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orbit: add ephemeris sampling and Hermite interpolation

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ApsisRS contributors 2026-09-26 23:11:00 +08:00
parent 48300342ab
commit f45f2ad713
7 changed files with 1077 additions and 0 deletions
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5
Cargo.lock generated
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@ -82,6 +82,11 @@ dependencies = [
[[package]] [[package]]
name = "apsis-ephemeris" name = "apsis-ephemeris"
version = "0.1.0" version = "0.1.0"
dependencies = [
"apsis-core",
"apsis-kepler",
"apsis-math",
]
[[package]] [[package]]
name = "apsis-event" name = "apsis-event"

5
crates/orbit/apsis-ephemeris/Cargo.toml
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@ -11,3 +11,8 @@ publish = false
workspace = true workspace = true
[dependencies] [dependencies]
apsis-core.workspace = true
apsis-math.workspace = true
[dev-dependencies]
apsis-kepler.workspace = true

297
crates/orbit/apsis-ephemeris/src/ephemeris.rs Normal file
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@ -0,0 +1,297 @@
//! The ephemeris table and its Hermite interpolation.
use crate::error::{EphemerisError, EphemerisErrorKind};
use apsis_math::Vec3;
/// One sampled state.
///
/// A sample is a position and velocity at a time, which is the form every
/// propagator in the workspace produces and the form the interpolation
/// needs.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Sample {
/// The time of the sample, in seconds from the caller's origin.
pub seconds: f64,
/// Position, in kilometers.
pub position: Vec3,
/// Velocity, in kilometers per second.
pub velocity: Vec3,
}
impl Sample {
/// Creates a sample.
pub const fn new(seconds: f64, position: Vec3, velocity: Vec3) -> Self {
Self {
seconds,
position,
velocity,
}
}
/// Returns `true` when the time, position, and velocity are finite.
pub fn is_valid(&self) -> bool {
self.seconds.is_finite() && self.position.is_finite() && self.velocity.is_finite()
}
/// Returns the distance from the origin, in kilometers.
pub fn radius(&self) -> f64 {
self.position.norm()
}
/// Returns the speed, in kilometers per second.
pub fn speed(&self) -> f64 {
self.velocity.norm()
}
}
/// Evaluates the Hermite cubic for one component.
///
/// `p0` and `p1` are the endpoint values, `v0` and `v1` the endpoint
/// derivatives with respect to time, `dt` the interval length, and `s` the
/// fraction along the interval in `[0, 1]`.
///
/// The four terms are the standard cubic Hermite basis: the first and third
/// carry the endpoints, the second and fourth the slopes. The derivative
/// terms are scaled by `dt` because the slopes are per unit time while `s`
/// is per unit interval.
pub fn hermite_position(p0: Vec3, v0: Vec3, p1: Vec3, v1: Vec3, dt: f64, s: f64) -> Vec3 {
let s2 = s * s;
let s3 = s2 * s;
let h00 = 2.0 * s3 - 3.0 * s2 + 1.0;
let h10 = s3 - 2.0 * s2 + s;
let h01 = -2.0 * s3 + 3.0 * s2;
let h11 = s3 - s2;
p0 * h00 + v0 * (h10 * dt) + p1 * h01 + v1 * (h11 * dt)
}
/// Evaluates the derivative of the Hermite cubic for one component.
///
/// This is the velocity that goes with [`hermite_position`]. It is the exact
/// derivative of that cubic, so the reconstructed state is internally
/// consistent: a caller that differentiates the position gets this velocity.
pub fn hermite_velocity(p0: Vec3, v0: Vec3, p1: Vec3, v1: Vec3, dt: f64, s: f64) -> Vec3 {
let s2 = s * s;
let dh00 = 6.0 * s2 - 6.0 * s;
let dh10 = 3.0 * s2 - 4.0 * s + 1.0;
let dh01 = -6.0 * s2 + 6.0 * s;
let dh11 = 3.0 * s2 - 2.0 * s;
// The chain rule turns the per interval derivative into a per second
// one, which divides the position terms by `dt` and leaves the velocity
// terms, already per second, untouched.
p0 * (dh00 / dt) + v0 * dh10 + p1 * (dh01 / dt) + v1 * dh11
}
/// Evaluates both the Hermite position and velocity.
pub fn hermite_state(p0: Vec3, v0: Vec3, p1: Vec3, v1: Vec3, dt: f64, s: f64) -> (Vec3, Vec3) {
(
hermite_position(p0, v0, p1, v1, dt, s),
hermite_velocity(p0, v0, p1, v1, dt, s),
)
}
/// A table of sampled states that answers queries by interpolation.
///
/// The table holds at least two samples in strictly increasing time order.
/// A query at a sample time returns that sample exactly; a query between two
/// samples returns the Hermite cubic through the four endpoint values. A
/// query outside the covered interval is refused rather than extrapolated,
/// because the interpolation has no information there and an extrapolated
/// orbit can diverge quickly.
