Global navigation satellite systems (GNSS) like GPS but also Galileo, GLONASS, and Beidou represent an important infrastructure to our society. They provide position and timing for numerous ...applications. A GNSS is a rather complex system consisting of around 30 satellites, a number of monitor stations, plus a control center. Billions of GNSS receivers represent the user segment. The receivers and the monitor stations receive a weak satellite radio signal and thus are susceptible to interference like jamming or spoofing. This paper outlines the core operation principles of satellite navigation to describe those kind of interference and to analyze the impacts. Different types of interference attacks result in different effects at the target receiver. The attack schemes are categorized within this paper. Reported incidents are summarized as well as the potential impact of GNSS vulnerability on critical infrastructure. Well known countermeasures at user receiver level or at system level are outlined and discussed for their suitability. The political and socioeconomic context of GNSS vulnerability is described emphasizing the large impact of a potential disruption of the GNSS service.
Coastal sea level variation as an indicator of climate change is extremely important due to its large socioeconomic and environmental impacts. The ground-based global navigation satellite system ...(GNSS)-reflectometry (GNSS-R) is becoming a reliable alternative for sea surface altimetry. We investigate the impact of antenna polarization and orientation on GNSS-R altimetric performance at different carrier frequencies. A one-year dataset of ground-based observations at the Onsala Space Observatory using a dedicated reflectometry receiver is used. Interferometric patterns produced by the superposition of direct and reflected signals are analyzed using the least-squares harmonic estimation (LS-HE) method to retrieve sea surface height. The results suggest that the observations from global positioning system (GPS) L1 and L2 frequencies provide similar levels of accuracy. However, the overall performance of the height products from the GPS L1 shows slightly better performance due to more observations. The combination of L1 and L2 observations (L12) improves the accuracy up to 25% and 40% compared to the L1 and L2 heights. The impacts of antenna orientation and polarization are also evaluated. A sea-looking left-handed circular polarization (LHCP) antenna shows the best performance compared to both zenith- and sea-looking right-handed circular polarization (RHCP) antennas. The results are presented using different averaging windows ranging from 15 min to 6 h. Based on a 6-h window, the yearly root mean squared errors (RMSEs) between GNSS-R L12 sea surface heights with collocated tide gauge observations are 2.4, 3.1, and 4.1 cm with the correlation of 0.990, 0.982, and 0.969 for LHCP sea-looking, RHCP sea-looking, and RHCP up-looking antennas, respectively.
This paper revises the precision of altimetric measurements made with signals of the Global Navigation Satellite Systems (GNSS) reflected (GNSS-R) off the sea surface. In particular, we investigate ...the performance of two different GNSS-R techniques, referred to here as the clean-replica and interferometric approaches. The former has been used in GNSS-R campaigns since the late 1990s, while the latter has only been tested once, in 2010, from an 18-m-high bridge in static conditions and estuary waters. In 2011, we conducted an airborne experiment over the Baltic Sea at 3-km altitude to test the interferometric concept in dynamic and rougher conditions. The campaign also flew a clean-replica GNSS-R instrument with the purpose of comparing both approaches. We have analyzed with detail the data sets to extract and validate models of the noise present in both techniques. After predicting the noise models and verifying these with aircraft data, we used them to obtain the precision of altimetric measurements and to extrapolate the performance analysis to spaceborne scenarios. The main conclusions are that the suggested noise model agrees with measured data and that the GNSS-R interferometric technique is at least two times better in precision than a technique based on using a clean replica of the publicly available GPS code. This represents a factor of at least four times finer along-track resolution. A precision of 22 cm in 65-km along-track averaging should be achievable using near-nadir interferometric GNSS-R observations from a low earth orbiter.
In this study, a new technique for evaluating the target satellite's relative states in the analysis of cluster satellite motion is presented. Within closed constellation architectures, various ...techniques for sensor selection are employed to analyze the motion of target satellites. Alongside Global Navigation Satellite Systems (GNSS)-based sensors, the utilization of visual-based sensors has witnessed significant adoption, particularly in tandem with advancements in technology. This study aims to address the computational complexity and challenges associated with line-ofsight vector calculations by proposing a directed and color-coded light source approach as a solution to overcome the difficulties encountered with visual-based sensors.
Spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R) has emerged as a pivotal tool with different land applications, prominently encompassing soil moisture estimation. In contrast to ...conventional radiometer satellites commonly used for this purpose, GNSS-R offers higher spatiotemporal coverage while maintaining cost-effectiveness. The potential of using Global Positioning System (GPS) reflections measured by the Cyclone Global Navigation Satellite System (CYGNSS) mission to retrieve soil moisture has been previously demonstrated. In 2019, Spire Global Inc. launched their first GNSS-R satellites, which now comprise a constellation of four CubeSats. These satellites track reflections from multi-constellation, encompassing GPS, Galileo, BeiDou Navigation Satellite System (BDS), and Quasi-Zenith Satellite System (QZSS). In this study, an analysis and validation of Spire GNSS-R L1B surface reflectivity for soil moisture retrieval within east Australia during an eight-month period in 2021 is presented. A comparison of the estimated Spire surface reflectivity to that of CYGNSS is performed, unveiling analogous behavioural patterns and biases across both missions. Soil moisture is estimated using observations from Spire GPS-only, Spire multi-constellation, and CYGNSS. The Soil Moisture Active and Passive (SMAP) retrievals are used as the reference, presuming a linear relationship between changes in soil moisture and changes in reflectivity. Our results indicate that the Spire GNSS-R mission can detect variations in soil moisture with a performance comparable to that of CYGNSS. A median unbiased root-mean-square difference (ubRMSD) of 0.062 m
3
.m
−3
is found for both Spire GPS and multi-constellation when using 9-km products and SMAP as the reference.
Stereo methods using GOES‐17 and Himawari‐8 applied to the Hunga Tonga‐Hunga Ha'apai volcanic plume on 15 January 2022 show overshooting tops reaching 50–55 km altitude, a record in the satellite ...era. Plume height is important to understand dispersal and transport in the stratosphere and climate impacts. Stereo methods, using geostationary satellite pairs, offer the ability to accurately capture the evolution of plume top morphology quasi‐continuously over long periods. Manual photogrammetry estimates plume height during the most dynamic early phase of the eruption and a fully automated algorithm retrieves both plume height and advection every 10 min during a more frequently sampled and stable phase beginning 3 hr after the eruption. Stereo heights are confirmed with Global Navigation Satellite System Radio Occultation bending angles, showing that much of the plume was lofted 30–40 km into the atmosphere. Cold bubbles are observed in the stratosphere with brightness temperature of ∼173 K.
Plain Language Summary
The Hunga Tonga‐Hunga Ha'apai volcano in the South Pacific erupted violently on 15 January 2022. A volcanic plume from the eruption was lofted into the stratosphere to altitudes that are unprecedented in the era of satellite observations. We observed the highest part of the plume at 55 km and tracked the motion of the plume in 3D in the vicinity of the volcano for a 7‐hr period, every 10 min, using imagery from the geostationary GOES‐17 and Himawari‐8 satellites that are positioned at different locations on the equator. The apparent shift in the plume as seen from two different vantage points contains information about the plume height and the apparent movement of the plume as it is repeatedly observed by one satellite contains information about the plume velocity. We confirmed our height observations using radio occultation measurements that NOAA uses to profile the atmosphere. Radio waves are normally bent as they pass through the atmosphere from satellite to satellite, nearly grazing the Earth's surface, but when radio waves pass through the volcanic plume, there is an anomalously large change in bending angle.
Key Points
The Hunga Tonga‐Hunga Ha'apai eruption of 15 January 2022, lofted material above 30 km to record‐breaking heights of ∼55 km
Our stereo‐winds code retrieved height and motion vectors from GOES‐17 and Himawari‐8 every 10 min shortly after the eruption
Radio occultation bending angles confirm plume altitudes
This literature review paper focuses on existing vulnerabilities associated with global navigation satellite systems (GNSSs). With respect to the civilian/non encrypted GNSSs, they are employed for ...proving positioning, navigation and timing (PNT) solutions across a wide range of industries. Some of these include electric power grids, stock exchange systems, cellular communications, agriculture, unmanned aerial systems and intelligent transportation systems. In this survey paper, physical degradations, existing threats and solutions adopted in academia and industry are presented. In regards to GNSS threats, jamming and spoofing attacks as well as detection techniques adopted in the literature are surveyed and summarized. Also discussed are multipath propagation in GNSS and non line-of-sight (NLoS) detection techniques. The review also identifies and discusses open research areas and techniques which can be investigated for the purpose of enhancing the robustness of GNSS.
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