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High Resolution Dynamics Limb Sounder


HIRDLS is a multi-channel, infrared radiometer designed to measure radiated thermal emissions from the atmospheric limb at various spectral intervals in the range (6 to 17) micrometers, chosen to correspond to specific gases and atmospheric "windows". The final output will be a set of global 3-D fields of atmospheric temperature, several minor constituents and geostrophic winds.

The instrument consists of an array of twenty-one detectors placed in the focal plane of the telescope. Each detector element corresponds to an atmospheric field of 0.19° (10 km) horizontal by 0.019° (1 km) vertical. The detector array will be cooled to 65 K by a Stirling-cycle mechanical cooler. A rotating chopper placed at an intermediate focus of the telescope chops the incoming radiation against a reflected view to space. The telescope views the atmosphere via a two-axis tilting mirror which provides altitude and azimuth scanning. A three-axis gyro mounts on the optical bench to provide relative pointing knowledge with high accuracy.

In-flight radiance calibration is checked at frequent intervals by tilting the scan mirror to view cold space (just above the atmospheric limb), and an internal blackbody reference target of accurately known temperature. Routine instrument functions, including scanning, in-flight calibration, thermostat heaters and a moveable sunshade will be controlled by stored programs in an on-board microprocessor. The entire instrument is packaged as a single module. Clear views to rearward (opposite the direction of the velocity vector), are needed for the optical beam. The instrument heat will be dissipated by radiation to space.