Global Navigation Satellite System Reflectometry Signals Research Articles

Detection Probability of Polarimetric GNSS-R Signals

Polarimetric Global Navigation Satellite System-Reflectometry (GNSS-R) is the next natural step for land monitoring using GNSS signals. The Soil Moisture Active Passive (SMAP) radar receiver has been retrieving polarimetric GNSS-R data using two orthogonal linearly polarized antennas since 2015, enabling the study of the polarimetric signature of GNSS-R signals on different Earth’s surfaces. Currently, new instruments and missions are using circularly polarized antennas to retrieve polarimetric GNSS-R information. In this manuscript, synthetic right and left-hand circularly polarized signals are reconstructed using the Stokes parameters of the SMAP L2C GNSS-R data. The signal-to-noise ratio (SNR) of the SMAP-R data at the equivalent RHCP, LHCP, H, and V polarization antennas is retrieved, and normalized to an arbitrary receiver with a noise figure of 2 dB. Appling the Albersheim model, we analyze the probability of detecting a reflection in a 0.5° Lat/Lon box. Results are presented for different configurations of coherent and incoherent integration times and antenna gains, for each possible antenna polarization. We present different receiver configurations capable of detecting more than 70% and 90% GNSS reflections over land. Results show that with a 10-dB antenna and a receiver with a coherent integration time of 4 ms, and an incoherent integration time of 1000 ms would suffice to detect 19.4%, 92.3%, 83.5%, and 79.4% for RHCP, LHCP, H-polarized, and V-polarized antenna, respectively. Detectability improves up to 57.4%, 99.3%, 96.3%, and 96.6% using a 14-dB antenna. Results are then generalized to L1 C/A GNSS-R signals.

A Hybrid Compact Polarimetry GNSS-R Analysis of the Earth’s Cryosphere

This manuscript provides the first Hybrid Compact Polarimetric (HCP) Global Navigation Satellite System – Reflectometry (GNSS-R) analysis of the cryosphere. By reconstructing the full-Stokes parameters and the child Stokes parameters from the reflectometry signals collected by the SMAP radar receiver, the sensitivity to the geophysical changes of the Earth’s landscapes of these opportunistic signals are analyzed. This analysis is focused over the Artic Sea ice formation zone, the northern latitudes presenting freeze and thawed transitions, and the Greenland ice sheet. We conduct a sensitivity analysis for each landscape. The signals analyzed over the Arctic Sea show correlation with sea ice concentration (SIC) of 0.77 for the second Stokes parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and 0.7 for the child Stokes parameter χ, which describes the ellipticity of the wave. The signals analyzed over the northern latitudes present a sensitivity to the two states (frozen and thawed). The analysis over Alaskan landscape shows a total reflectivity Γ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> difference between freeze and thaw states of ~8 dB with a standard deviation of ~1 dB. Greenland presents a challenge since available datasets are not from the same timeframe, despite of this our initial evaluation shows that there is a ~0.5 correlation between ice thickness and the third Stokes parameter <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> and the relative phase δ between the two linear electromagnetic components, h-pol and v-pol. This manuscript provides an extensive view of the HCP GNSS-R signals over the cryosphere and finds initial sensitivities to the seasonal geophysical changes of the different areas.

Calibration Strategy for Compact Polarimetric GNSS-R Instruments

Polarimetric Global Navigation Satellite System - Reflectometry (GNSS-R) is being proposed by different teams as a solution to directly estimate soil moisture. Up to date, two approaches using polarimetric GNSS-R have been proposed to estimate soil moisture, using the ratio between the right-hand and left-hand circularly polarized received signals, and using the ratio between the linear horizontal and vertical components at a set of incidence angles, known as Hybrid Compact Polarimetric (HCP) GNSS-R. In this manuscript, the necessary calibrations of a received HCP GNSS-R signal are presented. The Stokes parameters of the signal are required to compensate all non-idealities. A methodology to calibrate the receiver effects, scene-antenna polarimetric misalignment, Faraday rotation, and transmitter non-idealities is proposed. Finally, the calibration performance is evaluated using several statistical parameters and polarimetric ratio models based on two different soil moisture products. Results show a correlation coefficient increase of 4.7% with respect to the model derived from the Soil Moisture Active Passive (SMAP) soil moisture product, and a 6.5% improvement with respect to the model derived from the European Center for Medium-Range Weather Forecasts (ECMWF) Reanalysis v5 (ERA-5) monthly average soil moisture product. Furthermore, the proposed calibrations show an unbiased root-mean-square error reduction of 6.3% and 6.8% for the SMAP soil moisture model and the ERA-5 model, respectively. Due to the SMAP antenna and pointing design, SMAP-Reflectometry (SMAP-R) is implicitly insensitive to some of the corrections. More significant corrections should be expected for future polarimetric GNSS-R instruments as European Space Agency (ESA) HydroGNSS.

