ACE Weekly 12/28/2012 - 01/03/2013

All ACE spacecraft subsystems are performing as expected.

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Orbit/Attitude:

	Type        Attitude
	Date        01/03/2013
	DOY         003 2013
	Thrusters   2R 4R+ 4R-
	Duration    5:17 min
	Start       20:07:41z
	Stop        20:12:58z
	HGAStart    -8.89deg
	HGAStop     +8.99deg
	SunStart    14.36deg
	SunStop     16.34deg
	SpinStart   5.0785rpm
	SpinStop    5.0796rpm
	Nutation     0.07deg
	Firing      27 pulses
	FuelUsed      0.0906lbs
	FuelRemain  117.1468lbs
	FinalSCMass 1351.407lbs

The next attitude maneuver is scheduled for Tuesday 01/08/2012.  The
next station keeping maneuver (SK-64) is scheduled for 01/15/2012.

Tracking/range data since the last maneuver (12/28/2012) has only been
from one DSN antenna (DSS-27).  Without southern hemisphere
tracking/range data, there is more uncertainty in the spacecraft
ephemeris (but still within the limit for acquisition).  The increased
uncertainty may also impact station keeping maneuver planning and fuel
usage.  But as mentioned in the previous weekly report, station keeping
fuel usage is only ~10% of the total fuel usage.  Nevertheless, we are
pushing to obtain at least 1 hour of southern hemisphere
tracking/ranging from DSN each week.
 
	Normal ephem uncertainty       ~2.5 km
	uncertainty w/o southern range  10+ km
	limit for acquisition         3,000 km
	size of L1 orbit           ~150,000 km
	distance to L1           ~1,500,000 km

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OCRs:

DOY 363 (12/28/2012) MOCR 401  MTASS Attitude Software
The MTASS software replaces the old TPOCC/ADS attitude software for
generating the daily Attitude/Orbit Reports that are sent to the ACE
Science Center.  The solutions from the new system are more consistent
than the old TPOCC attitude software.  The following table shows the
error (daily deviation) from the old TPOCC/ADS system and the new MTASS
system.
	            TPOCC/ADS          MTASS
	Attitude:  0.5 degrees      0.05 degrees
	Spin:      0.01 rpm         0.001 rpm

During testing, we found that the daily drift due to solar radiation
pressure is roughly linear with the spacecraft sun angle and is
approximately 1/400 (daily drift ~ sun angle/400).  For example, with a
sun angle of 20 degrees, the attitude would change ~0.05 degrees per
day.

DOY 003 (01/03/2013) 1938-1946z & 2235-2242z  SIS-052  8 cmds
The SIS instrument team performed a 3 hour test with slightly raised
bias voltage to determine leakage currents.  The instrument team will
analyze the data to determine which Matrix strips can be re-enabled if
the bias voltage is set to a new value.  Note:  The third command in
this sequence takes 2:43 minutes to complete.  The fourth command was
sent before that time and was ignored by the instrument.  The fourth
command was then resent and successfully processed.

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Activities:

Data Capture:  100%  DOY 358-365 2012


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Anomalies:

None

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Average Sun Angles With Weekly Attitude Maneuvers

Dates         Avg Sun    Avg SEV   Sun-SEV (indicates extra s/c tilt)
-----------   -------    -------   ----------------------------------
10/18-10/23   11.0deg     5.1deg    5.9deg
10/23-10/28    9.3deg     3.3deg    6.0deg
10/28-11/06    7.3deg     2.0deg    5.3deg
11/06-11/13    8.4deg     3.8deg    4.6deg
11/13-11/20   10.3deg     6.3deg    4.0deg
11/20-11/27   12.7deg     8.6deg    4.1deg
11/27-12/04   14.9deg    10.1deg    4.8deg
12/04-12/11   16.0deg    10.9deg    5.1deg
12/11-12/18   16.8deg    11.3deg    5.5deg
12/18-12/27   16.5deg    10.7deg    5.8deg
12/27-01/03   16.0deg     9.1deg    6.9deg

There are 3 factors that influence the weekly sun angle.  They are:

1) The Sun-Earth-Vehicle angle.  The farther the spacecraft is from the
Sun-Earth line, the larger the spacecraft sun angle will be.  This is
the biggest influence on the spacecraft sun angle.

2) Spacecraft position in terms of ecliptic plane.  When the spacecraft
is in the ecliptic plane, the solar orbit is moving the spacecraft
attitude directly toward (or away) from the sun.  This lowers the
average sun angle (11/13-11/20 in above data).  When the spacecraft is
above or below the Sun-Earth line (10/23-10/28 in above data), the sun
angle can be kept at larger values.

3) Spacecraft motion relative to Earth.  When the spacecraft is moving
slower than the Earth in the solar orbit (bottom half of the current L1
orbit), then the spacecraft's antenna sweeps past Earth faster and sun
angle cannot be pushed as far.  When the spacecraft is moving faster
than the Earth (top half of the current L1 orbit), then the spacecraft's
antenna stays pointed at Earth longer and the sun angle can be pushed
further.  For the next ~2 months, the spacecraft is in the top half of
the orbit and the sun angles can be pushed higher.

When estimated sun angles were presented to the science team for the
SWEPAM proposal, these factors were averaged together to give an
*annual* average sun angle.  This information is provided here, just to
explain the weekly changes in Sun-SEV (i.e. extra s/c tilt).


The following is background information that will be included in each
weekly report.
The project has accepted the SWEPAM team proposal to keep the spacecraft
at larger sun angles with weekly attitude maneuvers.  The SWEPAM-Ion
instrument has a series of channel electron multipliers (CEMs) and
larger sun angles allows more responsive CEMs to measure the solar wind.
The maximum sun angle follows the Sun-Earth-Vehicle angle (SEV).  The
SEV angle is determined by the size/shape of the orbit around L1.  When
the spacecraft antenna is pointed directly towards earth, the
spacecraft's sun angle will be equal to the Sun-Earth-Vehicle angle.
With weekly maneuvers, the average sun angle can be kept 4-6deg more
than the SEV angle.  This results in the spacecraft antenna aspect angle
being kept between 5 and 9 degrees and never pointing directly back at
earth.  For reference, the SWEPAM team prefers sun angles above 13
degrees.  With the current size of the L1 orbit, the sun angle will be
above 13 degrees for ~45% of the time.