SATELLITE NAVIGATION FOR METEOROLOGICAL PURPOSES - INVERSE REFERENCING FOR NOAA-N AND ERS-1 IMAGERS WITH A 1-KM NADIR PIXEL SIZE

Iterative methods for inverse referencing from mean orbital elements o r osculating position and velocity, accounting for all necessary orbit al perturbations with respect to given nadir pixel size, are described . [Inverse referencing means that the geodetic coordinates of a point on the surface are given and the corresponding image coordinates (scan line number and pixel number) are obtained from satellite orbital ele ments or coordinates.] The idea is to treat a pixel like a satellite t racking station on the ground. This permits the use of existing softwa re for the computation of satellite ephemerides and orbit determinatio n. The time of culmination of a satellite over the pixel and the off-n adir angle at that moment have been computed. Two variants for such a computation have been tested. Numerical results for the NOAA-N meteoro logical satellites and ERS-1 are presented. The present state of our s oftware for inverse referencing should fulfil ordinary requirements po sed by meteorologists. For NOAA-N satellites, the accuracy achieved ro ughly the nadir pixel size. The main obstacle to an increase in accura cy is the low quality of the mean orbital elements usually available. For ERS-1, the accuracy may achieve a level of 100 m. A software packa ge, containing versions of the FORTRAN 77 programs PIXPO 3, PIXPO 4 an d PIXPOSC, for various data types, including US-2 line or TBUS mean el ements or a state vector, is available for scientific exchange.