This reprint of the Special Issue “GNSS-R Earth Remote Sensing from SmallSats” includes a compilation of research articles concerning the pioneering NASA CYGNSS mission and potential future ...constellations of GNSS-R SmallSats for Earth remote sensing. This reprint is composed of different sections, covering various scientific applications such as ocean wind speed, ocean altimetry, inland water bodies, and soil moisture content. The use of GNSS-R techniques by the future constellations of SmallSats could overcome several limitations of more classical remote sensing techniques; for example, passive microwave radiometry and synthetic aperture radar. Present and upcoming GNSS-R missions include BuFeng-1, Fengyun-3 series, Spire Global and Muon Space commercial constellations, ESA’s HydroGNSS and PRETTY, TASA TRITON, etc. The outcome of this reprint aims to leverage state-of-the-art procedures and techniques by present and future mission science teams.
High‐rate Global Navigation Satellite Systems (GNSS) has an increasing number of applications in geohazard monitoring. GNSS‐derived displacements can provide important information for magnitude ...estimation and fault slip inversion, which is critical for seismic and tsunamigenic hazard mitigation. In this paper, we propose a new approach to quickly capture coseismic displacements with a single GNSS receiver in real time. The new approach can overcome the convergence problem of precise point positioning, and also avoids the integration process of the variometric approach. Using the results of the 2011 Tohoku‐Oki earthquake, it is demonstrated that the proposed method can provide accurate displacement waveforms and permanent coseismic offsets at an accuracy of few centimeters, and can also reliably recover the moment magnitude and fault slip distribution.
Key Points
quickly capture seismic displacements with single GPS receiver in real‐time
It overcome convergence of PPP and integration process of variometric approach
It provides accurate waveforms,permanent offsets,magnitude ,slip distribution
The ability of spaceborne Global Navigation Satellite System (GNSS) bistatic radar receivers to sense changes in soil moisture is investigated using observations from the low Earth orbiting UK ...TechDemoSat‐1 satellite (TDS‐1). Previous studies using receivers on aircraft or towers have shown that ground‐reflected GNSS signals are sensitive to changes in soil moisture, though the ability to sense this variable from space has yet to be quantified. Data from TDS‐1 show a 7 dB sensitivity of reflected signals to temporal changes in soil moisture. If the effects of surface roughness and vegetation on the reflected signals can be quantified, spaceborne GNSS bistatic radar receivers could provide soil moisture on relatively small spatial and temporal scales.
Key Points
The first observations that spaceborne GNSS‐R can sense soil moisture are shown
GNSS‐R signals respond to the same land features consistently over time
Soil moisture could be estimated at short temporal scales if effects from roughness and vegetation are removed
Global Navigation Satellite System (GNSS) has drawn the attention of scientists and users all over the world for its wide-ranging Earth observations and applications. Since the end of May 2022, more ...than 130 satellites are available for fully global operational satellite navigation systems, such as BeiDou Navigation Satellite System (BDS), Galileo, GLONASS and GPS, which have been widely used in positioning, navigation, and timing (PNT), e.g., precise orbit determination and location-based services. Recently, the refracted, reflected, and scattered signals from GNSS can remotely sense the Earth’s surface and atmosphere with potential applications in environmental remote sensing. In this paper, a review of multi-GNSS for Earth Observation and emerging application progress is presented, including GNSS positioning and orbiting, GNSS meteorology, GNSS ionosphere and space weather, GNSS-Reflectometry and GNSS earthquake monitoring, as well as GNSS integrated techniques for land and structural health monitoring. One of the most significant findings from this review is that, nowadays, GNSS is one of the best techniques in the field of Earth observation, not only for traditional positioning applications, but also for integrated remote sensing applications. With continuous improvements and developments in terms of performance, availability, modernization, and hybridizing, multi-GNSS will become a milestone for Earth observations and future applications.
