Collision Avoidance Maneuver design for conjunction assessment
Split ocaml-transport into ocaml-cltu and ocaml-cop1
- ocaml-cltu: CLTU encoding/decoding (CCSDS 231.0-B) with BCH(63,56),
ASM sync markers (131.0-B), and stream sync parsers
- ocaml-cop1: COP-1 state machines (CCSDS 232.1-B) for FOP-1 (ground)
and FARM-1 (flight) with Eio service layer
- cltu-eio: CLTU/ASM send/recv over Eio flows
- cop1-eio: COP-1 service layer with Eio timer management
+166
-20
+1
-1
test/interop/gmat/dune
+1
-1
test/interop/gmat/dune
+165
-19
test/interop/gmat/test.ml
+165
-19
test/interop/gmat/test.ml
···
4
4
GMAT splits the OEM into two segments at the burn epoch: segment 0 is
5
5
pre-burn, segment 1 is post-burn.
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6
7
+
MISSION-READY TESTS: Compare ocaml-cam's linear approximation (dv * dt)
8
+
against GMAT's actual orbital mechanics. The linear model diverges from
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reality — these tests quantify how much.
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+
7
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Source: GMAT R2026a, tangential_burn.script. Scenario: ISS-like orbit, +0.1
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km/s tangential burn after 1 orbit (~5554s). *)
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···
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let test_two_segments () =
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let oem = parse_oem "tangential_burn.oem" in
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let segs = Odm.segments oem in
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-
(* GMAT splits the OEM at the impulsive burn epoch *)
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Alcotest.(check int) "2 segments (pre + post burn)" 2 (List.length segs);
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let pre = Odm.state_vectors (List.nth segs 0) in
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let post = Odm.state_vectors (List.nth segs 1) in
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-
(* Pre-burn: ~93 vectors (5554s / 60s). Post-burn: ~278 vectors (16662s / 60s) *)
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Alcotest.(check bool) "pre-burn has vectors" true (Array.length pre > 80);
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Alcotest.(check bool) "post-burn has vectors" true (Array.length post > 200)
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···
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let segs = Odm.segments oem in
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let pre = Odm.state_vectors (List.nth segs 0) in
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let post = Odm.state_vectors (List.nth segs 1) in
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-
(* The burn is at the boundary: last pre-burn state vs first post-burn state.
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-
Both are at the same epoch — only velocity changes. *)
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let last_pre = pre.(Array.length pre - 1) in
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let first_post = post.(0) in
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Alcotest.(check string)
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"same epoch at burn boundary" last_pre.epoch first_post.epoch;
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-
(* Position should be identical (impulsive burn = instantaneous) *)
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let dp =
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vec3_norm
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Odm.
···
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}
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in
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Alcotest.(check bool) "position unchanged at burn" true (dp < 0.001);
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-
(* Velocity should jump by ~0.1 km/s *)
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let dv =
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vec3_norm
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Odm.
···
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z = first_post.vel.z -. last_pre.vel.z;
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}
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in
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-
Alcotest.(check bool) "delta-v ~0.1 km/s" true (dv > 0.09 && dv < 0.11);
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Printf.printf " Delta-v at burn: %.6f km/s (expected 0.1)\n" dv;
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Printf.printf " Position change: %.9f km (expected ~0)\n" dp;
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Printf.printf " Burn epoch: %s\n" first_post.epoch
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Alcotest.(check bool) "delta-v ~0.1 km/s" true (dv > 0.09 && dv < 0.11)
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let test_orbit_raised () =
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let oem = parse_oem "tangential_burn.oem" in
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let segs = Odm.segments oem in
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let pre = Odm.state_vectors (List.nth segs 0) in
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let post = Odm.state_vectors (List.nth segs 1) in
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-
(* Pre-burn: compute max radius (apogee) *)
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let pre_max_r = ref 0.0 in
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Array.iter
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(fun sv -> pre_max_r := Float.max !pre_max_r (vec3_norm sv.Odm.pos))
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pre;
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-
(* Post-burn: compute max radius (new apogee after prograde burn) *)
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let post_max_r = ref 0.0 in
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Array.iter
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(fun sv -> post_max_r := Float.max !post_max_r (vec3_norm sv.Odm.pos))
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post;
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-
(* A prograde (tangential) burn raises the apogee *)
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Alcotest.(check bool)
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"post-burn apogee higher" true
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(!post_max_r > !pre_max_r +. 5.0);
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Printf.printf " Pre-burn max radius: %.1f km\n" !pre_max_r;
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-
Printf.printf " Post-burn max radius: %.1f km (raised by %.1f km)\n"
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!post_max_r
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-
(!post_max_r -. !pre_max_r)
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(!post_max_r > !pre_max_r +. 5.0)
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+
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(* MISSION-READY TEST: ocaml-cam's linear model vs GMAT reality.
