Carlos Perez-Gutierrez

Validation of the SMOS L2 Soil Moisture Data in the REMEDHUS Network (Spain)

The Level 2 soil moisture products from the Soil Moisture and Ocean Salinity (SMOS) mission have been re- leased. The data must be validated under different scenarios of biophysical and climatic conditions. For the current study, the data from January to December 2010 from 20 in situ soil moisture stations from the REMEDHUS soil moisture measurement station network (Spain) were used. A comparison analysis was carried out in terms of the soil moisture content, its spatial variability, and temporal stability. The results show an acceptable level of agreement (R = 0.73, RMSD = 0.069 m3 · m-3, and bias = 0.053 m3 · m-3) between the in situ and satellite data. A slight constant underestimation from the SMOS data set was detected. A centered (bias removed) root-mean-square difference was calculated to account for this persistent bias (RMSDc = 0.044 m3 · m-3). This result is close to the SMOS accuracy objective of 0.04 m3 · m-3. Two conclusions can be drawn: First, SMOS is close to meet the mission accuracy requirements in REMEDHUS, and second, SMOS is able to detect temporal anomalies and the temporal evolution of ground soil moisture, even though the soil moisture was slightly underestimated. Despite a noticeably reduced spatial variability among the SMOS grid cells, the remotely sensed soil moisture shows a spatial pattern of the soil moisture fields on the area scale, in agreement with the site-specific characteristics of REMEDHUS. No differences were found between the use of ascending and descending orbits. In addition, no differences were detected between the use of time-overpass values of in situ soil moisture and that of the daily average.

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.

Design and first results of an UAV-borne L-band radiometer for multiple monitoring purposes

UAV (unmanned Aerial Vehicle) platforms represent a challenging opportunity for the deployment of a number of remote sensors. These vehicles are a cost-effective option in front of manned aerial vehicles (planes and helicopters), are easy to deploy due to the short runways needed, and they allow users to meet the critical requirements of the spatial and temporal resolutions imposed by the instruments. L-band radiometers are an interesting option for obtaining soil moisture maps over local areas with relatively high spatial resolution for precision agriculture, coastal monitoring, estimation of the risk of fires, flood prevention, etc. This paper presents the design of a light-weight, airborne L-band radiometer for deployment in a small UAV, including the hardware and specific software developed for calibration, geo-referencing, and soil moisture retrieval. First results and soil moisture retrievals from different field experiments are presented.

Modeling of soil roughness using terrestrial laser scanner for soil moisture retrieval

The present work reports the bases of an ongoing research whose main objective is the development of a methodology to characterize surface roughness models using terrestrial laser scanning devices. The classical measurements take the profile as the valuable information on roughness variations but a brand new paradigm is applied here where an original three-dimensional, multi-scale framework leads towards an accurate characterization of patterns and roughness for different surfaces. Terrestrial laser scanners are able to provide a complete picture of the roughness properties over the spatial scale of a Synthetic Aperture Radar satellite resolution cell. The paper describes the methodology for measuring the roughness of different surfaces and analyzes parameters what can be used as ancillary data in soil moisture retrieval from satellite datasets.

Review of the CALIMAS Team Contributions to European Space Agency’s Soil Moisture and Ocean Salinity Mission Calibration and Validation

This work summarizes the activities carried out by the SMOS (Soil Moisture and Ocean Salinity) Barcelona Expert Center (SMOS-BEC) team in conjunction with the CIALE/Universidad de Salamanca team, within the framework of the European Space Agency (ESA) CALIMAS project in preparation for the SMOS mission and during its first year of operation. Under these activities several studies were performed, ranging from Level 1 (calibration and image reconstruction) to Level 4 (land pixel disaggregation techniques, by means of data fusion with higher resolution data from optical/infrared sensors). Validation of SMOS salinity products by means of surface drifters developed ad-hoc, and soil moisture products over the REMEDHUS site (Zamora, Spain) are also presented. Results of other preparatory activities carried out to improve the performance of eventual SMOS follow-on missions are presented, including GNSS-R to infer the sea state correction needed for improved ocean salinity retrievals and land surface parameters. Results from CALIMAS show a satisfactory performance of the MIRAS instrument, the accuracy and efficiency of the algorithms implemented in the ground data processors, and explore the limits of spatial resolution of soil moisture products using data fusion, as well as the feasibility of GNSS-R techniques for sea state determination and soil moisture monitoring.