A Novel Dual-Branch Neural Network Model for Flood Monitoring in South Asia Based on CYGNSS Data

Microwave remote sensing is widely applied in flood monitoring due to its independence from severe weather conditions, which usually restrict the usage of optical sensors. However, it is challenging to track the variation process of flood events in a timely manner by traditional active and passive microwave techniques, since they cannot simultaneously provide measurements with high spatial and temporal resolution. The emerging Global Navigation Satellite System Reflectometry (GNSS-R) technique with high spatio-temporal resolution offers a new solution to the dynamic monitoring of flood inundation. Considering the high sensitivity of GNSS-R signals to flooding, this paper proposes a dual-branch neural network (DBNN) with a convolution neural network (CNN) and a back propagation (BP) neural network for flood monitoring. The CNN module is used to automatically extract the abstract features from delay-Doppler maps (DDMs), while the BP module is fed with GNSS-R typical features, such as surface reflectivity and power ratio, as well as vegetation information from Soil Moisture Active Passive satellite (SMAP) data. In the experiments, the superiority of the DBNN method is firstly demonstrated by comparing it with the surface reflectivity and power ratio methods. Then, the spatio-temporal variation process of the 2020 South Asian flood events is analyzed by the proposed method based on Cyclone Global Navigation Satellite System (CYGNSS) data. The understanding of flood change processes could help enhance the capacity for resisting flood disasters.

An Analytical Formulation of the Correlation of GNSS-R Signals

We present an analytical formulation of the correlation coefficient of the electromagnetic fields scattered at near-specular direction by a rough or gently undulating surface and measured at two spatially separated positions occupied by a moving receiver at slightly different times. This allows us obtaining an explicit expression of the correlation time of the received signal in terms of radar and surface parameters. This work provides a contribution to the discussion, currently ongoing in the Global Navigation Satellite System Reflectometry (GNSS-R) scientific community, about the behavior of received signal fluctuations, especially when surface profile variations are such that the scattering is neither coherent nor completely incoherent. The scattering surface is here modeled as randomly rough, and the Kirchhoff Approximation (KA) or the first-order Small Slope Approximation (SSA1) is employed to compute the scattered field. In fact, the expression of the correlation coefficient is the same for both approximations. The obtained closed-form expression shows that, as the surface correlation length increases, the degree of coherence smoothly increases from the value obtained with the expression already available in literature for very rough surfaces to a value close to unity for gently undulating surfaces. The obtained behavior of correlation time as a function of surface parameters, system resolution and observation geometry is in agreement with numerical simulations available in literature. More in general, obtained analytical results are in agreement with the observed behavior of GNSS-R signals over flat land surfaces.

Single-Pass Soil Moisture Retrieval Using GNSS-R at L1 and L5 Bands: Results from Airborne Experiment

Global Navigation Satellite System—Reflectometry (GNSS-R) has already proven its potential for retrieving a number of geophysical parameters, including soil moisture. However, single-pass GNSS-R soil moisture retrieval is still a challenge. This study presents a comparison of two different data sets acquired with the Microwave Interferometer Reflectometer (MIR), an airborne-based dual-band (L1/E1 and L5/E5a), multiconstellation (GPS and Galileo) GNSS-R instrument with two 19-element antenna arrays with four electronically steered beams each. The instrument was flown twice over the OzNet soil moisture monitoring network in southern New South Wales (Australia): the first flight was performed after a long period without rain, and the second one just after a rain event. In this work, the impact of surface roughness and vegetation attenuation in the reflectivity of the GNSS-R signal is assessed at both L1 and L5 bands. The work analyzes the reflectivity at different integration times, and finally, an artificial neural network is used to retrieve soil moisture from the reflectivity values. The algorithm is trained and compared to a 20-m resolution downscaled soil moisture estimate derived from SMOS soil moisture, Sentinel-2 normalized difference vegetation index (NDVI) data, and ECMWF Land Surface Temperature.

