ACE Spin/Phase and Sunpulse Information
The ACE spacecraft coordinate system is documented in the
ACE Spacecraft Paper, in the ACE Book.
An online PDF version is available.
The Z-axis is the spin-axis, with positive pointing toward the Sun. See
page 262 of the paper.
The spacecraft spins about the Z-axis with sunward direction following
the sequence: +X, -Y, -X, +Y axes. The spacecraft spins at ~5rpm.
The J2000 (and GSE, etc) coordinate-systems are described in various places
online. The attitude vectors in the ACE attitude/orbit data just define the
direction of the spacecraft Z-axis in these various coordinate-systems.
To understand the spin-phase, one needs more than the attitude data;
One also needs the ACE sunpulsetime data from the Level-one HDF files,
or from the ASCII sunpulse files also provided by the ASC.
The sunpulse data is acquired by the sun-sensors. There are
two sets of sun-sensors, the +Z sensors (on the top-deck)
and the -X-Y sensors.
The +Z sensors are not currently in use. The -X-Y sensors are
on the -X-Y side of ACE, beside the star-scanner.
A possibly-useful document for visualizing this is online.
So, the X-axis of the -X-Y sun-sensors (also known as the sunpulse-line)
is +225 degrees from the official X-axis of the spacecraft. One should
be aware that the MAG team uses the sunpulse-line as the X-axis of
the spacecraft, a cause of some confusion between them and the SWEPAM
team (many years ago).
More info on the sun sensors is in sections 5.11 and 9.3 of the
"ACE Spacecraft C&DH Component Spec."
It is not online at the ASC currently, because of ITAR
restrictions. If you need it, contact the ASC.
The sunpulse-line is spinning around as the spacecraft spins.
The sunpulse occurs when the spacecraft-sun line, the sunpulse-line,
and the Z-axis of the spacecraft are coplanar (think awhile...)
The Sunpulsetime data are provided in the ACE level 1 data files, in
spacecraft clock units (floating-point). Since the spacecraft spins
at ~5rpm, these times are ~12 seconds apart.
In the ASCII sunpulse files, the sunpulse times are also provied in
ACEepoch units (seconds elapsed since 00:00:00 Jan 1 1996), and also
in UTC YEAR/Day_of_year.
You may obtain the ACE sunpulse data files
here.
Last Updated: 23 May, 2013
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