The IMAGE spacecraft spins about its Z axis. The spin is positive: looking into the Z axis, the spacecraft will rotate counter-clockwise. The nominal spin period is two minutes.

The spacecraft's CIDP sends 3600 evenly spaced synchronization (SYNC) pulses to HENA every spin, one SYNC pulse every 0.1°. A special double SYNC pulse signals nadir, which defines the beginning of a new spin. The nadir pulse occurs when the plane defined by the spacecraft +X axis and +/-Z axis points at the center of the earth. The HENA sensor will point at nadir 225° later.

A triple SYNC pulse is used to indicate sun position. This pulse occurs when the plane defined by the S/C +X axis and +/-Z axis encounters the center of the sun. The HENA sensor will see the sun 225° later. If the nadir and solar position are coincident, the nadir double SYNC pulse occurs, followed by the solar triple SYNC pulse one SYNC later. A sun pulse occurs every spin, even when the sun is eclipsed. Note: if the S/C spin axis is less than 30° off the sun, the sun never enters the HENA field of view, despite the sun pulse indication.

The HENA instrument divides the spin into 120 sectors. At the nominal spin rate, a sector is 1 second and covers three degrees. Sector zero and the new spin starts at nadir. HENA data products are labeled with the spin and sector that began their integration.

Return to HENA Software User's Guide. Report problems to John Hayes.