River slope, a crucial parameter in hydrological modeling, has historically been difficult to measure continuously on a regional or global scale. Satellite altimetry missions often have long revisit times, such as 10 to 20 days for the Surface Water and Ocean Topography (SWOT) mission. In this paper, a novel approach is presented utilizing spaceborne GNSS Reflectometry (GNSS-R) to measure river slopes with high accuracy and potentially short revisit times. Our Earth is enveloped in radio signals from over 100 GNSS satellites. These signals can be coherently reflected from river surfaces and detected by low Earth orbit (LEO) satellites with sufficient energy to estimate carrier phase. The carrier phase measurement captures water surface height variations, which can be extracted through modeling of the reflection signal propagation geometry and space environment effects to estimate river slopes. This study processes both the raw intermediate frequency (IF) data obtained by NASA's Cyclone GNSS (CYGNSS) microsatellites and the grazing-angle GNSS-R data generated by Spire Global nanosatellites to demonstrate the feasibility and performance of the GNSS-R based river slope retrieval. This paper focuses on selected river sections with width greater than similar to 500 meters. Detailed methodologies and error analyses are presented, indicating total uncertainty of approximately 0.38 cm/km plus ionospheric TEC model error for CYGNSS and 0.69 cm/km for Spire (with dual-frequency ionospheric correction) over an ideal 5-km river section at 30 degrees elevation angle. The retrieval results are validated in areas with nearby flat water surfaces (such as lakes or wide and slow river sections) and against in situ gauge measurements and satellite altimetry, consistently demonstrating the high accuracy and reliability of spaceborne GNSS-R for measuring river slopes.

This study demonstrates the utility of the Global Navigation Satellite System Reflectometry (GNSS-R) method for remote sensing of ice shelf surface deformation and roughness. The phase coherence of the reflected GNSS carrier signal is related to surface deformation and roughness: smoother, less deformed areas scatter the signal more coherently. More than 900000 tracks of Spire Global Inc. grazing angle GNSS-R data collected over Antarctica between 2021 and 2023 were processed to determine signal carrier phase coherence. Results are presented in the form of coherence maps, where each measurement has a spatial resolution of approximately five kilometers, and the Ross Ice Shelf (RIS) is used as a case study for more detailed examination. Clear visual patterns in signal coherence are shown to correspond with the locations of streaklines and deformation features on the surface. A GNSS-R coherence-based roughness proxy is introduced and shown to correlate with surface roughness derived from the reference elevation model of Antarctica digital elevation model (REMA DEM). This novel application of the GNSS-R method demonstrates the exciting potential to complement existing data sources and improve understanding of ice shelf structure and evolution.

The Total Electron Content (TEC) derived from Global Positioning System (GPS) radio occultation (RO) measurements contains residual bending errors that arise from signal refraction and dispersion in the ionosphere. Motivated by the need to ensure the accuracy of TEC retrievals, particularly under high solar activity conditions, we investigate the impact of ionospheric bending on the conventional dual-frequency TEC. This work marks a new attempt in assessing the residual bending error in TEC estimates using real RO measurements, in contrast to prior research efforts that relied primarily on simulation studies. We present an estimation method for the residual bending error based solely on measurement data, avoiding reliance on a priori knowledge of the true ionospheric state. Utilizing real RO observations from COSMIC-2, our analysis reveals that the residual bending error is largely influenced by the vertical gradient of TEC, with additional dependencies on solar activity and local time. Large residual bending errors were observed under high solar activity conditions, reaching up to 17.7 TECU in a 15-day dataset from January 2023. Residual bending errors in dual-frequency TEC were often found to exceed the 3 TECU threshold from COSMIC-2 accuracy requirements, suggesting the use of single-frequency TEC or the application of residual bending error correction as a potential alternative under certain conditions inducing high TEC gradients.

