The evolution of the PROP6 orbit determination program and related topics

The orbit generator of the orbit determination program PROP is documented and compared with the generators of other programs in use. A synopsis is given of all the other changes since PROP3, and of changes in the related FORTRAN programs PREP and ORES. Some basic information is given for another program in the same family, PRAP, and details are presented on the way in which the programs were implemented. Other topics discussed include the effect on PROP runs of the use of satellite observations based on UTC as reference, the effect of the earth's precession on a satellite's orbital elements, and various aspects of the differential correction process as used in PROP.