Feb. 6, 2000
Status 06 February 2000 As of Thursday, February 3, MISR has been operating under a set of commands that turns the cameras off on the orbit's night side and turns them back on again in daylight. These "ATC" or "absolute time commands" are uploaded to Terra ahead of time and are issued automatically by the spacecraft every orbit. MISR's nighttime "naps" last about 43 minutes within each 98-minute orbit. Earlier today, during one of MISR's dayside "wake-ups", a number of camera temperature indicators exceeded normal limits and the operations center commanded MISR to "safe" mode. However, this was a false alarm resulting from an idiosyncracy of MISR's engineering telemetry polling system. Because it takes a finite time for the readings to update, the data can appear to be incorrect for up to 24 seconds following a change of instrument configuration. This afternoon MISR was commanded back to normal operations and all telemetry looks fine. Overall, MISR continues to behave very well. We have observed two effects in the "dark" images that we attribute to enhanced charged particle concentrations in the South Atlantic Anomaly (SAA). The first consists of random bright points resulting from signals being generated in individual detector elements, most likely by proton radiation. These make the images look like a field of faint stars. The other occurs as a result of each camera having a circuit that stabilizes the overall background level from line to line. If a charged particle event occurs at a particular time, the stabilization circuit is fooled into thinking the overall background has brightened and compensates by temporarily making the image lines slightly darker. This effect looks like randomly placed horizontal stripes in the images. These two phenomena have led one of our software engineers to refer to the SAA effects as the "stars and stripes". Though these features are obvious now, most of them will be swamped by the vastly larger signal from earthlight once we open the cover. Simulations of the Terra thrusters which take into account the impingement of two thruster plumes on the solar array have successfully replicated the behavior that was observed during the aborted maneuver on January 11 and during last Wednesday's 20-second test burns. The "thruster pairing matrix", or on-board logic which determines which thrusters will be fired and at what time will be modified to take into account this new understanding of the propulsion system's behavior. In light of this, the "brute force" plan to achieve final orbit using many short burns has been abandoned; that approach has also been deemed risky because of the large number of communications contacts with the spacecraft that it requires. A sequence incorporating burns of duration up to five minutes has now been developed. It begins on February 10 and ends on February 24. We are planning to open MISR's cover on February 25. Since launch on December 18, Terra has completed over 700 orbits of the Earth, and has traveled a distance of more than 31 million kilometers. A straight path of this length would cover about three-fourths of the distance from Earth to Venus. 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