This study presents surface topography results for the Ross Ice Shelf in Antarctica, derived from space-borne Global Navigation Satellite System Reflectometry (GNSS-R) measurements. The spaceborne GNSS-R method utilizes reflected GNSS signals of opportunity collected on a platform in low-Earth orbit, allowing for widespread data coverage with improved temporal resolution compared to conventional remote sensing technologies. Remote sensing of the cryosphere is a relatively new application for GNSS-R, but the effectiveness of the method has been demonstrated for sea ice extent (J. Cartwright et al., “Sea Ice Detection Using GNSS-R Data From TechDemoSat-1,” JGR: Oceans, 2019) and altimetry (V. Nguyen et al., “Ini-tial GNSS Phase Altimetry Measurements From the Spire Satellite Constellation”, GRL, 2020), and more recently land ice altimetry (R. Buendia et al., “Ice sheet height retrievals from Spire grazing angle GNSS-R,” Remote Sensing of Environment, 2023). In this study, two separate GNSS-R processing techniques are used to determine the roughness and precise relative surface elevation of the reflecting ice shelf surface. Coherency of the reflected signal can be used to separate different types of terrain (e.g., mountainous areas versus flatter ice areas versus ocean) and as a proxy for the roughness of the surface at GNSS wavelengths. Coherency level can be quantified using circular length and circular kurtosis, defined respectively as the length of the sum of the phase rate unit vectors $\delta \phi$ and the kurtosis of $\delta\dot{\phi}$ . Both are close to 1 for coherent reflections and close to 0 otherwise, and an empirical threshold can be employed to differentiate coherent and incoherent reflections. Reflected signal coherency is a prerequisite for the second portion of the inves-tigation. For coherent reflections, we use carrier phase-based altimetry to retrieve precise surface height relative to a reference elevation model according to Equation 1, where $\Delta h$ is the retrieved height difference, $\theta$ is the elevation angle, and $\triangle\Phi_{\text{iono}}$ -free is the difference between the direct and reflected ionosphere free filtered residual phases. In addition to accounting for ionospheric effects using the dual-frequency signal, troposphere delay, cycle slips, clock and orbit errors, and dynamics are removed from $\Delta \Phi_{\text{iono}}$ -free before height calculation.

This study utilizes Global Navigation Satellite System Reflectometry (GNSS-R) to map the surface of the Ross Ice Shelf in Antarctica. Spaceborne GNSS-R is a relatively new method for cryosphere applications and utilizes reflected GNSS signals of opportunity collected on a platform in low-Earth orbit, allowing for widespread data coverage with better temporal resolution than conventional remote sensing technologies. Coherency of the reflected signal, computed using phase-based circular statistics, can be used as a proxy for surface roughness. Using 25819 Spire Global grazing angle reflectometry tracks, we generate a high-resolution map of coherency over the Ross Ice Shelf. This technique shows promise for identifying regions with similar surface characteristics and deformation history, as well as for detecting the ice shelf/sea ice border, large rifts, and flow lines.

Ionospheric monitoring is greatly benefitted by the utilization GNSS observations. GNSS-based estimates of ionospheric total electron content (TEC) are useful for the improvement of satellite navigation systems receiver measurement inaccuracies and for ensuring dependable navigation and positioning capabilities. Additionally, TEC estimation is a valuable measure in the study of space weather. This study investigates an innovative approach to refine TEC estimates by reducing measurement noise and multipath effects by analyzing single-frequency wideband GNSS signals. Traditionally, ionospheric TEC estimation relies on L1 and L2 dual-frequency GNSS pseudorange measurements. The large pseudorange noise associated with L1 and L2 narrowband signals may be smoothed by their corresponding carrier phase estimations. However, multipath effects and carrier cycle slips at low elevations can render the carrier phase measurements unusable. Consequently, many ground-based GNSS monitoring stations restrict measurements from low-elevation satellites. However, these low-elevation signals carry valuable ionospheric information, having traversed longer ionospheric paths.

