CALIBRATION OF MICAP L/C/K BAND INTERFEROMETRIC RADIOMETERS DURING IN-ORBIT COMMISSIONING PHASE

H. Liu¹, D. Han¹, C. Zhang¹, L. Niu¹, X. Tong¹, T. Wang¹, B. Li^2,1

¹National Space Science Center, Chinese Academy of Sciences, ²University of Chinese Academy of Sciences

COSM (Chinese Ocean Salinity Mission), the first Chinese mission dedicated for ocean salinity measurement, was successfully launched by CZ-4B carrier rocket from Taiyuan Satellite Launch Center in north China’s Shanxi Province, on Nov.14, 2004. COSM takes two payloads on board, namely LASMR (2-D L-band Aperture Synthesis Microwave Radiometer) and MICAP (Microwave Imager Combined Active and Passive). LASMR is an L-band two-dimensional synthetic aperture radiometer with Y-shape array, similar to SMOS/MIRAS instrument but with 56 dual-polarization antenna units and receivers whose least spacing is 0.82 wavelength. MICAP is a new type of multi-frequency active/passive payload proposed by NSSC, which consists of three one-dimensional synthetic aperture radiometers working at L/C/K band (1.4/6.9/18.7GHz) with 12/14/14 dual-polarization antenna feeds respectively (least spacing of each band is 0.618 wavelength), and an L-band (1.25GHz) digital beamforming scatterometer with 10 dual-polarization antenna feeds. All the feeds share a 3m × 5.5m solid deployable parabolic cylinder reflector made by carbon fiber, forming fan-beam shape secondary antenna pattern covering swath above 950km and same footprint projection geometry on ground. The designed performance specifications of L-band radiometer are 0.15K NeDT, 0.12K stability, and 50km×75km on ground boresight resolution, while C/K band target on 0.3K NeDT and 25 km×15km boresight resolution. The commissioning phase of COSM lasted about 10-month.

It is the first time that 1D/Multi-frequency interferometric radiometers have been put into operational in space. The multi-frequency feature of MICAP enables joint measurement on multiple physical parameters over ocean, including not only sea surface salinity, but also sea surface wind and sea surface temperature, which has been identified as a key feature of next generation SSS mission. It also brings potential benefits on soil moisture application and other cryosphere applications.

Detailed introduction of calibration, brightness temperature (TB) reconstruction and in-orbit performances of MICAP L/C/K band radiometers will be introduced in this paper, including:

1. The total power calibration of each single receiving channel (TA Calibration) is achieved by periodical dicky switching of the receiver inputs to match loads and internal noise diodes. External calibration with cold sky maneuver has also been carried out to mitigate the uncertainties of internal noise diodes.

2. The calibration of inter-channel cross-correlation outputs (Baseline Calibration) is achieved by dicky switching to correlated noise injection. All the correlation efficiency (or correlator complex gain) specifications show very good stability, which is less than 0.1% variation in amplitude and 0.3° variation in phase. So far, only less than 10 percent observation time has been used for calibration, TA and Baseline Calibration in total, thanks to the excellent thermal stability of the radiometer receivers achieved in-orbit.

3. A differential TB reconstruction approach has been used for MICAP 1D interferometric radiometers, utilizing dedicated calm ocean surface as reference strictly selected with constrain of localization and ocean/atmosphere/ionosphere states. This approach largely reduces the effects of element antenna pattern’s uncertainty and non-consistency on TB image reconstruction.

4. In commissioning phase, the tested results of MICAP instrument specifications have fulfilled all the mission requirements. As a new payload, MICAP features with its joint multi-frequency active/passive observation capability, in-orbit RFI detection & mitigation capability of L-band, and world-leading spatial resolution specification (15×25 km) of C-band. The in-orbit performance of MICAP instrument during commissioning phase ensures promising application over ocean, land and cryosphere.