GitHub - ElouanPetereau/sgp4: A Rust implementation of the SGP4 satellite propagation algorithm

The SGP4 algorithm, ported to Rust from the reference Celestrak implementation [1].

The code was entirely refactored to leverage Rust's algebraic data types and highlight the relationship between the implementation and the reference mathematical equations [2].

SGP4 can be called from JavaScript or Python via WebAssembly wrappers. See https://github.com/wasmerio/sgp4 to install and use SGP4 as a WAPM package.

The numerical predictions are almost identical to Celestrak's. The observed differences (less than 2 × 10⁻⁷ km for the position and 10⁻⁹ km.s⁻¹ for the velocity three and a half years after the epoch) are well below the accuracy of the algorithm.

We drew inspiration from the incomplete https://github.com/natronics/rust-sgp4 to write mathematical expressions using UTF-8 characters.

Documentation
Environments without std or alloc
Benchmark
Variables and mathematical expressions
- Variables
- Mathematical expressions
References

Documentation

The code documentation is hosted at https://docs.rs/sgp4/latest/sgp4/.

Examples can be found in this repository's examples directory:

examples/celestrak.rs retrieves the most recent Galileo OMMs from Celestrak and propagates them
examples/omm.rs parses and propagates a JSON-encoded OMM
examples/space-track.rs retrieves the 20 most recent launches OMMs from Space-Track and propagates them
examples/tle.rs parses and propagates a TLE
examples/tle_afspc.rs parses and propagates a TLE using the AFSPC compatibility mode
examples/advanced.rs leverages the advanced API to (marginally) accelerate the propagation of deep space resonant satellites

To run an example (here examples/celestrak.rs), use:

cargo run --example celestrak

To run the Space-Track example, you must first assign your Space-Track.org credentials to the fields identity and password (see lines 3 and 4 in examples/space-track.rs).

Environments without std or alloc

This crate supports no_std environments. TLE parsing and SGP4 propagation do not require alloc either. We use num-traits with libm for floating point functions when std is not available.

All serde-related features, in particular OMM parsing, require alloc.

Benchmark

The benchmark code is available at https://github.com/neuromorphicsystems/sgp4-benchmark. It compares two SGP4 implementations in different configurations:

cpp: the Celestrak implementation [1] in improved mode
cpp-afspc: the Celestrak implementation [1] in AFSPC compatibility mode
cpp-fastmath: the Celestrak implementation [1] in improved mode with the fast-math compiler flag
cpp-afspc-fastmath: the Celestrak implementation [1] in AFSPC compatibility mode with the fast-math compiler flag
rust: our Rust implementation in default mode
rust-afspc: our Rust implementation in AFSPC compatibility mode

This benchmark must not be confused with the code in this repository's bench directory. The latter considers only a small subset of the Celestrak catalogue (the tests recommended in [1]) and does not measure the original C++ implementation.

The present results were obtained using a machine with the following configuration:

CPU - Intel Core i7-8700 @ 3.20GHz
RAM - Kingston DDR4 @ 2.667 GHz
OS - Ubuntu 16.04
Compilers - Rust 1.44.1 and gcc 9.3.0

Accuracy measures the maximum propagation error of each implementation with respect to the reference implementation (cpp-afspc) over the full Celestrak catalogue (1 minute timestep over 24 hours).

implementation	maximum position error	maximum speed error
`cpp-afspc`	(reference)	(reference)
`cpp`	1.05 km	1.30 × 10⁻³ km.s⁻¹
`cpp-fastmath`	1.05 km	1.30 × 10⁻³ km.s⁻¹
`cpp-afspc-fastmath`	4.21 × 10⁻⁸ km	7.51 × 10⁻¹² km.s⁻¹
`rust`	1.05 km	1.30 × 10⁻³ km.s⁻¹
`rust-afspc`	4.19 × 10⁻⁸ km	7.46 × 10⁻¹² km.s⁻¹

The Rust and C++ fast-math errors have the same order of magnitude. In both cases, they can be attributed to mathematically identical expressions implemented with different floating-point operations.

Speed measures the time it takes to propagate every satellite in the Celestrak catalogue (1 minute timestep over 24 hours) using a single thread. 100 values are sampled per implementation.

implementation	minimum	Q1	median	Q3	maximum	relative difference
`cpp-afspc`	8.95 s	9.02 s	9.03 s	9.06 s	9.18 s	(reference)
`cpp`	8.95 s	9.01 s	9.04 s	9.06 s	9.25 s	+ 0 %
`cpp-fastmath`	7.67 s	7.74 s	7.77 s	7.79 s	7.90 s	- 14 %
`cpp-afspc-fastmath`	7.70 s	7.74 s	7.76 s	7.79 s	7.86 s	- 14 %
`rust`	8.36 s	8.41 s	8.43 s	8.45 s	8.53 s	- 7 %
`rust-afspc`	8.36 s	8.41 s	8.43 s	8.46 s	8.59 s	- 7 %

Rust fast-math support is a work in progress (see rust-lang/rust#21690). Similarly to C++, it should have a very small impact on accuracy while providing a substantial speed gain.

Variables and mathematical expressions

Variables

Each variable is used to store the result of one and only one expression. Most variables are immutable, with the exception of the variable (E + ω)ᵢ used to solve Kepler's equation and the state variables tᵢ, nᵢ and λᵢ used to integrate the resonance effects of Earth gravity.

The following tables list the variables used in the code and their associated mathematical symbol. Where possible, we used symbols from [2]. Partial expressions without a name in [2] follow the convention kₙ, n ∈ ℕ if they are shared between initialization and propagation, and pₙ, n ∈ ℕ if they are local to initialization or propagation.

