ACE Weekly 07/14/2010 - 07/20/2010 All ACE spacecraft subsystems are performing nominally. ======================================================================== Orbit/Attitude: Type Attitude Date 07/15/2010 DOY 196 2010 Thrusters 2R 4R+ 4R- Duration 15:44 min Start 16:29:58z Stop 16:45:42z HGAStart -7.60deg HGAStop +8.37deg SunStart 7.20deg SunStop 18.88deg SpinStart 5.0604rpm SpinStop 5.0651rpm Nutation 0.13deg Firing 80 pulses FuelUsed 0.2502lbs FuelRemain 134.9868lbs FinalSCMass 1369.247lbs The next attitude maneuver is scheduled for Friday 07/30/2010. ======================================================================= OCRs: DOY 200 (07/19/2010) GND-202 Old ACE MOC String 2 Dismantled String 2 in the old ACE MOC (Bldg 14 Rm W10F) has been turned off and dismantled. There are 3 activities that have not yet transitioned to the Multi-Mission Operations Center (MMOC). 1) Maneuver Planning (APU) 2)Attitude Determination (ADS) and 3)DSN CDR Download. Maneuver Planning and Attitude Determination are waiting for the final MTASS release. DSN CDR Download is waiting for the correct network configuration. DOY 201 (07/20/2010) MMOC-191 UPS-6 Replacement The MMOC is powered by 2 Uninterruptible Power Supplies (UPS). UPS-6 is being replaced on Wednesday 07/21/2010. Computers in the MMOC that use UPS-6 will either be powered down or switched to UPS-14 on Tuesday 07/20/2010. UPS-14 will be replaced at some point in the future. ======================================================================== Activities: Data Capture: 100% DOY 192-199 2010 ======================================================================== Anomalies: DOY 196 07/15/2010 S-ACE-0517 G10-0035 Incorrect Thruster Calibration The attitude maneuver plan generated using the GMAN (General Maneuver) software did not have the correct thruster calibration. The thruster calibration used was 0.675 and instead of 0.715. The target attitude was (134.70,22.99) and the resulting attitude was (135.42,22.57). IMPACT: The resulting attitude from the 07/15/2010 maneuver had a sun angle of 18.88� (original target=18.64�) and a High Gain Antenna angle of 8.37� (original target 7.52�). The postmaneuver downlink SNR was 15.4dB which was more than sufficient to maintain the downlink. The longer maneuver on 07/15/2010 will result in a shorter maneuver on 07/30/2010. Background Information As ACE orbits the sun at the L1 point, the attitude maneuvers are used to keep the spacecraft pointing towards the sun and the High Gain Antenna (HGA) pointing towards the earth. The sun angle constraint is 4�-20� and the HGA constraint is <8�. In 2002, ACE increased the HGA angle constraint and started performing longer attitude maneuvers. The FDF analysts noted an improved thruster performance and started adjusting the thruster calibration in the GMAN software. The FDF analysts fit the thruster calibration as a function of the number of thruster pulses to a line, which is roughly thruster calibration = 0.585 + (# pulses)/585 (the duplication of 585 is merely a coincidence ... and this is a rough fit) The previous attitude maneuver on 07/01/2010 had been a shorter maneuver with 54 pulses and a thruster calibration of 0.675. The attitude maneuver on 07/15/2010 was longer and should have had a higher thruster calibration. With a thruster calibration of 0.675, the GMAN software calculated a maneuver with 80 pulses. With a thruster calibration of 0.715, GMAN would have calculated a maneuver with 75 pulses. The 5 additional thruster pulses resulted in exceeding the target attitude with a post-maneuver HGA of 8.37 degrees. The HGA constraint of 8 degrees is used to give enough buffer to the precipitous drop-off in HGA performance between 10-11 degrees. On 07/15/2010, the premaneuver Signal to Noise Ratio (SNR) was 15.7db (HGA angle=7.6�) and the postmaneuver SNR was 15.4db (HGA angle=8.37�). This is an enviable SNR and the downlink was not impacted by the postmaneuver attitude. The upper sun angle constraint (20�) is for the ULEIS instrument. If the sun angle reaches 21.8 degrees, the ULEIS instrument shutter will be closed.