The fact that overall system responsivity seemed to be declining as a function of time was first mentioned in SABER's April 8, 2002 weekly report. After confirmation that water/ice buildup on the focal plane array was the likely and probable contributor to this decline, the SABER operations team was tasked with cycling the cooler. This process involved turning SABER off to allow the focal plane array to warm above the water/ice sublimation temperature, allowing the water/ice buildup to outgas, and then turning the cooler back on. Cycling the cooler is performed in a three step, three contact process. The first step is to disable both the cooler and SABER's adaptive scanning. The scan limits are hard seeded to 16.35 and 26.54 degrees to compensate for the oblateness of the Earth while adaptive scanning is off. It is important to note that SABER is left operational during a Cooler Power Cycle (CPC) so that the non-thermal channels may continue to take data throughout. The second step is to turn SABER's cryo-cooler back on. Because temperatures are relatively high at this point, it takes approximately 24 hours for SABER's focal plane to return to nominal operating temperatures. The third step is to re-enable the adaptive scan, which returns SABER to its nominal data collection state.
The purpose of the Lunar Observations is to gain more accurate near angle field of view (FOV) functions across all of SABER's detectors, and more specifically across channels 4 (O3), 5 (H20), and 6 (NO). This is accomplished during times when the moon crosses easily into SABER's view. For example, on the 22nd, 23rd, and 24th of January 2003 SABER naturally encounters the moon 24 times at tangent point altitudes between 100 and 280 km. Based upon times predicted by STK and verified by numerous other sources, SABER was commanded to jump out of its nominal data collection routine just before the moon passes into view and to do numerous repeated scans over the moon ("Lunar Observations"). After this, SABER will automatically return to its nominal data collection routine. On-orbit, at high tangent point altitudes, the moon can be considered to be a point source. Because of this, as SABER scans across the moon, the situation closely resembles the calibration techniques used in 1999 to calibrate SABER. Data gathered over this Lunar Observation period can therefore be used to draw far more accurate FOV functions than currently obtainable from the 1999 calibration sequences.
Investigation of slow scan data taken on January 24, 2004 has pointed to a potential difference between on-orbit and ground calibration values for the low-mid gain step for channels 5-7. In order to better calibrate these gain steps the scan mirror was pointed at several positions that provide sufficient signal in both mid and low gain for these channels. Channel 5 was pointed directly at the telescope, channel 6 into the telescope baffle adjacent to the IFC cavity, and channel 7 directly at the IFC. Each position was held for 10 seconds, with data taken for 10 iterations of 1 second each in low gain and 1 second in high gain. This data will be used along with the slow scan data to determine the correct gain step calibration values to use in processing.
An artifact of SABER executing from EEPROM (used for adding patches to operational code) when a routine memory dump command sequence was sent to SABER caused the state machine to restart SABER. The cause of this event is now entirely known, and avoidable. The possibility of SABER performing similarly in the future has been removed. There is a more detailed explanation in SABER document "TM2002-014.doc" (SABER Tech Memo 14), available from the operations team.
These maneuvers were performed to help remove scattering material from TIDI's optical path.
A small set of commands were tested on the SABER simulator and uploaded to the SABER instrument in order to analyze detector hysteresis. These commands performed "slews" (very fast mirror movements) from earth to space. In this type of scan, the detectors start at the earth and quickly get excited by the earth's albedo. After observing the earth's albedo for a short time, the mirror is "slewed" to space, effectively removing all signal from the detectors. Recording the amount of time after the mirror has reached space that it takes for the detectors' signal readings to drop to zero (or noise-levels) gives a helpful measure of a detector' hystereses effects. Although this analysis is still ongoing, the SABER operations team has prepared and tested modifications to the SABER data-collection routine that effectively remove concerns about hystereses.
Due to the expected and realized TIMED IRU (Inertial Reference Unit) performance declination, and because of concerns to conserve the lifetime of these valuable pointing tools - the TIMED community decided to convert the spacecraft from using these three-ring laser gyros to using solely the star trackers. The star trackers make up for in longevity what they may lack in accuracy. First the TIMED MOC performed a test to validate that the spacecraft could operate properly in this new star-tracker only mode. This test's success lead the team to make a permanent switch to "gyro-less" operations in late September of 2004.