#[derive(Debug, Clone, PartialEq)]
pub struct Ephemeris {
samples: Vec<Sample>,
}
impl Ephemeris {
/// Builds a table from samples in increasing time order.
///
/// Returns an error when there are fewer than two samples, when a sample
/// is not finite, or when the times do not strictly increase.
pub fn from_samples(samples: Vec<Sample>) -> Result<Self, EphemerisError> {
if samples.len() < 2 {
return Err(EphemerisError::new(EphemerisErrorKind::TooFewSamples)
.with_detail(format!("{} sample(s)", samples.len())));
}
for sample in &samples {
if !sample.is_valid() {
return Err(EphemerisError::new(EphemerisErrorKind::InvalidState)
.with_detail(format!("at {} s", sample.seconds)));
}
}
for pair in samples.windows(2) {
if pair[1].seconds <= pair[0].seconds {
return Err(
EphemerisError::new(EphemerisErrorKind::NotMonotonic).with_detail(format!(
"{} s follows {} s",
pair[1].seconds, pair[0].seconds
)),
);
}
}
Ok(Self { samples })
}
/// Returns the samples in time order.
pub fn samples(&self) -> &[Sample] {
&self.samples
}
/// Returns the number of samples.
pub fn len(&self) -> usize {
self.samples.len()
}
/// Returns `true` when the table is empty.
///
/// A table built through [`Ephemeris::from_samples`] is never empty, so
/// this is always `false` for one that exists; it is provided so that
/// generic code that accepts a slice can use the usual pair of methods.
pub fn is_empty(&self) -> bool {
self.samples.is_empty()
}
/// Returns the first sample time, in seconds.
pub fn start_time(&self) -> f64 {
self.samples.first().map_or(0.0, |sample| sample.seconds)
}
/// Returns the last sample time, in seconds.
pub fn end_time(&self) -> f64 {
self.samples.last().map_or(0.0, |sample| sample.seconds)
}
/// Returns the time the table covers, in seconds.
pub fn duration(&self) -> f64 {
self.end_time() - self.start_time()
}
/// Returns `true` when a time falls within the covered interval.
///
/// The interval is closed, so its endpoints are covered.
pub fn covers(&self, seconds: f64) -> bool {
seconds.is_finite() && seconds >= self.start_time() && seconds <= self.end_time()
}
/// Returns the nominal step, when the samples are evenly spaced.
///
/// Returns `None` when the spacing varies, which a caller checks before
/// indexing samples by arithmetic.
pub fn uniform_step(&self) -> Option<f64> {
let first = self.samples.windows(2).next()?[1].seconds - self.samples[0].seconds;
let uniform = self
.samples
.windows(2)
.all(|pair| (pair[1].seconds - pair[0].seconds - first).abs() <= 1e-9);
uniform.then_some(first)
}
/// Returns `true` when the samples are evenly spaced.
pub fn is_uniform(&self) -> bool {
self.uniform_step().is_some()
}
/// Returns the index of the interval that contains a time.
///
/// The index `i` is such that `samples[i].seconds <= seconds <=
/// samples[i + 1].seconds`. A time outside the interval returns `None`.
fn interval_of(&self, seconds: f64) -> Option<usize> {
if !self.covers(seconds) {
return None;
}
// A binary search for the last sample at or before the time. The
// partition point is the first sample strictly after it, so the
// interval starts one before, clamped to the last interval.
let after = self
.samples
.partition_point(|sample| sample.seconds <= seconds);
let index = after.saturating_sub(1);
Some(index.min(self.samples.len() - 2))
}
/// Returns the state at a time, interpolating between samples.
///
/// A time equal to a sample time returns that sample. Returns `None`
/// when the time is outside the covered interval.
pub fn state_at(&self, seconds: f64) -> Option<Sample> {
let index = self.interval_of(seconds)?;
let first = self.samples[index];
let second = self.samples[index + 1];
// At a sample the interpolation must reproduce the sample exactly,
// which avoids the round off of evaluating the cubic at its end.
if seconds == first.seconds {
return Some(first);
}
if seconds == second.seconds {
return Some(second);
}
let dt = second.seconds - first.seconds;
let s = (seconds - first.seconds) / dt;
let (position, velocity) = hermite_state(
first.position,
first.velocity,
second.position,
second.velocity,
dt,
s,
);
Some(Sample::new(seconds, position, velocity))
}
/// Returns the state at a time, or the nearest endpoint when the time is
/// outside the covered interval.
