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physics: keep frames limited to rotations

This commit is contained in:
ApsisRS contributors 2026-09-26 13:27:05 +08:00
parent efb69c77bf
commit bdbaa20a5a
5 changed files with 16 additions and 124 deletions
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17
crates/physics/apsis-frames/src/earth_rotation.rs
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@ -1,7 +1,6 @@
//! The Earth Rotation Angle and the rotation it defines between the
//! inertial and Earth-fixed frames.
use crate::frame::{Ecef, Eci};
use apsis_math::Mat3;
use core::f64::consts::PI;
@ -125,19 +124,3 @@ pub fn rotation_eci_to_ecef(angle: EarthRotationAngle) -> Mat3 {
pub fn rotation_ecef_to_eci(angle: EarthRotationAngle) -> Mat3 {
rotation_eci_to_ecef(angle).transpose()
}
impl Eci {
/// Converts this inertial vector to the Earth-fixed frame at the given
/// Earth Rotation Angle.
pub fn to_ecef(self, angle: EarthRotationAngle) -> Ecef {
Ecef::new(rotation_eci_to_ecef(angle) * self.0)
}
}
impl Ecef {
/// Converts this Earth-fixed vector to the inertial frame at the given
/// Earth Rotation Angle.
pub fn to_eci(self, angle: EarthRotationAngle) -> Eci {
Eci::new(rotation_ecef_to_eci(angle) * self.0)
}
}

41
crates/physics/apsis-frames/src/frame.rs
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@ -1,41 +0,0 @@
//! Frame marker types.
use apsis_math::Vec3;
/// A position or vector expressed in the Earth-centered inertial frame.
///
/// The frame is aligned with the mean equator and equinox of J2000.0. It is
/// treated as inertial here, so it does not rotate with the Earth.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Eci(pub Vec3);
/// A position or vector expressed in the Earth-centered Earth-fixed frame.
///
/// The frame rotates with the Earth, so a point fixed to the ground has
/// constant coordinates in it.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Ecef(pub Vec3);
impl Eci {
/// Creates an inertial frame vector from its components.
pub const fn new(vector: Vec3) -> Self {
Self(vector)
}
/// Returns the underlying vector.
pub const fn vector(self) -> Vec3 {
self.0
}
}
impl Ecef {
/// Creates an Earth-fixed frame vector from its components.
pub const fn new(vector: Vec3) -> Self {
Self(vector)
}
/// Returns the underlying vector.
pub const fn vector(self) -> Vec3 {
self.0
}
}

27
crates/physics/apsis-frames/src/lib.rs
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@ -1,8 +1,11 @@
//! Reference frame definitions and the transformations between them.
//! Reference frame definitions and the rotation between the inertial and
//! Earth-fixed frames.
//!
//! A reference frame fixes the orientation and origin that a vector is
//! expressed relative to. This crate defines the two frames used for
//! near-Earth work and the rotation between them.
//! expressed relative to. This crate names the two frames used for
//! near-Earth work and computes the rotation between them. Concrete
//! position types that carry a frame are defined in `apsis-coord`; this
//! crate works with the rotation matrix and the angle that defines it.
//!
//! The inertial frame is the Earth-centered inertial (ECI) frame aligned
//! with the mean equator and equinox of J2000.0. Its axes are fixed in
@ -11,16 +14,16 @@
//!
//! The rotating frame is the Earth-centered Earth-fixed (ECEF) frame. Its
//! third axis is the Earth's rotation axis and its first axis follows the
//! prime meridian, so a point fixed to the ground has constant
//! coordinates in it.
//! prime meridian, so a point fixed to the ground has constant coordinates
//! in it.
//!
//! The two are related by a rotation of [`EarthRotationAngle`] about the
//! common third axis. This crate models the Earth's rotation with the
//! Earth Rotation Angle, which is a function of UT1 only. It does not
//! apply precession or nutation, because an angle accurate to a few
//! hundredths of a degree is sufficient for pointing and coverage work and
//! the full model needs the orientation of the celestial pole, which is a
//! separate computation.
//! common third axis. This crate models the Earth's rotation with the Earth
//! Rotation Angle, which is a function of UT1 only. It does not apply
//! precession or nutation, because an angle accurate to a few hundredths of
//! a degree is sufficient for pointing and coverage work and the full model
//! needs the orientation of the celestial pole, which is a separate
//! computation.
//!
//! Source: the Earth Rotation Angle expression is from the International
//! Astronomical Union, as given in the *IERS Conventions (2010)*, equation
@ -43,10 +46,8 @@
#![deny(missing_docs)]
mod earth_rotation;
mod frame;
pub use earth_rotation::{
ERA_J2000_TURNS, ERA_RATE_TURNS_PER_DAY, EarthRotationAngle, rotation_ecef_to_eci,
rotation_eci_to_ecef, ut1_days_from_j2000,
};
pub use frame::{Ecef, Eci};

