Thermal measurements in the framework of SPARC

In this paper, a description of the thermal radiometric measurements carried out in the framework of the Spectra Barrax Campaign (SPARC) is presented. The purpose of these measurements is to retrieve biogeophysical parameters such as land surface emissivity and temperature to analyze the relationship of these parameters with fluorescence. The in situ data base was used to validate high and low resolution sensors such as Airborne Hyperspectral Scanner (AHS), Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER), Spinning Enhanced Visible and Infrared Imager (SEVIRI) and Advanced Along-Track Scanning Radiometer (AATSR), and a synergistic study was firstly developed between SEVIRI and ASTER and secondly between AATSR and CHRIS/PROBA (Compact High Resolution Imaging Spectrometer/ Project for On Board Autonomy). The thermal measurements were carried out basically using two multiband field radiometers (CIMEL CE 312-1 and 312-2) and several single band field radiometers (RAYTEK ST6, RAYTEK MID and EVEREST 3000.4ZLC), pointing at different targets (soil, vegetation, sky). Transects, angular and emissivity measurements were carried out and the results show the importance of thermal measurements to characterize the different surfaces over the test area. 1. REGION OF STUDY AND INSTRUMENTS The measurements campaign was developed in the Barrax field site, located in the south of Spain approximately 20 km away from Albacete (39°3’ N, 2°6’ W, 700m elevation). Fig.1 shows the different test parcels analysed. Radiometric measurements were carried out in the thermal infrared region with several instruments that include fixed FOV and single band or multi bands radiometers. In addition, thermocouples for thermometric measurements, a thermal camera, an emissivity box, a goniometric system and two black bodies (calibration sources) for calibration purposes were used. Details of the instruments are given in Fig.2. WATER GRASS BARE SOIL CUT WHEAT CORN ALFALFA GARLIC POTATOES VINEYARD Fig.1. Test parcels analysed during the field campaign. 2. MEASUREMENTS PROTOCOL Prior to the radiometric measurements, a calibration was carried out to compare each instrument with the reference blackbody sources and the thermocouple. 2.1 Transects Transects were taken over several samples surfaces. They were performed concurrently to the satellites and airborne flights over the studied samples. They were carried out taking temperature measurements with field radiometers, at regular steps (3 meters) along a walk performed within a well defined area. The transects were made half an hour before the scheduled satellite overpasses and ended half an hour later, and we made sure that the sky was clear while taking measurements. 2.2 Masts Thermal measurements were continuously recorded with radiometers located on fixed masts over determined areas (vineyard, bare soil and grass) and periods of time, in coincidence with the airborne/satellite overpasses. 2.3 Angular measurements A study was realized about the angular variation of the infrared brightness temperature of some representative samples at angles between 0o and 60o (at 10o increments). Several series of measurements for each sample were taken. Before and after every series, we measured the sky temperature pointing to the zenith. The two CIMEL radiometers were mounted together and were used simultaneously with the thermal camera to measure radiometric temperature over green grass and bare soil, in the solar and perpendicular plane and at day and night time under different observation angles. 2.4 Emissivity measurements: TES method The emissivity of representative samples was measured by means of the TES (temperature and emissivity separation) method [1] by pointing alternatively to the sample and the zenith using the CIMEL 312-2 radiometer in the Barrax test site. 2.5 Emissivity measurements: Box method The emissivity of representative samples was measured by means of the Box method [2]. The box allows the retrieval of surface emissivity from three different measurements; one with a cold lid (whose emissivity is e=0), another one with a hot lid (e=1), and the last is a measure of the temperature of the hot lid. Three different radiometers were used, namely CIMEL 312-1, EVEREST and RAYTEK MID. 2.6 Night campaign During the night of July, 19, angular and emissivity measurements were carried out over green grass and bare soil. CIMEL CE 312-1 8-13 μm 8.2-9.2 μm 10.3-11.3 μm 11.5-12.5 μm -80 to 60 oC ±0.1 oC FOV=10o CIMEL CE 312-2 8-13 μm 11-11.7 μm 10.3-11 μm 8.9-9.3 μm 8.5-8.9 μm 8.1-8.5 μm -80 to 60 oC ±0.1 oC FOV=10o RAYTEK ST6 8-14μm -30 to 100 oC ±0.5 oC FOV: 8o