Pressure Modulator Infrared Radiometer (PMIRR)
The Pressure Modulator Infrared Radiometer (PMIRR) was designed to make observations of the Martian atmosphere. Its objectives were to: (1) map the three-dimensional, time-varying thermal structure of the atmosphere from the surface to an altitude of 80 km; (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 (<= 30 km); (4) identify and map the spatial and temporal variation of H2O and CO2 condensates; (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.
To accomplish these goals, the PMIRR utilized narrow-band radiometric channels and two pressure modulation cells to measure atmospheric and surface emission in the thermal infrared and a visible channel to measure solar radiation reflected from the atmosphere and surface. The instrument consisted of a 6.0 cm diameter, f/11.9 Gregorian telescope with a total field-of-view of 1.70 x 0.95 degrees. Infrared radiation was measured in eight spectral intervals between 6--50 micrometers. All nine channels shared a common scan mirror and telescope primary and alternately viewed Mars and space at 800 Hz with a rotating double-sided mirror chopper.
The signal for three of the channels (1--3) was passed through a pressure modulator cell containing CO2 and for one channel (4) through a cell containing H2O. These cells were located in the optical paths of these channels and imposed a 50 Hz amplitude modulation on the chopped signal. Channels 1--5 used cooled HgCdTe photon detectors, while the remaining channels used ambient temperature deuterated triglycine sulfate pyroelectric detectors. Channels 6 and 7 each had two detectors, thus two fields of view; all others just one. Offset calibration for all channels was provided by the view to space, which is an excellent radiometric zero. Gain calibration for the thermal channels (1--8) was accomplished with an internal 300 K blackbody and for the solar channel (9) by observing an external diffusively scattering target, specifically an electric arc-sprayed aluminum disc which filled the instrument aperture.
Contact with Mars Observer was lost for unknown reasons on August 21, 1993, three days before scheduled orbit insertion, so no data were returned for this investigation. This experiment has been re-scheduled to fly on the Mars 98 Orbiter.