The Arctic-boreal zone is warming due to climate change. Current spaceborne remote sensing techniques and retrieval methodologies need to be complemented to improve systematic monitoring of the cryosphere. To that end, this article presents a new investigation of the use of the Global Navigation Satellite System Reflectometry (GNSS-R) remote sensing technique by a SmallSat constellation. A new freeze/thaw (F/T) seasonal multi-thresholding algorithm is developed using high-inclination orbit near-Nadir Spire Global GNSS-R data acquired through the National Aeronautics and Space Administration Commercial Smallsat Data Acquisition program. Five different soil surface reflectivity Γ models are proposed to account for the impact of vegetation cover and small-scale surface roughness on Earth-reflected GNSS signals. The sensitivity of the Γ models to F/T surface state transitions is evaluated, and the optimum model is selected to construct a seasonal scale factor. Then, a multi-thresholding matrix is obtained for F/T classification using a specific threshold for every surface grid cell. Results for the annual frozen soil duration (days yr-1) are compared with that by the Soil Moisture Active Passive mission. Additionally freezing and thawing periods are analyzed to determine when the moisture exchange with the atmosphere is locked, which is an important climatic factor. A novel metric is introduced to characterize the freeze intensity moving beyond classical F/T binary classifications. Results are evaluated using air and soil temperature, snow depth and temperature, and soil moisture content provided by the European Centre for Medium-Range Weather Forecasts ERA5-Land reanalysis product.

At present, it is speculated that the interannual variability of global CH4 is dominated by wetlands. More accurate CH4 emission models require improved capabilities for surface water extend monitoring by remote sensing sensors under dense vegetation. A new NASA Cyclone Global Navigation Satellite System (CYGNSS) surface water map data product has been produced, which can be used to further understand the water cycle in tropical latitudes. This new data product enables small inland water bodies detection under dense biomass up to ~ 400 tons/ha. Our product improves multi-spectral VisIR imagers aboard satellites such as e.g. Landsat & Sentinel-2, and C-band Synthetic Aperture Radar (SAR) images by e.g. Sentinel-1.The main objective of this work is the application of an improved version of the new CYGNSS retrieval algorithm to all the available Rongowai Global Navigation Satellite Systems Reflectometry (GNSS-R) airborne mission data to investigate the benefits of polarimetric measurements on inland water detection under dense biomass. The expected relevance of this study is high because tens of polarimetric GNSS-R SmallSats will be launched in the near future, including the European Space Agency (ESA) HydroGNSS mission and the Muon Space commercial constellation. On the other hand, SAR & microwave radiometry missions are limited by the required power consumption, instrument size and cost. In summary, we are developing a novel GNSS-R retrieval algorithm, useful for the remote sensing community, and with interest to further understand highly dynamic water surface processes.

Spatial and temporal patterns of landscape Freeze/Thaw (F/T) state transitions within the cryosphere are highly variable with impacts to climate, hydrological, ecological, and biogeochemical processes. F/T state has a strong impact on the seasonal amplitude and partitioning of surface energy exchange, while ecosystem responses to seasonal thaw are rapid, with evapotranspiration, soil respiration, and plant photosynthetic activity accelerating with warmer temperatures and the availability of liquid water. The annual thaw period also influences the vegetation growing season, while variability in F/T timing rules vegetation net primary production and Net Ecosystem CO2 Exchange (NEE) with the atmosphere. More than one third of the Earth’s land surface is covered by seasonal or permanent soil frost. Many agricultural, engineering, and environmental issues and applications are affected by F/T state. A frozen land surface tends to reduce or even to impede water infiltration, which may promote flooding, surface runoff, and soil erosion.In this work, Spire data through the NASA Commercial Smallsat Data Acquisition (CSDA) Program are used to develop novel Global Navigation Satellite Systems Reflectometry (GNSS-R) methods to determine the F/T state over the Arctic-Boreal region. The generated theoretical and experimental capabilities will be applied to analyze the spatial patterns and temporal dynamics with an improved spatio-temporal sampling as compared to Synthetic Aperture Radar (SAR) missions.

There are several established algorithms for processing Global Navigation Satellite System Reflected (GNSS-R) signals. The goal of these techniques is to generate delay Doppler maps (DDMs) of the magnitude distribution of the reflected/scattered signal power over the surface. The resulting DDMs are then used to estimate geophysical surface parameters such as ocean surface wind speed or near surface soil moisture. Each of these techniques have their unique application specific advantages and disadvantages. This presentation will present three of the most common GNSS-R DDM processing algorithms and compare them from the perspective of signal quality and calculation efficiency, and make recommendations for their suitability for various remote sensing applications and instrument design considerations.

