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