This was the first of the space instruments designed at Oxford. A nadir sounder, it used a "chopping" technique in which IR radiation entering the instrument, whether from the atmosphere or from the calibration views, was switched between two cells at a frequency of 10Hz. The gas cells contained CO2 at different pressures, which allowed different amounts of radiation to reach the detector. The detector, a thermistor bolometer, then measured this difference signal. The chief limitation of the instrument's performance (given the available technology) was the difficulty in maintaining a thermal balance within the instrument.
Also a nadir sounder, this was a development of the Nimbus 4 SCR. In this case the cells containing the different amounts of CO2 were switched into a single optical path between the atmosphere and the detector (frequency 1Hz). The signals were measured by a pyroelectric detector and the difference extracted on the ground. Some extra channels were added to increase the height range, to measure the temperature of the surface and cloud tops, and to sample water vapour concentrations. Performance was limited by gas leakage with time and contamination of the cell windows.
The PMR represented a departure from the earlier SCR design. Although still a nadir-sounder using selective chopping to produce a differenced signal, it employed only one gas cell to achieve this. Chopping was effected by modulating the pressure of the gas (~15Hz) rather than by changing the optical path, removing problems associated with balancing two optical paths. It also allowed measurements to be made at much lower pressures, extending the height range of the instrument to ~90km.
This was the final radiometer in the Nimbus series, subsequent. SAMS also used
the pressure modulation technique but was a limb-viewing instrument. This
improved the vertical resolution of the instrument, with some sacrifice of
horizontal resolution. In addition to CO2-filled cells (for temperature
sounding) SAMS also comprised cells filled with other gases, allowing several
atmospheric minor constituents to be observed. Global atmospheric
measurements were made using the following channels:
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