TIDI comprises three major subsystem: four identical telescopes, a Fabry-Perot interferometer with a CCD detector, and an electronics box. Light from the selected regions of the atmosphere is collected by the telescopes and fiber-optically coupled to the detection optics. The four fields of view are scrambled along with a calibration field input and converted to an array of five concentric circular wedges. This input then passes through a selected filter, then through a Fabry-Perot etalon, and is finally imaged onto a CCD via a circle to line imaging optic (CLIO) device. TIDI views emissions from OI 557.7 nm, O I 630.0 nm, OII 732.0 nm, 02(0-0), 02 (0-1), Na D, OI 844.6 nm, and OH to determine horizontal vector winds and temperature throughout the TIMED altitude range with a vertical resolution of 2.5 km at the lower altitudes and with an accuracy that approaches ~3 m/sec and ~3 K, respectively, under optimum viewing conditions. TIDI also makes spectral ratio observations to determine 02 densities and rotational temperatures. Throughtout the orbit the TIDI telescopes perform limb scans through the terrestrial airglow layers obtaining scans simultaneously in four orthogonal directions: two at 45 degrees forward but on either side of the satellite's velocity vector and two at 45 degrees rearward of the satellite. These four views provide the measurements necessary to construct the horizontally resolved vector winds as a function of altitude within the MLTI region along two parallel tracks, one on either side of the spacecraft. Each vertical scan consists of individual views 2.5 degrees (horizontal, along the limb) by 0.05 degrees (vertical, normal to the limb) in angular size. The vertical altitude resolution of the instrument is 2.5 km, but the altitude spacing between views will be adjusted to yield a measurement vertical resolution of half a scale height throughout the limb scan. The altitude step size will range from 2.5 km in the MLTI region to 25 km in the thermosphere. Each up/down acquisition cycle will take 100-200 seconds to complete, resulting in a nominal horizontal spacing between profiles of approximately 750 km along the orbit track. The exact time per vertical scan will depend on the mode being run and the integration or dwell time needed at each altitude step. Each up/down scan cycles through a sequence of filter tunings, selecting the optimal emissions to be viewed within each altitude range to allow rotational and/or Doppler temperatures as well as neutral winds to be retrieved. After launch it was discovered that the instrument suffers from a light leak that causes the background to be elevated and increases the uncertainty in the wind measurement. Modeling of the background has helped to eliminate any systematic effect. Water outgassing from the spacecraft or instrument has deposited as ice on some part of the optics and reduced the instrument's sensitivity. This problem has been reduced by two spacecraft rolls causing the optics to warm up and sublimate much of the ice. TIDI homepages are maintained by the University of Michigan at
http://tidi.engin.umich.edu/
and by the National Center for Atmospheric Research (NCAR) at
http://timed.hao.ucar.edu/tidi/