Operational Performance of the TIMED Doppler Interferometer (TIDI)

The TIMED Doppler Interferometer (TIDI) is a Fabry-Perot interferometer designed to measure winds in the mesosphere and thermosphere (60-180 km) as part of the TIMED mission. TIDI is a limb viewer and observes emissions from OI 557.7 nm and rotational lines in the O ₂(0-0) Atmospheric band. Wind measurement accuracies approach 3 ms ^-1 in the mesosphere and 15 ms ^-1 in the thermosphere. The TIDI instrument's performance during the first year and a half of operation is discussed in this paper. Many subsystems are working as designed. The thermal control system is holding the instrument temperatures at their desired set-points. The CCD detector is working as expected with no changes observed in the gain, bias or read noise. The instrument suffers from a light leak that causes the background to be elevated and increases the uncertainty in the wind measurement. Nothing can be done to eliminate this problem but modeling of the background has eliminated 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 which pointed the TIDI radiator to view more of the earth causing the optics to warm up and sublimate much of the ice.