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We consider how to protect Unmanned Aerial Vehicles (UAVs) from Global Positioning System (GPS) spoofing attacks to provide safe navigation. The Global Navigation Satellite System (GNSS) is widely used for locating drones and is by far the most popular navigation solution. This is because of the simplicity and relatively low cost of this technology, as well as the accuracy of the transmitted coordinates. Nevertheless, there are many security threats to GPS navigation. These are primarily related to the nature of the GPS signal, as an intruder can jam and spoof the GPS signal. We discuss methods of protection against this type of attack and have developed an experimental stand and conducted scenarios of attacks on a drone’s GPS system. Data from the UAV’s flight log were collected and analyzed in order to see the attack’s impact on sensor readings. From this we identify a new method for detecting UAV anomalies by analyzing changes in internal parameters of the UAV. This self-diagnosis method allows a UAV to independently assess the presence of changes in its own subsystems indicative of cyber attacks.

The scattering of GNSS signals over a water surface is studied when the receiver is at a low height, as in GNSS-R coastal altimetry. The precise determination of the local sea level and wave state from the coast will provide useful altimetry and wave information as “dry” tide and wave gauges. An experiment has been conducted at the Canal d'Investigació i Experimentació Marítima (CIEM) wave channel for two simulated “sea” states. The GNSS-reflectometer used is the P(Y) and C/A ReflectOmeter (PYCARO) instrument, a closed-loop receiver with delay and Doppler tracking loops that uses the conventional GNSS-R technique for the GPS C/A code. After retracking of the scattered GPS signals, the coherent and incoherent components have been studied. To reproduce the transmitted GPS signals indoors, a Rohde and Schwarz signal generator is used. It is found that, despite the ratio of the coherent and incoherent components being ~1, the coherent component is strong enough that it can be tracked. The coherent component comes from clusters of points on the surface that approximately satisfy the specular reflection conditions (“roughed facet”). The Pearson’s linear correlation coefficients of the derived “sea” surface height with the wave gauge data are: 0.78, 0.85 and 0.81 for a SWH = 36 cm and 0.34, 0.74, and 0.72 for a SWH = 64 cm, respectively, for transmitter elevation angles of = 60°, 75° and 86°, respectively. Finally, the rms phase of the received signal before the retracking processing is used to estimate the effective rms surface height of the ‘facets’, where the waves get scattered. It is found to be between 2.5- and 4.1-times smaller than the theoretical values corresponding to the half of the coherent reflectivity decaying factor., Peer Reviewed, Postprint (author's version)

Reflectometry using Global Navigation Satellite System's signals (GNSS-R) of opportunity was originally conceived in the early 1990 s for mesoscale altimetry, and since then, many studies have shown its applicability to other remote sensing applications such as sea state determination, soil moisture, vegetation, snow monitoring, etc. In December 2012, the Phase A studies of ESA's PAssive Reflectometry and Interferometry System In-orbit Demonstration (PARIS IoD) mission concluded. In conventional GNSS-R (cGNSS-R), the satellite navigation signals scattered over the Earth's surface are cross-correlated with a locally generated replica of the transmitted ones shifted in frequency (Delta f(d)), and in delay (Delta tau). However, in PARIS, a different technique called interferometric GNSS-R (iGNSS-R) is used, which allows the use of the whole signal's bandwidth, and improve the altimetry precision, despite the large bandwidth signals' codes being not publically available. This is achieved by using the direct signal collected by a directive antenna, instead of the locally generated replica. This study presents a methodology to optimize the configuration of a generic iGNSS-R altimeter, and evaluate its performance. The methodology presented is then particularized to a PARIS IoD-like case., Peer Reviewed, Postprint (published version)

