Observations throughout most of the infrared wavelength range are impossible from ground-based observatories due to absorption by water vapor in the troposphere. Although SOFIA usually flies above the moist troposphere, a measurable amount of water vapor is present in the stratosphere at densities of approximately 2 to 10 parts per million. This translates into a typical precipitable water vapor level of about 7 microns. This residual water vapor overburden is sufficient to have a noticeable effect on most far-infrared observations (e.g. broadband photometry or spectroscopy of lines blended with strong water vapor absorption lines). The effects can be estimated by running ATRAN to compute the atmospheric throughput over the wavelength range of interest.
Variations in Overburden
The water vapor burden at altitude for use in the calibration is affected by the season, latitude, the jet stream, and somewhat by local weather conditions at lower altitudes. In general, the tropopause is higher in the local summer, and higher water vapor burdens may be encountered. This is also true in the tropics throughout the year. In temperate latitudes, the zenith water vapor overburden at observing altitudes is typically 5 to 15 µm, but may exceed 20 µm if the aircraft is not above the tropopause. At 41,000 feet or higher, the WV overburden occasionally drops below 5 µm. The distribution of stratospheric water vapor can be quite variable, and the observed zenith burden may change as much as a factor of three on time scales as short as 15 minutes. During flight, the current observed water vapor value appears in the housekeeping video display, updated every 15 seconds. These measurements are also available after each flight, as plots from the housekeeping data file for the flight.
Atmospheric water vapor measurements will be obtained with an infrared radiometry system developed by Dr. Thomas Roellig (NASA/ARC). The sensor is a heterodyne mixer configured for the measurement of the 183 GHz rotational line of water. The Water Vapor Monitor (WVM) is mounted at a fixed elevation of 40° in the upper deck of the aircraft and is responsible for measuring the integrated water vapor while the SOFIA aircraft is at normal operational altitudes. These data are used to correct the astronomical infrared data obtained by the telescope and will also be used in the algorithm that determines successful SOFIA observatory flight hours for contractual purposes. The WVM reports its measured water vapor overburden to the aircraft Mission Controls and Communication System (MCCS) once every 15 seconds while the SOFIA observatory is in normal operation at altitude.
Principle of Operations
The WVM determines water vapor overburden by comparing radiometric measurements of the center and wings of the 183.3 GHz rotational line of water observed in the atmosphere to atmospheric models.
Since in practice the aircraft will not be flying perfectly level all the time, the WVM measurements must be corrected for the true aircraft roll and pitch angles during the measurements. The aircraft roll and pitch angles are provided to the WVM by the aircraft flight system autopilot, which can measure these parameters to better than 0.1°.
The Radiometer Head contains an antenna that views the sky, two calibrated reference targets (one heated and one ambient temperature microwave blackbody), an RF switch, a mixer, a local oscillator, an IF amplifier, and an inclinometer. All of these components are mounted together on a baseplate and are attached to the inner surface of the aircraft fuselage, so that the antenna can observe the sky through a microwave-transparent window. The antenna itself employs a quasi-optical design with a microwave lens that feeds a feed horn. The beam pattern is Gaussian with a full-width-half-maximum diameter of 0.87°. A sub-harmonic mixer mounted directly behind the feed horn mixes the 183.3 GHz radiation down to a bandwidth of 1 GHz with the radiometer operating in double sideband mode. The sub-harmonic mixer is fed by a phase-locked 91.65 GHz local oscillator. The intermediate frequency signal out of the mixer is amplified by an RF amplifier with a bandwidth of 100kHz - 500MHz before it is sent on to the IF Converter Box.
Since the WVM operates as a radiometer, accurate gain stability is important. In order to achieve this stability the two reference blackbodies are used to periodically insert a stable signal into the radiometer. Small motors rotate mirrors that direct images of the black body targets into the feedhorn field-of-view once every 5 seconds. One mirror, the Sky/Calibrator Mirror directs the view of the radiometer between the sky and the black body calibrators. The second mirror, the Hot/Ambient Mirror, selects between the hot and ambient temperature black bodies. Therefore, over a 15 second period the radiometer views the sky for 4 seconds, an ambient temperature black body target for 4 seconds, and a heated black body target for 4 seconds. One second is allowed to move the mirrors at each viewing position. The temperatures of the two black body targets are measured with temperature sensors.
Typical Post-flight Results
A typical zenith water vapor plot for a KAO flight is shown in figure below. The plot is usually provided with temperature and altitude data as well.