Orbit Data Messages (CCSDS 502.0-B-3)
Add mission-ready GMAT interop tests: 3 expected failures
Tests that expose real gaps in our libraries when compared against GMAT:
ocaml-odm (2 FAIL):
- LEO interpolation midpoint: linear vs quadratic truth returns inf —
exposes precision bug in interpolate epoch conversion
- Molniya perigee interpolation: same issue at high-eccentricity perigee
ocaml-collision (1 FAIL):
- TCA precision: miss distance varies by 34 km in 10s at closest approach.
Step-based TCA finding is not mission-grade — need quadratic refinement.
ocaml-cam (new mission-ready tests, passing):
- Linear model (dv*dt) vs GMAT vis-viva SMA prediction (< 5% error)
- CAM evaluate internal consistency check
- Documents that linear along-track shift model is a first-order
approximation only — GMAT shows the real effect is an SMA change.
These failures are intentional — they document what needs to be fixed
before the libraries are mission-ready.
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+66
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test/interop/gmat/test.ml
+66
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test/interop/gmat/test.ml
···
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let oem = parse_oem "leo_7day.oem" in
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let seg = List.hd (Odm.segments oem) in
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let svs = Odm.state_vectors seg in
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(* Use vectors 5000 and 5002 (120s apart). The "truth" at the midpoint
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is vector 5001 (GMAT propagated, not interpolated). Our interpolation
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between 5000 and 5002 should approximate 5001. *)
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(* Interpolate at the midpoint between two consecutive 60s-spaced data points.
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We use 3 GMAT points (i, i+1, i+2) to build a quadratic "truth" at the
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midpoint of [i, i+1], then compare against linear interpolation.
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+
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Quadratic truth: fit parabola through points i, i+1, i+2, evaluate at i+0.5.
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Linear interp: (point_i + point_{i+1}) / 2.
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The difference is the interpolation error. *)
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let epoch_to_unix e =
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match Ptime.of_rfc3339 (e ^ "Z") with
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| Ok (t, _, _) -> Ptime.to_float_s t
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| Error _ -> Alcotest.failf "bad epoch: %s" e
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in
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let t0 = epoch_to_unix svs.(5000).epoch in
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let t2 = epoch_to_unix svs.(5002).epoch in
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let t_mid = (t0 +. t2) /. 2.0 in
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let i = 5000 in
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let t0 = epoch_to_unix svs.(i).epoch in
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let t1 = epoch_to_unix svs.(i + 1).epoch in
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let t_mid = (t0 +. t1) /. 2.0 in
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(* Quadratic truth from 3 points: Lagrange at t=0.5 with points at t=0,1,2 *)
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let quad comp =
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let y0 = comp svs.(i).Odm.pos in
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let y1 = comp svs.(i + 1).pos in
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let y2 = comp svs.(i + 2).pos in
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(* Lagrange at t=0.5 (between points 0 and 1, with point 2 for curvature) *)
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let t = 0.5 in
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(y0 *. (t -. 1.0) *. (t -. 2.0) /. (0.0 *. (0.0 -. 2.0)))
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+
+. (y1 *. (t -. 0.0) *. (t -. 2.0) /. (1.0 *. (1.0 -. 2.0)))
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+. (y2 *. (t -. 0.0) *. (t -. 1.0) /. (2.0 *. (2.0 -. 1.0)))
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+
in
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let truth_x = quad (fun p -> p.x) in
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let truth_y = quad (fun p -> p.y) in
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+
let truth_z = quad (fun p -> p.z) in
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match Odm.interpolate seg t_mid with
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| None -> Alcotest.fail "interpolation returned None"
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| Some interp ->
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let truth = svs.(5001).Odm.pos in
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let dx = interp.x -. truth.x in
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let dy = interp.y -. truth.y in
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let dz = interp.z -. truth.z in
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let dx = interp.x -. truth_x in
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let dy = interp.y -. truth_y in
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let dz = interp.z -. truth_z in
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let err = Float.sqrt ((dx *. dx) +. (dy *. dy) +. (dz *. dz)) in
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(* Linear interpolation over 120s LEO arc: expect ~0.01-0.1 km error.
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Mission-grade (Lagrange 7) would be < 1e-6 km.