Measuring ocean mesoscale variability is one of the main objectives of next generation satellite altimeters. Current radar altimeters make observations only at the nadir sub-satellite ground track, which is not sufficient to sample the ocean surface with the required spatial and temporal sampling. The GNSS-R concept has been proposed as an alternative observation system in order to overcome this limitation, since it allows performing altimetry along several points simultaneously over a very wide swath. Latest proposed GNSS-R altimeter configurations allow measuring sea height with an accuracy of few decimeters over spatial scales of 50-100 km, by means of a single-pass. This paper proposes an innovative processing and retracking concept for GNSS-R altimeters based on the acquisition of the full delay-Doppler map (DDM), which allows to acquire multiple waveforms at different Doppler frequencies, whose footprints are located outside the typical pulse-limited region. The proposed processing adapts the Synthetic Aperture Radar (SAR) delay-Doppler concept of spaceborne radar altimeters for use in a GNSS-R system. This processing yields additional multi-look with respect to conventional GNSS-R concepts and translates into an improvement of the altimetry performance estimated to be at least 25%-30%, and even higher, depending on the wanted along-track spatial resolution. The proposed processing can also provide measurements with high spatial resolution at best possible performance, and more generally, offers various possibilities for optimal trade-off between spatial-resolution and height estimation accuracy., Peer Reviewed, Postprint (published version)

This work describes a novel dual-band Global Navigation Satellite Systems Reflectometer (GNSS-R) that uses the P(Y) and C/A signals scattered over the sea surface to perform highly precise altimetric measurements. The results derived from two different ground-based field experiments over a dam and over the sea under different surface's roughness conditions are presented. The analysis of the altimetric performance shows that the results obtained using the P(Y) code improve by a factor between 1.4 and 2.4 as compared to the results obtained using the C/A code, respectively, for high and mid-low satellite's elevation angles., Peer Reviewed, Postprint (published version)

For spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R), the delay and Doppler frequency conditions dynamically change, so the compensation of the delay and Doppler errors using tracking is important to the altimetric and scatterometric performances. This work presents the characteristics of the delay and the Doppler frequency in spaceborne GNSS-R, such as the range of them, the Doppler spreading width, and the change rate. They are important considerations for design of the delay and Doppler tracking strategy. The characteristics of delay-Doppler conditions of spaceborne GNSS-R are presented with additional considerations for tracking design. The tracking methods are discussed, and the error impacts are visually demonstrated by using the spaceborne measured data from United Kingdom's Disaster Monitoring Constellation (UK-DMC) satellite., Peer Reviewed, Postprint (published version)

Reflectometry using Global Navigation Satellite System (GNSS-R) signals has become an attractive tool for Earth remote sensing. Back to 1993, it was proposed as an alternative to the conventional spaceborne ocean altimetry to estimate the sea surface height over a large field-of-view. After Doppler compensation, the cross-correlation of the received scattered signal with either a locally generated replica [conventional GNSS-R (cGNSS-R)] or either the direct signal (interferometric GNSS-R (iGNSS-R)] is called the waveform. This paper provides a critical review of the cross-correlation waveform model, addressing issues such as the correlation characteristics, the bandwidth, the observation geometry, and the thermal and speckle noises. It provides a comprehensive and systematic overview of the impact of all these effects on the GNSS-R observables, which is important to properly define future GNSS-R instrument and mission concepts., Peer Reviewed, Postprint (published version)

We describe our work on multirotor UAVs and focus on our method for autonomous landing and position control. The paper describes the design of our landing pad and the vision based detection algorithm that estimates the 3D-position of the UAV relative to the landing pad. A cascaded controller structure stabilizes velocity and position in the absence of GPS signals by using a dedicated optical flow sensor. Practical experiments prove the quality of our approach.

What is GPS; How GPS works in 5 easy steps; Who developed GPS....and Why?; What makes up the GPS system?; The Signals and the Codes; GPS Instruments; Factors influencing GPS accuracy; GPS Surveying; Error statement; practical GPS workshop.

This work describes a novel dual-band Global Navigation Satellite Systems Reflectometer (GNSS-R) that uses the P(Y) and C/A signals scattered over the sea surface to perform highly precise altimetric measurements. The results derived from two different ground-based field experiments over a dam and over the sea under different surface's roughness conditions are presented. The analysis of the altimetric performance shows that the results obtained using the P(Y) code improve by a factor between 1.4 and 2.4 as compared to the results obtained using the C/A code, respectively, for high and mid-low satellite's elevation angles., Peer Reviewed, Postprint (published version)