We present two case studies of traveling ionospheric disturbances (TIDs) triggered by the 2022 Tonga volcanic eruption observed by low-cost CubeSat-based global navigation satellite system reflectometry (GNSS-R) measurements. The GNSS-R data used in this work are from Spire Global CubeSats. Our analysis shows that coherent GNSS signals reflected over the ocean can be used to derive precise ionospheric total electron content (TEC) measurements. The first case shows clear TID structures with a TEC disturbance magnitude of similar to 1 TEC unit (TECu) and horizontal wavelength of similar to 330 km over Northwest Australia on the day of Tonga volcanic eruption. The second case shows the TIDs with similar to 0.05 TECu disturbance magnitude and horizontal wavelength of similar to 240 km over East Russia the day after the eruption. The second case is likely a TIDs propagated outward from Tonga for the second time after it traversed around the Earth. The GNSS-R observed TID may be associated with the incident or reflection signal ray path. In this paper, the appropriate ray path was identified using simultaneous observations from ground receiver networks in the areas of both ray paths. The Tonga volcanic eruption on 15 January 2022 triggered various atmospheric and ionospheric waves which have been observed globally. There have been several studies showing traveling ionospheric disturbances (TIDs) triggered by this event using ground global navigation satellite system (GNSS) networks and GNSS radio occultation measurements. This study shows for the first time that the TIDs can also be observed using GNSS signals reflected from ocean surface and received by a side-looking antenna onboard a low-cost CubeSat. The dual-frequency GNSS reflection (GNSS-R) signals contain sufficient coherent energy to enable precise total electron content (TEC) estimations. This work presents two case studies of TIDs captured by receivers onboard a Spire Global CubeSat. The first case shows a TID with horizontal wavelength of similar to 330 km and TEC disturbance magnitude of similar to 1 TECu through the GNSS-R signals received over Northwest Australia. The second case shows a TID observed over East Russia with TEC perturbation magnitude of similar to 0.05 TECu and horizontal wavelength of similar to 240 km. The two case results are validated by nearly identical wave characteristics observed by local ground-based GNSS receivers in Australia and Japan respectively. Global navigation satellite system reflections over sea ice and calm oceans have sufficient coherent energy to offer precise total electron content (TEC) measurements The global navigation satellite system reflectometry (GNSS-R) derived TEC is the combination of TEC from incident ray and reflected ray The GNSS-R signal scans captured the traveling ionospheric disturbances on the event day and the day after of Tonga eruption

This paper presents recent processing results of ionospheric TEC enhancement obtained over the Arctic and Antarctic based on GNSS reflectometry (GNSS-R) measurements. The polar ionosphere is home for many interesting and dynamic processes. Yet, ionospheric observations at polar regions are sparse due to challenges in deploying instruments in the harsh environments. GNSS-R offers a promising new means to fill the data gap at high latitudes by utilizing coherently reflected signal over ice and received by LEO satellite equipped with side- or downward looking antennas. This paper presents real grazing angle GNSS-R data collected by Spire Global CubeSats to retrieve ionospheric TEC over the Arctic and Antarctic. This paper show examples of GNSS-R observations of ionospheric TEC enhancement over the Arctic and Antarctica.

This paper presents a preliminary study of a new TEC observation technique by exploiting single-frequency GNSS-R pseudorange measurements (especially from the wideband signal) collected by LEO satellites over equatorial oceans and high latitudes. Pseudorange estimation is more robust compared to that of carrier phase and can be obtained over Earth surfaces with more relaxed smoothness requirements. The GNSS-R pseudorange has contributions from geometric range of the reflection signal path, troposphere delay, satellite oscillator errors, and ionosphere delay. By utilizing precise orbit solutions for GNSS and the low-Earth orbit (LEO) satellites, GNSS satellite clock correction, ocean and sea ice surface height models, troposphere delay models, and by solving for LEO satellite clock offset using direct GNSS signals, we can estimate most of the pseudorange contributors. Then the residual is the measurement of ionospheric delay with some unmodeled/mis-modeled errors and noise. Such measurement can potentially provide ionospheric TEC measurement with an accuracy of a few TECU.