We propose a simple low-cost technique that enables civil global positioning system receivers and other civil global navigation satellite system (GNSS) receivers to reliably detect carry-off spooling ...and jamming. The technique, which we call the power-distortion detector, classilies received signals as interference-free, multipath-afflicted, spoofed, or jammed according to observations of received power and correlation function distortion. It does not depend on external hardware or a network connection and can be readily implemented on many receivers via a lirmware update. Crucially, the detector can with high probability distinguish low-power spooling from ordinary multipath. In testing against more than 25 high-quality empirical datasets yielding more than 900,000 separate detection tests, the detector correctly alarms on all malicious spooling or jamming attacks while maintaining a 0.6% single-channel false alarm rate.
Global navigation satellite system (GNSS) location engines on Android devices provide location and navigation utility to billions of people worldwide. However, these location engines currently have ...very limited protection from threats to their position, navigation, and time (PNT) solutions. External sources of radio frequency interference (RFI) can render PNT information unusable. Even worse, false signals or spoofing can provide a false PNT solution to Android devices. To mitigate this, four detection methods were developed and evaluated using native location parameters within Android: Comparing the GNSS and network locations, checking the Android mock location flag, comparing the GNSS and Android system times, and observing the automatic gain control (AGC) and carrier-to-noise density (C/N0) signal metrics. These methods provide a powerful means to significantly increase the robustness of the Android GNSS-based PNT solution and are implemented in the GNSSAlarm Android application to demonstrate real-time jamming and spoofing detection.
The Mw 6.4 right‐lateral Petrinja earthquake (2020, Croatia) is one of the largest continental earthquakes of central Europe for decades. The slip pattern of such events is usually difficult to ...retrieve with terrestrial geodesy due to limited monitoring means. This study takes advantage of a unique data set of dense measurements of coseismic displacement in the epicentral area, obtained by repeated measurements of benchmark networks designed for civilian purposes, and supplemented by far‐field continuous GNSS measurements. Elastic modeling of these data shows two coseismic slip patches that extend over a 15 × 10 km rupture plane, locally reaching the surface, and that a right‐lateral sub‐parallel secondary fault in the central part of the rupture likely accommodated part of the coseismic deformation. This study demonstrates that rapid re‐measurement of pre‐existing civilian networks offers unique coseismic constrains in the near‐field where InSAR or optical image correlation may decorrelate.
Plain Language Summary
The magnitude 6.4 Petrinja earthquake that stroke Croatia on 29 December 2020, is one of the largest earthquakes registered for decades in continental Europe. Large damage and surface ruptures were observed, suggesting that the earthquake occurred at a very shallow depth. The slip pattern for such moderate magnitude earthquakes is usually difficult to retrieve with terrestrial geodesy because of limited monitoring means and small deformations. In this study, we use a unique data set made of dense coseismic displacement estimates in the epicentral area obtained by repeated positioning measurements of benchmark networks designed for civilian purposes. The inversion of this displacement field completed by far‐field continuous GNSS measurements shows that the coseismic slip on the Petrinja‐Pokupsko fault is limited to depths no greater than 10 km and has reached the surface locally. A single‐fault model explains well the data, but the fit is largely improved with a sub‐parallel secondary fault in the central part of the rupture. This study demonstrates that quick remeasurement of pre‐existing civilian networks can offer unique constrains on the coseismic deformation and the associated fault geometry when spatial techniques such as InSAR or optical correlation may decorrelate.