///
/// This is for a caller that must produce a value for any time and is
/// content to hold the last known state rather than fail.
pub fn clamped_state_at(&self, seconds: f64) -> Option<Sample> {
if self.covers(seconds) {
return self.state_at(seconds);
}
if !seconds.is_finite() {
return None;
}
if seconds < self.start_time() {
self.samples.first().copied()
} else {
self.samples.last().copied()
}
}
/// Returns the sample nearest to a time, without interpolating.
pub fn nearest_sample(&self, seconds: f64) -> Option<Sample> {
if !seconds.is_finite() {
return None;
}
let index = match self.interval_of(seconds) {
Some(index) => {
let first = self.samples[index];
let second = self.samples[index + 1];
if (seconds - first.seconds).abs() <= (second.seconds - seconds).abs() {
index
} else {
index + 1
}
}
None if seconds < self.start_time() => 0,
None => self.samples.len() - 1,
};
Some(self.samples[index])
}
/// Returns the times of every sample, in order.
pub fn times(&self) -> Vec<f64> {
self.samples.iter().map(|sample| sample.seconds).collect()
}
/// Returns `true` when every state satisfies a predicate.
///
/// This is a convenience for a caller that wants to check a property of
/// the whole table, such as every radius being above the surface, once.
pub fn all_samples<F>(&self, predicate: F) -> bool
where
F: Fn(&Sample) -> bool,
{
self.samples.iter().all(predicate)
}
}

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crates/orbit/apsis-ephemeris/src/error.rs Normal file
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//! Errors reported while building or querying an ephemeris.
use core::fmt;
/// The category of an ephemeris error.
#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
#[non_exhaustive]
pub enum EphemerisErrorKind {
/// The sampling grid has fewer than two samples.
TooFewSamples,
/// A time or a step is not finite.
NotFinite,
/// A step is zero or negative.
NonPositiveStep,
/// The samples are not in increasing time order.
NotMonotonic,
/// A state produced by the sampler is not finite.
InvalidState,
/// A query time falls outside the covered interval.
OutOfRange,
}
impl fmt::Display for EphemerisErrorKind {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
let text = match self {
Self::TooFewSamples => "fewer than two samples",
Self::NotFinite => "a time is not finite",
Self::NonPositiveStep => "the step is not positive",
Self::NotMonotonic => "the samples are not in increasing time order",
Self::InvalidState => "the sampled state is not finite",
Self::OutOfRange => "the time is outside the covered interval",
};
f.write_str(text)
}
}
/// A failure while building or querying an ephemeris.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct EphemerisError {
kind: EphemerisErrorKind,
detail: Option<String>,
}
impl EphemerisError {
/// Creates an error with the given category.
pub fn new(kind: EphemerisErrorKind) -> Self {
Self { kind, detail: None }
}
/// Attaches a description of the value that caused the failure.
pub fn with_detail(mut self, detail: impl Into<String>) -> Self {
self.detail = Some(detail.into());
self
}
/// Returns the category of this failure.
pub const fn kind(&self) -> EphemerisErrorKind {
self.kind
}
/// Returns the detail text attached to this error, if any.
pub fn detail(&self) -> Option<&str> {
self.detail.as_deref()
}
}
impl fmt::Display for EphemerisError {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
match &self.detail {
Some(detail) => write!(f, "{}: {detail}", self.kind),
None => write!(f, "{}", self.kind),
}
}
}
impl core::error::Error for EphemerisError {}

74
crates/orbit/apsis-ephemeris/src/lib.rs
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@ -1 +1,75 @@
//! Ephemeris sampling and interpolation. //! Ephemeris sampling and interpolation.
//!
//! A propagator gives the state of a satellite at any instant, but calling
//! it millions of times is wasteful when the result is only needed to draw a
//! track or to search for a crossing. This crate records the state at a
//! grid of times, called samples, and answers queries between them by
//! interpolation.
//!
//! The two halves are deliberately independent:
//!
//! - [`Ephemeris`] is a table of times and states. It knows nothing about
//! where the states came from, so a table propagated by SGP4, computed by
//! two-body theory, or read from a file all work the same way.
//! - [`Sampler`] fills a table by calling a function that produces a state
//! from a time. In this workspace that function is normally
//! [`apsis_sgp4::Satellite::propagate_at`], but the sampler only requires
//! a closure, so a test can use an analytic function.
//!
//! Interpolation is Hermite cubic, which uses both the position and the
//! velocity at each end of an interval. Using the velocity is what makes it
//! accurate: a cubic through two positions alone has no information about
//! how fast the satellite is moving, while Hermite reconstruction matches
//! the slope at both ends and so reproduces a smooth orbit far better for
//! the same sample spacing. On a circular orbit the error is of order the
//! fourth power of the step, so halving the step cuts the error about
//! sixteen fold.
//!
//! Times are in seconds from an origin chosen by the caller, matching the
//! rest of the orbit layer. Positions are in kilometers and velocities in
//! kilometers per second.
//!
//! ```
//! use apsis_ephemeris::{Sample, Sampler};
//! use apsis_kepler::{GravityParameter, KeplerianElements};
//!
//! // A circular orbit at the Earth's equatorial radius.
