Global Navigation Satellite System Reflectometry Altimetry Research Articles

GNSS Reflectometry-Based Ocean Altimetry: State of the Art and Future Trends

For the past 20 years, Global Navigation Satellite System reflectometry (GNSS-R) technology has successfully shown its potential for remote sensing of the Earth’s surface, including ocean and land surfaces. It is a multistatic radar that uses the GNSS signals reflected from the Earth’s surface to extract land and ocean characteristics. Because of its numerous advantages such as low cost, multiple signal sources, and all-day/weather and high-spatiotemporal-resolution observations, this new technology has attracted the attention of many researchers. One of its most promising applications is GNSS-R ocean altimetry, which can complement existing techniques such as tide gauging and radar satellite altimetry. Since this technology for ocean altimetry was first proposed in 1993, increasing progress has been made including diverse methods for processing reflected signals (such as GNSS interferometric reflectometry, conventional GNSS-R, and interferometric GNSS-R), different instruments (such as an RHCP antenna with one geodetic receiver, a linearly polarized antenna, and a system of simultaneously used RHCP and LHCP antennas with a dedicated receiver), and different platform applications (such as ground-based, air-borne, or space-borne). The development of multi-mode and multi-frequency GNSS, especially for constructing the Chinese BeiDou Global Navigation Satellite System (BDS-3), has enabled more free signals to be used to further promote GNSS-R applications. The GNSS has evolved from its initial use of GPS L1 and L2 signals to include other GNSS bands and multi-GNSS signals. Using more advanced, multi-frequency, and multi-mode signals will bring new opportunities to develop GNSS-R technology. In this paper, studies of GNSS-R altimetry are reviewed from four perspectives: (1) classifications according to different data processing methods, (2) different platforms, (3) development of different receivers, and (4) our work. We overview the current status of GNSS-R altimetry and describe its fundamental principles, experiments, recent applications to ocean altimetry, and future directions.

Approximate maximum likelihood time-delay estimation for two closely spaced sources

The study of ground reflections of Global Navigation Satellite System (GNSS) signals, as in GNSS Reflectometry (GNSS-R) can lead to the receiver height estimation. The latter is estimated by comparing the time of arrival difference between the direct and reflected signals, also called path separation. In ground-based scenarios, this path separation can be very small, inducing important interference between paths, which makes it difficult to correctly obtain altimetry products. The path separation estimation can be obtained by a brute force dual source maximum likelihood estimator (2S-MLE), but this solution has a large computational cost. On the other hand, the path separation is so small that a number of approximations can be done. In this study, a third order Taylor approximation of the dual source likelihood criterion is proposed to reduce its complexity. The proposed algorithm performance is compared to the non approximated 2S-MLE for the estimation of the path separation, and to a standard single source processing for the estimation of the direct signal time-delay. These results, along with the corresponding lower bounds, prove that the proposed approach may be of interest for two applications: ground-based GNSS-R altimetry (or radar with low elevation targets) and GNSS multipath mitigation.

Computation of specular reflections on a sphere: Assessment and validation of algorithms based on special boundary cases

The calculation of specular reflections on a sphere is a common problem in the Geosciences in general and in Earth remote sensing in particular. Especially in Global Navigation Satellite System Reflectometry (GNSS-R), the interferometric or reflection-minus-direct radio propagation delay should be calculated to retrieve sea level in GNSS-R altimetry. However, the solution is not trivial, as can be attested by multiple algorithms reported in the literature, many reinvented independently, without the emergence of a preferred or standard algorithm. We propose their assessment and validation based on special boundary cases, at the spherical horizon and at the vertical direction (zenith), where independent closed-form trigonometric expressions exist. We implemented five different algorithms to compute spherical specular reflection points, including three analytical ones based on a quartic polynomial and two iterative methods. We also computed derived parameters, such as interferometric delay, grazing angle, slant distance, and arc length. We performed simulations for all algorithms considering one hundred different antenna heights above the surface, between 10 m and 1 km. Finally, we compared analytical and iterative algorithms results with truth values at the special boundary cases. On the horizon, we found that all algorithms presented negligible root-mean-square error (RMSE) only for interferometric delay, but all other parameters exhibited varying accuracies for each algorithm. The iterative methods had the worst results at the spherical horizon, reaching meter-level RMSE for reflection point position and slant distance, whereas Miller & Vegh algorithm had worst results at zenith, especially in terms of grazing angle, position coordinate and arc length. We found the algorithm of Fujimura et al. to be the most stable and accurate. We performed a relative intercomparison between Fujimura et al. and each other algorithms at 45° as an intermediary case. We confirmed Fermat's iterative algorithm had the worst performance. The algorithms of Martín-Neira and Helm exhibited a systematic effect on certain parameters, proportional to antenna height and inversely proportional to satellite elevation. Therefore, Fujimura et al. is the recommended algorithm to compute specular reflections on a sphere.