Initialization variables
Propagation variables
Third-body initialization variables
Third-body propagation variables

Initialization variables

The following variables depend solely on epoch elements.

variable	symbol	description
`Elements::datetime.year()`	`yᵤ`	Gregorian calendar year
`Elements::datetime.month()`	`mᵤ`	Gregorian calendar month in the range `[1, 12]`
`Elements::datetime.day()`	`dᵤ`	Gregorian calendar day in the range `[1, 31]`
`Elements::datetime.hour()`	`hᵤ`	Hours since midnight in the range `[0, 23]`
`Elements::datetime.minute()`	`minᵤ`	Minutes since the hour in the range `[0, 59]`
`Elements::datetime.second()`	`sᵤ`	Seconds since the minute in the range `[0, 59]`
`Elements::datetime.nanosecond()`	`nsᵤ`	Nanoseconds since the second in the range `[0, 10⁹[`
`epoch`	`y₂₀₀₀`	Julian years since UTC 1 January 2000 12h00 (J2000)
`d1900`	`d₁₉₀₀`	Julian days since UTC 1 January 1900 12h00 (J1900)
`d1970`	`d₁₉₇₀`	Julian days since UTC 1 January 1970 12h00 (J1970)
`c2000`	`c₂₀₀₀`	Julian centuries since UTC 1 January 2000 12h00 (J2000)
`geopotential.ae`	`aₑ`	equatorial radius of the earth in km
`geopotential.ke`	`kₑ`	square root of the earth's gravitational parameter in earth radii³ min⁻²
`geopotential.j2`	`J₂`	un-normalised second zonal harmonic
`geopotential.j3`	`J₃`	un-normalised third zonal harmonic
`geopotential.j4`	`J₄`	un-normalised fourth zonal harmonic
`kozai_mean_motion`	`n₀`	mean number of orbits per day (Kozai convention) at epoch in rad.min⁻¹
`a1`	`a₁`	semi-major axis at epoch (Kozai convention)
`p0`	`p₀`	partial expression of `𝛿₀` and `𝛿₁`
`d1`	`𝛿₁`	used in the Kozai to Brouwer conversion
`d0`	`𝛿₀`	used in the Kozai to Brouwer conversion
`B*`	`B*`	radiation pressure coefficient in earth radii⁻¹
`orbit_0.inclination`	`I₀`	angle between the equator and the orbit plane at epoch in rad
`orbit_0.right_ascension`	`Ω₀`	angle between vernal equinox and the point where the orbit crosses the equatorial plane at epoch in rad
`orbit_0.eccentricity`	`e₀`	shape of the orbit at epoch
`orbit_0.argument_of_perigee`	`ω₀`	angle between the ascending node and the orbit's point of closest approach to the earth at epoch in rad
`orbit_0.mean_anomaly`	`M₀`	angle of the satellite location measured from perigee at epoch in rad
`orbit_0.mean_motion`	`n₀"`	mean number of orbits per day (Brouwer convention) at epoch in rad.min⁻¹
`p1`	`p₁`	cosine of the inclination at epoch used in multiple expressions during initialization (`θ` in [2], renamed to avoid confusion with the sidereal time)
`p2`	`p₂`	partial expression of multiple initialization expressions
`a0`	`a₀"`	semi-major axis at epoch (Brouwer convention)
`p3`	`p₃`	perigee in earth radii
`p4`	`p₄`	height of perigee in km
`p5`	`p₅`	partial expression of `s`
`s`	`s`	altitude parameter of the atmospheric drag expression
`p6`	`p₆`	partial expression of the atmospheric drag
`xi`	`ξ`	partial expression of multiple initialization expressions
`p7`	`p₇`	partial expression of multiple initialization expressions
`eta`	`η`	partial expression of multiple initialization expressions and of the argument of perigee and mean anomaly in eccentric high altitude near earth propagation
`p8`	`p₈`	partial expression of multiple initialization expressions
`p9`	`p₉`	partial expression of multiple initialization expressions
`c1`	`C₁`	partial expression of multiple initialization and propagation expressions
`p10`	`p₁₀`	partial expression of multiple initialization expressions
`b0`	`β₀`	partial expression of multiple initialization expressions
`p11`	`p₁₁`	partial expression of multiple initialization expressions
`p12`	`p₁₂`	partial expression of multiple initialization expressions
`p13`	`p₁₃`	partial expression of multiple initialization expressions
`p14`	`p₁₄`	partial expression of multiple initialization expressions
`p15`	`p₁₅`	partial expression of multiple initialization expressions
`k14`	`k₁₄`	first order coefficient of the argument of perigee before adding solar and lunar perturbations
`c4`	`C₄`	partial expression of multiple initialization and propagation expressions (differs from the `C₄` expression in [2] by a factor B*)
`right_ascension_dot`	`Ω̇`	first order coefficient of the right ascension
`argument_of_perigee_dot`	`ω̇`	first order coefficient of the argument of perigee
`mean_anomaly_dot`	`Ṁ`	first order coefficient of the mean anomaly