//! let elements = KeplerianElements {
//! semi_major_axis: 7_000.0,
//! eccentricity: 0.0,
//! inclination: 0.9,
//! right_ascension: 0.0,
//! argument_of_perigee: 0.0,
//! true_anomaly: 0.0,
//! };
//! let state_at = |seconds: f64| {
//! let mean = 2.0 * core::f64::consts::PI * seconds
//! / elements.period(GravityParameter::WGS84);
//! let state = elements
//! .with_mean_anomaly(mean)
//! .to_state(GravityParameter::WGS84)
//! .expect("the orbit is valid");
//! Sample::new(seconds, state.position, state.velocity)
//! };
//!
//! let sampler = Sampler::new(0.0, 600.0, 60.0);
//! let ephemeris = sampler.sample(state_at).expect("the grid is usable");
//! assert_eq!(ephemeris.len(), 11);
//!
//! // A query between two samples is interpolated.
//! let state = ephemeris.state_at(30.0).expect("the time is covered");
//! assert!(state.position.norm() > 6_900.0);
//! ```
#![forbid(unsafe_code)]
#![deny(missing_docs)]
mod ephemeris;
mod error;
mod sampler;
pub use ephemeris::{Ephemeris, Sample, hermite_position, hermite_state, hermite_velocity};
pub use error::{EphemerisError, EphemerisErrorKind};
pub use sampler::Sampler;
pub use apsis_math::Vec3;

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crates/orbit/apsis-ephemeris/src/sampler.rs Normal file
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@ -0,0 +1,179 @@
//! Filling an ephemeris by calling a state function on a grid.
use crate::ephemeris::{Ephemeris, Sample};
use crate::error::{EphemerisError, EphemerisErrorKind};
use apsis_math::Vec3;
/// A uniform time grid that a state function is evaluated on.
///
/// A sampler describes when to sample and how finely. The grid is defined by
/// a start time, an end time, and a step, and the number of samples follows
/// from them: the samples are placed at the start and at every step up to
/// the end, with the end itself included whether or not it falls on the
/// grid. Including the end matters, because a caller that asks for coverage
/// up to a time expects the table to reach it.
#[derive(Debug, Clone, Copy, PartialEq)]
pub struct Sampler {
/// The time of the first sample, in seconds.
pub start_seconds: f64,
/// The time of the last sample, in seconds.
pub end_seconds: f64,
/// The spacing between samples, in seconds.
pub step_seconds: f64,
}
impl Sampler {
/// Creates a sampler over an interval with a step.
pub const fn new(start_seconds: f64, end_seconds: f64, step_seconds: f64) -> Self {
Self {
start_seconds,
end_seconds,
step_seconds,
}
}
/// Returns `true` when the sampler describes a usable grid.
///
/// The times must be finite, the end must be strictly after the start,
/// and the step must be finite and positive. A zero length interval is
/// refused because a table needs at least two distinct times and the
/// interpolation has no interval to work over.
pub fn is_valid(&self) -> bool {
self.start_seconds.is_finite()
&& self.end_seconds.is_finite()
&& self.step_seconds.is_finite()
&& self.step_seconds > 0.0
&& self.end_seconds > self.start_seconds
}
/// Returns the number of samples the grid produces.
///
/// This is the number of whole steps that fit in the interval plus one
/// for the start and one for the end when the end is off the grid.
/// Returns `None` when the sampler is not usable.
pub fn sample_count(&self) -> Option<usize> {
if !self.is_valid() {
return None;
}
let span = self.end_seconds - self.start_seconds;
let whole_steps = (span / self.step_seconds).floor();
// The end sample is added unless it already coincides with the last
// whole step within a rounding tolerance.
let remainder = span - whole_steps * self.step_seconds;
let extra = if remainder > self.step_seconds * 1e-9 {
1
} else {
0
};
Some(whole_steps as usize + 1 + extra)
}
/// Returns the times of the grid, in order.
///
/// Returns `None` when the sampler is not usable. The last time is the
/// end when the end does not fall on the grid, and the last grid point
/// otherwise.
pub fn times(&self) -> Option<Vec<f64>> {
let count = self.sample_count()?;
let mut times = Vec::with_capacity(count);
for index in 0..count {
let time = self.start_seconds + index as f64 * self.step_seconds;
if time >= self.end_seconds {
break;
}
times.push(time);
}
// The end is always included, whether or not it landed on the grid.
if times.last().copied() != Some(self.end_seconds) {
times.push(self.end_seconds);
}
Some(times)
}
/// Fills an ephemeris by evaluating a state function on the grid.
///
/// The function maps a time in seconds to a position and velocity. In
/// this workspace it is normally a closure over
/// [`apsis_sgp4::Satellite::propagate_at`], but any function works, so
/// the sampler can be tested against an analytic orbit.
