INTERORB 
                           AND RELATED 
                  GPS ORBITAL SOFTWARE PRODUCTS 
 
In order to prevent widespread duplication of effort, and to 
facilitate use of the National Geodetic Survey (NGS) Global 
Positioning System (GPS) ephemeris data products by 
nonprogrammers, a few fundamental Fortran 77 routines are 
available from NGS.  The following software programs provide for 
the creation of binary direct access files, and for retrieval and

interpolation of the GPS orbit data for the user's applications 
programs. 
 
INTERORB -- This main program is an untested driver to 
interpolate binary access orbit files with a record length of 52.

 
GPSDA -- This a main program which converts the standard GPS 
ephemeris ASCII file into a binary direct access file. 
 
SVPVF -- This is a subroutine to be called by the user's 
applications programs.  The applications program requests 
satellite position and velocity data at a specified time via 
SVPVF; SVPVF then retrieves the orbit data.  Internally SVPVF 
performs an iterpolation using LGPV9. 
 
LGPV9 -- This routine performs the interpolations for SVPVF. 
 
AJMJD -- This subroutine is to adjust a modified Julian date. 
 
GRCJL -- This subroutine converts a Gregorian date into a Julian 
date. 

GRCMJL -- This subroutine converts a Gregorian date into a
modified Julian date.
 
JLCMJL -- This subroutine converts a Julian date into a modified 
Julian date. 
 
MJLCGR -- This subroutine converts a modified Julian date into a 
Gregorian date. 
 
MJLCYD -- This subroutine converts a modified Julian date into 
the actual year and day from the beginning of that year. 
 
YDCGR -- This subroutine converts the actual year and the actual 
day of that year into a Gregorian date. 

 
June 30, 1989 
NOAA National Geodetic Survey 
William G. Kass 
301-443-0139