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Airborne Scanning Microwave Limb Sounder

Table 1. The designed A-SMALS performance parameters
Antenna aperture
20 cm
- Required for 2 km vertical resolution at 200km
Azimuth scan period
Elevation scan period
47 s
2 s
- Dictated by 10 km horizontal resolution and 12.6 km/minute air speed
Azimuth scan range
Elevation scan range

± 30°

- Provides 200 km horizontal scan at tangent point
- Provides 10 km vertical scan for allowing for 1 degree aircraft pitch variation
Vertical scans per horizontal scan
- Dictated by 10 km horizontal resolution adn 200 km scan width
Integration time
100 ms
Provides 1 km vertical resolution

Figure 1. A-SMLS block diagram
A-SMLS block diagram

Figure 2. The suite of A-SMLS measurements. In addition, temperature, pressure and cloud ice are also measured. For A-SMLS, measurements in white boxes are ‘standard’, others represent additional potential capability. Bars give the measurement time needed for an upper tropospheric spectral line radiance signal-to-noise of 10 for various situations as indicated by the legend. Both typical and enhanced values are shown, along with polluted boundary layer abundances that may be convectively transported into the upper troposphere, including soluble species that may be transported less readily. Vertical lines indicate horizontal resolution equivalent to the measurement time for A-SMLS. This figure is for a tropical ‘background’ atmosphere and 9 km height.
The suite of A-SMLS measurements

Figure 3. The A-SMLS configuration in the wing pod of the WB-57 aircraft
The A-SMLS configuration in the wing pod of the WB-57 aircraft

The National Research Council decadal survey for earth science identified the need for a Global Atmospheric Composition Mission (GACM) to address crucial issues on how changes in atmospheric composition affect the quality and well-being of life on earth. The baseline GACM instrument suite comprises UV/Vis and IR/SWIR spectrometers and an advanced microwave limb sounder working together to retrieve atmospheric composition worldwide with high spatial resolution. The Scanning Microwave Limb Sounder (SMLS) is designed to meet the measurement requirements of GACM by providing complete orbit-to-orbit retrieval of O3, N2O, temperature, water vapor, CO, HNO3, ClO, and volcanic SO2 in the upper troposphere and lower stratosphere. Unlike previous MLS instruments that only scanned the limb vertically leaving large orbit to orbit gaps, SMLS will simultaneously scan both in azimuth and elevation providing complete global coverage with 6 or more repeat measurements per day. SMLS will employ extremely sensitive, broadband, sideband-separating, SIS receivers centered at 230 and 640 GHz that provide the same precision as those on Aura MLS with a 100 fold reduction in integration time. SMLS will use a novel antenna design that provides high vertical resolution and enables rapid horizontal scanning of the field of view.

Since the late summer 2008, the development of the SMLS instrument technology has been underway within NASA Earth Science Technology Office’s Instrument Incubator Program. The objective of this development is to advance the core signal path technologies required for a microwave limb sounder with the capability to map the composition of the upper troposphere and stratosphere with 50x50x1 km spatial sampling and six times daily mid-latitude repeat coverage. The specific goals of this effort include:

the mitigation of the optics and calibration risks of the SMLS flight sensor design by constructing and testing an airborne prototype of the SMLS sensor and calibration system - A-SMLS - using prototype sideband-separating mixers, line sources, and advanced spectrometers and calibration targets;

the mitigation of the development risks of the cryogenics system by developing a flight-like cryostat and demonstrating an end-to-end prototype of the SMLS signal path from the antenna interface through the back-end electronics, and quantifying its stability, calibration accuracy, linearity, and sensitivity; and

the demonstration of the potential science measurement capability of SMLS through the A-SMLS science flights.

Figure 1 shows a block diagram of the signal path for A-SMLS, and Table 1 shows the expected A-SMLS performance parameters. Thermal emission from the limb is reflected onto a primary reflector by the scan mirror. The primary mirror has a projected aperture of 20 cm and a 20-dB edge taper in order to achieve a FWHM beam divergence of 10 milliradians and the corresponding vertical resolution of 2 km at a location 200 km in front of the aircraft. The primary mirror focuses the beam to a waist at the calibration mirror, allowing the system to view either a hot or cold load for calibration. The hot and cold load temperatures will be maintained by heaters and cryogens respectively. A polarizer splits the beam between Receiver 1 (R1, a Superconductor-Insulator-Superconductor (SIS) receiver) and Receiver 2 (R2, a more traditional double-sideband receiver). The 1st IF signals in R1 are demultiplexed, downconverted, and routed to seven polyphase spectrometers for analysis of different gaseous species over the spectral range from 225 to 234 GHz. The signals routed through R2 is for the monitoring of the 183 GHz water line. The suite of tropospheric products A-SMLS is capable of measuring are listed in Figure 2.

A-SMLS is planned to fly on the WB-57 aircraft at altitudes of 15 to 17 km to obtain a limb view covering 200 km of cross-track scan. It will be positioned in the WB-57 wing pod in order to obtain an unobstructed forward view. The fore-optics will be supported by a bulkhead attached to the WB57 Wing pod nose cone/pod interface. This A-SMLS configuration on WB-57 aircraft is graphically illustrated in Figure 3.

The A-SMLS instrument is planned to be completed in the later part of 2010, leading to the first balloon-borne engineering testing and science demonstration experiment in October 2010. The WB-57 integration and calibration is scheduled for the beginning of 2011, leading to the engineering checkout flight and science demonstration flight in the spring and early summer of 2011, respectively.

For more information on A-SMLS, please contact Paul Stek.

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