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To establish a relative scale against which all measurements can be compared, measurements are taken without any sample in the beam. The comparison with relative scale gives the “percent transmittance.” This technique produces a spectrum which eliminates all instrumental characteristics from the data. Thus, the spectrum seen is produced solely by the sample.
Sensor details
The detectors are housed in the IR-detector below the cooling unit. There are three detectors, each corresponding to three different spectral channels.
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The incoming light beams are splitted into different wavelengths and reflected into the IR-sensor and then the different semiconductors absorb them in their rispectable operating wavelengths. These portions of the reflected beam power electrons to move onto the the next conduction band, thus causing that particular semiconductor to produce a current. This current is then registered and analyzed.
Sensitivity
The sensitivity of the IMG varies according to the wavelength. Some differences in ground tests and on board tests were caused by unexpected changes in sensitivities. The longer wavelength limit was estimated to be 14.3 µm but it was extended to 16.6 µm, which might explain the differences in the tests. The change in sensitivity might have been caused by the lower temperature of the detector. Figure 4 shows the spectrum obtained from the IMG:
Range
The IMG is able to detect and monitor the vertical and spatial distribution of many greenhouse gases, such as CO2,CH4 and O3, through its spectrometer. Its wide range covers the infrared spectrum from 3.3 to 15 µm with a very high resolution of 0.1 cm-1.
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The detectors of the IMG are cooled to under 80 K with a Stirling-cycle cooler. The actual temperature may vary according to the thermal condition of the spacecraft. The vibrations caused by the cooler compressor are absorbed through a suspension mechanism. The expected lifetime of the cooler is 25000 hours, about 3 years.
Precision and Accuracy
The adjustment of the scanning mirror plays a large part in the accuracy of the results. A misalignment of the interferometer can be fixed by simply adjusting the angle of the mirror.This results in an accuracy of 0.0001 degrees over the entire scanning range.
The IMG also is able to measure the temperature and humidity in the troposphere with high accuracy. The retrieval accuracy has been found to be 1 K for temperature and water vapor concentration from 10 to 20%. These measurements are 2-3 times more accurate than satellite-borne radiometers used presently.
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Detection and monitoring of the vertical and spatial distribution of:
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