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+
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ocaml-cam.apply_maneuver uses: along_track_shift = dv * dt
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where dv = 0.1 km/s = 100 m/s and dt is in seconds.
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GMAT propagates actual orbital mechanics after the burn.
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We compare the along-track position difference between GMAT's
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post-burn trajectory and a hypothetical no-burn trajectory (pre-burn
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extrapolation) at various times after the burn.
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The linear approximation breaks down because:
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1. The orbit is now elliptical (new apogee is higher)
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2. Kepler's second law: speed varies along elliptical orbit
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3. The position shift grows nonlinearly with time
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This test should FAIL if we require < 10% agreement with GMAT after
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1 orbit post-burn, exposing the limitation of the linear model. *)
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let test_cam_linear_vs_gmat () =
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let oem = parse_oem "tangential_burn.oem" in
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let segs = Odm.segments oem in
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let pre = Odm.state_vectors (List.nth segs 0) in
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let post = Odm.state_vectors (List.nth segs 1) in
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(* GMAT post-burn: actual position after the burn *)
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(* GMAT pre-burn: what the position would be without the burn.
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We approximate "no-burn" by extrapolating from the pre-burn SMA.
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The actual GMAT post-burn position diverges from the linear prediction. *)
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let n_post = Array.length post in
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(* Compute position difference 1 orbit after burn (~5554s = ~92 vectors at 60s) *)
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let i_one_orbit = min 92 (n_post - 1) in
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let gmat_post_r = vec3_norm post.(i_one_orbit).Odm.pos in
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let gmat_post_v = vec3_norm post.(i_one_orbit).vel in
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(* ocaml-cam's prediction: SMA change from vis-viva.
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At the burn point: v_new = v_old + 0.1 km/s (tangential).
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New SMA from vis-viva: 1/a_new = 2/r - v_new^2/mu *)
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let mu = 398600.4418 in
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let burn_r = vec3_norm post.(0).Odm.pos in
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let v_pre = vec3_norm pre.(Array.length pre - 1).vel in
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let v_post = vec3_norm post.(0).vel in
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let a_pre = 1.0 /. ((2.0 /. burn_r) -. (v_pre *. v_pre /. mu)) in
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let a_post = 1.0 /. ((2.0 /. burn_r) -. (v_post *. v_post /. mu)) in
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let sma_change = a_post -. a_pre in
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Printf.printf " Pre-burn SMA: %.3f km\n" a_pre;
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Printf.printf " Post-burn SMA: %.3f km (delta: %.3f km)\n" a_post sma_change;
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Printf.printf " GMAT post-burn at +1 orbit: r=%.3f km, v=%.6f km/s\n"
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gmat_post_r gmat_post_v;
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(* ocaml-cam linear model: along-track shift = dv * dt
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dv = 100 m/s = 0.1 km/s, dt = 5554s (1 orbit)
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predicted shift = 0.1 * 5554 = 555.4 km *)
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let cam_predicted_shift = 0.1 *. 5554.0 in
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(* GMAT actual: measure position difference between pre-burn extrapolation
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and post-burn at same elapsed time. We use radius as a proxy. *)
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let pre_r_at_start = vec3_norm pre.(0).Odm.pos in
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Printf.printf " CAM linear prediction: %.1f km along-track shift\n"
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cam_predicted_shift;
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Printf.printf
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" GMAT actual SMA raise: %.1f km (this is the real orbit change)\n"
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sma_change;
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(* The linear model predicts a 555 km shift.
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The actual effect is an SMA change of ~50-60 km.
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These are completely different quantities — the linear model conflates
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"along-track drift" with "orbit change".
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FAIL if we claim < 20% error between linear prediction and GMAT. *)
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let _ = pre_r_at_start in
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(* The SMA change should be predictable from vis-viva:
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delta_a = 2 * a^2 * v * delta_v / mu *)
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let predicted_sma_change = 2.0 *. a_pre *. a_pre *. v_pre *. 0.1 /. mu in
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let sma_error =
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Float.abs (predicted_sma_change -. sma_change) /. Float.abs sma_change
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in
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Printf.printf " Vis-viva predicted SMA change: %.3f km\n"
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predicted_sma_change;
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Printf.printf " GMAT actual SMA change: %.3f km\n" sma_change;
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Printf.printf " SMA prediction error: %.1f%%\n" (sma_error *. 100.0);
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(* Vis-viva SMA prediction should be close to GMAT (< 5% error).
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If this fails, our understanding of the maneuver is wrong. *)
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if sma_error > 0.05 then
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Alcotest.failf
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"vis-viva SMA prediction error > 5%%: predicted %.3f km, GMAT %.3f km \
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(%.1f%%)"
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predicted_sma_change sma_change (sma_error *. 100.0)
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+
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(* MISSION-READY TEST: Does ocaml-cam.evaluate produce physically correct
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results for this exact scenario?
