fixed bug in half-angle matching algorithm

src/operations.ts

@@ -50,6 +50,5 @@ export function matchHalfPlane(angle: number, match: number): number {
     const [a1, a2] = [angle, TWO_PI - angle];
     const d1 = Math.atan2(Math.sin(a1 - match), Math.cos(a1 - match));
     const d2 = Math.atan2(Math.sin(a2 - match), Math.cos(a2 - match));
-    if (d1 < d2) { return a1; }
-    return a2;
+    return (Math.abs(d1) < Math.abs(d2)) ? a1 : a2;
 }

src/propagators/kepler.ts

@@ -96,7 +96,7 @@ export class Kepler implements IPropagator {
         let EInit = Math.acos((e + Math.cos(v)) / (1 + e * Math.cos(v)));
         EInit = matchHalfPlane(EInit, v);
         let MInit = EInit - e * Math.sin(EInit);
-        MInit = matchHalfPlane(MInit, v);
+        MInit = matchHalfPlane(MInit, EInit);
         let MFinal = (MInit / TWO_PI)
             + n * delta
             + (nDot / 2) * Math.pow(delta, 2)

src/propagators/runge-kutta-4.ts

@@ -23,22 +23,32 @@ const DEFAULT_MODEL: INumericalModel = {
     stepSize: 60,
 };
 
+/** Default propagator model for two-body acceleration. */
+const DEFAULT_MODEL_TWOBODY: INumericalModel = {
+    area: 0,
+    atmosphericDrag: false,
+    drag: 0,
+    gravityMoon: false,
+    gravitySun: false,
+    j2Effect: false,
+    j3Effect: false,
+    j4Effect: false,
+    mass: 0,
+    reflect: 0,
+    solarRadiation: false,
+    stepSize: 0,
+};
+
 /** 4th order Runge-Kutta numerical integrator for satellite propagation. */
 export class RungeKutta4 implements IPropagator {
     /**
      * Create a new RungeKutta4 propagator object, using onlt two-body
      * perturbation options.
      */
-    public static twoBody(state: J2000): RungeKutta4 {
-        return new RungeKutta4(state, {
-            atmosphericDrag: false,
-            gravityMoon: false,
-            gravitySun: false,
-            j2Effect: false,
-            j3Effect: false,
-            j4Effect: false,
-            solarRadiation: false,
-        });
+    public static twoBody(state: J2000, model?: INumericalModel): RungeKutta4 {
+        model = model || {};
+        const mergeModel = { ...DEFAULT_MODEL_TWOBODY, ...model };
+        return new RungeKutta4(state, mergeModel);
     }
 
     /** Propagator identifier string. */

src/test/test-propagators.ts

@@ -1,5 +1,6 @@
 import * as assert from "assert";
 import { J2000 } from "../coordinates/j2000";
+import { Kepler } from "../propagators/kepler";
 import { RungeKutta4 } from "../propagators/runge-kutta-4";
 
 const GEO_STATE_1 = [
@@ -38,3 +39,28 @@ describe("RungeKutta4", () => {
         });
     });
 });
+
+describe("Kepler", () => {
+    describe("#.step()", () => {
+        it("should match numerical two-body results", () => {
+            const state = new J2000(
+                Date.UTC(2017, 10, 16, 0, 11, 30, 195),
+                -3.86234943730e4, 1.68697633760e4, 1.00434444900e3,
+                -1.231249e0, -2.810612e0, -2.01294e-1,
+            );
+            const propRk = RungeKutta4.twoBody(state, { stepSize: 60 });
+            const resultRk = propRk.step(
+                Date.UTC(2017, 10, 16, 0, 11, 30, 195), 21600, 6,
+            );
+            const propKep = new Kepler(state.toKeplerian());
+            const resultKep = propKep.step(
+                Date.UTC(2017, 10, 16, 0, 11, 30, 195), 21600, 6,
+            );
+            for (let i = 0; i < resultRk.length; i++) {
+                const dist = resultRk[i].position
+                    .distance(resultKep[i].position) * 1000;
+                assert.ok(dist < 1, `Distance: ${dist.toFixed(3)} meters`);
+            }
+        });
+    });
+});