Larry L. Gordley

We present a new approach for measuring high-altitude winds and temperatures. We describe how gas filter correlation radiometry can be employed from low Earth orbit to simultaneously measure the Doppler shift and linewidth of emission spectra to infer both wind and kinetic temperature. Measurements are performed with a simple radiometer that images the limb emission of nitric oxide near 5.3 μm (to sense altitudes of >90 km and <50 km) and carbon dioxide near 4.4 μm (to sense altitudes of <110 km). Observations are made through a single low-pressure gas cell containing a mixture of these target gases. Profiles of temperature and wind can be measured day and night continuously from 25 to over 250 km with <2% uncertainty at intervals of 10 km along-track, far exceeding current capabilities. This approach, using a small, simple, moderately cooled IR camera, could provide unprecedented observations of atmospheric dynamics from the lower stratosphere into the middle thermosphere.

Space Dynamics Laboratory (SDL) recently designed, built, and delivered the Solar Occultation for Ice Experiment (SOFIE) instrument as the primary sensor in the NASA Aeronomy of Ice in the Mesosphere (AIM) instrument suite. AIM's mission is to study polar mesospheric clouds (PMCs). SOFIE will make measurements in 16 separate spectral bands, arranged in eight pairs between 0.29 and 5.3 μm. Each band pair will provide differential absorption limb-path transmission profiles for an atmospheric component of interest, by observing the sun through the limb of the atmsophere during solar occulation as AIM orbits Earth. A pointing mirror and imaging sun sensor coaligned with the detectors are used to track the sun during occulation events and maintain stable alignment of the sun on the detectors. This paper outlines the mission requirements and goals, gives an overview of the instrument design, fabrication, testing and calibration results, and discusses lessons learned in the process.

The SOFIE pointing control system (PCS) locates and tracks the top edge of the sun and periodically scans the solar disk for calibration. Primary hardware components are a steering mirror assembly (SMA), sun sensor, vibration isolation system (VIS), and associated electronics. The SMA has a 100-Hz control bandwidth and is capable of ±1.6 mechanical degree deflection in azimuth and elevation axes. The sun sensor uses a 1024x1024 pixel, radiation-hardened focal plane array and coarse and fine tracking algorithms to report the solar centroid and edge positions to the PCS. The PCS control law uses this information to command the SMA. A change in launch loads necessitated the development of the VIS, which features passive viscoelastic damping to protect the SMA. A rapid prototyping methodology was used to develop the control laws for the inner SMA feedback loop and outer PCS feedback loop. The methodology features integrated end-to-end modeling of structural dynamics, controls, and optics; automatic C-code synthesis from block diagrams; real-time hardware-in-the-loop (HIL) testing; and the ability to change control parameters "on the fly." Extensive testing of the PCS shows stable pointing performance of about 2 arcsec in the presence of 60-arcsec disturbances, compared to the requirement of 15 arcsec.

Microbolometer array technology is making rapid progress in the commercial and tactical military world. An array was used for uncooled IR imaging in a very successful Shuttle flight and more is planned. Applications for long missions with microbolometers are on the drawing board. A recent study concluded that an uncooled staring array, when applied to Mid IR atmospheric ozone limb profile measurement could dramatically improve earth coverage, vertical precision and mixing ratio accuracy. Imaging by a staring array also alleviates some of the problems of non-imaging approach. This paper describes the technique and strategy of ozone profiling with an uncooled microbolometer array, the derived key instrument requirements and a brief discussion on the instrument implementation approach.

The use of limb radiation measurements to infer atmospheric parameters continues to be a popular technique. The HALOE (Halogen Occultation Experiment) instrument is a gas correlation radiometer on board the UARS (Upper Atmosphere Research Satellite) that performs solar occultation measurements for inferring vertical profiles of HF, HCl, CH₄, NO, O₃, H₂O, NO₂, aerosol extinction and temperature. The first four gases and aerosol are inferred from gas correlation measurements. The remainder are inferred from broadband (> 20 cm^-1) radiometer measurements. The eruption of Mt. Pinatubo before the UARS launch presented a number of challenges for HALOE data processing. Although ideally the gas correlation technique is insensitive to aerosol, in practice the aerosol signature induces optical effects that must be accurately addressed. The inference of extinction profiles for modeling aerosol signature in the radiometer channels was found to require high vertical resolution. The impact due to vertical resolution and other optical effects on the retrieved results is discussed. Simulations and HALOE results are presented to demonstrate and validate the effects. It is found that the Pinatubo layering demands a vertical resolution on the order of 2 km or less to accurately model aerosol effects on broadband limb viewing radiometers.