Monique Dechambre

A new empirical model to retrieve soil moisture and roughness from C-band radar data

Abstract A new empirical model for the retrieval, at a field scale, of the bare soil moisture content and the surface roughness characteristics from radar measurements is proposed. The derivation of the algorithm is based on the results of three experimental radar campaigns conducted under natural conditions over agricultural areas. Radar data were acquired by means of several C-band space borne (SIR-C, RADARSAT) or helicopter borne (ERASME) sensors, operating in different configurations of polarization (HH or VV) and incidence angle. Simultaneously to radar acquisitions, a complete ground truth data base was built up with different surface condition measurements of the mean standard deviation (rms) height s , the correlation length l , and the volumetric surface moisture M v . This algorithm is more specifically developed using the radar cross-section σ 0 (HH polarization and 39° incidence angle off nadir), namely, σ 0 HH,39 , and the differential (HH polarization) radar cross-section Δ σ 0 = σ 0,23° − σ 0,39° in terms of an original roughness parameter, Z s , namely Z s = s 2 / l , and M v . A good agreement is observed between model outputs and backscattering measurements over different test fields. Eventually, an inversion technique is proposed to retrieve Z s and M v from radar measurements.

CAROLS: A New Airborne L-Band Radiometer for Ocean Surface and Land Observations

The “Cooperative Airborne Radiometer for Ocean and Land Studies” (CAROLS) L-Band radiometer was designed and built as a copy of the EMIRAD II radiometer constructed by the Technical University of Denmark team. It is a fully polarimetric and direct sampling correlation radiometer. It is installed on board a dedicated French ATR42 research aircraft, in conjunction with other airborne instruments (C-Band scatterometer—STORM, the GOLD-RTR GPS system, the infrared CIMEL radiometer and a visible wavelength camera). Following initial laboratory qualifications, three airborne campaigns involving 21 flights were carried out over South West France, the Valencia site and the Bay of Biscay (Atlantic Ocean) in 2007, 2008 and 2009, in coordination with in situ field campaigns. In order to validate the CAROLS data, various aircraft flight patterns and maneuvers were implemented, including straight horizontal flights, circular flights, wing and nose wags over the ocean. Analysis of the first two campaigns in 2007 and 2008 leads us to improve the CAROLS radiometer regarding isolation between channels and filter bandwidth. After implementation of these improvements, results show that the instrument is conforming to specification and is a useful tool for Soil Moisture and Ocean Salinity (SMOS) satellite validation as well as for specific studies on surface soil moisture or ocean salinity.

Analysis of RFI Issue Using the CAROLS L-Band Experiment

In this paper, different methods are proposed for the detection and mitigation of the undesirable effects of radio-frequency interference (RFI) in microwave radiometry. The first of these makes use of kurtosis to detect the presence of non-Gaussian signals, whereas the second imposes a threshold on the standard deviation of brightness temperatures in order to distinguish natural-emission variations from RFI. Finally, the third approach is based on the use of a threshold applied to the third and fourth Stokes parameters. All these methods have been applied and tested, with the cooperative airborne radiometer for ocean and land studies radiometer operating in the L-band, on the data acquired during airborne campaigns made in the spring of 2009 over the southwest of France. The performance of each approach, or of two combined approaches, is analyzed with our database. We thus show that the kurtosis method is well suited to detect pulsed RFI, whereas the method based on the second moment of brightness temperatures seems to be better suited to detect continuous-wave RFI in airborne brightness-temperature measurements.

Analysis of RFI Identification and Mitigation in CAROLS Radiometer Data Using a Hardware Spectrum Analyser

A method based on the use of a spectral analyzer has been developed in order to identify and mitigate radio frequency interference (RFI) in microwave radiometry. This method has been tested with L-band CAROLS airborne data highly corrupted by interferences. RFI is a major limiting factor in passive microwave remote sensing. Although the 1.4–1.427 GHz bandwidth is protected, RFI sources close to these frequencies may still corrupt radiometer measurements. In order to reduce RFI bad effects on the brightness temperature measurements, a new instrument called spectral analyzer has been added to the CAROLS radiometer system. A post processing algorithm based on a selective filtering with the division of bandwidth in subbands is proposed. Two discriminant analysis based on the computation of kurtosis and Mahalanobis distance have been compared, evaluated and validated in order to separate accurately the RF interference with natural signal.