`k0`	`k₀`	second order coefficient of the right ascension before adding perturbations
`k1`	`k₁`	partial expression of the second order coefficient of the mean anomaly
`k2`	`k₂`	partial expression of `aᵧₙ` in near earth propagation
`k3`	`k₃`	partial expression of `rₖ`, `ṙₖ` and `rḟₖ` in near earth propagation
`k4`	`k₄`	partial expression of `uₖ` in near earth propagation
`k5`	`k₅`	partial expression of the initial Kepler variable `p₃₈` in near earth propagation
`k6`	`k₆`	partial expression of multiple initialization expressions and of `rₖ` and `rḟₖ` in near earth propagation
`d2`	`D₂`	partial expression of multiple near earth initialization expressions and of the semi-major axis in near earth propagation
`p16`	`p₁₆`	partial expression of multiple near earth initialization expressions
`d3`	`D₃`	partial expression of multiple near earth initialization expressions and of the semi-major axis in near earth propagation
`d4`	`D₄`	partial expression of multiple near earth initialization expressions and of the semi-major axis in near earth propagation
`c5`	`C₅`	partial expression of multiple initialization and propagation expressions (differs from the `C₅` expression in [2] by a factor B*)
`k7`	`k₇`	sine of the mean anomaly at epoch
`k8`	`k₈`	partial expression of the mean anomaly third order coefficient in high altitude near earth propagation
`k9`	`k₉`	partial expression of the mean anomaly fourth order coefficient in high altitude near earth propagation
`k10`	`k₁₀`	partial expression of the mean anomaly fifth order coefficient in high altitude near earth propagation
`k11`	`k₁₁`	partial expression of the argument of perigee and mean anomaly in eccentric high altitude near earth propagation
`k12`	`k₁₂`	partial expression of the argument of perigee and mean anomaly in eccentric high altitude near earth propagation
`k13`	`k₁₃`	partial expression of the argument of perigee and mean anomaly in eccentric high altitude near earth propagation
`lunar_right_ascension_epsilon`	`Ωₗₑ`	lunar right ascension of the ascending node
`lunar_right_ascension_sine`	`sin Ωₗ`	sine of the lunar right ascension of the ascending node referred to the equator
`lunar_right_ascension_cosine`	`cos Ωₗ`	cosine of the lunar right ascension of the ascending node referred to the equator
`lunar_argument_of_perigee`	`ωₗ`	lunar argument of perigee
`sidereal_time_0`	`θ₀`	Greenwich sidereal time at epoch
`lambda_0`	`λ₀`	Earth gravity resonance variable at epoch
`lambda_dot_0`	`λ̇₀`	time derivative of the Earth gravity resonance variable at epoch
`p17`	`p₁₇`	partial expression of `𝛿ᵣ₁`, `𝛿ᵣ₂` and `𝛿ᵣ₃`
`dr1`	`𝛿ᵣ₁`	first Earth gravity resonance coefficient for geosynchronous orbits (`𝛿₁` in [2], renamed to avoid confusion with `𝛿₁` used in the Kozai to Brouwer conversion)
`dr2`	`𝛿ᵣ₂`	second Earth gravity resonance coefficient for geosynchronous orbits (`𝛿₂` in [2], renamed to match `𝛿ᵣ₁`)
`dr3`	`𝛿ᵣ₃`	third Earth gravity resonance coefficient for geosynchronous orbits (`𝛿₃` in [2], renamed to match `𝛿ᵣ₁`)
`p18`	`p₁₈`	partial expression of `D₂₂₀₋₁` and `D₂₂₁₁`
`p19`	`p₁₉`	partial expression of `D₃₂₁₀` and `D₃₂₂₂`
`p20`	`p₂₀`	partial expression of `D₄₄₁₀` and `D₄₄₂₂`
`p21`	`p₂₁`	partial expression of `D₅₂₂₀`, `D₅₂₃₂`, `D₅₄₂₁` and `D₅₄₃₃`
`f220`	`F₂₂₀`	partial expression of `D₂₂₀₋₁` and `D₄₄₁₀`
`g211`	`G₂₁₁`	partial expression of `D₂₂₁₁`
`g310`	`G₃₁₀`	partial expression of `D₃₂₁₀`
`g322`	`G₃₂₂`	partial expression of `D₃₂₂₂`
`g410`	`G₄₁₀`	partial expession of `D₄₄₁₀`
`g422`	`G₄₂₂`	partial expession of `D₄₄₂₂`
`g520`	`G₅₂₀`	partial expression of `D₅₂₂₀`
`g532`	`G₅₃₂`	partial expression of `D₅₂₃₂`
`g521`	`G₅₂₁`	partial expression of `D₅₄₂₁`
`g533`	`G₅₃₃`	partial expression of `D₅₄₃₃`
`d220₋1`	`D₂₂₀₋₁`	gravity resonance coefficient for Molniya orbits (the `Dₗₘₚₖ` expression in [2] is missing a factor `l - 2p + k` from the original equation in [4] with `k = -1` instead of `1`)
`d2211`	`D₂₂₁₁`	gravity resonance coefficient for Molniya orbits (the `Dₗₘₚₖ` expression in [2] is missing a factor `l - 2p + k` from the original equation in [4])
`d3210`	`D₃₂₁₀`	see `D₂₂₁₁`
`d3222`	`D₃₂₂₂`	see `D₂₂₁₁`
`d4410`	`D₄₄₁₀`	see `D₂₂₁₁`
`d4422`	`D₄₄₂₂`	see `D₂₂₁₁`
`d5220`	`D₅₂₂₀`	see `D₂₂₁₁`
`d5232`	`D₅₂₃₂`	see `D₂₂₁₁`
`d5421`	`D₅₄₂₁`	see `D₂₂₁₁`
`d5433`	`D₅₄₃₃`	see `D₂₂₁₁`

