Jan. 10, 2000
Status, 10 January 2000 The manufacturer of the Terra High Gain Antenna (HGA), Spar Aerospace, has completed laboratory testing of the transistors which they feared could get damaged by the South Atlantic Anomaly (SAA). Spar sent 1 million pulses of a few microseconds in duration through the devices--many, many more pulses than would be seen during the normal mission life--and longer in individual duration and with current levels about twice the value expected from proton irradiation. The transistors survived these tests. Engineers at Goddard performed similar tests with the same result. In light of this, normal operations of the HGA will resume this Thursday, January 13, and the plan is to keep the HGA powered on during passages through the SAA. In the event of a gimbal drive reset, a telemetry monitor will automatically restart the drive 7 minutes later. With experience, this time delay may be shortened. If a reset interrupts a download of data, a command to repeat the download will take place during the next contact through a TDRSS communications satellite. The anomaly in the propulsion system's power supply which occurred last week was traced to the failure of a device whose purpose is to put a resistive load on the power supply and keep it stable. There are four such devices, and although spacecraft engineers predicted that things would work fine with the loss of one of them, they recommended turning on a heater to keep the load on the power supply at the nominal level. This was done, and today's 11-second engineering test firing of Terra's propulsion system went very smoothly! The first long-duration ascent burn remains scheduled for tomorrow at 3:18 PM PST. The ASTER thermal-infrared (TIR) imager was turned on yesterday, completing the power-on of all science instruments (the other ASTER instruments, the visible and shortwave-infrared imagers, were turned on last Friday; CERES, MISR, MODIS, and MOPITT were all turned on earlier). Early this morning, the cooling system provided by the spacecraft for the ASTER TIR experienced an anomaly and shut down. The TIR itself was doing fine but was powered off for protection since its temperatures started to increase. The cooling system will be restarted tomorrow, and if necessary a backup system will be called into action. MISR's thermal engineer has completed a run of the computer model he built to predict instrument temperatures. The model predicts the maximum temperature of the optical bench in the current instrument configuration to be in the range of about 27-30 degrees Centigrade. Actual telemetry is showing the optical bench temperatures to be running about 27 degrees C at one location and 29 degrees C at another, with variation from day to night of about 1 degree C. What a nice result! All other engineering readings continue to look great. You can see earlier status reports by checking the "News" link of the MISR web site at
http://www-misr.jpl.nasa.gov.
David Diner