36
crates/physics/apsis-frames/tests/earth_rotation.rs
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@ -1,8 +1,8 @@
//! Tests for the Earth Rotation Angle and the frame rotations.
use apsis_frames::{
ERA_RATE_TURNS_PER_DAY, EarthRotationAngle, Ecef, Eci, rotation_ecef_to_eci,
rotation_eci_to_ecef, ut1_days_from_j2000,
ERA_RATE_TURNS_PER_DAY, EarthRotationAngle, rotation_ecef_to_eci, rotation_eci_to_ecef,
ut1_days_from_j2000,
};
use apsis_math::Vec3;
use core::f64::consts::PI;
@ -125,38 +125,6 @@ fn rotation_carries_the_second_axis_toward_the_first() {
assert!((rotated.norm() - 1.0).abs() < EPS);
}
#[test]
fn frame_conversions_round_trip() {
let angle = EarthRotationAngle::from_ut1_days(9.25);
let original = Eci::new(Vec3::new(7_000.0, -1_500.0, 3_200.0));
let ecef = original.to_ecef(angle);
let back = ecef.to_eci(angle);
assert!((back.vector() - original.vector()).norm() < 1e-9);
}
#[test]
fn frame_conversion_preserves_length() {
let angle = EarthRotationAngle::from_ut1_days(-3.5);
let vector = Vec3::new(6_878_137.0, 0.0, 0.0);
let converted = Eci::new(vector).to_ecef(angle);
assert!((converted.vector().norm() - vector.norm()).abs() < 1e-6);
}
#[test]
fn rotating_background_returns_a_ground_point_to_the_same_longitude() {
// A point on the equator at zero longitude in the Earth-fixed frame is
// rotated by minus the Earth Rotation Angle in the inertial frame.
let angle = EarthRotationAngle::from_ut1_days(1.5);
let ground = Ecef::new(Vec3::new(6_378_137.0, 0.0, 0.0));
let inertial = ground.to_eci(angle);
let expected = Vec3::new(
6_378_137.0 * angle.radians_positive().cos(),
6_378_137.0 * angle.radians_positive().sin(),
0.0,
);
assert!((inertial.vector() - expected).norm() < 1e-6);
}
#[test]
fn a_full_turn_returns_to_the_same_rotation() {
// The rotation depends on the angle modulo one turn, so angles that

19
crates/physics/apsis-frames/tests/frame.rs
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@ -1,19 +0,0 @@
//! Tests for the frame marker types.
use apsis_frames::{Ecef, Eci};
use apsis_math::Vec3;
#[test]
fn wrappers_preserve_the_vector() {
let vector = Vec3::new(1.0, 2.0, 3.0);
assert_eq!(Eci::new(vector).vector(), vector);
assert_eq!(Ecef::new(vector).vector(), vector);
}
#[test]
fn wrappers_compare_by_value() {
let vector = Vec3::new(-4.0, 5.5, 0.0);
assert_eq!(Eci::new(vector), Eci(vector));
assert_eq!(Ecef::new(vector), Ecef(vector));
assert_ne!(Eci::new(vector), Eci::new(Vec3::ZERO));
}