Feb. 2, 2000
Status 02 February 2000 This is a lengthy report so here's the short version: Plans to achieve final orbit for Terra are undergoing revision but are expected to firm up soon. Meanwhile, MISR continues to do well in science mode. The cameras, still operating with the cover closed, have detected the South Atlantic Anomaly. Large amounts of MISR data are flowing and giving our data processing system a good workout. For more details, read on! Two 20-second engineering burns of the Terra thrusters were successfully performed today. Spacecraft engineers suspected that a contributing factor to the anomalous roll that occurred on January 11 was that the plume from one of the thrusters was impinging on the solar array and providing an additional torque (rotational force) on the spacecraft. Today's results confirmed predictions that take this effect into account, and the behavior of the spacecraft controller that directs the thruster activity is now more readily understood. Factoring in this finding with a slip in the start of the ascent plan announced last week means that final orbit might not be reached before February 23. However, a more aggressive schedule which aims for an earlier date has not yet been ruled out. The MISR instrument continues to perform well. When we open the cover (soon!) and view the Earth, each picture element, or "pixel", in the charge-coupled device (CCD) detectors used in the cameras will generate a signal (essentially a flow of electrons) that is proportional to the amount of light falling upon it during each exposure--putting all these pixels together is how a complete picture is formed. CCD's are used in commercial digital cameras and camcorders, but the ones used in MISR are many, many times more sensitive and accurate. Other things besides light energy can cause CCD's to generate a signal, however, such as thermal energy (this is why we cool them) or the presence of electrically charged particles such as protons. Many of these charged particles come from the Sun, and the South Atlantic Anomaly (SAA) and the Van Allen Radiation Belts are locations where variations in the Earth's magnetic field cause these particles to become "trapped". In addition to being the source of the occasional Terra High Gain Antenna resets, we have found a definitive correlation between passage through the South Atlantic Anomaly and the frequency of random "hits", or fleeting bright points, in the MISR "dark" images. (The Terra star trackers, and many other space instruments, have also seen such an effect.) For MISR, the proportion of hits is small but noticeable, varying from about 1 per million pixels outside of the SAA to about 1 per thousand pixels inside the SAA. This means that even in the worst case, 99.9% of the data are unaffected, and most of the time the situation is a thousand times better. Portions of our ground data processing software already contain procedures that will "filter out" these random hits, and we are contemplating additional possibilities. Enough MISR data to fill about 35 CD-ROM's arrived in the first 24 hours after the start of "global" science mode, and data continue to flow into the Atmospheric Sciences Data Center and to JPL. Things are proceeding well, though the volume of data has brought to light some subtle pathologies not encountered during pre-launch testing. Our team is working diligently to address these as they are discovered. Most issues that have arisen are associated with handling non-typical situations, such as data gaps, missing spacecraft navigation or attitude information, and compensation for occasional instrument errors. A major focus at the moment is the software that turns the stream of bits sent out by the cameras into pictures, using information generated by MISR's flight computer to sort the data by camera, color, and location across the swath and within the orbit. Checkout of the software that combines spacecraft navigation data, camera geometric orientations, and solar position to compute illumination and view angle information is also underway. These supplementary data are essential for interpreting the MISR images. 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