GENERAL PROGRAM DESCRIPTION

The five ephemeris packages cited in Section Two have each been programmed in FORTRAN IV as stand-alone subroutines. They each access the two function subroutines ACTAN and FMOD2P and the deep-space equations access the function subroutine THETAG. The function subroutine ACTAN is a two argument (quadrant preserving) arctangent subroutine which has been specifically designed to return the angle within the range of 0 to 2π. The function subroutine FMOD2P takes an angle and returns the modulo by 2π of that angle. The function subroutine THETAG calculates the epoch time in days since 1950 Jan 0.0 UTC, stores this in COMMON, and returns the right ascension of Greenwich at epoch.

One additional subroutine DEEP is accessed by SDP4 and SDP8 to obtain the deep-space perturbations to be added to the main equations of motion.

The main program DRIVER reads the input NORAD 2-line element set in either G-card internal format or T-card transmission format and calls the appropriate ephemeris package as specified by the user. The DRIVER converts the elements to the units of radians and minutes before calling the appropriate subroutine. The ephemeris package returns position and velocity in units of Earth radii and minutes. These are converted by the DRIVER to kilometers and seconds for printout.

All physical constants are contained in the constants COMMON C1 and can be changed through the data statements in the DRIVER. The one exception is the physical constants used only in DEEP which are set in the data statements in DEEP.

In the following sections the equations and program listing are given for each ephemeris model. Every effort has been made to maintain a strict parallel structure between the equations and the computer code.