Earth’s inland water detection under thick biomass remains unresolved. Optical sensors are limited by night, clouds and upwelling biomass. The SWAMPS product (active + passive microwave sensors) has a coarse spatial resolution ~ 25 km. C-band Sentinel-1 radar is limited by the upwelling biomass and has a high revisit period. In this work, a new retrieval algorithm based on L1 Cyclone Global Navigation Satellite System (CYGNSS) mission data is developed and results show the capability of water detection under thick biomass ~ 450 ton/ha. The use of the recent IEEE Global Navigation Satellite Systems Reflectometry (GNSS-R) Standard would enable the application of this algorithm by all the present and future GNSS-R missions, and thus generating an unbeatable spatio-temporal sampling by this "virtual" constellation of SmallSats. The capability to accurately resolve surface water extend has an important impact on estimating global methane emissions to the atmosphere.

Earth’s inland water monitoring is probably the main promising application of Global Navigation Satellite Systems Reflectometry (GNSS-R) techniques. The ultimate spatial resolution under the coherent scattering regime deserves further investigation. The new Cyclone Global Navigation Satellite System (CYGNSS) raw Intermediate Frequency (IF) data product with a temporal resolution down to 2 ms could help to further understand this. In the framework of climate change the "water" is the "new gold". In-space water monitoring could help final users to make decisions with impact in several topics including geopolitics. The use of GNSS-R techniques by future constellations of SmallSats could overcome several limitations of more classical remote sensing techniques. In this work, a novel real-time inland water detector by a future GNSS-R receiver is presented. This detector, the so-called fast entropy Efast, shows the capability to detect small water bodies under thick biomass ~ 450 ton/ha in the Congo basin.

A new Cyclone Global Navigation Satellite System (CYGNSS) data product is described which is generated from the raw Intermediate Frequency (IF) data. The product includes several established signal coherence detectors, including the power-ratioPratio, complex zero-Doppler delay waveform and full entropyEfull, and a novel fast entropy detectorEfast. Both entropy detectors are provided with two temporal resolutions: 2 ms and 50 ms. Coherence performance is characterized using the phase derivative of the reflected signal at the peak of the delay waveform φpeak. Threshold values of the full entropy detector are determined which classify scattering into three regimes: incoherent, partially coherent, and coherent. Several scattered signal strength products are included: Signal-to-Noise RatioSNR, reflected powerPg, reflectivity Γ, and Normalized Bistatic Radar Cross-Section NBRCS. Each of these products is derived using a coherent integration time ofTC= 1 ms and incoherent integration times ofNinc= 1000, 500, 250, 100, 50, and 2 ms. Signal strength time series at the shorter (2 and 50 ms) times provides excellent detection of land-water transitions in heterogeneous scenes. Delay Doppler Maps (DDMs) are also generated with high delay (Δτ = 1/16 chip) and Doppler (Δf = 50 Hz) resolution. The behavior of each signal strength product as a coherence detector is examined using the full entropy method as a reference. Performance is characterized using Receiver Operating Characteristic (ROC) curves. The fast entropy method, which has much lower computational cost, is similarly characterized. This suite of coherence detection methods can be used to detect the presence of small inland water bodies.

NASA's Cyclone Global Navigation Satellite System (CYGNSS) mission has been in orbit for more than 5 years. Following the Earth Science Division 2020 Senior Review, NASA announced last year it is extending the CYGNSS mission through at least 30thSeptember 2023. The extended CYGNSS mission phase will focus on both ocean and land surface scientific investigations. One new recent application of CYGNSS is FreezelThaw (FIT) surface state detection and imaging [1]. The main objective of this presentation is the application of the retrieval algorithm developed in [1] over different high-interest target areas within the CYGNSS coverage to show the capability to provide long-term trending of FIT behaviour.