///
/// Returns an error when the sampler is not usable or when the function
/// produces a state that is not finite.
pub fn sample<F>(&self, mut state_at: F) -> Result<Ephemeris, EphemerisError>
where
F: FnMut(f64) -> Sample,
{
if !self.is_valid() {
return Err(
EphemerisError::new(EphemerisErrorKind::NonPositiveStep).with_detail(format!(
"start {}, end {}, step {}",
self.start_seconds, self.end_seconds, self.step_seconds
)),
);
}
let times = self
.times()
.ok_or_else(|| EphemerisError::new(EphemerisErrorKind::NotFinite))?;
let mut samples = Vec::with_capacity(times.len());
for time in times {
let sample = state_at(time);
if !sample.is_valid() {
return Err(EphemerisError::new(EphemerisErrorKind::InvalidState)
.with_detail(format!("at {time} s")));
}
// The time is set from the grid rather than trusted from the
// function, so the table stays strictly ordered even if the
// function returns a slightly different time.
samples.push(Sample::new(time, sample.position, sample.velocity));
}
Ephemeris::from_samples(samples)
}
/// Fills an ephemeris from a function that returns only a position.
///
/// The velocity is reconstructed from neighbouring positions by a
/// central difference. This is for a position-only source, such as a
/// tabulated file; where a velocity is available it should be used, so
/// prefer [`Sampler::sample`].
pub fn sample_positions<F>(&self, mut position_at: F) -> Result<Ephemeris, EphemerisError>
where
F: FnMut(f64) -> Vec3,
{
if !self.is_valid() {
return Err(EphemerisError::new(EphemerisErrorKind::NonPositiveStep));
}
let times = self
.times()
.ok_or_else(|| EphemerisError::new(EphemerisErrorKind::NotFinite))?;
let mut positions = Vec::with_capacity(times.len());
for &time in &times {
let position = position_at(time);
if !position.is_finite() {
return Err(EphemerisError::new(EphemerisErrorKind::InvalidState)
.with_detail(format!("at {time} s")));
}
positions.push(position);
}
// A central difference inside the interval and a one sided one at
// each end. The finite difference is second order, which matches the
// accuracy of the Hermite interpolation it feeds.
let count = positions.len();
let mut samples = Vec::with_capacity(count);
for (index, &position) in positions.iter().enumerate() {
let velocity = if count == 1 {
Vec3::new(0.0, 0.0, 0.0)
} else if index == 0 {
(positions[1] - positions[0]) / (times[1] - times[0])
} else if index == count - 1 {
(positions[count - 1] - positions[count - 2])
/ (times[count - 1] - times[count - 2])
} else {
(positions[index + 1] - positions[index - 1])
/ (times[index + 1] - times[index - 1])
};
samples.push(Sample::new(times[index], position, velocity));
}
Ephemeris::from_samples(samples)
}
}

441
crates/orbit/apsis-ephemeris/tests/ephemeris.rs Normal file
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//! Tests for the ephemeris table and its interpolation.
use apsis_ephemeris::{
Ephemeris, EphemerisErrorKind, Sample, Sampler, hermite_position, hermite_state,
hermite_velocity,
};
use apsis_kepler::{GravityParameter, KeplerianElements};
use apsis_math::Vec3;
/// The circular orbit the interpolation is checked against.
///
/// A circular orbit is the useful case: the state is analytic, so the error
/// of the interpolation is exactly the difference from a known value, and
/// the radius is constant, so a bad reconstruction shows up immediately.
fn circular() -> KeplerianElements {
KeplerianElements {
semi_major_axis: 7_000.0,
eccentricity: 0.0,
inclination: 0.9,
right_ascension: 0.5,
argument_of_perigee: 0.0,
true_anomaly: 0.0,
}
}
/// Returns the exact state of the circular orbit at a time.
fn exact_state(seconds: f64) -> Sample {
let elements = circular();
let mean = 2.0 * core::f64::consts::PI * seconds / elements.period(GravityParameter::WGS84);
let state = elements
.with_mean_anomaly(mean)
.to_state(GravityParameter::WGS84)
.expect("the circular orbit is valid");
Sample::new(seconds, state.position, state.velocity)
}
fn sample_table(step: f64, span: f64) -> Ephemeris {
Sampler::new(0.0, span, step)
.sample(exact_state)
.expect("the grid is usable")
}
#[test]
fn a_table_can_be_built_from_samples() {
let samples = vec![
Sample::new(0.0, Vec3::new(7_000.0, 0.0, 0.0), Vec3::new(0.0, 7.5, 0.0)),
Sample::new(
60.0,
Vec3::new(0.0, 7_000.0, 0.0),
Vec3::new(-7.5, 0.0, 0.0),
),
];
let ephemeris = Ephemeris::from_samples(samples).expect("two ordered samples");
assert_eq!(ephemeris.len(), 2);
assert!(!ephemeris.is_empty());
assert_eq!(ephemeris.start_time(), 0.0);
assert_eq!(ephemeris.end_time(), 60.0);
assert_eq!(ephemeris.duration(), 60.0);
}
#[test]
fn a_table_needs_two_samples() {
let error = Ephemeris::from_samples(Vec::new()).expect_err("an empty table is refused");
assert_eq!(error.kind(), EphemerisErrorKind::TooFewSamples);
let one = vec![Sample::new(0.0, Vec3::ZERO, Vec3::ZERO)];
let error = Ephemeris::from_samples(one).expect_err("one sample is refused");
assert_eq!(error.kind(), EphemerisErrorKind::TooFewSamples);
}
#[test]
fn samples_must_be_in_increasing_time_order() {
let samples = vec![
Sample::new(10.0, Vec3::ZERO, Vec3::ZERO),
Sample::new(5.0, Vec3::ZERO, Vec3::ZERO),
];
let error = Ephemeris::from_samples(samples).expect_err("the order is wrong");
assert_eq!(error.kind(), EphemerisErrorKind::NotMonotonic);
// Equal times are also refused, because a zero width interval has no
// interpolation to do.
