feat(decart): interp

internal/numerics/doc.go

@@ -1,11 +1,19 @@
 // Package numerics provides the numerical primitives used by the trajectory
-// engine: regular-grid multilinear interpolation, monotone bisection, and
-// a generic explicit Runge-Kutta-4 integrator with binary-search refinement
-// of a termination point.
+// engine: regular-grid multilinear interpolation, monotone bisection, spherical
+// kinematics, and a Runge-Kutta-4 integrator with binary-search refinement of a
+// termination point.
 //
-// The package has no dependencies on any domain type. State and derivative
-// types are generic, and all coordinate-wrap or unit-conversion semantics
-// live in the caller.
+// Positions are carried as GeoVec (degrees and metres) and advanced with
+// GeoStep, which rotates the position along a great circle. Rates are velocities
+// (Rate: metres per second east/north/up), never degrees per second — a
+// longitude derivative carries a 1/cos(lat) factor that diverges at the poles,
+// so the singularity is absent from the formulation rather than guarded against
+// by a threshold latitude. A step that reaches a pole continues down the far
+// side on its own.
+//
+// The package has no dependencies on any domain type. Longitude wrapping into
+// [0, 360) and the sphere geometry live here; unit conversions specific to a
+// data source stay in the caller.
 //
 // All algorithms are documented in docs/numerics.tex.
 package numerics