This paper presents Integrated Water Vapor (IWV) retrieval using spaceborne grazing-angle GNSS reflectometry (GNSS-R) data collected over the equatorial ocean. IWV is an important parameter for weather and climate modeling. There is a lack of observations over the oceans with short revisit times and high spatial resolution to capture the dynamic nature of IWV. This study demonstrates that GNSS signals, coherently reflected over calm ocean surfaces and received by a side-looking antenna at low elevation angles, can produce high precision range measurements from which IWV can be derived. This is achieved by using Spire Global’s grazing-angle GNSS-R signal carrier phase data collected over the Southeast Asia ocean region in 2022, at elevations between 5° and 8°. Detailed methodology of the retrieval process is presented, using coherent reflection tracks lasting more than 30 seconds corresponding to about 140 km along the ground track. The retrieval incorporates the VMF3 mapping functions to provide relative along-track IWV profiles which are compared with two IWV models, VMF3 and ERA5. To preserve the high spatial-temporal resolution of the GNSS-R measurements and ensure a fair comparison with models, we present the GNSS-R based IWV retrievals at the original 50-Hz (~100m spatial sampling) rate and a smoothed version at 5 seconds interval. The results highlight the dynamic nature of GNSS-R derived IWV measurements. Their deviations from the ERA5 model are statistically consistent over time, independent of the model’s fluctuations. Classification analysis shows 15% of GNSS-R derived IWV closely match either VMF3 and/or ERA5, suggesting that they provide an alternative measure of IWV variations. Meanwhile, 50% show no agreement, suggesting that GNSS-R captures unique IWV dynamics not fully represented by the models. This is supported by a comparison with IWV statistics from Sentinel-6 which reveals that GNSS-R retrievals follow the same patterns with slightly more dynamic variability than Sentinel-6. On-going studies are investigating additional direct validation between GNSS-R and other observations as well as application of this method in polar regions using signals reflected over sea ice.

In this paper, we compare water surface height profiles measured by the Ice, Cloud, and Elevation Satellite-2 (ICESat-2) ATL13 data product on April 25, 2023 that intersect with a GNSS-Reflectomety (GNSS-R) altimetry track collected on April 17, 2023 over the Tonlé Sap Lake in Cambodia. Overall, we find reasonable agreement between the two datasets, with the nearest segment of ICESat-2 and GNSS-R tracks having a correlation of 0.79. However, some deviations exist, with the GNSS-R track measuring 7 cm lower than the ICESat2 track at an intersection point, potentially due to different lake surface conditions at the time of each measurement. This comparison is motivated by the interest in applying GNSS-R altimetry for a broader study of large inland lakes. Our focus on the Tonlé Sap Lake is inspired by the lake’s key role in the region combined with the difficulty in monitoring it by other techniques due to its large size, frequent cloud cover, and surrounding dense mangrove forests. GNSS-R (operating in L-band) can penetrate these environmental factors while accumulating frequent and spatially diverse water surface measurements. In total, we find 53 GNSS-R tracks collected by Spire Global between 2021 and 2023 that pass our altimetry criteria. In contrast, we find only 27 ICESat-2 tracks (each laser counted individually) through 12 total passes, likely limited by cloud cover.

This paper describes a preliminary investigation into the relationship of Global Navigation Satellite System-Reflectometry (GNSS-R) phase-based coherency and signal-to-noise ratio (SNR) with the extent of surface water contained in the reflected the signal's spatial footprint. Our study focuses on 82 phase-based GNSS-R tracks collected by Spire Global grazing angle satellites in 2021 over Lake Okeechobee, FL. For each track, we calculate its spatial footprint (assumed equal to the First Fresnel Zone (FFZ)) and determine the amount of surface water within the FFZ using a lake mask derived from the 2021 Global Surface Water Explorer (GWSE) Seasonality product. The percentage of surface water in the signal footprint (denoted %F) is compared with the reflected signal's SNR and level of coherency, with coherency measured by the phase-rate circular length. Overall, we see a positive relationship between SNR, circular length, and %F. However, circular length appears to be a more robust indicator of water, with more consistent, high magnitudes once the scattering surface is dominantly water (>70%F). Further, circular length appears to maintain a high magnitude at different distances into Lake Okeechobee when the footprint is full of water, whereas SNR gradually decreases as the signal track approaches the center of the lake. This is most likely a result of wind roughening the lake surface farther away from the shore.

Global Navigation Satellite System (GNSS) coherent ocean reflections collected at grazing-angles by Spire Global’s CubeSats are explored to retrieve Sea Level Anomalies (SLA) over the Gulf of Mexico. Dual-frequency GPS carrier phase estimations are derived from the 50-Hz I and Q samples generated onboard the low Earth orbit CubeSats. First order ionospheric phase advancement, troposphere delay, and precise orbit solutions for GPS satellites and CubeSats mitigate the range measurement errors. Comparisons between collocated specular reflection tracks from various Spire CubeSats and GPS satellites configurations demonstrate the self-consistency of the resulting SLA solutions. The Spire retrievals are then validated against conventional altimetry SLA products. There is a Root Mean Square (RMS) difference of ~ 20 cm between the Spire and the gridded model mesoscale SLAs, unless the reflected elevations are very low. To better evaluate the performance of the Spire retrievals they are examined over sharp ocean topography. About 40% of coherent reflections with elevations above 12° detected the signature of the energetic Loop-Current during the 2020-2021 period. The Spire retrievals were also compared with collocated Sentinel-3 tracks. One track with elevations above 12° demonstrated high correlation and agreement with the Sentinel-3 retrievals. The other two low-elevation tracks showed large magnitude discrepancies, but similar trends once appropriate scaling factors are applied. The study demonstrates the potential for GNSS reflectometry’s application to SLA mapping while also highlighting where significant improvements in error mitigations are needed. A particularly large error source is the troposphere error for signals with lower elevations.