Flood Inundation Mapping by Combining GNSS-R Signals with Topographical Information

The Cyclone Global Navigation Satellite System (CYGNSS) mission collects near-global hourly, pseudo-randomly distributed Global Navigation Satellite System - Reflectometry (GNSS-R) signals in the form of signal-to-noise ratio (SNR) point data, which is sensitive to the presence of surface water, due to their operating frequency at L-band. However, because of the pseudo-random nature of these points, it is not possible to obtain continuous flood inundation maps at adequately high resolution. By considering topological indicators, such as height above nearest drainage (HAND) and slope of nearest drainage (SND), which indicate the probability of a certain area being prone to flooding, we hypothesize that combining static topographic information with the dynamic GNSS-R signals can result in large-scale, high-resolution flood inundation maps. Flood mapping was performed and validated with flood extent derived using available Sentinel-1A synthetic aperture radar (SAR) data for flooding in Kerala during August 2018, and North India during August 2017. The results obtained after thresholding indicate that the model exhibits a flooding accuracy ranging from 60% to 80% for lower threshold values. We observed significant overestimation error in mapping inundation across the flooding period, resulting in an optimal critical success index of 0.22 for threshold values between 17–19.

Simulation-Based Feasibility Analysis of Ship Detection Using GNSS-R Delay-Doppler Maps

In this article, we carry out a simulation analysis of ship detection via Global Navigation Satellite System-Reflectometry (GNSS-R) delay-Doppler map (DDM). The GNSS-R DDM simulator used here is a modified version of an algorithm conceived for the generation of GNSS-R DDMs of the sea surface. The new algorithm is based on an analytical model for the radar cross section of ships and is able to properly account for the presence of ship targets within the scene. The proposed GNSS-R DDM simulator is, then, exploited for assessing the viability of GNSS-R in ship detection applications at low incidence angles, where the adopted scattering models provide accurate results. The aim of the implemented simulation setup is to analyze what are the preferable conditions for ship detection using standard GNSS-R signal processing chain receiver and compare typical forward left-hand circularly polarized GNSS-R systems with nonstandard backward right-hand circularly polarized (RHCP) GNSS-R. The simulation study is two fold: first, detection performance is evaluated at spaceborne and airborne altitudes for both polarization channels under favorable detection conditions. Then, visibility of ship targets is assessed in terms of their location within the DDM. Simulation results show that ship detection is problematic when using satellite data, whereas interesting results are achieved at airborne altitudes, provided that the aircraft is approximately between the GNSS satellite and the target, and that appropriate RHCP polarization is probed. In such configurations, signal-to-noise-ratios larger than 10 dB are obtained with airborne receivers collecting the RHCP signal.

Increasing the Number of Sea Surface Reflected Signals Received by GNSS-Reflectometry Altimetry Satellite Using the Nadir Antenna Observation Capability Optimization Method

High spatial resolution Global Navigation Satellite System-Reflectometry (GNSS-R) sea surface altimetry is of great significance for extracting precise information from sea surface topography. The nadir antenna is one of the key payloads for the GNSS-R altimetry satellite to capture and track the sea surface GNSS reflected signal. The observation capability of the nadir antenna directly determines the number of received reflected signals, which, in turn, affects the spatial resolution of the GNSS-R altimetry. The parameters affecting the ability of the nadir antenna to receive the reflected signal mainly include antenna gain, half-power beam width (HPBW), and pointing angle. Thus far, there are rarely studies on the observation capability of GNSS-R satellite nadir antenna. The design of operational satellite antenna does not fully combine the above three parameters to optimize the design of GNSS-R nadir antenna. Therefore, it is necessary to establish a GNSS-R spaceborne nadir antenna observation capability optimization method. This is the key to improving the number of sea surface reflected signals received by the GNSS-R altimeter satellites, thereby increasing the spatial resolution of the altimetry. This paper has carried out the following research on this. Firstly, based on the GNSS-R geometric relationship and signal processing theory, the nadir antenna signal-to-noise ratio model (NASNRM) with the gain and the elevation angle at the specular point (SP) as the main parameters is established. The accuracy of the model was verified using TechDemoSat-1 (TDS-1) observations. Secondly, based on the theory of electromagnetic scattering, considering the influence of HPBW and pointing angle on the antenna footprint size, a specular point filtering algorithm (SPFA) is proposed. Combined with the results obtained by NASNRM, the number of available specular points (SPs) is counted. The results show that as the antenna gain and the nadir-pointing angle increase, the number of SPs can reach a peak and then gradually decrease. Thirdly, combined with NASNRM and SPSA, a nadir antenna observation capability optimization method (NAOCOM) is proposed. The nadir antenna observation capability is characterized through the reflected signal utilization, and the results obtained by the method are used to optimize the combination of nadir antenna parameters. The research shows that when the orbital height of the GNSS-R satellite is 635 km, the optimal combination of nadir antenna parameters is 20.94 dBi for the gain and 32.82 degrees for the nadir-pointing angle, which can increase the observation capability of the TDS-1 satellite nadir antenna by up to 5.38 times.