let samples = vec![
Sample::new(5.0, Vec3::ZERO, Vec3::ZERO),
Sample::new(5.0, Vec3::ZERO, Vec3::ZERO),
];
let error = Ephemeris::from_samples(samples).expect_err("equal times are refused");
assert_eq!(error.kind(), EphemerisErrorKind::NotMonotonic);
}
#[test]
fn a_non_finite_sample_is_refused() {
let samples = vec![
Sample::new(0.0, Vec3::ZERO, Vec3::ZERO),
Sample::new(60.0, Vec3::new(f64::NAN, 0.0, 0.0), Vec3::ZERO),
];
let error = Ephemeris::from_samples(samples).expect_err("the state is not finite");
assert_eq!(error.kind(), EphemerisErrorKind::InvalidState);
}
#[test]
fn a_query_at_a_sample_returns_it_exactly() {
let ephemeris = sample_table(60.0, 600.0);
for sample in ephemeris.samples() {
let queried = ephemeris
.state_at(sample.seconds)
.expect("the sample time is covered");
// The interpolation must not perturb a sample, so the returned
// state is bit for bit the stored one.
assert_eq!(queried, *sample, "at {} s", sample.seconds);
}
}
#[test]
fn a_query_between_samples_is_interpolated() {
let ephemeris = sample_table(60.0, 600.0);
let midpoint = ephemeris.state_at(30.0).expect("the time is covered");
assert_eq!(midpoint.seconds, 30.0);
// The interpolated position differs from both endpoints.
assert_ne!(midpoint.position, ephemeris.samples()[0].position);
assert_ne!(midpoint.position, ephemeris.samples()[1].position);
// And it is a plausible radius for the orbit.
assert!((midpoint.radius() - 7_000.0).abs() < 1.0);
}
#[test]
fn the_interpolation_is_accurate_on_a_circular_orbit() {
// Hermite cubic on a smooth orbit: the position error over a step of
// sixty seconds should be far below a kilometer.
let ephemeris = sample_table(60.0, 600.0);
let mut worst = 0.0_f64;
for index in 0..600 {
let time = f64::from(index) + 0.5;
let interpolated = ephemeris.state_at(time).expect("covered");
let truth = exact_state(time);
worst = worst.max(interpolated.position.distance(truth.position));
}
assert!(
worst < 1e-3,
"the worst position error is {worst} km, which is too large"
);
}
#[test]
fn halving_the_step_cuts_the_error_by_about_sixteen() {
// A quartic method: the error scales as the fourth power of the step.
// This is the property that makes the interpolation worth using, and it
// fails if the velocity is dropped or mis-scaled.
let error_for = |step: f64| {
let ephemeris = sample_table(step, 600.0);
let mut worst = 0.0_f64;
// Query at the midpoints between samples, where the error is
// largest.
let mut time = step / 2.0;
while time < 600.0 {
let interpolated = ephemeris.state_at(time).expect("covered");
let truth = exact_state(time);
worst = worst.max(interpolated.position.distance(truth.position));
time += step;
}
worst
};
let coarse = error_for(120.0);
let fine = error_for(60.0);
assert!(coarse > 0.0, "the coarse error should be non-zero");
let ratio = coarse / fine;
assert!(
(8.0..32.0).contains(&ratio),
"the error ratio is {ratio}, expected about sixteen"
);
}
#[test]
fn the_interpolated_velocity_matches_the_position_derivative() {
// The velocity is the analytic derivative of the position cubic, so a
// numerical derivative of the position must agree with it.
let ephemeris = sample_table(60.0, 600.0);
let time = 137.0;
let h = 1e-4;
let before = ephemeris.state_at(time - h).expect("covered").position;
let after = ephemeris.state_at(time + h).expect("covered").position;
let numeric = (after - before) / (2.0 * h);
let analytic = ephemeris.state_at(time).expect("covered").velocity;
assert!(
numeric.distance(analytic) < 1e-6,
"numeric {} against analytic {}",
numeric,
analytic
);
}
#[test]
fn a_query_outside_the_interval_is_refused() {
let ephemeris = sample_table(60.0, 600.0);
assert!(ephemeris.state_at(-1.0).is_none());
assert!(ephemeris.state_at(600.1).is_none());
assert!(ephemeris.state_at(f64::NAN).is_none());
assert!(ephemeris.state_at(f64::INFINITY).is_none());
// The endpoints are covered.