Propagation variables

The following expressions depend on the propagation time t.

variable	symbol	description
`t`	`t`	minutes elapsed since epoch (can be negative)
`p22`	`p₂₂`	right ascension of the ascending node with neither Earth gravity resonance nor Sun and Moon contributions
`p23`	`p₂₃`	argument of perigee with neither high altitude drag effects, Earth gravity resonance nor Sun and Moon contributions
`orbit.inclination`	`I`	inclination at epoch plus `t` without the short-period effects of Earth gravity
`orbit.right_ascension`	`Ω`	right ascension of the ascending node at epoch plus `t` without the short-period effects of Earth gravity
`orbit.eccentricity`	`e`	eccentricity at epoch plus `t` without the short-period effects of Earth gravity
`orbit.argument_of_perigee`	`ω`	argument of perigee at epoch plus `t` without the short-period effects of Earth gravity
`orbit.mean_anomaly`	`M`	mean anomaly at epoch plus `t` without the short-period effects of Earth gravity
`orbit.mean_motion`	`n`	mean motion at epoch plus `t` without the short-period effects of Earth gravity
`a`	`a`	semi-major axis
`p32`	`p₃₂`	partial expression of `aᵧₙ`
`p33`	`p₃₃`	partial expression of `rₖ`, `ṙₖ` and `rḟₖ`
`p34`	`p₃₄`	partial expression of `uₖ`
`p35`	`p₃₅`	partial expression of the initial Kepler variable `p₃₈`
`p36`	`p₃₆`	partial expression of `rₖ` and `rḟₖ`
`p37`	`p₃₇`	partial expression of `aᵧₙ` and the initial Kepler variable `p₃₈`
`axn`	`aₓₙ`	normalized linear eccentricity projected on the line of nodes
`ayn`	`aᵧₙ`	normalized linear eccentricity projected on the normal to the line of nodes
`p38`	`p₃₈`	initial Kepler variable (`U` in [2], renamed to avoid confusion with the true anomaly plus argument of perigee `u`)
`ew`	`(E + ω)ᵢ`	Kepler variable used in an iterative process to estimate the eccentric anomaly `E`
`delta`	`Δ(E + ω)ᵢ`	correction to the Kepler variable at iteration `i`
`p39`	`p₃₉`	eccentricity at epoch plus `t`
`pl`	`pₗ`	semi-latus rectum
`p40`	`p₄₀`	normalized linear eccentricity projected on the semi-minor axis
`r`	`r`	radius (distance to the focus) without the short-period effects of Earth gravity
`r_dot`	`ṙ`	radius time derivative without the short-period effects of Earth gravity
`b`	`β`	semi-minor axis over semi-major axis
`p41`	`p₄₁`	partial expression of `p₄₂` and `p₄₃`
`p42`	`p₄₂`	sine of `u`
`p43`	`p₄₃`	cosine of `u`
`u`	`u`	true anomaly plus argument of perigee without the short-period effects of Earth gravity
`p44`	`p₄₄`	`sin(2 u)`, partial expression of `uₖ`, `Ωₖ` and `ṙₖ`
`p45`	`p₄₅`	`cos(2 u)`, partial expression of `rₖ`, `Iₖ` and `rḟₖ`
`p46`	`p₄₆`	partial expression of `rₖ`, `uₖ`, `Iₖ` and `Ωₖ`
`rk`	`rₖ`	radius (distance to the focus)
`uk`	`uₖ`	true anomaly plus argument of perigee
`inclination_k`	`Iₖ`	inclination at epoch plus `t`
`right_ascension_k`	`Ωₖ`	right ascension at epoch plus `t`
`rk_dot`	`ṙₖ`	radius time derivative
`rfk_dot`	`rḟₖ`	radius times the true anomaly derivative
`u0`	`u₀`	x component of the position unit vector
`u1`	`u₁`	y component of the position unit vector
`u2`	`u₂`	z component of the position unit vector
`prediction.position[0]`	`r₀`	x component of the position vector in km (True Equator, Mean Equinox (TEME) of epoch reference frame)
`prediction.position[1]`	`r₁`	y component of the position vector in km (True Equator, Mean Equinox (TEME) of epoch reference frame)
`prediction.position[2]`	`r₂`	z component of the position vector in km (True Equator, Mean Equinox (TEME) of epoch reference frame)
`prediction.velocity[0]`	`ṙ₀`	x component of the velocity vector in km.s⁻¹ (True Equator, Mean Equinox (TEME) of epoch reference frame)
`prediction.velocity[1]`	`ṙ₁`	y component of the velocity vector in km.s⁻¹ (True Equator, Mean Equinox (TEME) of epoch reference frame)
`prediction.velocity[2]`	`ṙ₂`	z component of the velocity vector in km.s⁻¹ (True Equator, Mean Equinox (TEME) of epoch reference frame)
`p24`	`p₂₄`	mean anomaly without drag contributions in near earth propagation
`p25`	`p₂₅`	partial expression of `ω` and `M` in near earth propagation
`p26`	`p₂₆`	mean anomaly with elliptic correction and without drag contributions in near earth propagation
`p27`	`p₂₇`	non-clamped eccentricity in near earth propagation
`p28`	`p₂₈`	semi-major axis with resonance correction in deep space propagation
`p29`	`p₂₉`	mean anomaly with resonance correction in deep space propagation
`p31`	`p₃₁`	non-clamped eccentricity in deep space propagation
`sidereal_time`	`θ`	sidereal time at epoch plus `t`
`delta_t`	`Δt`	time step used in the integration of resonance effects of Earth gravity in min (either `720` or `-720`)
`lambda_dot`	`λ̇ᵢ`	resonance effects of Earth gravity variable's time derivative at epoch plus `i Δt`
`ni_dot`	`ṅᵢ`	mean motion time derivative at epoch plus `i Δt`
`ni_ddot`	`n̈ᵢ`	mean motion second time derivative at epoch plus `i Δt`
`ResonanceState::t`	`tᵢ`	resonance effects of Earth gravity integrator time (`i Δt`)
`ResonanceState::mean_motion`	`nᵢ`	mean motion time derivative at epoch plus `Δt i`
`ResonanceState::lambda`	`λᵢ`	resonance effects of Earth gravity variable at epoch plus `i Δt`
`p30`	`p₃₀`	non-normalised `Ω` in Lyddane deep space propagation

Third-body initialization variables

The contribution of the Sun and the Moon to the orbital elements are calculated with a unique set of expressions. src/third_body.rs provides a generic implementation of these expressions. Variables specific to the third body (either the Sun or the Moon) are annotated with x. In every other file, these variables are annotated with s if they correspond to solar perturbations, and l if they correspond to lunar perturbations.

The aₓₙ, Xₓₙ, Zₓₙ (n ∈ ℕ), Fₓ₂ and Fₓ₃ variables correspond to the aₙ, Xₙ, Zₙ, F₂ and F₃ variables in [2]. The added x highlights the dependence on the perturbing third body.

The following variables depend solely on epoch elements.

