Pressure Modulated Infrared Radiometer (PMIRR)

Description

The Pressure Modulator Infrared Radiometer (PMIRR) is a multispectral scanning atmospheric sounder. Its primary scientific objectives are to: (1) map the three-dimensional, time-varying thermal structure of the atmosphere; (2) map atmospheric dust loading and its global, vertical, and temporal variation; (3) map the seasonal and spatial variation of the vertical distribution of atmospheric water vapor in the lower atmosphere; (4) identify and map the spatial and temporal variation of H2O and CO2 condensate clouds; (5) measure atmospheric pressure and monitor the seasonal and spatial variation of surface pressure; (6) monitor radiative balance in the polar regions; and, (7) derive surface temperatures, albedo, and thermal inertia.

PMIRR is designed to detect atmospheric and surface emission in the visible and thermal infrared to measure solar radiation reflected from the atmosphere and surface. It utilizes nine spectral channels, one broadband visible channel and eight thermal infrared channels from 6 to 50 micrometers. Two pressure (density) modulation cells are used to ensure high spectral discrimination in the 6.7 micrometer water vapor band and the 15 micrometer carbon dioxide bands. These cells are located in the optical paths of these channels. All nine channels share a common 2-axis scan mirror and telescope primary. The instrument is mounted on the bottom (nadir-pointing side) of the Mars Climate Orbiter. It has a passive radiative cooler which is opened at the start of the mapping orbit. The radiator is used to keep a cold focal plane assembly cooled to 80 K to ensure low noise. A solar target is mounted on the instrument to calibrate the visible channel.

Once Mars Climate Orbiter is in its mapping orbit and the focal plane assembly is cooled, PMIRR will begin its observations. The instrument takes observations in three fields-of-view, stepping from the limb to the surface (nadir) to allow construction of a three-dimensional map of atmospheric radiation. Nominally the aft limb is used, but the instrument is also capable of viewing out of the spacecraft orbit plane. This can be used near the polar regions, where PMIRR is capable of tracking out of the orbit plane to measure the bi-direction reflectance distribution function to estimate the polar surface albedo. Side limb viewing can also be used to acquire observations at different local times. The vertical resolution is 5 km, and the water vapor detection threshold at the surface is approximately 10^13/cubic centimeter, corresponding to an atmospheric column abundance of about 4 precipatable microns of water.