assert!(ephemeris.covers(0.0));
assert!(ephemeris.covers(600.0));
assert!(!ephemeris.covers(-1e-9));
}
#[test]
fn a_clamped_query_returns_the_nearest_endpoint() {
let ephemeris = sample_table(60.0, 600.0);
let before = ephemeris.clamped_state_at(-100.0).expect("clamped");
assert_eq!(before, ephemeris.samples()[0]);
let after = ephemeris.clamped_state_at(10_000.0).expect("clamped");
assert_eq!(after, ephemeris.samples()[ephemeris.len() - 1]);
// Inside the interval the clamped query is the ordinary one.
let inside = ephemeris.clamped_state_at(30.0).expect("covered");
assert_eq!(inside, ephemeris.state_at(30.0).expect("covered"));
assert!(ephemeris.clamped_state_at(f64::NAN).is_none());
}
#[test]
fn the_nearest_sample_is_found() {
let ephemeris = sample_table(60.0, 600.0);
assert_eq!(ephemeris.nearest_sample(0.0).expect("found").seconds, 0.0);
assert_eq!(ephemeris.nearest_sample(29.0).expect("found").seconds, 0.0);
assert_eq!(ephemeris.nearest_sample(31.0).expect("found").seconds, 60.0);
// A midpoint ties toward the earlier sample.
assert_eq!(ephemeris.nearest_sample(30.0).expect("found").seconds, 0.0);
// Outside the interval the nearest end is returned.
assert_eq!(
ephemeris.nearest_sample(-100.0).expect("found").seconds,
0.0
);
assert_eq!(
ephemeris.nearest_sample(10_000.0).expect("found").seconds,
600.0
);
assert!(ephemeris.nearest_sample(f64::NAN).is_none());
}
#[test]
fn a_uniform_grid_is_recognized() {
let uniform = sample_table(60.0, 600.0);
assert!(uniform.is_uniform());
assert_eq!(uniform.uniform_step(), Some(60.0));
let uneven = Ephemeris::from_samples(vec![
Sample::new(0.0, Vec3::ZERO, Vec3::ZERO),
Sample::new(60.0, Vec3::ZERO, Vec3::ZERO),
Sample::new(150.0, Vec3::ZERO, Vec3::ZERO),
])
.expect("ordered");
assert!(!uneven.is_uniform());
assert_eq!(uneven.uniform_step(), None);
}
#[test]
fn the_times_of_the_samples_are_listed() {
let ephemeris = sample_table(60.0, 300.0);
assert_eq!(
ephemeris.times(),
vec![0.0, 60.0, 120.0, 180.0, 240.0, 300.0]
);
}
#[test]
fn a_property_of_every_sample_can_be_checked() {
let ephemeris = sample_table(60.0, 600.0);
assert!(ephemeris.all_samples(|sample| sample.radius() > 6_000.0));
assert!(!ephemeris.all_samples(|sample| sample.radius() > 8_000.0));
}
// The sampler's grid.
#[test]
fn the_grid_includes_both_ends() {
// The end falls on the grid.
let sampler = Sampler::new(0.0, 300.0, 60.0);
assert_eq!(sampler.sample_count(), Some(6));
assert_eq!(
sampler.times().expect("usable"),
vec![0.0, 60.0, 120.0, 180.0, 240.0, 300.0]
);
// The end does not fall on the grid, so it is appended.
let sampler = Sampler::new(0.0, 250.0, 60.0);
assert_eq!(sampler.sample_count(), Some(6));
assert_eq!(
sampler.times().expect("usable"),
vec![0.0, 60.0, 120.0, 180.0, 240.0, 250.0]
);
}
#[test]
fn a_single_step_grid_has_two_samples() {
let sampler = Sampler::new(10.0, 70.0, 60.0);
assert_eq!(sampler.sample_count(), Some(2));
assert_eq!(sampler.times().expect("usable"), vec![10.0, 70.0]);
}
#[test]
fn a_zero_length_interval_is_refused() {
// A table needs at least two distinct times, so a zero length grid has
// no interval to interpolate over and is rejected outright rather than
// producing a table that cannot answer a query.
let sampler = Sampler::new(5.0, 5.0, 60.0);
assert!(!sampler.is_valid());
assert_eq!(sampler.sample_count(), None);
assert!(sampler.times().is_none());
assert!(sampler.sample(exact_state).is_err());
}
#[test]
fn an_unusable_sampler_is_refused() {
for sampler in [
Sampler::new(0.0, 600.0, 0.0),
Sampler::new(0.0, 600.0, -60.0),
Sampler::new(0.0, 600.0, f64::NAN),
Sampler::new(f64::NAN, 600.0, 60.0),
Sampler::new(0.0, f64::INFINITY, 60.0),
Sampler::new(600.0, 0.0, 60.0),
] {
assert!(!sampler.is_valid(), "{sampler:?}");
assert_eq!(sampler.sample_count(), None);
assert!(sampler.times().is_none());
assert!(sampler.sample(exact_state).is_err());
}
}
#[test]
fn a_bad_state_from_the_function_is_reported() {
let sampler = Sampler::new(0.0, 120.0, 60.0);
let error = sampler
.sample(|seconds| {
if seconds > 0.0 {
Sample::new(seconds, Vec3::new(f64::NAN, 0.0, 0.0), Vec3::ZERO)
} else {
Sample::new(seconds, Vec3::ZERO, Vec3::ZERO)
}
})
.expect_err("the state is not finite");
assert_eq!(error.kind(), EphemerisErrorKind::InvalidState);
}
#[test]
fn the_sampler_overrides_a_bad_time_from_the_function() {
// A function that returns a wrong time must not corrupt the table: the
// grid time is used instead.