Correction of Atmospheric Delay Error of Airborne and Spaceborne GNSS-R Sea Surface Altimetry

Improving the measurement accuracy is a necessary condition for sea surface altimetry using the Global Navigation Satellite System Reflectometry (GNSS-R). The ionosphere and troposphere delay the transmission of satellite signals, which directly affect the measuring accuracy. The influence of the atmospheric environment on GNSS-R altimetry differs from different platforms. By analyzing and sorting out the altimetry data of airborne and spaceborne platforms, this paper studies the variation law of signal delay in the altimetry process from the point of view of mathematical geometry, which provides an example for improving the precision of GNSS-R altimetry measurements. Firstly, in order to facilitate data analysis, this paper constructed an altimetry model with the GNSS satellite position, specular reflection point position, receiver position as nodes, classified direct signals, and reflected signals. Secondly, calculate ionospheric puncture point coordinates , and interpolate GIM products provided by IGS using time and puncture point coordinates to obtain the VTEC value in the vertical direction of the puncture point, which was converted into the path direction STEC by projection function, the ionospheric delay of each part was obtained in this way. The tropospheric delay of each part is considered for the along-path component and the geometric component, the delay of along-path component was calculated by the UNB3m model, and the delay of geometric component was calculated by the equation provided by Nikolaidou (Nikolaidou et al., 2021). Thirdly, by comparing the sea surface height inversion results with or without atmospheric delay correction with the mean sea surface height provided by DTU15, the measurement accuracy with atmospheric delay correction is obviously improved. The study results of the influence of atmospheric delay on the altimetry experiments precision error of airborne and spaceborne platforms show that the error magnitude is consistent with the existing literature. In the airborne experiment, the influence of the ionosphere is negligible and the troposphere has sub-meter influence on altimetry results, among which the tropospheric along-path delay component occupies a high proportion. The geometric delay component has a high correlation with the satellite elevation angle and its influence on the measurement accuracy decreases with the elevation angle increase. The effect of this factor can be effectively weakened by setting a high satellite cutoff angle. In the spaceborne experiment, the effect of atmospheric delay on altimetry results fluctuates in the range of 3∼5 m when the satellite elevation angle is greater than 60°. In this paper, the method of calculating signal atmospheric delay through geometric relation to improving the measurement accuracy can provide an example for the atmospheric delay correction of GNSS-R ocean altimetry with high precision and spatial resolution in future research.

Analysis and Mitigation of Crosstalk Effect on Coastal GNSS-R Code-Level Altimetry Using L5 Signals from QZSS GEO

Coastal Global Navigation Satellite System Reflectometry (GNSS-R) can be used as a valuable supplement for conventional tide gauges, which can be applied for marine environment monitoring and disaster warning. Incidentally, an important problem in dual-antenna GNSS-R altimetry is the crosstalk effect, which means that the direct signal leaks into the down-looking antenna dedicated to the reflected signals. When the path delay between the direct and reflected signals is less than one chip length, the delay waveform of the reflected signal is distorted, and the code-level altimetry precision decreases consequently. To solve this problem, the author deduced the influence of signal crosstalk on the reflected signal structure as the same as the multipath effect. Then, a simulation and a coastal experiment are performed to analyze the crosstalk effect on code delay measurements. The L5 signal transmitted by the Quasi-Zenith Satellite System (QZSS) from a geosynchronous equatorial orbit (GEO) satellite is used to avoid the signal power variations with the elevation, so that high-precision GNSS-R code altimetry measurements are achieved in the experiment. Theoretically and experimentally, we found there exists a bias in proportion to the power of the crosstalk signals and a high-frequency term related to the phase delay between the direct and reflected signals. After weakening the crosstalk by correcting the delay waveform, the results show that the RMSE between 23-h sea level height (SSH) measurements and the in-situ observations is about 9.5 cm.