variable	symbol	description
`third_body_inclination_sine`	`sin Iₓ`	sine of the inclination of the Sun (`sin Iₛ`) or the Moon (`sin Iₗ`)
`third_body_inclination_cosine`	`cos Iₓ`	cosine of the inclination of the Sun (`cos Iₛ`) or the Moon (`cos Iₗ`)
`delta_right_ascension_sine`	`sin(Ω₀ - Ωₓ)`	sine of the difference between the right ascension of the ascending node of the satellite at epoch and the Sun's (`sin(Ω₀ - Ωₛ)`) or the Moon's (`sin(Ω₀ - Ωₗ)`)
`delta_right_ascension_cosine`	`cos(Ω₀ - Ωₓ)`	cosine of the difference between the right ascension of the ascending node of the satellite at epoch and the Sun's (`cos(Ω₀ - Ωₛ)`) or the Moon's (`cos(Ω₀ - Ωₗ)`)
`third_body_argument_of_perigee_sine`	`sin ωₓ`	sine of the argument of perigee of the Sun (`sin ωₛ`) or the Moon (`sin ωₗ`)
`third_body_argument_of_perigee_cosine`	`cos ωₓ`	cosine of the argument of perigee of the Sun (`sin ωₛ`) or the Moon (`cos ωₗ`)
`third_body_mean_anomaly_0`	`Mₓ₀`	mean anomaly at epoch of the Sun (`Mₛ₀`) or the Moon (`Mₗ₀`)
`ax1`	`aₓ₁`	partial expression of multiple `Xₓₙ` and `Zₓₙ` expressions
`ax3`	`aₓ₃`	partial expression of multiple `Xₓₙ` and `Zₓₙ` expressions
`ax7`	`aₓ₇`	partial expression of multiple `aₓ₂` and `aₓ₅`
`ax8`	`aₓ₈`	partial expression of multiple `aₓ₂` and `aₓ₅`
`ax9`	`aₓ₉`	partial expression of multiple `aₓ₄` and `aₓ₆`
`ax10`	`aₓ₁₀`	partial expression of multiple `aₓ₄` and `aₓ₆`
`ax2`	`aₓ₂`	partial expression of multiple `Xₓₙ` and `Zₓₙ` expressions
`ax4`	`aₓ₄`	partial expression of multiple `Xₓₙ` and `Zₓₙ` expressions
`ax5`	`aₓ₅`	partial expression of multiple `Xₓₙ` and `Zₓₙ` expressions
`ax6`	`aₓ₆`	partial expression of multiple `Xₓₙ` and `Zₓₙ` expressions
`xx1`	`Xₓ₁`	partial expression of multiple `Zₓₙ` expressions, `kₓ₀`, `kₓ₁` and `ėₓ`
`xx2`	`Xₓ₂`	partial expression of multiple `Zₓₙ` expressions, `kₓ₀`, `kₓ₁` and `ėₓ`
`xx3`	`Xₓ₃`	partial expression of multiple `Zₓₙ` expressions, `kₓ₀`, `kₓ₁` and `ėₓ`
`xx4`	`Xₓ₄`	partial expression of multiple `Zₓₙ` expressions, `kₓ₀`, `kₓ₁` and `ėₓ`
`xx5`	`Xₓ₅`	partial expression of multiple `Zₓₙ` expressions
`xx6`	`Xₓ₆`	partial expression of multiple `Zₓₙ` expressions
`xx7`	`Xₓ₇`	partial expression of multiple `Zₓₙ` expressions
`xx8`	`Xₓ₈`	partial expression of multiple `Zₓₙ` expressions
`zx31`	`Zₓ₃₁`	partial expression of `Zₓ₃`, `kₓ₈` and `ω̇ₓ`
`zx32`	`Zₓ₃₂`	partial expression of `Zₓ₂`, `kₓ₇` and `ω̇ₓ`
`zx33`	`Zₓ₃₃`	partial expression of `Zₓ₃`, `kₓ₈` and `ω̇ₓ`
`zx11`	`Zₓ₁₁`	partial expression of `kₓ₃` and `İₓ`
`zx13`	`Zₓ₁₃`	partial expression of `kₓ₃` and `İₓ`
`zx21`	`Zₓ₂₁`	partial expression of `kₓ₁₁` and `Ω̇ₓ`
`zx23`	`Zₓ₂₃`	partial expression of `kₓ₁₁` and `Ω̇ₓ`
`zx1`	`Zₓ₁`	partial expression of `kₓ₅` and `Ṁₓ`
`zx3`	`Zₓ₃`	partial expression of `kₓ₅` and `Ṁₓ`
`px0`	`pₓ₀`	partial expression of multiple `kₓₙ` expressions and `Ṁₓ`
`px1`	`pₓ₁`	partial expression of multiple `kₓₙ` expressions and `İₓ`
`px2`	`pₓ₂`	partial expression of multiple `kₓₙ` expressions and `ω̇ₓ`
`px3`	`pₓ₃`	partial expression of multiple `kₓₙ` expressions and `ėₓ`
`kx0`	`kₓ₀`	`Fₓ₂` coefficient of `δeₓ`
`kx1`	`kₓ₁`	`Fₓ₃` coefficient of `δeₓ`
`kx2`	`kₓ₂`	`Fₓ₂` coefficient of `δIₓ`
`kx3`	`kₓ₃`	`Fₓ₃` coefficient of `δIₓ`
`kx4`	`kₓ₄`	`Fₓ₂` coefficient of `δMₓ`
`kx5`	`kₓ₅`	`Fₓ₃` coefficient of `δMₓ`
`kx6`	`kₓ₆`	`sin fₓ` coefficient of `δMₓ`
`kx7`	`kₓ₇`	`Fₓ₂` coefficient of `pₓ₄`
`kx8`	`kₓ₈`	`Fₓ₃` coefficient of `pₓ₄`
`kx9`	`kₓ₉`	`sin fₓ` coefficient of `pₓ₄`
`kx10`	`kₓ₁₀`	`Fₓ₂` coefficient of `pₓ₅`
`kx11`	`kₓ₁₁`	`Fₓ₃` coefficient of `pₓ₅`
`third_body_dots.inclination`	`İₓ`	secular contribution of the Sun (`İₛ`) or the Moon (`İₗ`) to the inclination
`third_body_right_ascension_dot`	`Ω̇ₓ`	secular contribution of the Sun (`Ω̇ₛ`) or the Moon (`Ω̇ₗ`) to the right ascension of the ascending node
`third_body_dots.eccentricity`	`ėₓ`	secular contribution of the Sun (`ėₛ`) or the Moon (`ėₗ`) to the eccentricity
`third_body_dots.agument_of_perigee`	`ω̇ₓ`	secular contribution of the Sun (`ω̇ₛ`) or the Moon (`ω̇ₗ`) to the argument of perigee
`third_body_dots.mean_anomaly`	`Ṁₓ`	secular contribution of the Sun (`Ṁₛ`) or the Moon (`Ṁₗ`) to the mean anomaly

Third-body propagation variables

The following variables depend on the propagation time t.

variable	symbol	description
`third_body_mean_anomaly`	`Mₓ`	mean anomaly of the Sun (`Mₛ`) or the Moon (`Mₗ`)
`fx`	`fₓ`	third body true anomaly
`fx2`	`Fₓ₂`	partial expression of the third body long-period periodic contribution
`fx3`	`Fₓ₃`	partial expression of the third body long-period periodic contribution
`third_body_delta_eccentricity`	`δeₓ`	long-period periodic contribution of the Sun (`δeₛ`) or the Moon (`δeₗ`) to the eccentricity
`third_body_delta_inclination`	`δIₓ`	long-period periodic contribution of the Sun (`δIₛ`) or the Moon (`δIₗ`) to the inclination
`third_body_delta_mean_mootion`	`δMₓ`	long-period periodic contribution of the Sun (`δMₛ`) or the Moon (`δMₗ`) to the mean motion
`px4`	`pₓ₄`	partial expression of the long-period periodic contribution of the Sun (`pₛ₄`) or the Moon (`pₗ₄`) to the right ascension of the ascending node and the argument of perigee
`px5`	`pₓ₅`	partial expression of the long-period periodic contribution of the Sun (`pₛ₅`) or the Moon (`pₗ₅`) to the right ascension of the ascending node

Mathematical expressions

UT1 to Julian conversion

The epoch (Julian years since UTC 1 January 2000 12h00) can be calculated with either the AFSPC formula:

y₂₀₀₀ = (367 yᵤ - ⌊7 (yᵤ + ⌊(mᵤ + 9) / 12⌋) / 4⌋ + 275 ⌊mᵤ / 9⌋ + dᵤ
        + 1721013.5
        + (((nsᵤ / 10⁹ + sᵤ) / 60 + minᵤ) / 60 + hᵤ) / 24
        - 2451545)
        / 365.25

or the more accurate version of the same formula:

y₂₀₀₀ = (367 yᵤₜ₁ - ⌊7 (yᵤₜ₁ + ⌊(mᵤₜ₁ + 9) / 12⌋) / 4⌋ + 275 ⌊mᵤₜ₁ / 9⌋ + dᵤₜ₁ - 730531) / 365.25
        + (3600 hᵤₜ₁ + 60 minᵤₜ₁ + sᵤₜ₁ - 43200) / (24 × 60 × 60 × 365.25)
        + nsᵤₜ₁ / (24 × 60 × 60 × 365.25 × 10⁹)

