Enric Valencia

Land Geophysical Parameters Retrieval Using the Interference Pattern GNSS-R Technique

In the past years, the scientific community has placed a special interest in remotely sensing soil moisture and vegetation parameters. Radiometry and radar techniques have been widely used for years. Global Navigation Satellite Systems opportunity signals Reflected (GNSS-R) over the earths surface are younger, but they have already shown their potential to perform these observations. This paper presents a GNSS-R technique, based on Global Positioning System (GPS) measurements, that allows the retrieval of several geophysical parameters from land surfaces. This technique measures the power of the interference signal between the direct GPS signal and the reflected one after scattering over the land, so it is called Interference Pattern Technique (IPT). This paper presents the results obtained after applying the IPT for topography, soil moisture, and vegetation height retrievals over vegetation-covered soils.

Soil Moisture Retrieval Using GNSS-R Techniques: Experimental Results Over a Bare Soil Field

This paper presents a new technique to retrieve soil moisture using global navigation satellite signals reflected over the soil surface using the measurement of the power fluctuations of the signal created by the interference between the direct GPS signal and the one reflected over the soil surface. As a function of the elevation angle, power fluctuations at vertical polarization pass through a notch, which is related to the soil moisture content, while horizontal polarization exhibits a very weak dependence. Experimental results of the measurements obtained over a bare soil field are presented and discussed.

Correction of the Sea State Impact in the L-Band Brightness Temperature by Means of Delay-Doppler Maps of Global Navigation Satellite Signals Reflected Over the Sea Surface

This paper presents an efficient procedure based on 2-D convolutions to obtain delay-Doppler maps (DDMs) of Global Navigation Satellite Signals reflected (GNSS-R) over the sea surface and collected by a spaceborne receiver. Two DDM-derived observables (area and volume) are proposed to link the sea-state-induced brightness temperature to the measured normalized DDM. Finally, the requirements to use Global Positioning System reflectometry to accurately correct for the sea state impact on the L-band brightness temperature (quantization levels, decimation, truncation, and noise impact) are analyzed in view of its implementation in the Passive Advanced Unit instrument of the Spanish Earth Observation Satellite (SeoSAT/INGENIO) project.

Altimetry with GNSS-R interferometry: first proof of concept experiment

The Global Navigation Satellite System Reflectometry (GNSS-R) concept was conceived as a means to densify radar altimeter measurements of the sea surface. Until now, the GNSS-R concept relied on open access to GNSS transmitted codes. Recently, it has been proposed that the ranging capability of the technique for ocean altimetric applications can be improved by using all the signals transmitted in the bandwidth allocated to GNSS, which includes open access as well as encrypted signals. The main objective of this study is to provide experimental proof of this enhancement through a 2-day experiment on the Zeeland Bridge (The Netherlands). In the experiment, we used a custom built GNSS-R system, composed of high gain GPS antennas, calibration subsystem, and an FPGA-based signal processor which implemented the new concepts, an X-band radar altimeter and a local geodetic network. The results obtained indicate that the new approach produces a significant improvement in GNSS-R altimetric performance.

Using GNSS-R Imaging of the Ocean Surface for Oil Slick Detection

Even though reflectometry using Global Navigation Satellite Systems signals (GNSS-R) was first envisioned for mesoscale ocean altimetry, nowadays a number of different applications have been developed (ocean scatterometry, ice monitoring, soil moisture retrieval, etc.). Recently, imaging of the ocean surface from a GNSS-R spaceborne receiver has been proposed by treating the measured delay-Doppler Maps (DDM) as a blurred image of the surfaces scattering coefficient in the delay-Doppler domain. Thus, by deconvolving the DDM with the GNSS code Woodward ambiguity function (WAF), and appropriate domain transform, an image of the surfaces scattering coefficient distribution can be obtained. In this work this technique is applied to oil slick detection. A realistic scenario is simulated, and the performance of the GNSS-R imaging technique is evaluated on the retrieved image. Error below 10% in the retrieved scattering coefficient distribution is achieved (except for regions affected by deconvolution artifacts), and the image resolution of the order of 2 km is comparable to that of a synthetic aperture radar (SAR) system with equivalent specifications.