A Random Model and Simulation for Generating GNSS Ocean Reflected Signals

In recent years, Global Navigation Satellite System Reflectometry (GNSS-R) has been studied as a new technique for ocean remote sensing, and the simulation of GNSS-R signals has played a very important role. In this letter, a random model for ocean reflected signal simulation of GNSS-R is presented to provide a useful method for generating reflected signals. The study is conducted by means of a superimposed result of multiple random reflected signals based on a previous fixed model. Reflected signals are randomly selected with different delay bins and different random amplitudes. To generate GNSS-R signals, a software GNSS-R simulator based on the proposed random model is developed. A receiver is also used to process the simulated signals to demonstrate the availability of the proposed model and developed simulator. The analysis shows good agreement between the produced and theoretical delay waveforms with the correlation coefficient of 0.998. To further verify the significance of the random model, the simulated reflected signals are compared with the airborne experimental data collected in Yantai, China. The comparative results show that the random model has better statistical and stochastic characteristics.

Classifying Inundation in a Tropical Wetlands Complex with GNSS-R

The use of global navigation satellite system reflectometry (GNSS-R) measurements for classification of inundated wetlands is presented. With the launch of NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission, space-borne GNSS-R measurements have become available over ocean and land. CYGNSS covers latitudes between ±38°, providing measurements over tropical ecosystems and benefiting new studies of wetland inundation dynamics. The GNSS-R signal over inundated wetlands is driven mainly by coherent scattering associated with the presence of surface water, producing strong forward scattering and a distinctive bistatic scattering signature. This paper presents a methodology used to classify inundation in tropical wetlands using observables derived from GNSS-R measurements and ancillary data. The methodology employs a multiple decision tree randomized (MDTR) algorithm for classification and wetland inundation maps derived from the phased-array L-band synthetic aperture radar (PALSAR-2) as reference for training and validation. The development of an innovative GNSS-R wetland classification methodology is aimed to advance mapping of global wetland distribution and dynamics, which is critical for improved estimates of natural methane production. The results obtained in this manuscript demonstrate the ability of GNSS-R signals to detect inundation under dense vegetation over the Pacaya-Samiria Natural Reserve, a tropical wetland complex located in the Peruvian Amazon. Classification results report an accuracy of 69% for regions of inundated vegetation, 87% for open water regions, and 99% for non-inundated areas. Misclassification of inundated vegetation, primarily as non-inundated area, is likely related to the combination of two factors: partial inundation within the GNSS-R scattering area, and signal attenuation from dense overstory vegetation, resulting in a low signal.

An Assessment of Non-geophysical Effects in Spaceborne GNSS Reflectometry Data From the UK TechDemoSat-1 Mission

An assessment of non-geophysical effects in spaceborne global navigation satellite system reflectometry (GNSS-R) data from the UK TechDemoSat-1 (TDS-1) mission is presented. TDS-1 was launched in July 2014 and provides the first new spaceborne GNSS-R data since the pioneering UK-disaster monitoring constellation experiment in 2003. Non-geophysical factors evaluated include ambient L-band noise, instrument operating mode, and platform-related parameters. The findings are particularly relevant to users of uncalibrated GNSS-R signals for the retrieval of geophysical properties of the Earth surface. Substantial attitude adjustments of the TDS-1 platform are occasionally found to occur that introduce large uncertainties in parts of the TDS-1 GNSS-R dataset, particularly for specular points located outside the main beam of the nadir antenna where even small attitude errors can lead to large inaccuracies in the geophysical inversion. Out of eclipse however, attitude adjustments typically remain smaller than 1.5°, with larger deviations of up to 10° affecting less than 5% of the overall sun-lit data. Global maps of L1 ambient noise are presented for both automatic and programmed gain modes of the receiver, revealing persistent L-band noise hotspots along the Equator that can reach up to 2.5 dB, most likely associated with surface reflection of signals from other GNSS transmitters and constellations. Sporadic high-power noise events observed in certain regions point to sources of human origin. Relevant conclusions of this study are that platform attitude knowledge is essential and that radiometric calibration of GNSS-R signals should be used whenever possible. Care should be taken when considering using noise measurements over the equatorial oceans for calibration purposes, as ambient noise and correlated noise in delay–Doppler maps both show more variation than might be expected over these regions.

