Vincenzo Sabatelli

Efficiency test of solar collectors: uncertainty in the estimation of regression parameters and sensitivity analysis

The results obtained from efficiency tests conducted on a flat plate solar collector, according to the ISO 9806/1 test procedure, have been used to determine the uncertainty in the curve fitting parameters. The said standard, though requiring certain levels of accuracy in the measuring process, doesnot provide any method to determine the uncertainty of the efficiency curve parameters. The methodology used in the present paper (not provided by the ISO standard) allows solving the above mentioned problem and evaluating not only the parameters and their uncertainties but also the reliability of the test procedure and its goodness toward fitness. In order to evaluate the effects of measurement errors on the values of the uncertainty in estimated parameters, a sensitivity analysis has also been conducted. Strong dependence of some uncertainties, involving a larger accuracy level in the estimation of the measured parameters, is a clear indication of the present investigations.

Use of a C-Band Ground-Based Scatterometer to Monitor Surface Roughness and Soil Moisture Changes

Eight experiments on remote sensing of soil moisture and surface roughness were carried out over bare fields with a microwave C-band scatterometer from 1998 until 2001. This device is able to provide backscattering coefficients in the range of +10 dB and −40 dB for incidence angles between 10° and 60°. The objective is to assess the conditions (of roughness and incidence angle) in which it is possible to separate the effects of roughness and soil moisture, thus allowing a reliable estimation of soil moisture from backscattering coefficients. In particular, starting with measurements carried out at different incidence angles, we experimented with an approach to normalize the backscattering coefficients to a reference angle with the principal aim of comparing data sets acquired in different conditions and putting in evidence the radar response to soil moisture variations. A sensitivity analysis, performed over the whole acquired data set, confirmed that the dependence of backscattering coefficients on the incidence angle is influenced more by surface roughness than by soil moisture, as indicated also by theoretical models and other similar data sets. A significant result is that a simple model for comparing data acquired with different incidence angles works better for rough surfaces: only in this case an acceptable correlation between backscattering coefficients and soil moisture is retained. In order to better understand this behavior, the experiments carried out in 2001 were designed to acquire radar measurements on a test site where soil moisture was controlled with artificial irrigation and constantly monitored during the dry-down phase. This allowed a direct estimation of the relationship between radar responses and soil moisture, a quantitative evaluation of the sensitivity of our device and a test for the model developed using the previous acquisitions.

Retrieval of soil moisture profile by combined C-band scatterometer data and a surface hydrological model

The objective of this work is to develop a method to use radar scatterometer data and a hydrological model in order to retrieve soil behaviour at a level greater than C-band microwave penetration depth. For microwave measurements a C-band FM-CW scatterometer has been employed in two campaigns; the device is able to provide backscattering coefficients in the range of+10 dB and -40 dB for incidence angles between 10° and 60°. Subsequently, microwave scatterometer data have been analysed to estimate their sensitivity to the soil moisture patterns of topsoil comparing them with ground truth measurements. For the validation of these radar data, a coupled heat and moisture balance model has been run to predict the hydrological behaviour of the same topsoil starting from point ground truth measurements. In a second run, soil moisture values derived from scatterometer data should have been used for the initialisation of the model. First attempts have been carried out to propagate the surface physical parameters to unreachable soil layers, such as vertical soil moisture profiles.

Temporal monitoring of soil surface state by means of a 5.3 GHz ground-based scatterometer

This paper analyses eight different remote sensing campaigns carried out from 1998 to 2001, pointing out the backscattering coefficients behaviour in dependence both to soil moisture and roughness. Our study indicates a clear dependence of backscattering coefficients on soil moisture with an average sensitivity of 0.25 dB/gr/cm3. In a subsequent step these data sets are utilised to validate an inversion procedure based on a Bayesian algorithm aimed at extracting soil moisture information from backscattering coefficients. After a first run, a priori soil moisture information deriving from the simulation of a hydrological model is introduced leading to an improvement both in extracted soil moisture values and in their uncertainties.

Multifrequency polarimetric microwave scatterometer based on a vector network analyzer

In order to test a multi-frequency polarimetric scatterometer based on a Vector Network Analyzer, calibration measurements have been performed over point targets. Two trihedral corner reflectors with different dimensions have been employed. The radar cross sections have been measured at different frequency bands (L, C and X) and for different look angles between 23 degree(s) and 50 degree(s). Satisfactory results have been obtained in all three bands, however in the L-band the electromagnetic smog, due to mobile phones and airport radars, caused some difficulties in the extinction of the radiometric information. Other calibration tests have been planned before using the instrument as a ground-truth data acquisition device on the test-sites envisaged for the spaceborne SRTM and ENVISAT SAR missions.

C-band polarimetric scatterometer for soil studies

The aim of this study is to evaluate the performances of a polarimetric scatterometer. This sensor can measure the module of the electromagnetic backscattering matrix elements. The knowledge of this matrix permits the computation of all the possible polarisation combinations of transmitted and received signals through a Polarisation Synthesis approach. Scatterometer data are useful for monitoring a large number of soil physical parameters. In particular, the sensitivity of a C-band radar to different growing conditions of vegetation depends on the wave polarisation. As consequences, the possibility of acquiringi both polarisation components presents a great advantage in the vegetarian studies. In addition, this type of ground sensor can permit a fast coverage of the areas of interest. A first test of the polarimetric scatterometer has been performed over an asphalt surface, which has a well-known electromagnetic response. Moreover, a calibration procedure has been tested using both passive (Trihedral Corner Reflector, TCR) and active (Active Radar Calibrator, ARC) radar calibrator.

Sensitivity to soil moisture and roughness by a 5.3 GHz ground-based scatterometer

Six experiments on remote sensing of soil moisture and surface roughness were carried out over bare fields with a microwave scatterometer at a frequency of 5.3 GHz during February 1998 and January-April 2000. Other two experiments in May and June 2001 were conducted under controlled field condition for putting in evidence the radar response sensitivity to soil moisture. Data analysis indicates that a clear dependence of backscattering coefficients on soil moisture variations, with an average sensitivity of 0.25 dB/gr/cm3, when other parameters as roughness and incidence angle remain constant. Regarding surface roughness, a rougher field shows more suitable characteristics for inversion purposes. In fact, backscattering coefficients retain a good sensibility on soil moisture content, after the removal of incidence angle effects. These remote-sensing campaigns are part of an extensive activity where angular and polarization microwave signatures for airborne and ground based radars are collected on bare soils in different soil moisture and roughness conditions.

Microwave polarimetric scatterometer as a tool for monitoring topsoil moisture

Microwave remote sensing is a good tool for topsoil moisture monitoring due to the large difference between dielectric properties of dry soil and water. The aim of this work is to exploit microwave remote sensing techniques to collect data on soil water content of large areas rapidly and without direct soil samples analysis. To this purpose, a frequency modulated - continuous wave C-band microwave polarimetric radar has been built. The device has two transmitting channels that illuminate the soil with orthogonal linearly polarized electromagnetic waves. Two receiving channels detect the linearly polarized waves backscattered from the same target. This scatterometer can measure the module of the soil electromagnetic scattering matrix elements. The knowledge of this matrix permits the computation of all the possible polarization combinations of backscattering normalized Radar Cross Section (RCS) through a polarization synthesis approach. A Fourier analysis of this signal extracts the scattering matrix values of different-in-range resolution cells. The height and the incidence angle of this microwave sensor can be varied within large intervals; this allows the measurements of the RCS in various configurations that should give insights on soil moisture parameter extraction under different conditions.