Before each OM science exposure a reference tracking frame is obtained. On this frame suitable guide stars are identified by the onboard software and tracking windows defined around these.
The guide stars are used to calculate the OM attitude solution for the
process of shift & add of the tracking frames of the science windows. The
reference tracking frame taken first provides the coordinate reference
point. OM guide star tracking windows are small windows (
in-memory pixels), defined as such so as to read out a minimum of data as
fast as possible and take away a minimum of OM's processing capacity
from the science observations.
Images of star tracking windows are not down-linked to the ground, but processed onboard. Only the x and y position of the tracking stars in units of in-memory pixels, the number of counts from the stars per tracking frame and the pointing offset information is transmitted as part of the telemetry stream. The number and the properties of OM star tracker windows for a science observation are determined by an onboard algorithm and therefore not user-selectable.