The CYclone Global Navigation Satellite System (CYGNSS) End-to-End Simulator (E2ES) is updated with a new module capable of accurately simulating coherent forward scattering from inland water bodies. This module uses a Global Navigation Satellite Systems Reflectometry (GNSS-R) model that can account for both coherent and incoherent scattering due to small scale surface roughness. The model is developed based on the Huygens-Kirchhoff principle. One principle application is to support the development of wetland extent retrieval algorithms over heterogenous scenes. In this abstract, a water mask corresponding to a lake surrounded by mountains and vegetation is considered. A modeled time series of reflected power is generated and results are compared to actual CYGNSS raw IF data. The final goal is to support scientific studies related to the capabilities of CYGNSS for inland water body monitoring.

Freeze/Thaw (F/T) surface state retrieval is important to further understand hydrological patterns and climate change. This article investigates the use of Earth-reflected Global Positioning System (GPS) L-band signals as collected by the National Aeronautics and Space Administration NASA’s Cyclone Global Navigation Satellite System (CYGNSS) mission for F/T surface state retrieval over a target area in South America, covering the Andes Mountains and the Argentinian Pampas. In the study, CYGNSS responsiveness to changes in surface permittivity is leveraged to detect transitions of F/T surface state, at an improved spatio-temporal sampling as compared to traditional Remote Sensing missions. A Seasonal-Threshold Algorithm (STA) is developed and validated using surface temperature data as provided by the European Centre for Medium-Range Weather Forecast (ECMWF) ERA5-Land numerical reanalysis model. Then, the monthly evolution of CYGNSS-derived F/T surface state maps is evaluated and an inter-comparison with the Soil Moisture Active Passive (SMAP) F/T data product is performed.

Soil Freeze/Thaw (F/T) transition monitoring is essential for quantifying climate change and hydrologic dynamics over cold regions. In this work, we investigated the use of Earth-reflected Global Positioning System (GPS) L-band signals as collected by the CYclone Global Navigation Satellite System (CYGNSS) mission for F/T retrieval over high altitude areas. CYGNSS can potentially detect the freezing and thawing of soil surface due to changes of dielectric constant when frozen, at an improved spatio-temporal sampling as compared to traditional microwave radiometry missions. A feasibility study has been performed over the Andes Mountains to further advance on this topic. Sensitivity in CYGNSS data to F/T transitions was demonstrated by comparing reflectivity differences between Summer and Winter to soil and snow temperature as provided by the European Centre for Medium-Range Weather Forecast (ECMWF) ERA 5-Land numerical reanalysis model. Finally, F/T retrievals with CYGNSS showed a better performance than those derived with the Soil Moisture Active Passive (SMAP) mission.

A new CYclone Global Navigation Satellite System (CYGNSS) data product is generated using all the available raw Intermediate Frequency (IF) tracks. In so doing, calibrated Delay Doppler Maps (DDMs) are produced using the CYGNSS raw IF processor, and the CYGNSS End-to-End Simulator (E2ES) to generate Look-Up- Tables (LUTs) to bin-by-bin correct the whole DDMs. Such calibrated DDMs are delivered with shorter incoherent integration time$(\boldsymbol{N}_{\mathbf{inc}})$, finer delay and Doppler resolution, and a wider range of delay and Doppler values as compared to the L1b CYGNSS data product, currently available in the Physical Oceanography Distributed Active Archive Center (PODAAC). This work aims to foster the use of this new data product, so as to improve the performance of CYGNSS for a wide range of scientific applications over land and ocean.

In February 2019 a Project Authorization Request was approved by the Institute of Electrical and Electronics Engineers (IEEE) Standards Association with the title “Standard for Global Navigation Satellite System Reflectometry (GNSS-R) Data and Metadata Content”. A Working Group has been assembled to draft this standard with the purpose of unifying and documenting GNSS-R measurements, calibration procedures, and product level definitions. The Working Group (http://www.grss-ieee.org/community/technical-committees/standards-or-earth-observations/) includes members, collaborators, and contributors from academia, international space agencies, and private industry. In a recent face-to-face meeting held during the ARSI+KEO 2019 Conference, the need was recognized to develop a standard with a wide range of operations, providing procedure guidelines independently of constraints imposed by current limitations on geophysical parameters retrieval algorithms. As such, this effort aims to establish the fundamentals of a potential virtual network of satellites providing inter-comparable data to the scientific community.