Coastal Altimetry Using Interferometric Phase From GEO Satellite in Quasi-Zenith Satellite System

Global navigation satellite system reflectometry (GNSS-R) altimetry has great potential to provide high spatial–temporal resolution sea surface heights (SSHs) at low cost. Interferometric phase measurements between direct and reflected signals can be used for altimetry retrieval to achieve high-precision solutions. The motions of the medium Earth orbit (MEO) satellites cause interferometric phase change rapidly, which would increase the probability of occurrence of the phase unwrapping errors than the case of the geosynchronous Earth orbit (GEO) satellite. In order to overcome this problem, we propose a coastal GNSS-R altimetry algorithm using the signals from Quasi-Zenith Satellite System (QZSS) GEO satellite. Precise SSH variations can be achieved using the interferometric phase measurements without ambiguity fixed. We also perform coastal experiments on a trestle using a specialized GNSS-R setup to verify our algorithm. It is composed of an intermediate frequency (IF) data collector and two antennas. The up-looking antenna is used to receive direct signals, while the down-looking antenna receives the signals reflected from the sea surface. Raw IF data sampled at 62 MHz are collected and processed to derive interferometric carrier phase delay measurements using a self-developed software-defined receiver. Approximately 7 h of reflector heights are retrieved at 1-min intervals and the solutions are evaluated via comparison with measurements provided by a 26-GHz altimetry radar located near the GNSS-R setups. The results show that the root mean square error (RMSE) of sea level estimation is about 1.4 cm by using the QZSS GEO data.

An open-source low-cost sensor for SNR-based GNSS reflectometry: design and long-term validation towards sea-level altimetry

Monitoring sea level is critical due to climate change observed over the years. Global Navigation Satellite System Reflectometry (GNSS-R) has been widely demonstrated for coastal sea-level monitoring. The use of signal-to-noise ratio (SNR) observations from ground-based stations has been especially productive for altimetry applications. SNR records an interference pattern whose oscillation frequency allows retrieving the unknown reflector height. Here we report the development and validation of a complete hardware and software system for SNR-based GNSS-R. We make it available as open source based on the Arduino platform. It costs about US$200 (including solar power supply) and requires minimal assembly of commercial off-the-shelf components. As an initial validation towards applications in coastal regions, we have evaluated the system over approximately 1 year by the Guaíba Lake in Brazil. We have compared water-level altimetry retrievals with independent measurements from a co-located radar tide gauge (within 10 m). The GNSS-R device ran practically uninterruptedly, while the reference radar gauge suffered two malfunctioning periods, resulting in gaps lasting for 44 and 38 days. The stability of GNSS-R altimetry results enabled the detection of miscalibration steps (10 cm and 15 cm) inadvertently introduced in the radar gauge after it underwent maintenance. Excluding the radar gaps and its malfunctioning periods (reducing the time series duration from 317 to 147 days), we have found a correlation of 0.989 and RMSE of 2.9 cm in daily means. To foster open science and lower the barriers for entry in SNR-based GNSS-R research and applications, we make a complete bill of materials and build tutorials freely available on the Internet so that interested researchers can replicate the system.

Space-Borne GNSS-R Ionospheric Delay Error Elimination by Optimal Spatial Filtering.

Global Navigation Satellite System Reflectometry (GNSS-R) technology is a new and promising remote sensing technology, especially satellite-based GNSS-R remote sensing, which has broad application prospects. In this work, the ionospheric impacts on space-borne GNSS-R sea surface altimetry were investigated. An analysis of optimal values for spatial filtering to remove ionospheric delays in space-borne GNSS-R altimetry was conducted. Considering that there are few satellite-borne GNSS-R orbit observations to date, simulated high-resolution space-borne GNSS-R orbital data were used for a comprehensive global and applicable study. The curves of absolute bias in relation to the bilateral filtering points were verified to achieve the minimum absolute bias. The optimal filtering points were evaluated in both statistical probability density and quantile analysis to show the reliability of the selected values. The proposed studies are helpful and valuable for the future implementation of high-accuracy space-borne GNSS-R sea surface altimetry.