Abstract This paper studies a new concept of using global navigation satellite system (GNSS) signals coherently reflected over relatively smooth ocean and ice surfaces from very low elevation angles (below ∼8°) and received by low Earth orbit (LEO) satellites to retrieve the tropospheric information. This approach can provide horizontal profiles of tropospheric zenith delay and total column water vapor (TCWV) with centimeter‐level high precision and spatial resolutions of tens of km by ∼1 km, depending on the elevation angle, with a sampling spacing of ∼100 m. This approach can potentially be applied to most sea ice and calm ocean areas and provide tropospheric sensing data, which can complement and augment existing observation systems. A few case studies are conducted in this paper using the Spire grazing‐angle GNSS‐R data. The retrieved TCWV is compared to ERA5 products and the Sentinel‐3 Ocean and Land Color Instrument measurements and shows promising performances. The errors associated with the GNSS‐R tropospheric measurements are also discussed.

<p>In this work, we investigate a novel method to estimate the tropospheric wet delay, and further the vertically integrated water vapor (VIWV) and its horizontal gradients, using the coherent-reflection GNSS signals received by a CubeSat in the low-Earth orbit (LEO).  It can complement existing observation approaches over some polar and ocean regions where GNSS signals can be coherently reflected.</p> <p>The precise altimetry using coherent-reflection GNSS signal carrier phase measurements has gained popularity over the past couple of decades for the observation of ocean, sea ice, lake, and river surfaces.  The troposphere delay error is found to be a major error source for GNSS-R phase altimetry, especially at a low elevation angle.  However, if we can model the reflection surface elevation relatively well, then the estimated residual phase from GNSS-R signal can be dominantly contributed by the mis-modeled tropospheric wet delay, and GNSS-R signal can become a new data source for tropospheric water vapor sensing.  The GNSS-R approach can observe the horizontal gradients of VIWV along the specular point (SP) track, as the SP moves at high speeds of ~5 km/s, and the spatial resolution is 10s of km by ~1 km.</p> <p>In the presentation, we will provide examples using Spire Global’s grazing-angle GNSS-R data and comparisons with the ECMWF reanalysis VIWV data.  We will also discuss the applicable regions, performance, and error mitigations of the proposed method in estimating tropospheric wet delay and issues to be addressed in the further retrieval of VIWV.</p>

This paper studies a new concept of using GNSS signals coherently reflected over relatively smooth ocean and ice surfaces from very low elevation angles (below ˜8 ° ) and received by low Earth orbit (LEO) satellites to retrieve the tropospheric information. This approach can provide horizontal profiles of tropospheric zenith delay and total column water vapor (TCWV) with centimeter-level high precision and spatial resolutions of 10s of km by ˜1km, depending on the elevation angle, with a sampling spacing of ˜100m. This approach can potentially be applied to most sea ice and calm ocean areas and provide tropospheric sensing data, which can complement and augment existing observation systems. A few case studies are conducted in this paper using the Spire grazing-angle GNSS-R data. The retrieved TCWV is compared to ERA5 products and the Sentinal-3 OLCI measurements and shows promising performances. The errors associated with the GNSS-R tropospheric measurements are also discussed.

The atmospheric water vapor contributes to ~60% of the atmospheric greenhouse effect under clear sky and is important to climate and weather modeling. The atmospheric water vapor is usually measured as vertically integrated water vapor (IWV), and it has been difficult to measure especially over polar ice and ocean. This paper presents a pilot study to use the dual-frequency GPS signals reflected off sea ice and collected by Spire Global CubeSats at grazing angles to estimate the troposphere delay and further retrieve the IWV. Results from two closely located reflection tracks are presented and compared to the ERA5 products. The GNSS-R results show good consistency with ERA5 and higher precision. The methodology and limitations of this proposed novel approach are also presented in this paper.

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.

This paper investigates using Spire’s grazing-angle GNSS-R data to detect the existence of surface water in the lower Mississippi region. The Signal-to-Noise Ratio (SNR) of the reflected L2 GPS signals acquired using the Spire Global constellation are analyzed over several tracks covering the Mississippi river, along with Normalized Difference Vegetation Index (NDVI) from Sentinel-2 Multispectral Imager (MSI) and backscattering coefficients from Sentinel-1 Synthetic Aperture Radar (SAR) C-band acquisitions. The specular points are segregated on the basis of thresholds based on the GNSS data and associated remote sensing data with correctly classifying the points over water with a true positive rate between 55-70% for L2 SNR thresholds above noise floor.

