The GPM Common Calibrated Brightness Temperature Product

The Global Precipitation Measurement (GPM) project will provide a core satellite carrying the GPM Microwave Imager (GMI) and will use microwave observations from a constellation of other satellites. Each partner with a satellite in the constellation will have a calibration that meets their own requirements and will decide on the format to archive their brightness temperature (Tb) record in GPM. However, GPM multi-sensor precipitation algorithms need to input intercalibrated Tb's in order to avoid differences among sensors introducing artifacts into the longer term climate record of precipitation. The GPM Common Calibrated Brightness Temperature Product is intended to address this problem by providing intercalibrated Tb data, called "Tc" data, where the "c" stands for common. The precipitation algorithms require a Tc file format that is both generic and flexible enough to accommodate the different passive microwave instruments. The format provides detailed information on the processing history in order to allow future researchers to have a record of what was done. The format is simple, including the main items of scan time, latitude, longitude, incidence angle, sun glint angle, and Tc. It also provides a quality flag, spacecraft orientation, spacecraft location, orbit, and instrument scan type (cross-track or conical). Another simplification is to store data in real numbers, avoiding the ambiguity of scaled data. Finally, units and descriptions will be provided in the product. The format is built on the concept of a swath, which is a series of scans that have common geolocation and common scan geometry. Scan geometry includes pixels per scan, sensor orientation, scan type, and incidence angles. The format includes 3 space saving methods: first rounding variables written as floats to their needed accuracy to achieve good compression, second writing sun glint angle as a one byte variable, and third storing only unique incidence angles but allowing access via a mapping array. Two programs are used to create a Tc product. Program One reads the input data which may be antenna temperature (Ta) or Tb and could be an orbit with an arbitrary starting point or a half orbit. Program One then writes a "base" file which is a standard orbit and written in a base format which preserves all the information from the input. Program Two reads the base file, calibrates the radiance information, and outputs Tc in the standard format. The use of two programs allows researchers to insert experimental calibrations in Program Two without having to read the input to Program One or orbitize the data. The Tc algorithms and data formats are being tested using the pre-GPM Precipitation Processing System (PPS) software to generate formats and I/O routines. In the test, data from SSM/I, SSMI/S, TMI, AMSR-E, and WindSat are being processed and written as Tc products.