GNSS-R Derived Centimetric Sea Topography: An Airborne Experiment Demonstration

Carreño-Luengo, Hugo; Park, Hyuk; Camps, A.; Fabra, Fran; Rius, A.

doi:10.1109/jstars.2013.2257990

Cited by 38 publications

(23 citation statements)

References 15 publications

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“…The simulation uses an altimetric precisionδ code of 20 cm based on code delay retrieval as given by Cardellach and Rius (2008). This precision is anticipated for the GEROS-ISS mission and a rather conservative assumption compared to precisions reported for recent airborne experiments (Carreno-Luengo et al 2013;Semmling et al 2014). Therefore, a precisionδ phase of 3 cm for carrier phase retrieval, based on observations described in Semmling et al (2014) is used in addition.…”

Section: Precision Of Reflection Eventsmentioning

confidence: 99%

Potential of space-borne GNSS reflectometry to constrain simulations of the ocean circulation

et al. 2015

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The Agulhas current system transports warm and salty water masses from the Indian Ocean into the Southern Ocean and into the Atlantic. The transports impact past, present and future climate on local and global scales. The size and variability, however, of the respective transports are still much debated. In this study, an idealized model based twin experiment is used to study whether sea surface height (SSH) anomalies estimated from reflected signals of the Global Navigation Satellite System (GNSS-R) can be used to determine the internal water mass properties and transports of the Agulhas region. A space-borne GNSS-R detector on the International Space Station (ISS) is assumed and simulated. The detector is able to observe daily SSH fields with a spatial resolution of 1-5 • . Depending on reflection geometry, the precision of a single SSH observation is estimated to reach 3 cm (20 cm) when the carrier phase (code delay) information of the reflected GNSS signal is used. The average precision over the Agulhas region is 7 cm (42 cm). The proposed GNSS-R measurements surpass the radar-based satellite altimetry missions in temporal and spatial resolution but are less precise. Using the estimated GNSS-R characteristics, measurements of SSH are generated by sampling a regional nested general circulation model of the South African oceans. The artificial observations are subsequently assimilated with a 4DVAR adjoint data assimilation method into the same ocean model but with a different initial state and forcing. The assimilated and the original, i.e., the sampled model state are compared to systematically identify improvements and degradations in the model variables that arise due to the assimilation of GNSS-R based SSH observations. We show that SSH and the independent, i.e., not assimilated model variables velocity, temperature and salinity improve by the assimilation of GNSS-R based SSH observations. After the assimilation of 90 days of SSH observations, improvements in the independent variables cover the whole water column. Locally, up to 39 % of the original model state are recovered. Shorter assimilation windows result in enhanced reproduction of the observed and assimilated SSH but are accompanied by an insufficient or wrong recovery of sub-surface water properties. The assimilation of real GNSS-R observations, when available, and consequently the estimation of Agulhas water mass properties and the leakage of heat and salt into the Atlantic will benefit from this model based study.

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Section: Precision Of Reflection Eventsmentioning

confidence: 99%

Potential of space-borne GNSS reflectometry to constrain simulations of the ocean circulation

et al. 2015

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“…Despite many models having been studied, including the small slope approximation (SSA) model [25] and the two-scale composite model (TSM) [26], in the case of the GNSS-R, the geometrics optics limit of the Kirchhoff model (KGO) is the one most widely used [4,8,27,28] because of its simplicity and its capability to reproduce the cross-polar experimental data in the forward direction. The scattering of electromagnetic waves from the sea is strongly affected by its roughness, being the total scattered field the combination of many electromagnetic waves coming from multiple individual scatterers on the surface.…”

Section: Number Of Specular Points Inside the Scattering Areamentioning

confidence: 99%

“…Assuming that coherent scattering is negligible, the bistatic scattering coefficient was derived under the geometric optics limit, for a sea surface model with Gaussian distribution of the slopes, and a final expression of the "waveform" was derived. During the last decade, additional experimental [5][6][7][8] and theoretical [9][10][11][12] works have been performed to investigate the feasibility of this bistatic radar system to perform accurate ocean altimetry, usually with open-loop receivers, and using a model of the scattering geometry to center the delay and Doppler tracking windows.…”

