Driver Program

The DRIVER is the entry point for the entire propagation system. It reads TLE data from standard input, selects the appropriate model based on orbital period, and dispatches to the correct subroutine.

WGS-72 Constants

The DATA statement on lines 13–17 defines all physical and geopotential constants. These are WGS-72, not WGS-84 — deliberately, because NORAD TLEs are fitted to the SGP4/WGS-72 model.

*     DRIVER                                             3 NOV 80
*
*     WGS-72 PHYSICAL AND GEOPOTENTIAL CONSTANTS
*         CK2= .5*J2*AE**2        CK4=-.375*J4*AE**4
*
      DOUBLE PRECISION EPOCH,DS50
      COMMON/E1/XMO,XNODEO,OMEGAO,EO,XINCL,XNO,XNDT2O,XNDD6O,BSTAR,
     1          X,Y,Z,XDOT,YDOT,ZDOT,EPOCH,DS50
      COMMON/C1/CK2,CK4,E6A,QOMS2T,S,TOTHRD,
     1          XJ3,XKE,XKMPER,XMNPDA,AE
      COMMON/C2/DE2RA,PI,PIO2,TWOPI,X3PIO2
      DATA IHG/1HG/
      DATA DE2RA,E6A,PI,PIO2,QO,SO,TOTHRD,TWOPI,X3PIO2,XJ2,XJ3,
     1     XJ4,XKE,XKMPER,XMNPDA,AE/.174532925E-1,1.E-6,
     2     3.14159265,1.57079633,120.0,78.0,.66666667,
     4     6.2831853,4.71238898,1.082616E-3,-.253881E-5,
     5     -1.65597E-6,.743669161E-1,6378.135,1440.,1./
      DIMENSION ISET(5)
      CHARACTER*80 ABUF(2)
      DATA (ISET(I),I=1,5)/3HSGP,4HSGP4,4HSDP4,4HSGP8,4HSDP8/

Constant	Value	Description
`DE2RA`	0.0174532925	Degrees to radians
`E6A`	1.0E-6	Kepler iteration tolerance
`PI`	3.14159265	$\pi$
`QO`	120.0	Atmospheric parameter $q_0$ (km above Earth radius)
`SO`	78.0	Atmospheric parameter $s$ (km above Earth radius)
`TOTHRD`	0.66666667	$\frac{2}{3}$
`XJ2`	1.082616E-3	2nd zonal harmonic $J_2$
`XJ3`	-2.53881E-5	3rd zonal harmonic $J_3$
`XJ4`	-1.65597E-6	4th zonal harmonic $J_4$
`XKE`	0.0743669161	$\sqrt{GM}$ in Earth-radii $^{1.5}$ /min
`XKMPER`	6378.135	Earth equatorial radius (km)
`XMNPDA`	1440	Minutes per day
`AE`	1.0	Earth radius in Earth radii (normalization)

TLE Parsing

The driver reads two card formats: T (transmission/public two-line) and G (internal three-card G-card format used within NORAD):

*     READ IN MEAN ELEMENTS FROM 2 CARD T(TRANS) OR G(INTERN) FORMAT
      READ (5,706) ABUF
      READ(ABUF(1),707)    ITYPE
      IF(ITYPE.EQ.IHG) GO TO 5
      READ (ABUF,702) EPOCH,XNDT2O,XNDD6O,IEXP,BSTAR,IBEXP,XINCL,
     1       XNODEO,EO,OMEGAO,XMO,XNO
      GO TO 7
    5 READ(ABUF,701) EPOCH,XMO,XNODEO,OMEGAO,EO,XINCL,XNO,XNDT2O,
     1       XNDD6O,IEXP,BSTAR,IBEXP

The IHG Hollerith constant (1HG) stores the character “G” in an integer for comparison — a pre-CHARACTER*N technique from FORTRAN 66.

Model Selection

Deep-space classification is determined by checking if the orbital period exceeds 225 minutes:

*     INPUT CHECK FOR PERIOD VS EPHEMERIS SELECTED
*     PERIOD GE 225 MINUTES   IS DEEP SPACE
      A1=(XKE/XNO)**TOTHRD
      TEMP=1.5*CK2*(3.*COS(XINCL)**2-1.)/(1.-EO*EO)**1.5
      DEL1=TEMP/(A1*A1)
      AO=A1*(1.-DEL1*(.5*TOTHRD+DEL1*(1.+134./81.*DEL1)))
      DELO=TEMP/(AO*AO)
      XNODP=XNO/(1.+DELO)
      IF((TWOPI/XNODP/XMNPDA) .GE. .15625) IDEEP=1

Where .15625 = 225/1440 — 225 minutes as a fraction of a day. The computed GO TO on line 72 dispatches to the selected model:

   10 GO TO (21,22,23,24,25), IEPT
   21 CALL SGP(IFLAG,TSINCE)
   22 CALL SGP4(IFLAG,TSINCE)
   23 CALL SDP4(IFLAG,TSINCE)
   24 CALL SGP8(IFLAG,TSINCE)
   25 CALL SDP8(IFLAG,TSINCE)

Output Conversion

The models compute position/velocity in Earth radii and Earth radii per minute. The driver converts to km and km/s:

   60 X=X*XKMPER/AE
      Y=Y*XKMPER/AE
      Z=Z*XKMPER/AE
      XDOT=XDOT*XKMPER/AE*XMNPDA/86400.
      YDOT=YDOT*XKMPER/AE*XMNPDA/86400.
      ZDOT=ZDOT*XKMPER/AE*XMNPDA/86400.

Compatibility Notes

Two changes from the original printed source:

CHARACTER*80 ABUF(2) — the original CHARACTER ABUF*80(2) placed the length specifier between variable name and array dimension
READ for DECODE — the original used the DEC/VAX DECODE statement, a non-standard extension for internal reads