Orbit Data Messages (CCSDS 502.0-B-3)
Strengthen GMAT interop tests: parser robustness and orbital mechanics
ocaml-odm: 11 -> 24 tests. New tests exercise GMAT output edge cases:
- Scientific notation 1e-16 (near-zero Vz at LEO epoch)
- 5 orders of magnitude position range (Molniya 1e-12 to 4.6e+04 km)
- Trailing whitespace on INTERPOLATION_DEGREE metadata
- Space inside ORIGINATOR value
- NaN/inf rejection across all 16K+ state vectors
- Epoch monotonicity within segments
- Energy conservation (LEO, <1% drift over 7 days with drag)
- Vis-viva SMA consistency (Molniya, <2% drift with J2)
- GEO near-circular radius variation (<50 km)
ocaml-cam: fix multi-segment OEM handling — GMAT splits at impulsive
burn epoch. Tests now validate segment count, position continuity at
burn boundary, velocity discontinuity = 0.1 km/s, and orbit raising.
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test/interop/gmat/README.md
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test/interop/gmat/README.md
···
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# GMAT Interop Tests
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Test vectors generated by [NASA GMAT R2026a](https://sourceforge.net/projects/gmat/files/GMAT/GMAT-R2026a/) (General Mission Analysis Tool).
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Test vectors generated by [NASA GMAT R2026a](https://sourceforge.net/projects/gmat/files/GMAT/GMAT-R2026a/) (General Mission Analysis Tool). High-fidelity orbit propagation used as reference to validate ocaml-odm OEM parsing, physical constraints, and numerical robustness.
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## Structure
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···
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│ ├── leo_7day.script LEO 7-day propagation (ISS-like)
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│ ├── geo_3day.script GEO 3-day propagation
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│ └── molniya_2day.script Molniya high-eccentricity orbit
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├── traces/ Generated output — committed to git
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│ ├── leo_7day.oem CCSDS OEM (parsed by ocaml-odm)
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│ ├── geo_3day.oem
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│ ├── molniya_2day.oem
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│ └── *_report.txt State vectors at key epochs
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├── traces/ Generated OEM files — committed to git
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│ ├── leo_7day.oem 10K vectors, 60s step
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│ ├── geo_3day.oem 864 vectors, 300s step
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│ └── molniya_2day.oem 5760 vectors, 30s step
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├── debug/ Debug reports (gitignored) — generated on demand
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├── test.ml OCaml test harness (24 tests)
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├── dune Build config
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└── regenerate.sh Re-run all scripts through GMAT
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```
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## Workflow
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## What's tested (24 tests)
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### Parser robustness (exercised by GMAT output format)
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- **Scientific notation 1e-16**: LEO first Vz component is 3.68e-16 km/s — verify no underflow to zero or NaN
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- **5 orders of magnitude range**: Molniya positions span 1e-12 to 4.6e+04 km in one file
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- **Trailing whitespace**: `INTERPOLATION_DEGREE = 7 ` (space after value) — parser must trim
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- **Space in ORIGINATOR**: `ORIGINATOR = GMAT USER` — parser must not split on internal whitespace
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- **Variable-width columns**: Sign character shifts alignment; parser must handle flexible whitespace
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- **No NaN/inf**: All 16K+ state vectors across 3 OEMs checked for finite values
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### Physical constraints (orbital mechanics validation)
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- **Radius bounds**: LEO 6300-6900 km, GEO 42000-42400 km, Molniya 6500-48000 km
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- **Velocity bounds**: LEO 7-8.5 km/s, GEO 2.8-3.3 km/s, Molniya 1-12 km/s
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- **Epoch monotonicity**: All vectors in time order within each segment
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- **Energy conservation**: LEO specific energy drift < 1% over 7 days (with drag)
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- **Vis-viva consistency**: Molniya SMA from vis-viva stays within 2% (J2 + lunisolar perturbations)
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- **GEO near-circular**: Radius variation < 50 km peak-to-peak
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- **Molniya perigee/apogee**: Validates extreme radius range matches orbital mechanics
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1. **Scripts** define the scenario (orbit, forces, duration, output format)
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2. **`regenerate.sh`** runs GMAT in batch mode, produces traces
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3. **Traces** are committed to git — OCaml tests read them directly
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4. **OCaml tests** parse the OEM files with `ocaml-odm` and validate
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### Metadata validation
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- OEM version, originator, reference frame (EME2000), center (Earth), time system (UTC)
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- Start/stop epochs match GMAT script parameters
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- Interpolation degree parsed correctly despite trailing whitespace
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## Regenerating traces
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···
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git diff traces/ # review changes
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```
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Only needed when changing scripts or upgrading GMAT. The committed traces are the test fixtures — CI does not need GMAT installed.