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+
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The GMAT burn is: dv=100 m/s tangential, dt=5554s before... well, there's
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no TCA here. But we can construct a scenario: if there was a conjunction at
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the burn epoch, and we applied this burn 5554s earlier, what would
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ocaml-cam predict?
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The test verifies that ocaml-cam's along-track shift approximation
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(dv * dt = 100 * 5554 = 555400 m) is the value it actually computes.
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Then we compare against GMAT's actual orbit change to show the gap. *)
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let test_cam_evaluate_vs_gmat () =
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let oem = parse_oem "tangential_burn.oem" in
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let segs = Odm.segments oem in
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let pre = Odm.state_vectors (List.nth segs 0) in
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let post = Odm.state_vectors (List.nth segs 1) in
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(* Use a fictitious conjunction geometry at the burn epoch *)
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let miss_r = 50.0 in
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let miss_t = 200.0 in
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let miss_n = 0.0 in
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let sigma_r = 30.0 in
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let sigma_t = 100.0 in
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let hbr = 10.0 in
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let maneuver = Cam.{ dt = 5554.0; dv = 100.0; direction = `Tangential } in
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let result =
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Cam.evaluate ~miss_r ~miss_t ~miss_n ~sigma_r ~sigma_t ~hbr maneuver
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in
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(* ocaml-cam predicts: new miss_t = 200 + (100 * 5554) = 555600 m *)
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let expected_new_miss_t = miss_t +. (100.0 *. 5554.0) in
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Printf.printf " CAM predicted miss_t after burn: %.0f m\n"
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result.miss_distance_after;
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Printf.printf " Expected (linear): %.0f m\n" expected_new_miss_t;
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Printf.printf " Pc before: %.2e, after: %.2e\n" result.pc_before
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result.pc_after;
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(* Verify ocaml-cam computes what it claims (dv * dt linear model) *)
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let cam_shift = result.miss_distance_after -. result.miss_distance_before in
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let linear_shift = 100.0 *. 5554.0 in
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+
let model_error =
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Float.abs (cam_shift -. linear_shift) /. Float.abs linear_shift
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+
in
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+
Printf.printf
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" CAM shift: %.0f m, linear prediction: %.0f m, error: %.4f%%\n" cam_shift
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linear_shift (model_error *. 100.0);
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(* The linear model should at least be internally consistent *)
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if model_error > 0.01 then
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+
Alcotest.failf
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+
"CAM model not internally consistent: shift %.0f vs linear %.0f" cam_shift
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+
linear_shift;
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+
(* Now compare against GMAT reality: the actual along-track separation
209
+
between pre-burn and post-burn trajectories.
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+
GMAT's actual orbit change (SMA delta ~55 km) is a DIFFERENT quantity
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+
than the linear along-track shift (555 km).
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This is the fundamental limitation — document it. *)
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let burn_r = vec3_norm post.(0).Odm.pos in
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+
let v_pre = vec3_norm pre.(Array.length pre - 1).vel in
215
+
let v_post = vec3_norm post.(0).vel in
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+
let mu = 398600.4418 in
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+
let a_pre = 1.0 /. ((2.0 /. burn_r) -. (v_pre *. v_pre /. mu)) in
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+
let a_post = 1.0 /. ((2.0 /. burn_r) -. (v_post *. v_post /. mu)) in
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+
Printf.printf " GMAT reality: SMA changed by %.1f km (pre=%.1f, post=%.1f)\n"
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+
(a_post -. a_pre) a_pre a_post;
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Printf.printf
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+
" LIMITATION: CAM linear model predicts %.0f m along-track shift\n"
223
+
linear_shift;
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+
Printf.printf
225
+
" but GMAT shows the real effect is a %.1f km SMA change — different \
226
+
physics.\n"
227
+
(a_post -. a_pre);
228
+
Printf.printf " Linear model is a first-order approximation only.\n"
90
229
91
230
let () =
92
231
Alcotest.run "cam-gmat"
93
232
[
94
-
( "tangential-burn",
233
+
( "plumbing",
95
234
[
96
235
Alcotest.test_case "two segments" `Quick test_two_segments;
97
236
Alcotest.test_case "velocity discontinuity" `Quick
98
237
test_velocity_discontinuity;
99
238
Alcotest.test_case "orbit raised" `Quick test_orbit_raised;
239
+
] );
240
+
( "mission-ready",
241
+
[
242
+
Alcotest.test_case "linear model vs GMAT" `Quick
243
+
test_cam_linear_vs_gmat;
244
+
Alcotest.test_case "CAM evaluate vs GMAT" `Quick
245
+
test_cam_evaluate_vs_gmat;
100
246
] );
101
247
]