Common initialization

a₁ = (kₑ / n₀)²ᐟ³

      3      3 cos²I₀ - 1
 p₀ = - J₂ ---------------
      4      (1 − e₀²)³ᐟ²

𝛿₁ = p₀ / a₁²

𝛿₀ = p₀ / (a₁ (1 - ¹/₃ 𝛿₁ - 𝛿₁² - ¹³⁴/₈₁ 𝛿₁³))²

n₀" = n₀ / (1 + 𝛿₀)

p₁ = cos I₀

p₂ = 1 − e₀²

k₆ = 3 p₁² - 1

a₀" = (kₑ / n₀")²ᐟ³

p₃ = a₀" (1 - e₀)

p₄ = aₑ (p₃ - 1)

p₅ = │ 20      if p₄ < 98
     │ p₄ - 78 if 98 ≤ p₄ < 156
     │ 78      otherwise

s = p₅ / aₑ + 1

p₆ = ((120 - p₅) / aₑ)⁴

ξ = 1 / (a₀" - s)

p₇ = p₆ ξ⁴

η = a₀" e₀ ξ

p₈ = |1 - η²|

p₉ = p₇ / p₈⁷ᐟ²

C₁ = B* p₉ n₀" (a₀" (1 + ³/₂ η² + e₀ η (4 + η²))
     + ³/₈ J₂ ξ k₆ (8 + 3 η² (8 + η²)) / p₈)

p₁₀ = (a₀" p₂)⁻²

β₀ = p₂¹ᐟ²

p₁₁ = ³/₂ J₂ p₁₀ n₀"

p₁₂ = ¹/₂ p₁₁ J₂ p₁₀

p₁₃ = - ¹⁵/₃₂ J₄ p₁₀² n₀"

p₁₄ = - p₁₁ p₁ + (¹/₂ p₁₂ (4 - 19 p₁²) + 2 p₁₃ (3 - 7 p₁²)) p₁

k₁₄ = - ¹/₂ p₁₁ (1 - 5 p₁²) + ¹/₁₆ p₁₂ (7 - 114 p₁² + 395 p₁⁴)

p₁₅ = n₀" + ¹/₂ p₁₁ β₀ k₆ + ¹/₁₆ p₁₂ β₀ (13 - 78 p₁² + 137 p₁⁴)

C₄ = 2 B* n₀" p₉ a₀" p₂ (
     η (2 + ¹/₂ η²)
     + e₀ (¹/₂ + 2 η²)
     - J₂ ξ / (a p₈) (-3 k₆ (1 - 2 e₀ η + η² (³/₂ - ¹/₂ e₀ η))
     + ³/₄ (1 - p₁²) (2 η² - e₀ η (1 + η²)) cos 2 ω₀)

k₀ =  ⁷/₂ p₂ p₁₁ p₁ C₁

k₁ = ³/₂ C₁

Ω̇ = │ p₁₄            if n₀" > 2π / 225
    │ p₁₄ + (Ω̇ₛ + Ω̇ₗ) otherwise

ω̇ = │ k₁₄            if n₀" > 2π / 225
    │ k₁₄ + (ω̇ₛ + ω̇ₗ) otherwise

Ṁ = │ p₁₅            if n₀" > 2π / 225
    │ p₁₅ + (Ṁₛ + Ṁₗ) otherwise

Near earth initialization

Defined only if n₀" > 2π / 225 (near earth).

       1 J₃
k₂ = - - -- sin I₀
       2 J₂

k₃ = 1 - p₁²

k₄ = 7 p₁² - 1

     │   1 J₃        3 + 5 p₁
k₅ = │ - - -- sin I₀ --------    if |1 + p₁| > 1.5 × 10⁻¹²
     │   4 J₂         1 + p₁
     │   1 J₃         3 + 5 p₁
     │ - - -- sin I₀ ----------- otherwise
     │   4 J₂        1.5 × 10⁻¹²

High altitude near earth initialization

Defined only if n₀" > 2π / 225 (near earth) and p₃ ≥ 220 / (aₑ + 1) (high altitude).

D₂ = 4 a₀" ξ C₁²

p₁₆ = D₂ ξ C₁ / 3

D₃ = (17 a + s) p₁₆

D₄ = ¹/₂ p₁₆ a₀" ξ (221 a₀" + 31 s) C₁

C₅ = 2 B* p₉ a₀" p₂ (1 + 2.75 (η² + η e₀) + e₀ η³)

k₁₁ = (1 + η cos M₀)³

k₇ = sin M₀

k₈ = D₂ + 2 C₁²

k₉ = ¹/₄ (3 D₃ + C₁ (12 D₂ + 10 C₁²))

k₁₀ = ¹/₅ (3 D₄ + 12 C₁ D₃ + 6 D₂² + 15 C₁² (2 D₂ + C₁²))

Elliptic high altitude near earth initialization

Defined only if n₀" > 2π / 225 (near earth), p₃ ≥ 220 / (aₑ + 1) (high altitude) and e₀ > 10⁻⁴ (elliptic).

                    J₃ p₇ ξ  n₀" sin I₀
k₁₂ = - 2 B* cos ω₀ -- ----------------
                    J₂        e₀

        2 p₇ B*
k₁₃ = - - -----
        3 e₀ η

Deep space initialization

Defined only if n₀" ≤ 2π / 225 (deep space).

e₁₉₀₀ = 365.25 (t₀ + 100)

sin Iₛ = 0.39785416

cos Iₛ = 0.91744867

sin(Ω₀ - Ωₛ) = sin Ω₀

cos(Ω₀ - Ωₛ) = cos Ω₀

sin ωₛ = -0.98088458

cos ωₛ = 0.1945905

Mₛ₀ = (6.2565837 + 0.017201977 e₁₉₀₀) rem 2π

Ωₗₑ = 4.523602 - 9.2422029 × 10⁻⁴ e₁₉₀₀ rem 2π

cos Iₗ = 0.91375164 - 0.03568096 Ωₗₑ

sin Iₗ = (1 - cos²Iₗ)¹ᐟ²

sin Ωₗ = 0.089683511 sin Ωₗₑ / sin Iₗ

cos Ωₗ = (1 - sin²Ωₗ)¹ᐟ²

ωₗ = 5.8351514 + 0.001944368 e₁₉₀₀
                    0.39785416 sin Ωₗₑ / sin Iₗ
     + tan⁻¹ ------------------------------------------ - Ωₗₑ
             cos Ωₗ cos Ωₗₑ + 0.91744867 sin Ωₗ sin Ωₗₑ

sin(Ω₀ - Ωₗ) = sin Ω₀ cos Ωₗ - cos Ω₀ sin Ωₗ

cos(Ω₀ - Ωₗ) = cos Ωₗ cos Ω₀ + sin Ωₗ sin Ω₀

Mₗ₀ = (-1.1151842 + 0.228027132 e₁₉₀₀) rem 2π

Third body perturbations

Defined only if n₀" ≤ 2π / 225 (deep space).