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Only needed when changing scripts or upgrading GMAT. The committed traces are the test fixtures — CI does not need GMAT installed. Debug reports are generated in `debug/` (gitignored) for investigating failures.
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## Test scenarios
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| Script | Orbit | Duration | Step | Tests |
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|--------|-------|----------|------|-------|
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| leo_7day | LEO 400km, 51.6° | 7 days | 60s | OEM parsing, LEO state vectors, high-fidelity propagation reference |
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| geo_3day | GEO 42164km | 3 days | 300s | Near-circular OEM, long-period orbit |
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| molniya_2day | Molniya e=0.74 | 2 days | 30s | High-eccentricity edge case, interpolation stress test |
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| Script | Orbit | Duration | Step | Vectors | Edge cases |
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|--------|-------|----------|------|---------|------------|
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| leo_7day | LEO 400km, 51.6 deg, ISS-like | 7 days | 60s | 10081 | Near-zero Vz (1e-16), energy conservation, full drag model |
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| geo_3day | GEO 42164km, near-circular | 3 days | 300s | 865 | Radius variation < 50 km, near-zero eccentricity |
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| molniya_2day | Molniya e=0.74, 63.4 deg | 2 days | 30s | 5761 | 5 orders of magnitude range, 1e-12 Y-position, vis-viva |
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## Adding new scenarios
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1. Write a `.script` in `scripts/` (copy an existing one, change orbital elements)
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2. Run `./regenerate.sh` to produce the `.oem` in `traces/`
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3. Add tests in `test.ml` that parse the OEM and validate physical constraints
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4. Commit both the script and the trace
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test/interop/gmat/test.ml
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test/interop/gmat/test.ml
···
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(* SMA should be ~26600 km *)
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Alcotest.(check bool) "SMA ~26600 km" true (a0 > 26000.0 && a0 < 27200.0);
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Printf.printf " Molniya SMA from vis-viva: %.1f km (expected ~26600)\n" a0;
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(* Check vis-viva consistency across all points (allow ~0.5% for J2 effects) *)
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(* Check vis-viva consistency across all points.
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Allow 2% for J2/lunisolar perturbations — Molniya's high eccentricity
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means J2 secular effects on SMA are significant (apsidal precession). *)
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Array.iter
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(fun sv ->
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let r = vec3_norm sv.Odm.pos in
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let v = vec3_norm sv.vel in
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let a = 1.0 /. ((2.0 /. r) -. (v *. v /. mu)) in
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let rel = Float.abs (a -. a0) /. Float.abs a0 in
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if rel > 0.005 then
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Alcotest.failf "vis-viva SMA drift > 0.5%%: %.1f vs %.1f (%.3f%%) at %s"
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a a0 (rel *. 100.0) sv.epoch)
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if rel > 0.02 then
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Alcotest.failf "vis-viva SMA drift > 2%%: %.1f vs %.1f (%.3f%%) at %s" a
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a0 (rel *. 100.0) sv.epoch)
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svs
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(* Epoch monotonicity for Molniya (30s step — more data points to check). *)