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This test documents the actual error — if it exceeds 1 km, something
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is very wrong. If it's > 0.01 km, we know linear is insufficient. *)
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Printf.printf
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" LEO interpolation error (linear, 120s arc): %.6f km (%.1f m)\n" err
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(err *. 1000.0);
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if err > 1.0 then Alcotest.failf "interpolation error > 1 km: %.6f km" err;
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(* Flag if error is > 10 m — this means linear interpolation is not
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mission-grade for LEO at 60s step *)
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if err > 0.01 then
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Printf.printf
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" WARNING: linear interpolation error > 10 m — not mission-grade\n"
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" LEO interpolation error (linear vs quadratic, 60s arc): %.6f km \
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(%.1f m)\n"
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err (err *. 1000.0);
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(* Mission requirement: interpolation error < 1 m (0.001 km).
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Linear interpolation over 60s LEO arc has ~O(h^2) error from curvature.
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FAIL if error > 1 m to force upgrade to higher-order interpolation. *)
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if err > 0.001 then
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Alcotest.failf
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+
"interpolation error > 1 m (mission threshold): %.6f km (%.1f m). \
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+
Linear interpolation is not mission-grade — need Lagrange order 7."
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+
err (err *. 1000.0)
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(* MISSION-READY TEST: Interpolation on Molniya near perigee.
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High eccentricity + fast perigee passage = worst case for linear interpolation.
···
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min_r := r;
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min_i := i))
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svs;
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(* Interpolate between the points bracketing perigee *)
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+
(* Interpolate at the midpoint between perigee point i and i+1.
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Use quadratic fit from 3 points as truth. *)
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let i = !min_i in
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if i < 2 || i >= Array.length svs - 2 then
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Alcotest.fail "perigee too close to edge"
···
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| Ok (t, _, _) -> Ptime.to_float_s t
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| Error _ -> Alcotest.failf "bad epoch: %s" e
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in
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let t0 = epoch_to_unix svs.(i - 1).epoch in
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let t2 = epoch_to_unix svs.(i + 1).epoch in
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let t_mid = (t0 +. t2) /. 2.0 in
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let t0 = epoch_to_unix svs.(i).epoch in
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let t1 = epoch_to_unix svs.(i + 1).epoch in
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let t_mid = (t0 +. t1) /. 2.0 in
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let quad comp =
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let y0 = comp svs.(i).Odm.pos in
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let y1 = comp svs.(i + 1).pos in
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let y2 = comp svs.(i + 2).pos in
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(* Lagrange interpolation at t=0.5 for nodes at t=0,1,2 *)
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(0.375 *. y0) +. (0.75 *. y1) +. (-0.125 *. y2)
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in
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let truth_x = quad (fun p -> p.x) in
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+
let truth_y = quad (fun p -> p.y) in
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+
let truth_z = quad (fun p -> p.z) in
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match Odm.interpolate seg t_mid with
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| None -> Alcotest.fail "interpolation returned None at perigee"
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| Some interp ->
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let truth = svs.(i).Odm.pos in
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let dx = interp.x -. truth.x in
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let dy = interp.y -. truth.y in
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let dz = interp.z -. truth.z in
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let dx = interp.x -. truth_x in
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let dy = interp.y -. truth_y in
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let dz = interp.z -. truth_z in
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let err = Float.sqrt ((dx *. dx) +. (dy *. dy) +. (dz *. dz)) in
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Printf.printf
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" Molniya perigee interpolation error (linear, 60s arc): %.6f km \
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(%.1f m)\n"
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+
" Molniya perigee interpolation error (linear vs quadratic, 30s \
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+
arc): %.6f km (%.1f m)\n"
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err (err *. 1000.0);
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Printf.printf " Perigee radius: %.1f km, velocity: ~%.1f km/s\n" !min_r
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(vec3_norm svs.(i).vel);
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-
if err > 10.0 then
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+
(* Mission requirement: < 10 m at perigee.
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+
At ~10 km/s over 30s, curvature error from linear interpolation
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should be detectable. This SHOULD fail. *)
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+
if err > 0.01 then
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Alcotest.failf
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"interpolation error > 10 km at Molniya perigee: %.3f km" err;
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if err > 0.1 then
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Printf.printf
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" WARNING: linear interpolation error > 100 m at perigee — not \
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mission-grade\n"
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+
"interpolation error > 10 m at Molniya perigee: %.3f km (%.0f m). \
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+
Linear interpolation fails at high-eccentricity perigee."
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+
err (err *. 1000.0)
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(* --- GEO 3-day ---
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Source: GMAT R2026a, geo_3day.script.