Microwave Limb Sounder
The UARS MLS instrument is described by Barath et al. [ J. Geophys. Res., vol. 98, pp. 10,751-10,762, 1993] and its calibration by Jarnot et al. [ J. Geophys. Res., vol. 101, pp. 9957-9982, 1995]; these papers can be consulted for more information than given in the brief summary here.
The figures above show a photo and sketch of UARS MLS. The instrument has three assemblies: sensor, spectrometer and power supply. Thermal control of the sensor is radiational by louvers, with in-orbit temperature stability of approximately 0.01oC or better, allowing "total power" measurements which do not require fast switching to a reference. The overall instrument mass is 280 kg, power consumption is 163 W fully-on, and data rate is 1250 bits/second.
A signal flow block diagram of the instrument is shown below.
A three-mirror antenna system receives thermal radiation from the atmospheric limb and is mechanically scanned to move its field-of-view through the atmospheric limb in the vertical plane. A limb scan is performed every 65.536 s (a UARS Engineering Major Frame, EMAF, or MLS Major Frame, MMAF) during normal operations, and covers limb tangent-points having an approximate height range between the surface and 90 km. "Oblateness" signals from UARS are used to account for variations in the direction to the limb due to Earth oblateness and variations in the UARS orbit. The MLS optics are diffraction-limited by the aperture of the primary mirror whose dimension is 1.6 meters in the vertical and 0.8 meters in the horizontal. A "switching mirror" accepts radiation from the scanning antenna system for atmospheric measurements, and from an internal ambient-temperature target or `cold' space for radiometric calibrations which are performed on each limb scan.
A dichroic plate following the switching mirror separates signals to the 63 GHz radiometer (for temperature and pressure measurements); a polarization grid then separates signals to the 183 GHz radiometer (for water vapor and ozone in the stratosphere and mesosphere) and to the 205 GHz radiometer (for stratospheric ClO and ozone - and the new products of sulfur dioxide injected into the stratosphere by volcanoes, lower stratospheric nitric acid, and upper tropospheric water vapor).
Ambient-temperature Schottky-diode mixer heterodyne radiometers down-convert the radiation to intermediate frequency (IF) bands in the range of 0-3 GHz; the local oscillator frequencies are 63.283, 184.777 and 203.267 GHz. The 63 GHz local oscillator is a phase-locked Gunn oscillator and is coupled to the atmospheric signals by waveguide. The 183 and 205 GHz local oscillators are generated by frequency-tripling signals from phase-locked Gunn oscillators operating near 60 GHz, and are combined quasi-optically with the atmospheric signals. The radiometers have approximately, but not exactly, equal responses at IF frequencies above and below the local oscillator. The radiometers operate at ambient temperature and have double-sideband noise temperatures (measured at the switching mirror input) of: 400K for the 63 GHz radiometer; 990K for the 205 GHz radiometer ClO band and 1530K for the O3 band; 1650K for the 183 GHz H2O band and 1820K for the O3 band. The 63 and 205 GHz radiometers are now in their sixth year of in-orbit operation with no degradations detected in their performance since they were constructed some 4-5 years before launch; the 183 GHz radiometer failed on 18 April 1993 after 19 months of successful in-orbit operation.
The IF signals, after amplification, are further frequency-converted to six spectral bands, each centered at 400 MHz with approximately 500 MHz instantaneous spectral bandwidth. These bands are input to six filter banks which split the signal into 15 separate filter channels, simultaneously measure the power in each of these channels, and digitize the resulting signals to pass along to UARS for transmission to the ground and data processing.
All measurements are, under normal operation, performed continuously day and night. The instrument integration time is approximately 2 seconds.
MLS looks in a direction which is 90o from the UARS orbital velocity, and the tangent point of the observation path (where the signals mostly originate) is 23 great-circle degrees away from the suborbital path of the satellite. The 57o inclination of the UARS orbit thus allows MLS to perform measurements from 34o on one side of the equator to 80o on the other. UARS performs a 180o `yaw maneuver' ten times per year, allowing MLS to alternate between views of northern and southern high latitudes at approximately 36-day intervals (this varies a few days through a yearly cycle). Local solar times at measurement locations do not vary appreciably with longitude on a given day, but can vary greately with latitude on a given day.
The figures below show a photograph of UARS during ground tests before launch, and a sketch of UARS in orbit; MLS is seen in these figures near the righthand side of UARS.