Forward scattering properties of Earth-reflected Global Navigation Satellite Systems (GNSS) signals are evaluated over land surfaces. The CYclone Global Navigation Satellite System (CYGNSS) End-to-End Simulator (E2ES) is updated. A GNSS-Reflectometry (GNSS-R) model capable of evaluating both the incoherent and the coherent scattering terms is developed based on the Huygens-Kirchhoff principle. The coherent term is studied for incidence angles θi~ [0, 80]° over a wide range of surface height standard deviation σ = [1, 4] cm and Vegetation Optical Depth VOD = [0, 1.5]. This model is validated over a lake with space-borne data as generated using the CYGNSS raw IF processor. Results demonstrate the impact of higher order Fresnel zones on the spatial resolution of GNSS-R over heterogenous areas, showing “ringing” fluctuations in the reflected power near high contrast boundaries. We expect to contribute to the accurate interpretation of datasets over inland water bodies.

Understanding the effects of Earth's surface topography in Global Navigation Satellite Systems Reflectometry (GNSS-R) spaceborne data is essential to correct experimental observations so as to achieve accurate Soil Moisture Content (SMC) determination and biomass monitoring. A comprehensive set of observables obtained from Delay Doppler Maps (DDMs) generated on-board Cyclone Global Navigation Satellite Systems (CyGNSS) are evaluated as a function of several topographic parameters derived from a Digital Elevation Model (DEM). Terrain Ruggedness Index (TRI) and profile curvature have a moderate-to-strong impact on CyGNSS-derived trailing edge (TE) and reflectivity Γ for low elevation angles θe~ [20, 30]° (r_tri_Γ ~ -0.39, r_tri_TE ~ 0.39, r_curv_Γ ~ 0.41, r_curv_TE ~ -0.38). On the other hand, local slopes have an influence on Γ and TE for a constant curvature level, while both observables are independent of the slopes for a constant TRI.

A performance assessment of the Cyclone Global Navigation Satellite System (CyGNSS) mission for biomass studies is presented in this paper over Congo and Amazon tropical rainforests. The functional correlation between CyGNSS-derived trailing edge TE and Avitabile et. al. Above Ground Biomass (AGB) pan-tropical data is evaluated using ICESat/GLAS-derived Canopy Height (CH), SMAP-derived Vegetation Optical Depth (VOD) and Polarization Index (PI). The main objective is to investigate the sensitivity of CyGNSS to biomass over dense forest canopies. The potential advantage of CyGNSS as compared to classical monostatic radar missions could rely on the increasing signal attenuation by the upwelling vegetation cover that reduces the coherent scattering component σ coh,0 . This term can only be collected in a bistatic radar geometry. Ignoring measurement noise, preliminary results show that the use of TE allows to estimate AGB up to ~400 ton/ha and ~250 ton/ha over Congo and Amazon rainforests at low elevation angles θ e ~ [20, 40] °. The analysis using the SMAP-derived microwave vegetation parameters shows that: a) VOD performs well even over rainforests, and b) PI could depend on the CH levels.

Earth's surface bistatic reflectivity ΓLHCP,CyGNSS profiles are obtained using Global Navigation Satellite Systems Reflectometry (GNSS-R) products from NASA's Cyclone Global Navigation Satellite Systems (CyGNSS). The focus of this study is to evaluate the influence of the GNSS satellites' elevation angle θe on ΓLHCP,CyGNSS. Specific target areas with differentiated scattering media are selected to characterize these profiles as a function of the land-cover type and the wind-speed regimes for ocean-surfaces. In the former case, the main interest is to further understand the function ΓLHCP,CyGNSS=f(θe,...) in preparation of the potential application of the so-called Tau-Omega model to GNSS-R. In the latter case, the objective is to analyze the impact of the coherent scattering component in GNSS-R observables. Preliminary results show ΓLHCP,CyGNSS increases ~ 3.7 dB over land and ~ 5.3 dB ocean, as θe moves from~ [70], [90]° to ~ [20], [40]°.

This work presents an assessment on the correlation between CyGNSS-derived Global Navigation Satellite Systems Reflectometry (GNSS-R) bistatic reflectivity Γrland SMAP-derived brightness temperature TB, over land surfaces. This parametric-study is performed as a function of Soil Moisture Content (SMC), and vegetation opacity τ. Several target areas are selected to evaluate potential differentiated geophysical effects on “active” (as many transmitters as navigation satellites are in view), and passive approaches. Although microwave radiometry has potentially a better sensitivity to SMC, the spatial resolution is poor ~ 40 km. On the other hand, GNSS-R bistatic coherent radar footprint is limited by half of the first Fresnel zone which provides about ~ 150 m of spatial resolution (depending on the geometry). The synergetic combination of both techniques could provide advantages with respect to active monostatic Synthetic Aperture Radar (SAR).

