The Mission


On-board Spin FIT module


Phase Shift: Given a spin rate of 3 seconds, the use of 128 Hz data for spin fitting puts an apparent phase shift of 360/(3*128) or roughly 0.9 degrees into the results. While this meets the 1.0 degree requirement, the phase shift correction can be determined on the ground using the spin pulse time data relative to the 1Hz tick which is the basis of the 128 Hz data.

Sunward-aligned signal: Based upon data from numerous missions, the type of Electric Field sensors THEMIS is using will likely experience an apparent sunward-aligned signal. This error signal is due to the electron population shifting between probe and spacecraft bodies. In addition, any shadowing of the electric field sensor from the spacecraft body will cause a voltage spike, whose width is roughly 2 * arcsin (0.7/25) meters or about 3 degrees. Both issues point out that the Spin Fit data points shall be taken at consistent points through the spin. It has been tradition to make the first point the sunward aligned point, and let it be rejected immediately in the SPIN procedure.

Missing Sun-pulses The sun pulse is not available when the spacecraft is in occultation. Spin Fit data continues to be acquired during shadows, "free-wheeling" with the last known spin period. As soon as sun pulse data becomes available, at shadow exit, the Spin Fit data resynchronizes with the sun pulse times.