ACE Weekly 09/10/2014 - 09/16/2014 All ACE spacecraft subsystems are performing as expected. ======================================================================== Orbit/Attitude: Type Attitude Date 09/16/2014 DOY 259 2014 Thrusters 2R 4R+ 4R- Duration 9:29 min Start 16:03:49z Stop 16:13:18z ra,decStart 158.87,-7.73 ra,decStop 167.07,-11.30 ra,decExpect 167.47,-11.48 Deviation 0.43 deg (-4% short) HGAStart -10.34 deg HGAStop +10.80 deg SunStart 18.38 deg SunStop 15.58 deg SpinStart 4.9848 rpm SpinStop 4.9873 rpm Nutation 0.18 deg Firing 48 pulses FuelUsed 0.1518 lbm FuelRemain 104.0776 lbm FinalSCMass 1338.337 lbm The next attitude maneuver is scheduled for Monday 09/22/2014. The past few maneuvers continue to be slightly short. Analysis has found that the thruster catbed temperature is not heating as fast as normal during short maneuvers. And the slower the catbed heating, the shorter the maneuver. The decreased heating rate can happen at the beginning of the maneuver or the middle or the end or during the entire maneuver. Recent maneuvers (only -4% to -5% short) show that catbed heating is returning to normal. The interesting observation is that 4R- and 4R+ thruster catbeds are synchronous in their decreased heating rate but not synchronous with the 2R decreased heating rate. With the station-keeping maneuvers also showing asynchronous heating rates, the current theory is that fuel flow is the issue (1A,1R,4A,4R-,4R+ using the +X fuel line; 2A,2R,3A,3R-,3R+ use the -X fuel line). The intermittent nature of this problem has current speculation that nitrogen (no bladder between pressurant & fuel) bubbles had entered the fuel line. The speculation continues that when the nitrogen bubbles pass through the internal/external line juncture, they temporarily decrease the pressure down the fuel line to the thruster. ======================================================================== OCRs: None ======================================================================== Activities: Data Capture: 100% DOY 250-257 2014 ======================================================================== Anomalies: DOY 253 09/10/2014 S-ACE-0684 G14-0011 DSN Earth Receive Time Another -1 second ERT delay occurred on 9/10/2014. IMPACT: Since the ERT deviation is short (~3 minutes), it has negligible impact on the clock calibration report sent to ASC and NOAA SWPC. Analysis of the -1 second ERT delay has found that the problem is only with the SLE (Space Link Extension) ERT. The DSN IDR files (Intermediate Data Recorder) contain valid Earth Receive Times. The SLE ERTs and IDR ERTs are occasionally different by 1 millisecond and this contributes to the problem. ACE telemetry frames are generated slightly faster than once a second. So every ~6 minutes, the ERT for the next frame will be 0.999 seconds later instead of the usual 1.000 seconds. When the IDR ERT transitions from xx:xx:xx.000 to xx:xx:xx.999, the SLE ERT does not change. s/c clock IDR ERT time SLE ERT time 537825380 11:30:02.000 11:30:02.000 537825381 11:30:03.000 11:30:03.000 537825382 11:30:04.000 11:30:04.000 <--- 537825383 11:30:04.999 11:30:04.000 <--- 537825384 11:30:05.999 11:30:05.000 537825385 11:30:06.999 11:30:06.000 When the SLE ERT transitions from xx:xx:xx.000 to xx:xx:xx.999, the SLE ERT is fixed when it advances 1.999 seconds s/c clock IDR ERT time SLE ERT time 537825629 11:34:10.999 11:34:10.000 537825630 11:34:11.999 11:34:11.000 537825631 11:34:12.999 11:34:12.000 <--- 537825632 11:34:13.999 11:34:13.999 <--- 537825633 11:34:14.999 11:34:14.999 537825634 11:34:15.999 11:34:15.999 This has been happening ever since ACE transitioned to SLE in 2010. This was only noticed recently while checking the other problem where the ERT was behind by 1-3 telemetry frames after a bit rate change. Interestingly, there is only a 10-15% chance of seeing the -1 second ERT problem during an ACE pass each week (since the spacecraft clock is only minimally fast). It was not at all likely to see this problem 6 times in 9 weeks! DOY Date Station ERT behind duration DSN DR# 197 07/14/2014 DSS-65 -1 second 15:30:44 to 15:38:16z 201 07/20/2014 DSS-15 -1 second 16:11:48 to 16:14:45z N109669 207 07/26/2014 DSS-24 -1 second 14:45:48 to 14:48:30z N109670 224 08/12/2014 DSS-24 -1 second 18:51:18 to 18:55:17z N109699 230 08/18/2014 DSS-24 -1 second 15:55:42 to 15:58:35z N109705 253 09/10/2014 DSS-65 -1 second 11:30:04 to 11:34:13z N109722