Instrument Science and Calibration

Contact Robert Jarnot

Good Aura MLS science quality is dependent upon accurate calibration knowledge in three key areas: radiometric, spectral and field-of-view responses. To achieve this required careful attention to instrument design and fabrication, careful pre-launch performance verification, and comprehensive pre-launch calibrations.

In-orbit tests are performed periodically to monitor such important calibration data as the radiance differences between cold space views via the Switching Mirror and Antenna. In over 5 years of operation these tests have shown remarkable stability, with overall changes typically smaller than 20 mK in all bands.

Another valuable in-orbit calibration has come from the brief opportunities during which MLS can scan across the Moon. These measurements have sometimes required coordinated spacecraft maneuvers to allow the relatively narrow MLS field-of-views to scan across the appropriate portion of the lunar disk, and have significantly lowered the uncertainties on the pre-launch measurements of boresight (pointing) differences between the various radiometers, as well as indicated that the shapes of the antenna FOV's have remained stable.

Another area in which in-orbit data has added to our knowledge of measurement accuracy is in the area of standing waves on radiometric calibration views. Tracking (as opposed to scanning) the lunar disk has provided quantitative information on the impact of standing waves on radiometric calibration accuracy that was not available from pre-launch calibration data.

Signal levels seen when observing cold space via the main GHz antenna indicate no significant changes to the reflectivity or emissivity of the 3 antenna mirrors. The observed positions of atmospheric spectral lines as seen by the 30+ spectrometers on MLS indicate the absence of frequency drifts within the instrument.

The one area in which signals have been changing for the duration of the mission is that the signal powers to the nineteen 25-channel spectrometers have steadily decreased. These decreases arise from radiation effects on low-dropout voltage regulators that were added to the spectrometers late in the instrument fabrication phase.

The dropping signal levels have been periodically compensated by changes to programmable attenuator settings, and the regular in-orbit radiometric calibration sequences completely compensate for these small drifts. Extrapolating the declining signal levels indicates that these degradations are unlikely to be a life-limiting factor for the data from any measurement band.