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The partners in the GIFTS project completed the Engineering Development Unit (EDU) in May 2005. The EDU allows researchers and engineers to test GIFTS technologies and concepts.
The Fourier Transform Spectrometer (FTS) is the defining element of GIFTS. While the concept of an FTS with image capabilities is not new, the combination of technologies will allow vast improvements in spatial spectral coverage with an exceedingly accurate and sensitive system. To simultaneously satisfy the required level of spectral, spatial, and temporal resolution, the system needs to make many measurements at the same timeóa concept called parallel sensing. The Large area Focal Plane Array (LFPA) facilitates parallel sensing and gives GIFTS the measuring power of more than 16,300 individual FTSs.
GIFTS has several features that give the system an edge over other geostationary hyperspectral sounders. The final product is small with a low mass and reasonable power requirements. While compact and convenient, GIFTS still provides the spectral and spatial resolution required to make it a revolutionary geostationary satellite.
GIFTS Technologies put together
To take measurements, Michelson interferometers use a "scene" mirror that moves through a programmable sequence. The mirror alternately reflects information from three calibration sources (see slide ###) and the atmosphere. The reflection then bounces off a series of mirrors within the instrument and into a beam splitter. Here, a mirror runs back and forth on a track and interacts with the incoming data. Part of the information goes to the IR detector arrays and the rest bounces to the visible detector arrays. The cryogenic component helps maintain alignment over long periods. The remote alignment assembly allows personnel on the ground to adjust the instrumentís alignment when necessary.
The FPA consist of complex detectors that generate power when exposed to radiant energy such as that which GIFTS measures. The large active area of the detector arrays, 128 x 128, permits simultaneous sampling, which means that GIFTS operates at a power equivalent to more than 16,300 individual FTS.
The temperature management system has two stages. The first maintains the detector arrays at 140 K while the second keeps the optics at a chilly 60 K. The cryogenic cooling system improves spectral resolution and accuracy. The external structure of GIFTS also contributes to temperature management. The cold instrument design minimizes internal scattering of the radiation.
The input telescope was built using conventional materials and advanced techniques to reduce mass. The design not only ensures the smallest and lightest instrument possible, but it also helps minimizes the effects of sunlight. As radiation travels from the telescope to the interferometer and the visible imaging array, mirrors align the beam so that its edges are parallel. A flip-in mirror allows the system to view internal calibration sources, or blackbodies, which were developed at SSEC.
After passing through the FTS, IR radiation bounces to the aft optics, a multi-mirror refocusing system that directs the image from the detector to the interferometerís fixed mirror. In the aft optics, IR radiation is separated into long-wave and short- to midwave radiation. These two bands then travel to the long-wave detector array or the short to midwave detector array respectively.
The GIFTS laser provides the optical frequency standard for the FTS by referencing the IR spectrum in which GIFTS takes atmospheric soundings. The laser is small and consumes very little power.
SSEC maintains a tradition of developing highly accurate infrared calibration sources, known as blackbodies, for remote sensing instruments that look both inward at the earth and outward into the atmosphere. For its calibration, GIFTS uses two blackbodies run at two different temperatures in combination with views of deep space. To support state-of-the-art weather forecasts, temperature measurements of the atmosphere from space must be determined with an uncertainty that does not exceed 1 Kelvin. For the GIFTS instrument to make these kinds of measurements the absolute temperature of the blackbody cavity must be known to within 0.1 K and the cavity emissivity must be known to within 0.2% (assuming the cavity emissivity is 0.996). GIFTS Engineering Development Unit significantly exceeds these requirements, and serves as a prototype for future planned climate missions that have more stringent calibration performance requirements. For this system, the spectral emissivity of the delivered blackbodies is better than 0.999 with an absolute uncertainty of better than 0.1%, and the absolute temperature measurement uncertainty is less than 0.06 K (GIFTS would even maintain this accuracy through the end of a seven-year mission in a geostationary orbit).
The external structure of GIFTS has been designed to help control the thermodynamics inside the instrument. Each structural surface panel is a composite radiator with heat pipes along the inside. The thermal system maintains consistent internal temperatures, which vary from instrument to instrument, with different amounts of sunlight on each panel.
If GIFTS is launched into space aboard a satellite, it will be mounted on the satellite surface either on a side face or at the lowest point, the nadir face. This image shows the system as if it were mounted on the nadir face.