GNSS‐R nonlocal sea state dependencies: Model and empirical verification

Global Navigation Satellite System Reflectometry (GNSS-R) is an active, bistatic remote sensing technique operating at L-band frequencies. GNSS-R signals scattered from a rough ocean surface are known to interact with longer surface waves than traditional scatterometery and altimetry signals. A revised forward model for GNSS-R measurements is presented which assumes an ocean surface wave spectrum that is forced by other sources than just the local near-surface winds. The model is motivated by recent spaceborne GNSS-R observations that indicate a strong scattering dependence on significant wave height, even after controlling for local wind speed. This behavior is not well represented by the most commonly used GNSS-R scattering model, which features a one-to-one relationship between wind speed and the mean-square-slope of the ocean surface. The revised forward model incorporates a third generation wave model that is skillful at representing long waves, an anchored spectral tail model, and a GNSS-R electromagnetic scattering model. In comparisons with the spaceborne measurements, the new model is much better able to reproduce the empirical behavior.

Online soil moisture retrieval and sharing using geospatial web-enabled BDS-R service

Global navigation satellite system-reflectometry (GNSS-R) signals have recently being used in the monitoring of soil moisture (SM) due to their high availability and widespread existence. GNSS-R signal-based online SM retrieval and real-time product sharing has advantages over other methods because it features the instant and automatic retrieval of SM, and low-latency SM product publishing and sharing, enabling a notable decrease in human and material resource consumption and a timely SM product delivery; however, prior to this manuscript, there has not been a study describing how to do it. This paper uses one type of GNSS-R signals, the Beidou navigation satellite system-reflectometry (BDS-R) signal, as the data source and proposes a geospatial web-enabled BDS-R (GWEB) service. This method tackles the problems of SM retrieval and sharing by implementing the original BDS-R signal publishing, the online SM retrieval, and the SM product sharing. In the GWEB service, a web processing service is adopted to encapsulate the SM retrieval algorithm, and a sensor observation service is applied to publish the original BDS-R observations and the SM products. Signals of the Baoxie BDS-R SM monitoring station in Wuhan, China for the period of November 17, 2014 to November 24, 2014 are used as input in the validation experiment, and the SM products are demonstrated in the sharing platform. Meanwhile, the BDS-R derived SM products are compared with the corresponding SM values measured by ground soil hygrometers, indicating the feasibility of the service for online SM retrieval and sharing.

GNSS-R Wind-Dependent Polarimetric Signature Over the Ocean

Global Navigation Satellite System-Reflectometry (GNSS-R) is a promising and innovative remote sensing technique. In this letter, an investigation on the polarimetric characteristics of GNSS-R signals is performed. The study is conducted by means of an ad hoc simulator based on a facet approach to approximate a 3-D time-domain generated sea surface. To best mimic a realistic configuration, the simulator uses both recorded GPS and Galileo signals as incident waves. The effect of the incoherent averaging in mitigating the speckle is shown. Finally, a relationship between the wind speed and the polarimetric ratio is analyzed.

Detection of buried objects using reflected GNSS signals

The use of reflected Global Navigation Satellite System (GNSS) signals for sensing the Earth has been growing rapidly in recent years. This technique is founded on the basic principle of detecting GNSS signals after they have been reflected off the Earth's surface and using them to determine the properties of the reflecting surface remotely. This is the so-called GNSS reflectometry (GNSS-R) technique. In this paper, a new application regarding the detection of metallic buried objects is analyzed and it is validated through several experimental campaigns. Although the penetration depth of GNSS signals into the ground is not optimal and depends on the soil moisture, GNSS signals can likely interact approximately with the first 10 cm of the ground and therefore can be reflected back by any metallic object buried on the first terrain layer. A very light and low-cost GNSS receiver prototype based on a software-defined radio approach was developed. This receiver can be used as a payload on board small drones or unmanned aerial systems to detect metallic objects (mines or other explosive devices). A signal processing tool based on an open-loop GNSS signal acquisition strategy was developed. The results of two experiments which show the possibility of using GNSS-R signals to detect buried metallic objects and to provide an estimate of their dimensions are discussed.