Why are ephemeris data used?
Ephemeris data is a component part of the information downloaded in the telemetry data and is used during geometric corrections of the image. The data describes the conditions under which the data has been collected, and may also be used to correct the data before it is analysed. Geometric correction involves modelling the relationship between the image and systems of ground coordinates. Two kinds of geometric errors can be discerned: systematic geometric errors non-systematic geometric errors. Data on sensors and ephemeris information are modelled and applied to the raw imagery as part of systematic corrections. Thus, Eurimage systematic corrected (L1 G) products are free of systematic errors. Non-systematic errors caused primarily by satellite altitude and attitude require image to map rectification, and this process can use Ground Control Points (GCPs) to tie image pixel coordinates to their corresponding map coordinates. A sub pixel registration can necessitate a non-systematic err