The Validation of the Weight Function in the Leading-Edge-Derivative Path Delay Estimator for Space-Based GNSS-R Altimetry

Global Navigation Satellite System Reflectometry (GNSS-R) altimetry uses GNSS-reflected signals off a water body to measure the height of the reflecting surface. The maximum value of the leading-edge derivative (LED) is an important path delay estimator in GNSS-R altimetry. The LED assumes that the weight function representing the power contribution per unit delay is monotonically decreasing with delay. This article focuses on the validation of the LED in space-based GNSS-R altimetry. Two problems of the weight function were studied. First, the monotonicity condition that the weight function requires to meet was theoretically analyzed, reaching the conclusion that the weight function needs to be monotonically nonincreasing. Second, whether the weight function could meet the monotonicity condition was simulated using the orbits of the COSMIC, GRACE, and GPS satellites, and the antenna gain map of the TechDemoSat-1 (TDS-1) mission. Results showed that the monotonicity of the weight function is largely dominated by Doppler, and meets the monotonicity condition because over 97% events decrease with delay. Antenna gain is the only factor that may disobey the monotonicity condition, but has no significant impact on the monotonicity of the weight function for this article. Therefore, the LED is valid for space-based GNSS-R altimetry as studied in this article.

Improved GNSS-R Ocean Surface Altimetry With CYGNSS in the Seas of Indonesia

Ocean surface altimetry with GNSS reflectometry (GNSS-R) has the potential to improve the observation and retrieval of mesoscale ocean current flows by increasing the spatial density of ocean surface observations. The leading challenges in current GNSS-R altimetry investigations include precise delay retracking, correction of ionospheric effects, and spacecraft receiver positioning. Here, we present improved methods to account for these challenges. A reflection-model-based approach is presented for delay retracking that uses simulated delay-Doppler maps (DDMs) to retrieve the specular delay from measured DDMs. Global ionosphere maps are used to estimate the group delay effect along the direct and reflection paths. Precise orbit determination techniques are used to improve estimates of the receiving spacecraft position and reduce systematic intersatellite biases. We analyze altimetric height retrievals from two cases, strongly coherent or diffuse reflections. We study the residual systematic error signals with a focus on understanding the errors identified above. The strong coherent observations have narrow, symmetric delay waveforms and provide higher than average signal-to-noise ratio. The DDMs of diffuse observations, originate from relatively rough ocean surfaces and have the characteristic horse-shoe shape. Only measurements from the ocean and seas surrounding Indonesia are considered as this area provides a significant number of strongly coherent reflections. More than 50 000 single-point observations and week-long averaged results are presented to quantify the noise and systematic characteristics of the retrieved surface heights.

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.

Impact of Receiver Dynamics on Space-Based GNSS-R Altimetry

It is a challenge to achieve high accuracy altimetric results using space-based global navigation satellite system reflectometry (GNSS-R). This paper focuses on improving the accuracy of space-based GNSS-R altimetric measurements by considering receiver dynamics in the geometric model. The receiver dynamics has negligible impact on the geometric model of low-platform GNSS-R because the receiver movement is quite small during the time period between receiving direct and reflected signals. However, in the case of space-based GNSS-R, the receiver has much higher dynamics, thus the receiver dynamics cannot be ignored. This paper analyzes the altimetric error caused by the receiver dynamics in space-based GNSS-R. Theoretical studies and simulations were conducted to evaluate the magnitude of the altimetric error caused by receiver dynamics. The results show that the dynamics of a low earth orbit receiver can lead to errors of the order of several meters, depending on the trajectory angle of the receiver. The results using data from the TDS-1 mission conclude that neglecting receiver motion causes an average error of 3.4 m. This agrees with the result obtained from a theoretical analysis. Therefore the geometric model that takes into account receiver dynamics can improve the accuracy of space-based GNSS-R altimetric measurements of the order of a few meters compared to neglecting receiver dynamics.