Key Points
Real Time Kinematics measurements of civilian networks capture near‐field coseismic displacements of the 2020 strike‐slip Petrinja earthquake
Time‐series from regional continuous GNSS stations constrain coseismic deformation in the far‐field
Elastic modeling argues for two slip patches on the main fault and for significant slip on a parallel secondary fault
This paper presents the temporal evolution of Global Navigation Satellite System Reflectometry (GNSS-R) ocean wind speed retrieval performance during three years of the UK TechDemoSat-1 (TDS-1) ...mission. TDS-1 was launched in July 2014 and provides globally distributed spaceborne GNSS-R data over a lifespan of over three years, including several months of 24/7 operations. TDS-1 wind speeds are computed using the NOC Calibrated Bistatic Radar Equation algorithm version 0.5 (C-BRE v0.5), and are evaluated against ERA5 high resolution re-analysis data over the period 2015–2018. Analyses reveal significant temporal variability in TDS-1 monthly wind speed retrieval performance over the three years, with the best performance (~2 m∙s−1) achieved in the early part of the mission (May 2015). The temporal variability of retrieval performance is found to be driven by several non-geophysical factors, including TDS-1 platform attitude uncertainty and spatial/temporal changes in GPS transmit power from certain satellites. Evidence is presented of the impact of the GPS Block IIF Flex mode on retrieved GNSS-R wind speed after January 2017, which results in significantly underestimated ocean winds over a large region covering the North Atlantic, northern Indian Ocean, the Mediterranean, the Black Sea, and the Sea of Okhotsk. These GPS transmit power changes are shown to induce large negative wind speed biases of up to 3 m∙s−1. Analyses are also presented of the sensitivity of TDS-1 wind speed retrieval to platform attitude uncertainty using statistical simulations. It is suggested that a 4° increase in attitude uncertainty can produce up to 1 m∙s−1 increase in RMSE, and that TDS-1 attitude data do not fully reflect actual platform attitude. We conclude that the lack of knowledge about the GNSS-R nadir antenna gain map and TDS-1 platform-attitude limits the ability to determine the achievable wind speed retrieval performance with GNSS-R on TDS-1. The paper provides recommendations that accurate attitude knowledge and a good characterisation of GNSS-R nadir antenna patterns should be prioritised for future GNSS-R missions.
•TDS-1 GNSS-R wind retrieval performance has large variability over 3 years lifetime.•Retrieval performance reflects attitude uncertainty and GPS transmit power changes.•Spatial and temporal changes in GPS transmit power can bias winds by 3 m∙s−1.
The validity of the results obtained within different permanent GNSS reference station networks (GNSS Network) must be periodically controlled using criteria that are generally known from statistical ...analyzes or prescribed by International Standards. Procedures for evaluating the uncertainty of measurements are defined in accordance with the purpose of the GNSS Network. The authors of this paper want to point out the need to establish requirements for periodical and systematical control of GNSS coordinates within the same permanent GNSS Network and control of GNSS coordinates between different permanent GNSS Networks measured on the same/unique point on the ground. This paper presents control procedures for three permanent GNSS reference station Networks established and operating in the Republic of Serbia. Special attention is on the analysis of data consistency within one permanent GNSS Network and the mutual consistency of GNSS data between different networks. The paper aims to promote reliance on the different GNSS Networks and contains suggestions on how GNSS Networks may prove that they are performing competently and that they can provide valid results for field measurements. Particularly highlighted is the need to plan and implement measures related to increasing the effectiveness of the GNSS system, achieving improved results, and preventing negative effects while performing field measurements. The paper presents the results for comparison, selected according to the rules for creating a Digital Cadastral Map features, i.e., points, lines, and polygon. The results for comparing point features are the GNSS coordinates. The results for comparing line features are the lengths of the line, i.e., distances, and the results for comparing polygon features are the areas of the polygons.
Lane-level vehicle self-localization is a challenging and significant issue arising in autonomous driving and driver-assistance systems. The Global Navigation Satellite System (GNSS) and onboard ...inertial sensor integration are among the solutions for vehicle self-localization. However, as the main source in the integration, GNSS positioning performance is severely degraded in urban canyons because of the effects of multipath and non-line-of-sight (NLOS) propagations. These GNSS positioning errors also decrease the performance of the integration. To reduce the negative effects caused by GNSS positioning error, this paper proposes to employ an innovative GNSS positioning technique with the aid of a 3-D building map in the integration. The GNSS positioning result is used as an observation, and this is integrated with the information from the onboard inertial sensor and vehicle speedometer in a Kalman filter framework. To achieve stable performance, this paper proposes to evaluate and consider the accuracy of the employed GNSS positioning method in dynamic integration. A series of experiments in different scenarios is conducted in an urban canyon, which can demonstrate the effectiveness of the proposed method using various evaluation and comparison processes.
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