Soil moisture downscaling activities at the REMEDHUS Cal/Val site and its application to SMOS

The European Space Agency (ESA) Soil Moisture and Ocean Salinity (SMOS) mission will provide accurate global surface soil moisture estimates. However, due to technological limitations, the spatial resolution of SMOS observations is limited to 40-km. This resolution is adequate for many global applications, but restricts the use of the data in regional studies over land, where a resolution of 1–10 km is needed. Different downscaling experiments have been carried out at the REMEDHUS Cal/Val site within the GPS and RAdiometric Joint Observations one-year field experiment (GRAJO) to explore the possibility of enhancing the spatial resolution of SMOS-derived soil moisture estimates. This paper investigates the use of visible/infrared remote sensing data to improve the spatial resolution of passive microwave soil moisture estimates, and presents an approach to down-scale airborne observations acquired with the UPC Airborne RadIomEter at L-band (ARIEL) using LANDSAT imagery. Although a full validation of the method was not possible, preliminary results indicate that with this approach it is feasible to improve the spatial and radiometric resolution of the soil moisture estimates. The comparison with ground-based soil moisture demonstrates the strength of the link between visible/infrared satellite data and soil moisture status, and the potential of adapting this downscaling technique to SMOS-derived soil moisture estimates.

The GPS and RAdiometric Joint Observations experiment at the REMEDHUS site (Zamora-Salamanca region, Spain)

GRAJO (GPS and RAdiometric Joint Observations) is a longterm field experiment over land which is being conducted since November 2008 at the REMEDHUS site, Zamora, Spain. REMEDHUS has been identified as a cal/val site for ESAs SMOS mission. The objectives of GRAJO are multiple: (i) validate and calibrate SMOS-derived soil moisture, (ii) study the variability of soil moisture within the SMOS footprint, (iii) test pixel disaggregation techniques to improve the spatial resolution of SMOS observations, (iv) determine the optical depth and vegetation water content of barley and grass and assess their influence on soil moisture estimates from radiometric and GNSS-R measurements, and (v) characterise the soil roughness factor. This paper presents an overview of the GRAJO experiment, describing the setup and measurements strategy.

GNSS-R Delay-Doppler Maps over land: Preliminary results of the GRAJO field experiment

Within the ESAs SMOS CAL/VAL activities, the GPS and Radiometric Joint Observations (GRAJO) field experiment was conducted. Apart from contributing to the SMOS CAL/VAL, the main purpose of the GRAJO experiment was to study the synergy of L-band radiometry and GNSS Reflectometry for soil moisture retrieval. Long-term experiments under controlled conditions. During one of these intensive short term experiments, the griPAU instrument (a Delay-Doppler Map GNSS-R receiver) was set up. The first results derived from the griPAU measurements are presented.

Soil moisture and vegetation height retrieval using GNSS-R techniques

Global Navigation Satellite Signals Reflections (GNSS-R) techniques are currently being used for remote sensing purposes retrieving geophysical parameters over different types of surfaces. Over the ocean, sea state information can be retrieved to improve the ocean salinity retrieval. Furthermore, over land these techniques can be used to retrieve soil moisture. This paper presents the theoretical and experimental results of using GNSS-R to retrieve soil moisture when vegetation is present. The particular technique being applied in this study is the Interference Pattern Technique (IPT) that measures the interference pattern of the GPS direct and reflected signals, after reflecting over the surface.

Comparison of a multilateral-based acquisition with Terrestrial Laser Scanner and profilometer technique for soil roughness measurement

Soil roughness parameters have been traditionally retrieved using altimetric profiles acquired from profilometer devices. Measuring soil roughness using the technology called Terrestrial Laser Scanner (TLS) is an innovative method because of it is possible to get a three-dimensional surface more realistic than previous approaches. This paper shows a comparison between both techniques for different kind of soil roughness. A multilateral-based acquisition is proposed for TLS measurement. Analyses are made with profiles from the same soil samples acquired using a profilometer and a TLS. Profile-to-profile comparison got altimetric correlation between both devices and how soil roughness parameters are affected with. Results offer a straight relation (R=0.92 globlally) between both datasets. The oblique angle used by multilateral acquisition in TLS reports a smoother value than vertical acquisition by profilometer