The following variables are evaluated for two third bodies, the Sun (solar perturbations s) and the Moon (lunar perturbations l). Variables specific to the third body are annotated with x. In other sections, x is either s or l.

aₓ₁ = cos ωₓ cos(Ω₀ - Ωₓ) + sin ωₓ cos Iₓ sin(Ω₀ - Ωₓ)

aₓ₃ = - sin ωₓ cos(Ω₀ - Ωₓ) + cos ωₓ cos Iₓ sin(Ω₀ - Ωₓ)

aₓ₇ = - cos ωₓ sin(Ω₀ - Ωₓ) + sin ωₓ cos Iₓ cos(Ω₀ - Ωₓ)

aₓ₈ = sin ωₓ sin Iₓ

aₓ₉ = sin ωₓ sin(Ω₀ - Ωₓ) + cos ωₓ cos Iₓ cos(Ω₀ - Ωₓ)

aₓ₁₀ = cos ωₓ sin Iₓ

aₓ₂ = aₓ₇ cos i₀ + aₓ₈ sin I₀

aₓ₄ = aₓ₉ cos i₀ + aₓ₁₀ sin I₀

aₓ₅ = - aₓ₇ sin I₀ + aₓ₈ cos I₀

aₓ₆ = - aₓ₉ sin I₀ + aₓ₁₀ cos I₀

Xₓ₁ = aₓ₁ cos ω₀ + aₓ₂ sin ω₀

Xₓ₂ = aₓ₃ cos ω₀ + aₓ₄ sin ω₀

Xₓ₃ = - aₓ₁ sin ω₀ + aₓ₂ cos ω₀

Xₓ₄ = - aₓ₃ sin ω₀ + aₓ₄ cos ω₀

Xₓ₅ = aₓ₅ sin ω₀

Xₓ₆ = aₓ₆ sin ω₀

Xₓ₇ = aₓ₅ cos ω₀

Xₓ₈ = aₓ₆ cos ω₀

Zₓ₃₁ = 12 Xₓ₁² - 3 Xₓ₃²

Zₓ₃₂ = 24 Xₓ₁ Xₓ₂ - 6 Xₓ₃ Xₓ₄

Zₓ₃₃ = 12 Xₓ₂² - 3 Xₓ₄²

Zₓ₁₁ = - 6 aₓ₁ aₓ₅ + e₀² (- 24 Xₓ₁ Xₓ₇ - 6 Xₓ₃ Xₓ₅)

Zₓ₁₃ = - 6 aₓ₃ aₓ₆ + e₀² (-24 Xₓ₂ Xₓ₈ - 6 Xₓ₄ Xₓ₆)

Zₓ₂₁ = 6 aₓ₂ aₓ₅ + e₀² (24.0 Xₓ₁ Xₓ₅ - 6 Xₓ₃ Xₓ₇)

Zₓ₂₃ = 6 aₓ₄ aₓ₆ + e₀² (24 Xₓ₂ Xₓ₆ - 6 Xₓ₄ Xₓ₈)

Zₓ₁ = 2 (3 (aₓ₁² + aₓ₂²) + Zₓ₃₁ e₀²) + p₁ Zₓ₃₁

Zₓ₃ = 2 (3 (aₓ₃² + aₓ₄²) + Zₓ₃₃ e₀²) + p₁ Zₓ₃₃

pₓ₀ = Cₓ / n₀"

        1 pₓ₀
pₓ₁ = - - ---
        2 β₀

pₓ₂ = pₓ₀ β₀

pₓ₃ = - 15 e₀ pₓ₂

Ω̇ₓ = │ 0                               if I₀ < 5.2359877 × 10⁻²
     │                                 or I₀ > π - 5.2359877 × 10⁻²
     │ - nₓ pₓ₁ (Zₓ₂₁ + Zₓ₂₃) / sin I₀ otherwise

kₓ₀ = 2 pₓ₃ (Xₓ₂ Xₓ₃ + Xₓ₁ Xₓ₄)

kₓ₁ = 2 pₓ₃ (Xₓ₂ Xₓ₄ - Xₓ₁ Xₓ₃)

kₓ₂ = 2 pₓ₁ (- 6 (aₓ₁ aₓ₆ + aₓ₃ aₓ₅) + e₀² (- 24 (Xₓ₂ Xₓ₇ + Xₓ₁ Xₓ₈) - 6 (Xₓ₃ Xₓ₆ + Xₓ₄ Xₓ₅)))

kₓ₃ = 2 pₓ₁ (Zₓ₁₃ - Zₓ₁₁)

kₓ₄ = - 2 pₓ₀ (2 (6 (aₓ₁ aₓ₃ + aₓ₂ aₓ₄) + Zₓ₃₂ e₀²) + p₁ Zₓ₃₂)

kₓ₅ = - 2 pₓ₀ (Zₓ₃ - Zₓ₁)

kₓ₆ = - 2 pₓ₀ (- 21 - 9 e₀²) eₓ

kₓ₇ = 2 pₓ₂ Zₓ₃₂

kₓ₈ = 2 pₓ₂ (Zₓ₃₃ - Zₓ₃₁)

kₓ₉ = - 18 pₓ₂ eₓ

kₓ₁₀ = - 2 pₓ₁ (6 (aₓ₄ aₓ₅ + aₓ₂ aₓ₆) + e₀² (24 (Xₓ₂ Xₓ₅ + Xₓ₁ Xₓ₆) - 6 (Xₓ₄ Xₓ₇ + Xₓ₃ Xₓ₈)))

kₓ₁₁ = - 2 pₓ₁ (Zₓ₂₃ - Zₓ₂₁)

