SGP4 - SGP4/SDP4 Orbit Propagator (Spacetrack Report #3)#

Pure OCaml implementation of the standard algorithm for predicting satellite positions from Two-Line Element (TLE) data. Used by NORAD, NASA, and most space agencies.

The algorithm handles:

• SGP4: Near-Earth objects (orbital period < 225 minutes)
• SDP4: Deep-space objects (orbital period >= 225 minutes)

Installation#

Install with opam:

$ opam install sgp4

If opam cannot find the package, it may not yet be released in the public opam-repository. Add the overlay repository, then install it:

$ opam repo add samoht https://tangled.org/gazagnaire.org/opam-overlay.git
$ opam update
$ opam install sgp4

Usage#

open Sgp4

(* Parse TLE and propagate *)
let tle_string =
  "ISS (ZARYA)\n\
   1 25544U 98067A   21073.51041667  .00001264  00000-0  29621-4 0  9993\n\
   2 25544  51.6442  21.2413 0002180  62.8608  47.5454 15.48915328273145"

let propagate_iss minutes_since_epoch =
  match parse_tle_string tle_string with
  | Error e -> Error e
  | Ok tle -> (
      match init tle with
      | Error e -> Error e
      | Ok state -> propagate state tle minutes_since_epoch)

Testing#

• Vallado test suite: 33 satellites from Vallado et al. (2006), verified against python-sgp4 tcppver.out
• GMAT interop: High-fidelity reference ephemeris from NASA GMAT R2026a (20x20 gravity, MSISE90 drag, SRP, lunisolar). Validates OEM parsing and physical bounds. See test/interop/gmat/.

Reference#

• Hoots & Roehrich, "Spacetrack Report No. 3", 1980
• Vallado's improvements (2006)