ACE Weekly 05/15/2012 - 05/21/2012

All ACE spacecraft subsystems are performing as expected.

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Orbit/Attitude:

No maneuvers were completed this week.  The next attitude maneuver and
Station-Keeping #61 are scheduled for Friday 05/25/2012.

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OCRs:

None

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Activities:

Data Capture:  100%  DOY 134-141 2012

WS1, the 18-meter antenna in White Sands, NM, is now getting range data
consistent with DSN's Goldstones stations (+6.8 meter bias).  There are
still several outstanding issues, including: 1) during previous testing
the MMOC needed to disconnect & reconnect in-between sending commands
for them to be radiated, 2) WS1 handling of downlink data rate changes
and 3) updating MMOC procedures and tools: clock calibration, schedule
retrieval, TLM & CMD conversion tools, data forwarding to NOAA SWPC &
ASC and pass procedures.

Background Information on Downlink Data Rate Changes
For normal operations, ACE transmits at 2 data rates.  The low data rate
(498bps) provides Real-Time Solar Wind (RTSW) data for NOAA SWPC's
(Space Weather Prediction Center) network of ~12m antennas.  During the
~3.5 hour DSN pass (34m antennas), the spacecraft is switched to the
high data rate (87648bps) for the recorded science data.  There are pros
and cons for making the data rate change at the beginning of the pass
via ground command or onboard time-tag command.

Onboard Time-tag Command
Using an onboard time-tag command to switch to the high data rate before
the DSN pass means that the ground station only needs to configure for
one data rate.  This was done from 03/20/1998 to 11/02/2004.  However,
if there are station problems or if the DSN pass has been rescheduled,
then NOAA SWPC wouldn't be receiving any ACE data (the network of ~12m
antennas cannot process the high rate data).  If the DSN pass had been
rescheduled and the onboard timer was not reloaded, then the DSN station
would need to configure for the low data rate at the beginning of the
pass and then switch to the high data rate after the ground command had
been sent.

Ground Command
Starting on 11/03/2004, each DSN pass started at the low data rate and
was commanded from the ground to the high data rate.  This provided more
consistent operations and reduced data loss by NOAA SWPC (~5
hours/year).  But, it also required a DSN operator to reconfigure the
ground equipment and it delayed the science data playback by 2-4
minutes.

Why Mention This Now?
To WS1's credit, they are updating and testing their equipment so that
it can determine which of the two data rates it is receiving.  (DSN is
also looking at reducing the work performed by the station operators
which includes data rate changes and other activities.)  Dealing with
the data rate change is just one of many decisions that goes into
configuring ACE for WS1.  And the length to which it is discussed in
this week's report shouldn't be used as a gauge of its importance.  It
just provides a little background information on ACE.

Some other notes:  In 2011, there were 9 DSN passes with problems at
beginning of track that delayed activities from 13-120 minutes and 3
passes that were rescheduled at the last minute.  This averages to only
1 problem per month where onboard data rate commands at the beginning of
the DSN pass would have an impact.  Also, in all scenarios, the command
to switch from the high data rate to the low data rate at the end of the
DSN pass is always done via onboard time-tag command.

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Anomalies:

Update on AR#S12-0001  S-ACE-0639  SWEPAM-E Overcount Alarm
The SWEPAM-Electron Overcount Alarm has flagged continuously since DOY
135.  The instrument team is working the next course of action, but
their time is limited.  IMPACT:  The instrument automatically reduces
voltages when the overcount alarm occurs.