Recent results from CYGNSS measurements over land show the importance of coherent scattering. It is envisioned that future GNSS-R instruments will have the ability to separate and detect coherent scattering and downlink complex-valued coherent DDM measurements. This additional information will allow carrier phase altimetry, the separation of coherent and non-coherent scattered power, and the evaluation of geo-physical phenomena at along-track resolutions 10x greater than current spaceborne instruments, such as TDS-1 or CYGNSS. In this paper, we will present a prospective design for the on-board detection of coherent reflections. We also investigate how to achieve enhanced along-track resolution.

Positioning with cellular signals has been gaining attention in urban and indoor environments, where global navigation satellite system signals have limited availability due to interference, blockage, or multipath. However, accurate and reliable tracking of cellular signals under highly dynamic urban channel conditions remains a challenging task. This article presents a cellular long-term evolution (LTE) signal tracking algorithm implemented by an adaptive multipath estimating delay lock loop (AMEDLL) to achieve carrier phase synchronization and time-of-arrival (TOA) tracking under severe multipath propagation conditions. The analytical expression of the coherently integrated correlation result over multiple slots with the LTE cell-specific reference signal is derived. A multipath estimator along with a simple yet efficient multipath estimation monitoring approach is developed to estimate the parameters of all detected multipath signals. Several heuristic monitoring criteria based on historical multipath parameter estimations are established to enable adaptive adjustment of the estimated path number. Real LTE signals are collected in an urban environment for the signal tracking performance evaluation. This article presents two case studies with varying levels of multipath effects and signal power to illustrate the effectiveness of the developed signal tracking algorithm. Instead of a TOA truth reference, open-loop carrier phase estimations are used to analyze the TOA tracking error. Our analyses demonstrate that the AMEDLL-based tracking algorithm provides improved TOA estimation accuracy over the existing super-resolution-algorithm-based and delay-lock-loop-based tracking schemes.

Spaceborne global navigation satellite system-reflectometry (GNSS-R) has the potential to provide a new data source for inland water surface observations. In this paper, we present several case studies for the Mississippi River surface gradient observations using the GNSS-R data from Spire's CubeSats. The retrieved river surface slopes are in a range from 2.2 to 16.6 cm/km, and the lower river sections tend to be more flat. Higher phase noise is observed from Spire's GNSS-R data compared previous studies using CY GNSS data. It is likely due to the Spire CubeSat antenna design and the low elevation angles of the reflection signals. A lower elevation angle corresponds to a larger footprint of up to several kilometers and thus may introduce strong multipath interference in the measurements. The presented results also confirmed the GNSS-R observation of superelevation phenomenon that was first discussed in [1]. The ionosphere effects and L2 signal's competitive contributions to the retrieval noise and ionospheric correction are also discussed.

This paper presents an onboard complex waveform or Delay Doppler map (DDM) processing approach for spaceborne coherent-reflection GNSS signal reflected off complex terrain. A digital elevation map (DEM) is needed for the processing of land-reflection signals. For onboard processing, the slope of the land surface is usually not considered or accurately modeled due to the computation limit, and this will cause mis-modeling of the SP location and errors in the code phase and Doppler frequency models for signal reflected off complex terrain, such as in the mountains. Discrepancies of up to 3 chips of GPS C/A code are shown in this paper for the code phase models from the processing results of CYGNSS raw data. The complex waveform/DDM processing presented in this paper can be configured for various ranges and spacings of the code phase and Doppler frequency shifts to allow various levels of model uncertainties. An example is presented for GPS signal reflected off the Himalaya region. The complex waveform processing result shows coherent reflection measurements that may not be captured by the normally applied open loop (OL) tracking approach, and the result also includes probable observations of glacier extent and surface variations.