr (dtec.bw) of Germany. Within SeRANIS, the research satellite ATHENE-1 is currently under development and will house amongst other experiments a payload that is designed to capture digitized signals containing possible GNSS interference signals.
The second project is MOGSI (Spaceborne Monitoring of GNSS Signal Interference), co-funded by the German Aerospace Agency under the project reference 50NA2309D. Within MOGSI, a payload that consists of two modules is going to be implemented and tested in the laboratory. On the one hand, a payload module for monitoring of GNSS interferences is designed and on the other hand a secure space-born Galileo PRS (Public Regulated Service) receiver is implemented which delivers accurate and integrity proofed PVT information that severs as key information for the interference monitoring application.
At first, the paper introduces the general concept of monitoring of GNSS interference from space. It highlights all the relevant functional building blocks which consists of the whole processing chain on the satellite from the earth-oriented antenna towards the Analogue-to-Digital Converter (ADC) and storage unit of the captured interference signals. This chapter concludes with an overview of necessary communication links for interacting between the satellite and the ground station and exchange of information between satellites in a formation for the case of a multiple satellite scenario.
In the following chapter, the practical design considerations are mentioned. This consists of a preliminary design for a satellite constellation, including space segment and ground segment, which would be capable to demonstrate GNSS interference monitoring from space. Different constraints in terms of processing capabilities (available hardware resources for the detection and geo-localization algorithm), satellite internal raw I/Q storage capacities, datalink rates and number of ground stations and antenna designs are investigated, and a set of useful parameters / choices are highlighted.
The further chapters of this paper put a special emphasis in the sensitive detection and accurate geo-location of emitters that operate in the navigation bands. These emitters have the aim to deliberately interferer (jam) or deceive (spoof) authentic GNSS signals. Some examples of already known jamming and spoofing incidents are mentioned and the underlying typical signal structures of jamming signals are highlighted.
Even though LEO-based interference monitoring offers the advantage of an obstruction-free Line-of-Sight (LOS) to ground-based interferences, the large propagation distance between the satellite and interferences reduces the received power level. A high sensitivity detection therefore enables the detection of larger magnitude of interferences. A sensitive and robust detection in real-time onboard then enhances the utilization of the downlink bandwidth in limiting the transmission to only raw samples containing interferences and enables detailed characterization and localization of these interferences through post-processing. The detection performance of algorithms across different domains were evaluated under different types of interferences namely Continuous (CW), linear and non-linear chirp, and multiple interferences. Single satellite-based detection using Periodogram, Short-Time-Fourier-Transform (STFT), Smoothed-Pseudo-Wigner-Ville-Distribution (SPWVD) and dual satellite based cross-correlation detection methods are considered.
Detection sensitivity and computational complexity are evaluated through an interference signal generation and processing environment developed in Matlab. The interference signal generation module models the signal characteristics of interferences, link budget along with the dynamics of the interference source and LEO satellites.
The emitter coordinates can be derived from the interference signal parameter of interest which are in principle the Delay- and Doppler- information. In case of a two satellites scenario, the Delay and Doppler difference between the two received signal snapshots can be extracted by cross-correlating these signals with each other and performing a peak search for different Delay and Doppler offsets. For a two satellite formation with varying distances the narrow-band Cross-Ambiguity-Function (CAF) can be used for the differential Time-of-Arrivial (TDoA) and differential Frequency-of-Arrival (FDoA) estimation in a two step geolocation approach. The changing behaviours with respect to differential delay, Doppler and Doppler rate are shown for this specific scenario and the constraints in using the narrow-band CAF for geolocation from LEO are highlighted.
A lower bound of the achievable geolocation errors is derived based on Cramer-Rao-Lower-Bounds (CRLB) of Time- and Frequency measurement errors. For one interference signal, the geo-location accuracies are evaluated and shown for different emitter locations. For the analysis of the achievable accuracy of the emitter location, these CRLBs are used for modelling the Delay and Doppler errors, which are dependent on the SNR of the received signals at the satellites, the integration time, the Root-Mean-Square (RMS) duration and the RMS bandwidth of the interference signal and the Front-End bandwidth. The geolocation accuracy error is derived from the estimator co-variance matrix of the Weighted-Least-Squares (WLS) algorithm. The co-variance matrix of the TDoA and FDoA measurement errors are derived from the CRLBs of the Delay and Doppler errors. The distance root-mean-squared (drms) errors of the WLS estimator for a grid of different emitter locations are for each emitter position evaluated separately. The drms errors are transformed for each emitter location individually into a local East-North (EN) coordinate system. The centers of the EN coordinate systems are the true emitter locations. This chapter concludes with highlighting the most promising satellite constellation in terms of minimizing the overall geo-localization error for a certain kind of interference signal.
This paper concludes with the highlighted proposal of a LEO satellite formation which is useful for an accurate estimation of earth-bounded GNSS interference locations within the nadir-oriented antenna spot beam. The constraints in using the narrow-band CAF for TDoA and FDoA estimation under highly dynamic transmission links between the emitter and different satellites were evaluated and the theoretical performance bounds of geo-localization errors for certain type of GNSS interference signals were derived.