Use of External Sources for Calibration and Validation of L-band Missions

Abstract:

During the past 5 years two microwave remote sensing space borne missions have been launched at L-band (1.4GHz) responsible for making global soil moisture and sea-surface salinity measurements. The Soil Moisture Ocean Salinity (SMOS) mission was launched by ESA in November 2009 and the Aquarius mission was launched by NASA in June 2011. Another L-band mission the Soil Moisture Active Passive (SMAP) mission is slated to launch no earlier than January 2015. All of these missions are responsible for making accurate and precise measurements of global geophysical phenomenon. All missions have internal calibration sources and front-end losses that can be subject to drifts, offsets and glitches. In order to completely calibrate the brightness temperatures, external calibration sources such as modeled ocean brightness temperature and modeled galactic background (cold-sky) temperatures are used. Ocean brightness temperatures are a very stable source, but in some cases (such as Aquarius) it is undesirable to calibrate measured brightness temperatures to modeled brightness temperatures. Cold-sky brightness temperatures are extremely stable to 0.1K level. Cold-sky measurements though are infrequent in nature and are also impacted by back-lobe interference from the Earth that needs to be calibrated. In addition to noise-diode drifts, front-end losses, back-lobe interference there are other issues such as antenna spillover, antenna cross-pol contamination, RF back-end offsets that need to be considered when considering external calibration. These limitations put a requirement on the nature of additional calibration sources. In the following talk, we will present additional sources required for L-band calibration such as Antarctic ice-sheets, Amazonian rain-forests, cold-sky measurements with land crossings that need to be measured in order to back-out various inter-dependent calibration terms. We will present modeling issues of the rain-forest at L-band (due to transmissivity properties at low-frequencies) as well as suitability of the Antarctic regions for calibration. We will also discuss the suitability of the calibration sources with respect to absolute and relative calibration.