Low-Cost GNSS-R Altimetry on a UAV for Water-Level Measurements at Arbitrary Times and Locations.

Together with direct Global Navigation Satellite System (GNSS) signals, the signals reflected at the water surface can be received by an unmanned aerial vehicle (UAV). From the range difference between two GNSS signal paths, the height of the UAV above the water level can be geometrically estimated using the weighted least squares method, called GNSS reflectometry (GNSS-R) altimetry. Experimental low-cost GNSS-R altimetry flights with a UAV were conducted at the coast of Lake Biwa, Japan. Although the height estimated by the GNSS-R altimeter included large short-term noises up to 8 m amplitude, it agreed well with the UAV altitude measured by the post-processed kinematic positioning. By selecting better weight functions in the least square method and using sufficient temporal averaging, the GNSS-R altimetry achieved accuracy in the order of 0.01 m if a sufficient number of GNSS satellites with high elevation angles were available. The dependency of the results on the weight functions is also discussed.

Single-Pass Sub-Meter Space-Based GNSS-R Ice Altimetry: Results From TDS-1

Space-based Global Navigation Satellite System Reflectometry (GNSS-R) altimetry remains an open challenge. This paper reports on space-based GNSS-R altimetry using 40-s period of intermediate frequency recording from the TechDemoSat-1 mission. This recording is unique because one GPS signal is reflected from ice. The waveforms that are used to determine path delay are generated by 1 ms coherent integration. Pseudoranges are smoothed every 0.5 s by linear models before calculating the path delay. Altimetric results are compared to DTU10 mean sea surface heights, with good agreement being obtained. The RMS difference of 4.4 m is much smaller than reported in the current literature. Very good altimetric precision of better than 1 m (0.96 m) is achieved with a spatial resolution of 3.8 km. This result validates the potential of space-based GNSS-R altimetry.

Accurate Pseudorange Estimation by Means of Code and Phase Delay Integration: Application to GNSS-R Altimetry

This article presents a new estimator of the pseudorange between a global navigation satellite systems (GNSS) satellite and a stand-alone receiver, and its application to GNSS-reflectometry (GNSS-R) altimetry. In our approach, we use as independent observations the difference between the code delay provided by successive acquisitions of the GNSS signal and the code delay truncated using the phase and Doppler information. We show that the mean value of these observations is an estimate of the first pseudorange of the observed time series. The ability of the proposed estimator to provide the centimeter accuracy is theoretically evaluated. In ground-based altimetry applications using GNSS signals, the direct pseudorange and the pseudorange associated to the reflected signal can be assumed to have the same dynamic evolution. Based on this assumption, the proposed estimate uses as observations the difference between the code delays obtained by successive acquisitions of both signals, without using phase measurement. The assessment of the method for ground-based GNSS-R is realized with real data.

Analysis of GNSS-R Altimetry for Mapping Ocean Mesoscale Sea Surface Heights Using High-Resolution Model Simulations

The capability of global navigation satellite system reflectometry (GNSS-R) altimetry to map mesoscale sea surface height (SSH) fields is analyzed using synthetic measurements derived from a high-resolution (1/10°) numerical model of the North Pacific. As an example, we consider the GPS and GLONASS constellation transmitters and assume six reflection-capable receivers onboard the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites in high-inclination (720 km, 72°) orbits. An individual GNSS-R measurement has a ∼10-km footprint. The SSH measurement error is simulated as a function of the incidence angle and the error in the delay measurement between the transmitter and receiver. The delay measurement error is assumed to have Gaussian white noise distribution with a root-mean-square error (RMSE) of 1.0 or 2.0 m. Two days of synthetic measurements are used to reconstruct SSH fields using a two-dimensional variational algorithm. For the 1.0-m delay error, the basin-wide RMSE of the mapped field is 2.3 cm and the spatial correlation between the mapped and true mesoscale fields is larger than 0.9. For the 2.0-m delay error, the basin-wide RMSE is 3.8 cm and the spatial correlation is larger than 0.8. Spectral and synoptic analyses suggest that two days of measurements can reproduce mesoscale features down to 100 km. The result demonstrates the ability of GNSS-R altimetry to suppress large measurement errors due to the high density of measurements and the potential to constrain mesoscale features down to scales beyond what the constellation of existing nadir-altimeters allows.