Section: Introductionmentioning

confidence: 99%

Empirical Results of a Surface-Level GNSS-R Experiment in a Wave Channel

Carreño-Luengo

Camps

2015

Remote Sensing

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Abstract:The scattering of GNSS signals over a water surface is studied when the receiver is at a low height, as in GNSS-R coastal altimetry. The precise determination of the local sea level and wave state from the coast will provide useful altimetry and wave information as "dry" tide and wave gauges. An experiment has been conducted at the Canal d'Investigació i Experimentació Marí tima (CIEM) wave channel for two simulated "sea" states. The GNSS-reflectometer used is the P(Y) and C/A ReflectOmeter (PYCARO) instrument, a closed-loop receiver with delay and Doppler tracking loops that uses the conventional GNSS-R technique for the GPS C/A code. After retracking of the scattered GPS signals, the coherent and incoherent components have been studied. To reproduce the transmitted GPS signals indoors, a Rohde and Schwarz signal generator is used. It is found that, despite the ratio of the coherent and incoherent components being ~1, the coherent component is strong enough that it can be tracked. The coherent component comes from clusters of points on the surface that approximately satisfy the specular reflection conditions ("roughed facet"). The Pearson's linear correlation coefficients of the derived "sea" surface height with the wave gauge data are: 0.78, 0.85 and 0.81 for a SWH = 36 cm and 0.34, 0.74, and 0.72 for a SWH = 64 cm, respectively, for transmitter elevation angles of θ e = 60°, 75° and 86°, respectively. Finally, the rms phase of the received signal before the retracking processing is used to estimate the effective rms surface height of the 'facets', where the waves get scattered. It is found to be between 2.5-and 4.1-times smaller than the theoretical values corresponding to the half of the coherent reflectivity decaying factor.

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“…Geoid-induced lake surface undulations were resolved using an airship setup (approximately 500-m altitude) [6]. Topographic variations of the sea surface were detected based on aircraft observations [7], [8] (altitudes of 3000 and 3500 m, respectively). Antarctic ice topography was retrieved with submeter precision from reflections acquired, by chance, with a spaceborne radio occultation setup [9].…”

Section: Introductionmentioning

confidence: 99%

A Phase-Altimetric Simulator: Studying the Sensitivity of Earth-Reflected GNSS Signals to Ocean Topography

Semmling

Leister

Saynisch

et al. 2016

IEEE Trans. Geosci. Remote Sensing

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This paper presents a simulation study on Global Navigation Satellite System (GNSS) reflections focusing on a phase altimetric method for ocean topography retrieval. It examines carrier phase residuals of Earth-reflected GNSS signals in preparation for the GNSS Reflectometry Radio Occultation and Scatterometry experiment aboard the International Space Station (GEROS-ISS). The residuals' sensitivity to ocean topography (maximum of 2-m amplitude variation of global sea level) is shown. A trigonometric approach to determine the specular reflection point is proposed. Reflection events are simulated assuming different low Earth orbit receivers and GNSS-type transmitters. Suitable events for phase altimetry are assumed between 5 • and 30 • elevation lasting between 10 and 15 min with ground tracks length of > 3000 km. Typical along-track footprints (1 s integration time) have a length of about 5 km. Within the assumed elevation range the coherent footprint ellipse has a major axis between 1 and 6 km. A Master-Slave sampling is proposed to approximate large-scale delay and Doppler variations of the reflected signal (Slave channel) relative to the direct signal (Master channel). Slave residuals of an example event are simulated to retrieve a small-scale phase delay for ocean topography inversion. The signal-to-noise ratio restricts the quality of the topography results. Height precision on sub-decimeter level for 30-dB SNR is degraded up to a meter level for 20-dB SNR. Ionosphere-free linear combination allows keeping the precision level. Troposphere refraction degrades precision particularly at the low elevation limit. Precision improves toward higher elevations. The tolerance to ocean roughness decreases in the same way.

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GNSS-R Derived Centimetric Sea Topography: An Airborne Experiment Demonstration

Cited by 38 publications

References 15 publications

Potential of space-borne GNSS reflectometry to constrain simulations of the ocean circulation

Potential of space-borne GNSS reflectometry to constrain simulations of the ocean circulation

Empirical Results of a Surface-Level GNSS-R Experiment in a Wave Channel

A Phase-Altimetric Simulator: Studying the Sensitivity of Earth-Reflected GNSS Signals to Ocean Topography

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