Towards validation of SMOS using airborne and ground data over the Murrumbidgee Catchment

Abstract

With the launch of the European Space Agency's Soil Moisture and Ocean Salinity (SMOS) satellite scheduled for mid 2009, the first long-term space-borne passive microwave observations at L-band (∼ 1.4 GHz) will soon be available. Consequently, SMOS will be the first mission dedicated to global mapping of near-real-time surface soil moisture information. Though space-borne microwave instruments have measured global data at high frequencies (e.g. C- and X-band) for the last 20 years, this innovative L-band radiometer will use a new synthetic aperture concept that will provide observations at multiple incidence angles. Consequently, the observed brightness temperature data and derived soil moisture product must be validated. To achieve this, intensive field campaigns are being planned world-wide to support the satellite mission with reliable data from i) passive microwave airborne observations at L-band, ii) detailed ground measurements of surface soil moisture content and associated environmental parameters, and iii) long-term soil moisture monitoring network data from anchor sites (e.g. Murrumbidgee in Australia, Valencia in Spain, Upper Danube in Germany etc.). With the SMOS launch likely to take place in the later part of 2009, Australia is particularly well positioned for conducting the first intensive SMOS validation campaign during its spring. The Australian Airborne Cal/val Experiment for SMOS (AACES) will provide one of the most comprehensive assessments world-wide, due to its combined airborne and in-situ data collection strategy across an extensive transect of the Murrumbidgee catchment in south-eastern Australia. This area is unique as it comprises a distinct variety of topographic, climatic and land cover characteristics, and therefore represents an excellent validation site for the land component of this satellite mission. Moreover, a large database of previous campaign measurements, continuous soil moisture monitoring stations, and meteorological data over the past seven years is available for this region. A total of four airborne campaigns are planned to cover a 100 km x 500 km (more than 20 SMOS pixels) transect of the Murrumbidgee catchment in its entirety at 1 km resolution using an L-band radiometer. The primary airborne instruments will include the Polarimetric L-band Multibeam Radiometer (PLMR) and thermal infrared sensors, supported by surface soil moisture content, soil temperature and rainfall data from the Murrumbidgee monitoring network. This will be further complemented by intensive soil moisture observations with the Hydraprobe Data Acquisition System (HDAS), short-term soil moisture and temperature monitoring stations with additional leaf wetness and thermal infrared measurements, and extensive vegetation characterisation. The four separate month-long campaigns are planned to extend across a two year timeframe, enabling the effects of seasonal variation in vegetation condition and land cover change to be assessed in addition to soil moisture. Consequently, issues related to snow cover, litter, vegetation dynamics etc. will be assessed in relation to soil moisture retrieval. This paper outlines the airborne campaigns and related ground monitoring for the first SMOS validation campaign, together with some of the major science questions to be addressed. Persons interested in participating in these campaigns are encouraged to contact the authors