NASA's Soil Moisture Active Passive (SMAP) mission has been tuned to perform a Global Navigation Satellite Systems Reflectometry (GNSS-R) experiment. The motivation of this study is to assess the capabilities of GNSS-R for soil moisture determination, biomass monitoring and cryosphere studies. The use for first time of the Polarimetric Ratio (PR) from a spaceborne platform shows significant sensitivity to soil moisture. Additionally, the investigation for first time of the leading and trailing edges width sensitivity to Above Ground Biomass (AGB) and rough topography shows promising results. Better understanding of these effects in the reflected waveforms will improve the development of retrieval algorithms.

Global Navigation Satellite Systems Reflectometry (GNSS-R) ocean applications includes scatterometry and altimetry. In this work, an investigation is performed on polarimetric scatterometry over ocean surface using data from a GNSS-R experiment on-board the Soil Moisture Active Passive (SMAP) mission, and on ocean surface topography from the Cyclone Global Navigation Satellite System (CyGNSS) mission using new retrieval algorithms. The former one provides global coverage because of the Sun Synchronous Orbit (SSO), while the latter one focuses on tropical latitudes providing a spatial sampling of 32 swaths. First results from SMAP over the Artic Sea show clearly sea ice effects on the reflected waveforms.

Global Navigation Satellite Systems Reflectometry (GNSSR) is a sort of multi-static radar using navigation signals as signals of opportunity. Due to the large number of transmitting satellites multiple scattering points over the Earth's surface can be simultaneously tracked, which provides wide-swath and improved spatio-temporal sampling over current space-borne radar altimetry missions. The lack of experimental datasets from space covering signals from multiple constellations (GPS, GLONASS, Galileo and Beidou) at dual-band (L1 and L2), and dual-polarization (Right and Left Hand Circular Polarization: RHCP and LHCP), over the ocean, land, and cryosphere remains a bottleneck to further develop these techniques.3Cat-2 is a 6 unit (3 × 2 elementary blocks of 10 × 10 × 10 cm3) cubesat mission designed and implemented at the Universitat Politècnica de Catalunya-BarcelonaTech. It will be launched in late July 2016. During one of the two stratospheric balloon flights of the3Cat-2 payload, the P(Y) and C/A ReflectOmeter (PYCARO), it was found that the received signal power was nearly constant with the balloon height. This revealed the presence of a strong coherent component. This work presents the derivation of a unified equation to account for both incoherent and coherent scattering contributions.

The empirical results of a global navigation satellite systems reflectometry (GNSS-R) experiment onboard the Balloon EXperiments for University Students (BEXUS) 17 stratospheric balloon performed north of Sweden over boreal forests show that the power of the reflected signals is nearly independent of the platform height for a high coherent integration time Tc= 20 ms. This experimental evidence shows a strong coherent component in the forward scattered signal, as compared with the incoherent component, that allows to be tracked. The bistatic coherent reflectivity is also evaluated as a function of the elevation angle, showing a decrease of ~6 dB when the elevation angle increases from 35° to 70°. The received power presents a clearly multimodal behavior, which also suggests that the coherent scattering component may be taking place in different forest elements, i.e., soil, canopy, and through multiple reflections canopy-soil and soil-trunk. This experiment has provided the first GNSS-R data set over boreal forests. The evaluation of these results can be useful for the feasibility study of this technique to perform biomass monitoring that is a key factor to analyze the carbon cycle.

Scientific evaluation of the3Cat-2 payload (PYCARO reflectometer) has been performed from the BEXUS 19 stratospheric balloon flight with an apogee of ~ 27,000 m over boreal forests and lakes. The payload was configured in closed-loop mode during this flight. Results show the first-ever multi-constellation Global Navigation Satellite Systems Reflectometry (GNSS-R) measurements at dual-band and dual-polarization.

3Cat-2 Assembly, Integration and Verification (AIV) activities of the Engineering Model (EM) and the Flight Model (FM) are being carried out at present. The Attitude Determination and Control System (ADCS) and Flight Software (FSW) validation campaigns will be performed at Universitat Politècnica de Catalunya (UPC) during the incomings months. An analysis and verification of the3Cat-2 key mission requirements has been performed. The main results are summarized in this work.

