The soil moisture active passive experiments (SMAPEx): toward soil moisture retrieval from the SMAP mission

Abstract

NASA’s Soil Moisture Active Passive (SMAP) mission will carry the first combined spaceborne L-band radiometer and Synthetic Aperture Radar (SAR) system with the objective of mapping near-surface soil moisture and freeze/thaw state globally every 2–3 days. SMAP will provide three soil moisture products: i) high-resolution from radar (∼3 km), ii) low-resolution from radiometer (∼36 km), and iii) intermediate-resolution from the fusion of radar and radiometer (∼9 km). The Soil Moisture Active Passive Experiments (SMAPEx) are a series of three airborne field experiments designed to provide prototype SMAP data for the development and validation of soil moisture retrieval algo- rithms applicable to the SMAP mission. This paper describes the SMAPEx sampling strategy and presents an overview of the data collected during the three experiments: SMAPEx-1 (July 5–10, 2010), SMAPEx-2 (December 4–8, 2010) and SMAPEx-3 (September 5–23, 2011). The SMAPEx experiments were con- ducted in a semi-arid agricultural and grazing area located in southeastern Australia, timed so as to acquire data over a seasonal cycle at various stages of the crop growth. Airborne L-band brightness temperature (∼1 km) and radar backscatter (∼10 m) observations were collected over an area the size of a single SMAP footprint (38 km×36 km at 35◦latitude) with a 2–3 days revisit time, providing SMAP-like data for testing of radiometer-only, radar-only and combined radiometer-radar soil moisture retrieval and downscaling algorithms. Airborne observations were sup- ported by continuous monitoring of near-surface (0–5 cm) soil moisture along with intensive ground monitoring of soil moisture, soil temperature, vegetation biomass and structure, and surface roughness.