Numerical Backscattering Analysis for Rough Surfaces Including a Cloddy Structure

In recent years, the presence of a new type of agricultural-surface tillage used for the sowing of wheat and corn has been observed with increasing frequency. It illustrates less roughly ploughed soils, with a greater quantity of small clods distributed over the soil surface. In this paper, a new description of such rough agricultural surfaces is proposed. It is based on a composite model, including a classical surface represented by an exponential correlation function, together with a random cloddy structure. This description enables volumetric structures to be introduced over the soils surface. A numerical moment-modeling method, based on integral equations, is used to evaluate the contribution of clods to the radar backscattering behavior of agricultural surfaces. It is found that the presence of clods explains the very small correlation lengths which are often found in cloddy agricultural fields. The classical approach, in which the surface is described by a correlation function only based on two statistical parameters, rms height and correlation length, overestimates the backscattering coefficients when compared with an approach that includes the clods. This overestimation is often observed with real radar data for such fields.

Combined airborne radio-instruments for ocean and land studies (CAROLS)

The CAROLS, L band radiometer, is built and designed as a copy of EMIRAD II radiometer of DTU team. It is a Correlation radiometer with direct sampling and fully polarimetric (i.e 4 Stockes). It will be used in conjunction with other airborne instruments (in particular the C-Band scatterometer (STORM) and IEEC GPS system, Infrared CIMEL radiometer, one visible camera), in coordination with in situ field campaigns for SMOS CAL/VAL. The instruments are implemented on board the French research airplane ATR42. A validation campaign with four flights was made over south west of France, Hourtin Lake and Bay of Biscay (Atlantic Ocean) in September 2007. In order to qualify the radiometer data, different types of aircraft movements were realized: circle flights, wing and nose wags. Simultaneously to flights, different ground measurements were made over continental surfaces and ocean. First results show a good quality of data over ocean surfaces. For continental surfaces, important Radio-Frequency Interferences (RFI) were observed over a large part of the studied region.

A new empirical model to inverse soil moisture and roughness using two radar configurations

A new empirical model for the retrieval, at a field scale, of the bare soil moisture content and the surface roughness characteristics from radar measurements is proposed. The derivation of the algorithm is based on the results of 3 experimental radar campaigns conducted under natural conditions over agricultural areas. Radar data were acquired by means of several C-band space borne (SIR-C, RADARSAT) or helicopter borne (ERASME) sensors. This algorithm is more specifically developed using the radar cross-section /spl sigma//sup 0/ (HH polarization and 39/spl deg/ incidence angle off nadir), namely, /spl sigma//sub 0(HH,39)/, and the differential (HH polarization) radar cross-section /spl Delta//spl sigma//sub 0/=/spl sigma//sub 0,23/spl deg//-/spl sigma//sub 0,39/spl deg// in terms of an original roughness parameter, Zs, and Mv. An inversion technique is proposed to retrieve Zs and Mv from radar measurements.

Observations of forests using a helicopter-borne nadir-looking radar: a new method for morphometric purposes

The authors present an original pattern recognition method for the observation of forests using a nadir-looking ranging scatterometer. The radar configuration provides vertical profiles of the vegetation. Radar data were not processed in a classical way, i.e., the calculation of a radar cross section, but through pattern recognition methods applied to the raw data. The aim of the study was to search for typical patterns related to three types of forests. A small subsample of the data was determined for methodology validation purposes. Some results are also reported for a much larger data set. Morphodecomposition, a new methodology presented for the first time in these types of applications, is able to recognize the main features of the data, not only the types of forests involved, but also subtle morphological patterns of the signals. Morphodecomposition provides a morphometric methodology applicable to other geophysical univariate sampled signals.

Wisdom GPR measurements in a cold artificial and controlled environment

The WISDOM (500MHz - 3GHz) GPR is one of the instruments that have been selected as part of the Pasteur payload of ESAs 2018 ExoMars Rover mission. One of the main scientific objectives of the mission is to characterize the nature of the shallow sub-surface on Mars and WISDOM has been designed to explore the first ~ 3 meters of the sub-surface with a vertical resolution of a few centimetres. In order to illustrate and quantify the WISDOM performance, measurements in cold artificial and controlled conditions have been performed by the prototype. The objectives of this experiment were the detection of home made internal layering and the possible detection of a wedge included in the permafrost filled with different material with known characteristics. Some results and comparisons with a simple simulation are presented here and show the WISDOM performances.

Radio frequency interferences investigation using the airborne L-band full polarimetric radiometer CAROLS

In the present paper, different methods are proposed for the detection and mitigation of the undesirable effects of radio frequency interference (RFI) in microwave radiometry. The first of these makes use of kurtosis to detect the presence of non-Gaussian signals, whereas the second imposes a threshold on the standard deviation of brightness temperatures, in order to distinguish natural emission variations from RFI. Finally, the third approach is based on the use of a threshold applied to the third and fourth Stokes parameters. All of these methods have been applied and tested, with a CAROLS radiometer operating in the L-band, on data acquired during airborne campaigns made in spring 2009 over the South West of France. The performance of each, or of two combined approaches is analyzed with our database. We thus show that the kurtosis method is well adapted to pulsed RFI, whereas the method based on the second moment is well adapted to continuous-wave RFI.