This paper presents an updated evaluation of the coherency of grazing-angle ocean reflected GPS signals received on low-cost Spire CubeSat receivers. It is based on almost a year of dual-frequency carrier-phase observations sampled at 50 Hz from 2020. This dataset contains more than 2000 ocean reflections every week in 2020. As expected, coherency increases for low grazing angles: by a factor 20 from grazing angles above 20° to below 10° in the Indonesian Seas. Coherency also increases for smooth ocean surface conditions: by a factor 80 for Significant Wave Height (SWH) decreasing from 1.5 m to 0.5 m using grazing angles below 10°. The results provide clear relationships between coherency and various ocean surface parameters. Overall coherency is more sensitive to SWH with an almost exponential trend, followed by mean wave period, and less by wind speed which shows a relatively constant behavior for wind speeds < 4 m/s. The mean wave direction relative to the specular point along-track direction has no effect on coherency for wave periods less than 5s.

This article assesses the coherency of Global Navigation Satellite System (GNSS) signals reflected off the oceans and sea ice under grazing angle geometries and received aboard low Earth orbit (LEO) CubeSats for precision altimetry applications. The coherency is characterized as a function of ocean surface conditions and reflected signal parameters based on Spire Global CubeSat data collected from January to April 2019. The data contain 50-Hz GPS L1 and L2 carrier phase estimations obtained by open-loop tracking. Indicators based on the circular statistics of the excess-phase noise are developed to identify coherent and semicoherent reflections. Based on these indicators, we found that ~1% and 44% of GPS reflections over the ocean and sea ice, respectively, have potential for precision altimetry. The coherent and semicoherent reflection rates reach 23% in areas less than 200 km from the coastline and under calm sea conditions. Over young sea ice over the Arctic, this rate can be as high as 70%. There is a strong relationship between coherency and signal strength, and the coherency occurrence rate improves as the grazing angle decreases. The quality of the L1 and L2 coherent reflections is similar over sea ice, while, for reflections over the ocean, L1 signals are predominantly noisier and less coherent than the L2 signals. Using a postprocessing filtering method, the semicoherent reflections can achieve a similar level of altimetry precision as that of the coherent ones, thereby increasing the along-track length of the retrieved altimetry profile.

Coherent GNSS reflections over land predominantly occur over surface water bodies. This study presents a method to jointly use carrier phases and signal strengths of reflected signals to identify coherent reflections and applies it to the 50-Hz GNSS-R measurements from Spire Global Cubesats and CYGNSS microsatellites to map inland water bodies. A coherence detector was first developed using the circular statistics of carrier phase noises, identifying the input samples as coherent, semi-coherent, or incoherent. For any given track of data, we used this coherence detector to iteratively assess the coherency levels of the samples by a moving time window, then derived the coherency levels with the highest confidence. The circular statistics-based semi-coherent reflections with signal strengths above the prescribed threshold were regarded as coherent. The specular reflection points of the coherent reflections represent the locations of surface water. This method was applied to the Spire data to obtain the surface water extents for 1951 lakes and the CYGNSS data for 113 lakes. Compared to Global Surface Water Explorer observations, around 90% of the disagreements of the Spire data-based surface water boundaries are less than 0.73 km with a mean of 0.28 km and a standard deviation of 0.24 km. As for CYGNSS, ~90% of the disagreements are less than 0.43 km with a mean value of 0.18 km and a standard deviation of 0.16 km. The possible error sources are mainly fractional surface water, nearly flat and saturated ground surface, background land cover, and GNSS-R geometry.

Precise carrier phase measurements from coherent reflections of Global Navigation Satellite Systems (GNSS) signals are poised to enable new capabilities in Earth remote sensing. However, there is still much to learn about the reflection conditions that produce coherence. For land reflections especially, the degree to which various surface characteristics impact coherence is unclear. In this paper, we explore the dependence of coherence on four surface characteristics: maximum water extent, surface topography, soil moisture, and vegetation water content. To quantify the level of coherence, the phase is tracked from two sets of CYGNSS raw IF data and the circular length of the changes in phase is calculated. A strong link is found between coherence and the presence of surface water; the impact of the other surface characteristics on coherence is still under examination.

This paper presents assessment on the coherency of reflected GPS signals received on a LEO satellite for high accuracy ocean altimetry applications. We analyze the coherent signal levels dependence on sea surface conditions and signal parameters based on GPS reflection data collected by Spire cubeSats between January and April 2019. The data contains carrier phase estimations obtained from an open-loop tracking process sampled at 50Hz for GPS carrier frequency L1 and L2. A conservative coherency quality test based on two circular statistics of the carrier phase measurements, circular-length and circular-kurtosis, is applied to the data. The results show that ~ 1% and ~40% of the events contain sufficient coherent energy for high precision altimetry applications over ocean and sea ice respectively.

This paper presents a support vector machine (SVM) -based detection method to identify coherent reflections in GNSS-R measurements. The data was collected by Spire low-Earth orbit (LEO) satellites during January to April, 2019, and it contains the In-phase (I) and Quadrature (Q) correlation outputs at 50 Hz from open-loop (OL) tracking process. The coherence detection is investigated based on 1100 labelled 1-second data segments. The detection results show that the SVM-based method achieves a detection accuracy of 98.66% and outperforms several other algorithms.