Improved Calibration of Space-based Passive Microwave Cross-track Sounders

A suite of sensors scheduled to fly onboard the NPOESS Preparatory Project (NPP) satellite in 2011 will both continue and improve the environmental data records provided by operational and research missions over the last 40 years. The Cross-track Infrared and Microwave Sounding Suite (CrIMSS), consisting of the Cross-track Infrared Sounder (CrIS) and the first space-based, Nyquist-sampled cross-track microwave sounder, the Advanced Technology Microwave Sounder (ATMS), will provide atmospheric vertical profile information needed to improve numerical weather and climate modeling. A critical aspect of the CrIMSS success will be the calibration of the ATMS brightness temperatures, i.e., sensor data records. The complete calibration of passive microwave sounders starts with a full prelaunch characterization and continues throughout the intensive calibration and validation period. This presentation will describe the complete calibration procedure with emphasis on calibrating the NPP ATMS sensor data records.

Pre-launch testing of ATMS has characterized the principal calibration parameters and has enabled predictions of on-orbit performance with high levels of confidence. The prelaunch radiometric calibration of ATMS consists of using a Compact Antenna Test Range (CATR); thermal vacuum chamber; vibration testing; and electromagnetic and radio frequency interference testing. A more limited performance assessment of ATMS is planned during the spacecraft environmental testing.

Postlaunch calibration and validation consists of four phases: activation, functional evaluation and optimization, Intensive Cal/Val (ICV), and long-term monitoring. The ICV will end approximately 180 days after launch. This paper will describe the various calibration and validation tasks in the four phases and the team responsible. Some of the tasks include optimal space view selection; geolocation accuracy evaluation; RFI evaluation and mitigation; simultaneous nadir overpasses of other microwave sensors; on-orbit spacecraft maneuvers; simulation comparisons with radiosondes and numerical weather prediction models; and aircraft underflights.

We review an approach for on-orbit FOV calibration of the ATMS satellite instrument using vicarious calibration sources with high spatial frequency content (the Earth's limb, for example). The antenna beam is slowly swept across the target of interest and a constrained deconvolution approach is used to recover antenna pattern anomalies. The Earth's limb can also be exploited as an additional radiometric calibration source.