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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