Identification of spaceborne microwave radiometer calibration sites for satellite missions

The first dedicated soil moisture satellite mission will be the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) mission. This satellite, scheduled for launch in the second half of 2009, has a new type of satellite design that is based on the radio-astronomy technique of simulating a large antenna from a number of smaller ones placed some distance apart. Because of its unique design and the fact it is sensing in a currently unutilized frequency range makes it critical that on-orbit calibration targets be included in the calibration strategy. Consequently, targets such as the Antarctic, cold oceans, tropical forests and deserts are being considered. However, the large footprint size of passive microwave observations means that large scale homogeneous regions must be identified for calibration purposes. Moreover, these sites must also be either stable through time or the temporal variation easily described by models. In order to satisfy the calibration accuracy required by SMOS for soil moisture retrieval, such sites should be characterized with a brightness temperature uncertainty of less than 4K. A field experiment has been undertaken in November 2008 in the Australian Arid Zone to explore the suitability of three potential on-orbit calibration targets for SMOS. These sites were chosen for their assumed spatial homogeneity in terms of surface conditions (soil moisture and temperature, vegetation, soil type etc.), and consequently their expected microwave response. Each site covers an area of approximately 50km x 50km, being the approximate size of a satellite footprint. These sites include i) Wirringula Hills, a station to the north-east of Coober Pedy that is characterized by a dense cover of gibber; ii) Lake Eyre, characterized by a predominantly moist material under a layer of salt crust; and iii) Simpson Desert, characterized by sand dunes orientated in a north-south direction. The data collected during this field campaign consists of both airborne and ground-based measurements. The airborne data includes passive microwave emissions obtained with an L-band airborne radiometer, thermal infrared observations, and optical data. The ground data collection consisted of surface soil moisture measurements at targeted locations along sections of the high-resolution flight tracks, and station measurements of soil moisture and temperature profiles to a depth of 40cm. Additionally, there were soil core samples to a depth of 2m, surface characterization and surface roughness measurements. The airborne data were collected at two different resolutions, 1km and 50m. This paper presents the results from airborne observations made during this campaign, and discusses their significance in relation to the calibration of SMOS. Of the three study sites assessed, Wirringula Hills appears to be the most promising, having a spatial variability in brightness temperatures of less than 4K at H polarisation. In comparison, the Simpson Desert had a spatial variability of about 10K, and the moist region of Lake Eyre had a spatial variability of about 13K. Moreover, Lake Eyre was found to also have considerable spatial heterogeneity, making it unsuitable for calibration at the spatial resolution of SMOS.