The PEACE instrument consists of two sensors, HEEA (High Energy Electron Analyser) and LEEA (Low Energy Electron
Analyser), and a data processing unit, the DPU. The sensors are Top Hat electrostatic analysers. The basic measurement
made by PEACE is the number of counted electrons per accumulation time. The satellite spin period, Tspin
, is measured as the time taken between two detections of the
Sun by the spacecraft sun sensor (which is expected to be 4 seconds ±10%).
Within the PEACE DPU, the spin period is subdivided into 1024 equal parts, of duration Tacc
, the PEACE accumulation time. The instrument measurement cycle
is thus adjusted to match the spin period so that PEACE collects data from a full
4pi solid angle (i.e. provides all-sky coverage) during each and every spin. For a nominal 4 second spin, Tacc
~ 3.9 msec.
In reality this is calculated from the duration of the previous spin
provided by the spacecraft housekeeping.