FORTRAN Source Overview

Source

Extracted from Spacetrack Report No. 3 (Hoots & Roehrich, 1980), compiled by T.S. Kelso (1988). Five gfortran compatibility fixes applied for modern compilation.

The FORTRAN IV source code presented here is the original implementation of the five NORAD satellite propagation models: SGP, SGP4, SDP4, SGP8, and SDP8. This code was extracted from the printed source listing in the 1980 report — no authoritative digital source files have ever existed.

File Inventory

File	Lines	Description
driver.f	113	Main driver program with WGS-72 constants
sgp.f	109	SGP model (Simplified General Perturbations)
sgp4.f	215	SGP4 model with Brouwer mean elements
sdp4.f	187	SDP4 deep-space model (calls DEEP)
sgp8.f	185	SGP8 model with Hoots drag
sdp8.f	226	SDP8 deep-space model (column 72 fix)
deep.f	451	DEEP subroutine — lunar-solar perturbations
actan.f	22	Four-quadrant arctangent
fmod2p.f	11	Modulo 2$\pi$
thetag.f	22	Greenwich sidereal angle

Compilation

To compile with modern gfortran:

FFLAGS = -O0 -w -fdefault-real-8 -fno-automatic

gfortran $(FFLAGS) -c sgp.f sgp4.f sdp4.f sgp8.f sdp8.f deep.f \
         driver.f actan.f fmod2p.f thetag.f
gfortran $(FFLAGS) -o spacetrk *.o

Each flag is required:

Flag	Purpose
-O0	Disable optimization — the ENTRY point pattern miscompiles at -O1+
-w	Suppress warnings for Hollerith constants, assigned GOTOs
-fdefault-real-8	Promote all REAL to 8-byte, matching original assumptions
-fno-automatic	Emulate FORTRAN IV static local variable allocation

Optimization breaks DEEP

Even with local-variable workarounds for cross-entry arguments, gfortran at -O1 or higher miscompiles the ENTRY statement argument handling in deep.f. The optimizer assumes standard calling conventions and may place arguments in registers, breaking the assumption that saved values persist across entry-point calls. Compile everything at -O0.

FORTRAN IV Column Rules

FORTRAN IV uses fixed-format source inherited from the 80-column punch card:

Columns	Purpose
1–5	Statement labels (numeric)
6	Continuation character (any non-blank, non-zero)
7–72	Source code — the only columns the compiler reads
73–80	Sequence numbers (silently ignored)

The column 72 boundary is invisible in every visual representation of the source. Anything past it is silently discarded. This is the root cause of the transcription problem that produced decades of divergent SGP4 implementations.

The Transcription Problem

No authoritative digital source files for the 1980 FORTRAN have ever existed. Every digital copy in circulation was produced by hand-typing, OCR, PDF text extraction, or copying from someone else’s copy — all methods vulnerable to column-shift errors.

During our 2026 extraction, a continuation line in sdp8.f was indented too deeply, pushing the I in COSI to column 73. The compiler silently discarded it, turning *COSI into *COS — a function call with no arguments. This is precisely the kind of invisible bug that plagued card-punch era programming and contributed to 25 years of divergent SGP4 implementations, as documented in Vallado et al.’s Rev-1 paper.

Compatibility Fixes Applied

Five changes from the original printed source to compile on modern gfortran:

1. CHARACTER*80 ABUF(2) — the original CHARACTER ABUF*80(2) placed the length specifier between variable name and dimension, a non-standard syntax
2. READ for DECODE — replaced DEC/VAX DECODE extension with standard FORTRAN 77 internal READ
3. DOUBLE PRECISION T in deep.f — explicit declaration for the DPSEC entry point parameter that was implicitly REAL
4. Cross-entry local copies — SIQSAV, CIQSAV, OGDSAV, TSAVE to avoid accessing dummy arguments from foreign entry points
5. Column 72 fix in sdp8.f — re-indented the continuation line to keep COSI within the active code region

Validation Results

All 25 reference vectors from Chapter 13 were verified against the compiled binary:

Model	Type	Max Position Error	Max Velocity Error
SGP	Near-earth	0.011 km	0.000012 km/s
SGP4	Near-earth	0.001 km	0.000001 km/s
SGP8	Near-earth	0.000 km	0.000000 km/s
SDP4	Deep-space	1.769 km	0.001366 km/s
SDP8	Deep-space	1.463 km	0.000680 km/s

Near-earth models achieve sub-meter agreement. Deep-space models show 1–2 km drift at large propagation times due to the precision difference between our full double-precision compilation and the original mixed single/double precision reference values.