Reflectometry using Global Navigation Satellite System's signals of opportunity (GNSS-R) was originally conceived in the early 90s for mesoscale altimetry, and since then, many studies have shown its applicability to other remote sensing applications. In 2002, the Remote Sensing Lab at the Universitat Politècnica de Catalunya started working in Global Navigation Satellite Reflectometry (GNSS-R) to perform a more accurate correction of the sea state when retrieving sea surface salinity from L-band microwave radiometer data (e.g. SMOS). During these 12 years, the lab has developed ground-based, airborne, balloon, and spaceborne GNSS-R payloads for a number of different applications. This paper provides an overview of the families of instruments, and the design choices made.

Universitat Politècnica de Catalunya (UPC) - BarcelonaTech Remote Sensing Laboratory focus on the development of breakthrough concepts. The3Cat-2 mission is on the synergy of GNSS reflectometry and the CubeSat concept. Scientifically valuable mission data will improve our understanding on the Earth's environment. In particular, ocean currents need further investigation because of the spatio-temporal evolution of the mesoscale phenomena. The spacecraft development work includes two stratospheric flights in the frame of the REXUS/BEXUS programme coordinated by the European Space Agency (ESA). Coherent and incoherent scattering of Earth-reflected GNSS signals as sensed by the P(Y) & C/A ReflectOmeter (PYCARO) payload will be evaluated using the main state-of-the art Global Navigation Satellite Systems Reflectometry (GNSS-R) methodologies. Future spaceborne activities could take advantage of it, including the GNSS REflectometry, Radio Occultation and Scatterometry on-board International Space Station (GEROS-ISS) experiment.

Global Navigation Satellite System Reflectometry (GNSS-R) has been emerged as a new opportunity for satellite remote sensing. From the bistatic measurement of GNSS-R, various geophysical parameters can be retrieved. This work shows the possibility of GNSS-R to monitoring the tropical cyclone. For case study, the GEROS mission is considered, which plans to install GNSS-R instrument on International Space Station. From the simulated DDM, the scattering coefficient maps are restored, and the eye of cyclone is detected.

This study presents the feasibility of sea waves monitoring using the P(Y) & C/A ReflectOmeter (PYCARO) instrument which employs a novel Global Navigation Satellite Sys tems Reflectometry (GNSS-R) correlation technique. To that end, an experiment over the wave channel surface of the civil engineering school of UPC (Barcelona, Spain) is performed using a signal generator to simulate the GPS satellites at different elevation angles. Results are compared to these derived from a wave probe sensor. The experimental evidence of sea waves determination opens a new scientific application of this remote sensing methodology.

3Cat-2 is a 6 U CubeSat mission designed to perform ocean altimetry by means of Global Navigation Satellite Systems Reflectometry (GNSS-R). In this paper3Cat-2 mission is presented. The main payload is the novel dual-band altimeter P(Y) & C/A ReflectOmeter (PYCARO) which has already been tested from a pier and from an aircraft over the Mediterranean Sea. In addition, an experiment from the DLR/SNSB Balloon-borne EXperiments for University Students (BEXUS) stratospheric balloon in North Sweden will be performed on October 2013 for the ultimate optimization of the payload parameters.

PAU/PARIS End-to-end Performance Simulator (P2EPS) has been developed for studying and design tools for GNSS-R space mission. For the purpose of accuracy and various scenario, P2EPS has recently improved in many features. Especially, the wide range of the simulation inputs enables to simulate the upcomming GNSS-R space missions. This paper presents the recent improvement of the P2EPS.

The ultimate accuracy and precision of conventional1 and interferometric2Global Positioning Satellite System Rerflectometry (GNSS-R) techniques for mesoscale ocean altimetry are still a matter of debate in the scientific community. The results obtained depend on the techniques used to identify the point of the specular delay in the waveform, and to perform the different delay corrections to derive the geometric delay. Also, the geometric model assumed for the scenario determines the accuracy of the altimeter range. This work presents the results of two ESA-sponsored airborne experiments using conventional GNSS-R showing subdecimeter altimetric precision with the Global Positioning System (GPS) L1 C/A code only. The Relative Mean Dynamic Topography (RMDT) obtained in both experiments is compared with results derived from traditional radar altimetry provided by Jason 2. The Root Mean Square (RMS) of the RMDT difference between both measurement systems is 48 cm for the first flight, and 198 cm for the second flight. Additionally, results from the second flight experiment show the capability of the proposed technique to retrieve sea slope measurements by superposing the ground track with EM96 geoid undulations.