An Efficient Algorithm to the Simulation of Delay–Doppler Maps of Reflected Global Navigation Satellite System Signals

A new and efficient algorithm to compute delay-Doppler maps is presented. It improves by more than an order of magnitude the required computation time and memory resources. This approach is based on the derivation of explicit expressions of the space coordinates as a function of the delay offset and Doppler shift. Using this technique, the limitation posed by the number of sampling points of the observed surface is drastically attenuated, and a wide range of scenarios from low- to medium-height airborne-to-spaceborne scenarios can now be simulated with standard desktop computers.

Ocean Surface's Scattering Coefficient Retrieval by Delay–Doppler Map Inversion

Global navigation satellite system reflectometry of signals is a promising technique to remotely sense a number of Earths geophysical parameters, and it has been proposed for ocean monitoring applications such as mesoscale altimetry or sea state monitoring. So far, the following two main approaches have been considered to retrieve a sea state descriptor from measured delay waveforms or delay-Doppler (DD) maps (DDMs): 1) fitting the measurements to a model tuned with the desired parameter or 2) directly linking a property of the measurements with the parameter to be retrieved (e.g., volume of the normalized DDM). However, these approaches provide a single sea state descriptor related to the overall glistening zone where sea state conditions may not be homogeneous. In this letter, the relationship between the physical space and the DD domains is exploited, and a method to retrieve the bistatic scattering coefficient distribution over the observation ocean surface from measured DDMs is proposed.

Sea-State Determination Using GNSS-R Data

Global Navigation Satellite Systems (GNSS) signals can be used to infer geophysical data related to the surface where they scatter. When dealing with the sea surface, its state influences the GNSS scattered signals and, therefore, the GNSS reflectometry (GNSS-R) observables. The aim of the Advanced L-band Emissivity and Reflectivity Observations of the Sea Surface 2008 field experiment was to gather experimental data to study the relationship of the GNSS-R delay-Doppler maps (DDMs) and the sea state. This work describes the field campaign and the main results obtained, where among them is the use of the DDM volume as a roughness descriptor weakly affected by the GPS satellite geometry.

Using DDM Asymmetry Metrics for Wind Direction Retrieval From GPS Ocean-Scattered Signals in Airborne Experiments

Reflectometry of signals of opportunity such as those emitted by a global navigation satellite system, known as GNSS-R, has been developed over the past years as a technique with great potential for ocean scatterometry, among other applications. Different approaches have been proposed to use GNSS-R for remote sensing of ocean surface roughness. One of them is based on deriving some descriptor/metric from the measured delay-Doppler map (DDM) and directly relating it to a geophysical property of the scattering surface. For instance, different descriptors have been proposed in the literature to measure the DDM spreading caused by the increase in ocean surface mean square slopes due to surface winds. In this paper, a new descriptor based on the DDM is proposed for wind direction retrieval. This descriptor, designated as the skewness angle φ1,skew, measures the asymmetry in the DDM power distribution along the Doppler frequency axis, and it was modeled as a function of wind direction by means of a simulation study. Then, that model was validated using real GNSS-R data from an airborne experiment. After validation, the DDM skewness model was successfully used for wind direction retrieval, with a resulting rms error on the order of 20 °.

Vegetation Water Content Estimation Using GNSS Measurements

Global Navigation Satellite Systems (GNSS) opportunity signals reflected at or near the Earths surface have already shown their potential to perform retrievals of a number of geophysical parameters. Radio occultations using GNSS signals are also used for atmospheric sensing. This letter presents a GNSS technique to retrieve vegetation water content (VWC). This technique measures the received powers of the GPS signals in open sky and under the vegetation layer. From these two powers, the attenuation due to the vegetation is computed, which is related to the VWC. This letter presents the results obtained after deploying the instrument in a walnut-tree stand for 11 months.