let sampler = Sampler::new(0.0, 120.0, 60.0);
let ephemeris = sampler
.sample(|_| Sample::new(999.0, Vec3::new(7_000.0, 0.0, 0.0), Vec3::ZERO))
.expect("the states are finite");
assert_eq!(ephemeris.times(), vec![0.0, 60.0, 120.0]);
}
#[test]
fn positions_can_be_sampled_without_velocities() {
// The velocity is reconstructed by finite differences, so a
// position-only source still produces a usable table.
let ephemeris = Sampler::new(0.0, 600.0, 60.0)
.sample_positions(|seconds| exact_state(seconds).position)
.expect("the positions are finite");
assert_eq!(ephemeris.len(), 11);
// The reconstructed velocity is close to the true one.
let sample = ephemeris.state_at(300.0).expect("covered");
let truth = exact_state(300.0);
assert!(
sample.velocity.distance(truth.velocity) < 1e-2,
"the reconstructed velocity {} is too far from {}",
sample.velocity,
truth.velocity
);
// The interpolation is still usable, though far less accurate than with
// exact velocities: the finite difference velocity is only second order
// accurate, so its error feeds straight into the Hermite reconstruction.
// The error is a few kilometers over a one minute step, against
// millimeters when the velocity is known.
let mut worst = 0.0_f64;
for index in 0..600 {
let time = f64::from(index) + 0.5;
let interpolated = ephemeris.state_at(time).expect("covered");
worst = worst.max(interpolated.position.distance(exact_state(time).position));
}
assert!(worst < 5.0, "the worst position error is {worst} km");
}
#[test]
fn position_sampling_reports_a_bad_position() {
let sampler = Sampler::new(0.0, 120.0, 60.0);
let error = sampler
.sample_positions(|seconds| {
if seconds > 0.0 {
Vec3::new(f64::NAN, 0.0, 0.0)
} else {
Vec3::ZERO
}
})
.expect_err("the position is not finite");
assert_eq!(error.kind(), EphemerisErrorKind::InvalidState);
}
// The Hermite basis functions directly.
#[test]
fn the_hermite_basis_reproduces_its_endpoints() {
let p0 = Vec3::new(1.0, 2.0, 3.0);
let p1 = Vec3::new(4.0, 5.0, 6.0);
let v0 = Vec3::new(0.1, 0.2, 0.3);
let v1 = Vec3::new(0.4, 0.5, 0.6);
let dt = 60.0;
// At s = 0 the result is p0 and the velocity is v0.
assert_eq!(hermite_position(p0, v0, p1, v1, dt, 0.0), p0);
assert_eq!(hermite_velocity(p0, v0, p1, v1, dt, 0.0), v0);
// At s = 1 the result is p1 and the velocity is v1.
assert_eq!(hermite_position(p0, v0, p1, v1, dt, 1.0), p1);
assert_eq!(hermite_velocity(p0, v0, p1, v1, dt, 1.0), v1);
}
#[test]
fn the_hermite_state_returns_both_parts() {
let p0 = Vec3::new(1.0, 0.0, 0.0);
let p1 = Vec3::new(2.0, 0.0, 0.0);
let (position, velocity) = hermite_state(p0, Vec3::ZERO, p1, Vec3::ZERO, 10.0, 0.5);
// Both endpoints are stationary, so the cubic is the S-curve that
// passes through the midpoint with the slope the endpoints imply.
assert_eq!(position, Vec3::new(1.5, 0.0, 0.0));
// The slope at the midpoint is 1.5 / dt, not zero: the curve rises from
// zero slope at one end to zero slope at the other.
assert_eq!(velocity, Vec3::new(0.15, 0.0, 0.0));
}
#[test]
fn a_sample_reports_its_radius_and_speed() {
let sample = Sample::new(0.0, Vec3::new(3.0, 4.0, 0.0), Vec3::new(0.0, 0.0, 12.0));
assert!((sample.radius() - 5.0).abs() < 1e-12);
assert!((sample.speed() - 12.0).abs() < 1e-12);
}
#[test]
fn an_error_displays_its_kind() {
let error = Ephemeris::from_samples(Vec::new()).expect_err("empty");
assert!(error.to_string().contains("two samples"), "{error}");
}