A study of the electromagnetic bias for GNSS-R ocean altimetry using the choppy wave model

Global Navigation Satellite System-Reflectometry (GNSS-R) altimetry involves measuring reflections of Global Positioning System transmissions from the Earth’s surface (a bistatic radar configuration.) These reflected signals carry information about the Earth’s surface, including the sea surface height. The electromagnetic (EM) bias is a significant error source when measuring sea surface height with GNSS-R due to the non-symmetric properties of sea waves. Although previous studies of the EM bias have been conducted for traditional backscatter altimetry, information on the EM bias in the bistatic configurations important for GNSS-R is limited. Of particular interest is the influence of the bistatic geometry on the EM bias. This paper presents a study of the EM bias in GNSS-R altimetry. The study employs a Monte Carlo procedure with numerical nonlinear hydrodynamic simulations coupled with a physical optics method for EM scattering from the sea surface to produce a deterministic set of sea surface profiles and the corresponding GNSS-R waveforms. In this initial study, “choppy wave” model is used for nonlinear surface description in order to improve further the computational efficiency and one-dimension surface analysis is chosen for the simple computations. The influence of the bistatic configuration on the EM bias properties has analyzed, and it is shown that the EM bias varies approximately as a cosine function of the incident angle and that “short wave” effects make important contributions.

Improvement of Data Precision and Spatial Resolution of cGNSS-R Altimetry Using Improved Device With External Atomic Clock

The applications of Global Navigation Satellite System Reflectometry (GNSS-R) altimetry will mainly rely on the precision of this remote-sensing system. Comparing with nadir measurement of satellite altimetry and water gauges, GNSS-R altimetry makes off-nadir measurements with high spatial and temporal resolution come true. To enhance the data precision and spatial resolution, an improved cGNSS-R altimetry system, including one uplooking geodetic GNSS receiver, one downward left-handed circularly polarized GNSS-R receiver, and one tamed atomic clock, was adopted. The objective of the introduced atomic clock is to provide an accurate and synchronous time frequency signal for both receivers, and receiver clock errors can be removed in the regular single difference geodetic processing. Therefore, the unknown parameters in the computation of cGNSS-R were reduced to only one, caused by the phase subtraction between two receivers. By this proposed method, water level height can be derived from one GNSS satellite's signals, and the spatial resolution will be improved greatly due to more observations. An experimental water level measurement over a smooth inland lake is presented with GNSS-R carrier-phase processing. Then, GNSS-R-derived local water level height is compared with in situ observations. A high degree of agreement is found in these two independent data, and the standard deviation of the derived water level height is better than 1 cm at 1-Hz sample.

A first comprehensive evaluation of China’s GNSS-R airborne campaign: part II—river remote sensing

The Global Navigation Satellite System Reflectometry (GNSS-R) has been proven to be a powerful technique for retrieving geophysical parameters of ocean and land. Airborne GNSS-R is an important experimental platform, because it is not only needed as validation for spaceborne application, but also possesses the advantages to be capable of remote sensing of small and medium scale targets, such as rivers and lakes. This paper presents an overview of China’s airborne GNSS-R campaign conducted on May 30, 2014, in Henan. The campaign has two objectives, i.e.: (1) to examine the capability of a GNSS-R payload developed by National Space Science Center, Chinese Academy of Sciences (NSSC, CAS) for airborne observations and (2) to study the algorithms for soil moisture and river remote sensing, including altimetry and flow velocity measurement. A previous paper has presented results of soil moisture retrieval as part I, and in this paper, initial results of the Yellow River remote sensing are presented as part II. This paper presents the river altimetry results and explores a new potential application of GNSS-R technology, which is used to detect the flow velocity of the river. The river surface height results observed by code delay altimetry method were consistent with the height results of GPS dual-frequency differential positioning altimetry. The GNSS-R altimetry results showed that decimeter level heights were achieved in 1-min sliding average. Comparing with in situ measurements, the GNSS-R flow velocity result was reasonable; the error was about 0.027m/s, which indicated the validity and feasibility of using GNSS-R technique to detect river flow velocity.