Alternative blackbody configurations for infrared calibration of future GOES imagers and sounders

摘要

The Imagers and the Sounders on the present generation of GOES spacecraft each have a full-aperture blackbody for onboard infrared (IR) calibration. Each instrument interrupts its Earth-viewing operations at regular intervals and views this blackbody by rotating its scan mirror. IR Radiation from this blackbody follows the same path through the entire optical train as radiation from the Earth. The instrument also observes space to measure background generated by the instrument. The difference between signals from the blackbody and from space is used in conjunction with the measured temperature of the blackbody to compute the gain for each IR channel of the instrument. The GIFTS is an experimental, next-generation IR sounder with a Michelson interferometer. It performs on-board gain calibration by sequentially viewing two internal blackbodies that operate at different temperatures, measuring the difference between the resultant signals. A flip mirror is used to insert the blackbodies into the optical beam after the beam has been converged by the telescope. The apertures of the GIFTS blackbodies are much smaller than that of the telescope. This approach allows the blackbodies to have deep cavities and minimizes the time lag and the momentum disturbance in the calibration process. On the other hand, GIFTS is unable to directly measure long-term changes in the throughput of the optical elements that are not included in the calibration path: the scan mirror and two telescope mirrors. The GIFTS approach is capable of high radiometric precision, but requires augmentation by occasional end-to-end IR calibration measurements to achieve the required absolute accuracy over the lifetime of an operational GOES mission.