İₓ = pₓ₁ nₓ (Zₓ₁₁ + Zₓ₁₃)

ėₓ = pₓ₃ nₓ (Xₓ₁ Xₓ₃ + Xₓ₂ Xₓ₄)

ω̇ₓ = pₓ₂ nₓ (Zₓ₃₁ + Zₓ₃₃ - 6) - cos I₀ Ω̇ₓ

Ṁₓ = - nₓ pₓ₀ (Zₓ₁ + Zₓ₃ - 14 - 6 e₀²)

Resonant deep space initialization

Defined only if n₀" ≤ 2π / 225 (deep space) and either:

0.0034906585 < n₀" < 0.0052359877 (geosynchronous)
8.26 × 10⁻³ ≤ n₀" ≤ 9.24 × 10⁻³ and e₀ ≥ 0.5 (Molniya)

The sidereal time θ₀ at epoch can be calculated with either the IAU formula:

c₂₀₀₀ = y₂₀₀₀ / 100

θ₀ = ¹/₂₄₀ (π / 180) (- 6.2 × 10⁻⁶ c₂₀₀₀³ + 0.093104 c₂₀₀₀²
     + (876600 × 3600 + 8640184.812866) c₂₀₀₀ + 67310.54841) mod 2π

or the AFSPC formula:

d₁₉₇₀ = 365.25 (y₂₀₀₀ + 30) + 1

θ₀ = 1.7321343856509374 + 1.72027916940703639 × 10⁻² ⌊d₁₉₇₀ + 10⁻⁸⌋
     + (1.72027916940703639 × 10⁻² + 2π) (d₁₉₇₀ - ⌊d₁₉₇₀ + 10⁻⁸⌋)
     + 5.07551419432269442 × 10⁻¹⁵ d₁₉₇₀² mod 2π

λ₀ = │ M₀ + Ω₀ + ω₀ − θ₀ rem 2π if geosynchronous
     │ M₀ + 2 Ω₀ − 2 θ₀ rem 2π  otherwise

λ̇₀ = │ p₁₅ + (k₁₄ + p₁₄) − θ̇ + (Ṁₛ + Ṁₗ) + (ω̇ₛ + ω̇ₗ) + (Ω̇ₛ + Ω̇ₗ) - n₀" if geosynchronous
     │ p₁₅ + (Ṁₛ + Ṁₗ) + 2 (p₁₄ + (Ω̇ₛ + Ω̇ₗ) - θ̇) - n₀"                otherwise

Geosynchronous deep space initialization

Defined only if n₀" ≤ 2π / 225 (deep space) and 0.0034906585 < n₀" < 0.0052359877 (geosynchronous orbit).

p₁₇ = 3 (n / a₀")²

𝛿ᵣ₁ = p₁₇ (¹⁵/₁₆ sin²I₀ (1 + 3 p₁) - ³/₄ (1 + p₁))
          (1 + 2 e₀²) 2.1460748 × 10⁻⁶ / a₀"²

𝛿ᵣ₂ = 2 p₁₇ (³/₄ (1 + p₁)²)
     (1 + e₀² (- ⁵/₂ + ¹³/₁₆ e₀²)) 1.7891679 × 10⁻⁶

𝛿ᵣ₃ = 3 p₁₇ (¹⁵/₈ (1 + p₁)³) (1 + e₀² (- 6 + 6.60937 e₀²))
      2.2123015 × 10⁻⁷ / a₀"²

Molniya deep space initialization

Defined only if n₀" ≤ 2π / 225 (deep space) and 8.26 × 10⁻³ ≤ n₀" ≤ 9.24 × 10⁻³ and e₀ ≥ 0.5 (Molniya).

p₁₈ = 3 n₀"² / a₀"²

p₁₉ = p₁₈ / a₀"

p₂₀ = p₁₉ / a₀"

p₂₁ = p₂₀ / a₀"

F₂₂₀ = ³/₄ (1 + 2 p₁ + p₁²)

G₂₁₁ = │ 3.616 - 13.247 e₀ + 16.29 e₀²                     if e₀ ≤ 0.65
       │ - 72.099 + 331.819 e₀ - 508.738 e₀² + 266.724 e₀³ otherwise

G₃₁₀ = │ - 19.302 + 117.39 e₀ - 228.419 e₀² + 156.591 e₀³      if e₀ ≤ 0.65
       │ - 346.844 + 1582.851 e₀ - 2415.925 e₀² + 1246.113 e₀³ otherwise

G₃₂₂ = │ - 18.9068 + 109.7927 e₀ - 214.6334 e₀² + 146.5816 e₀³ if e₀ ≤ 0.65
       │ - 342.585 + 1554.908 e₀ - 2366.899 e₀² + 1215.972 e₀³ otherwise

G₄₁₀ = │ - 41.122 + 242.694 e₀ - 471.094 e₀² + 313.953 e₀³      if e₀ ≤ 0.65
       │ - 1052.797 + 4758.686 e₀ - 7193.992 e₀² + 3651.957 e₀³ otherwise

G₄₂₂ = │ - 146.407 + 841.88 e₀ - 1629.014 e₀² + 1083.435 e₀³   if e₀ ≤ 0.65
       │ - 3581.69 + 16178.11 e₀ - 24462.77 e₀² + 12422.52 e₀³ otherwise

G₅₂₀ = │ - 532.114 + 3017.977 e₀ - 5740.032 e₀² + 3708.276 e₀³ if e₀ ≤ 0.65
       │ 1464.74 - 4664.75 e₀ + 3763.64 e₀²                    if 0.65 < e₀ < 0.715
       │ - 5149.66 + 29936.92 e₀ - 54087.36 e₀² + 31324.56 e₀³ otherwise

G₅₃₂ = │ - 853.666 + 4690.25 e₀ - 8624.77 e₀² + 5341.4 e₀³         if e₀ < 0.7
       │ - 40023.88 + 170470.89 e₀ - 242699.48 e₀² + 115605.82 e₀³ otherwise

G₅₂₁ = │ - 822.71072 + 4568.6173 e₀ - 8491.4146 e₀² + 5337.524 e₀³  if e₀ < 0.7
       │ - 51752.104 + 218913.95 e₀ - 309468.16 e₀² + 146349.42 e₀³ otherwise

G₅₃₃ = │ - 919.2277 + 4988.61 e₀ - 9064.77 e₀² + 5542.21 e₀³      if e₀ < 0.7
       │ - 37995.78 + 161616.52 e₀ - 229838.2 e₀² + 109377.94 e₀³ otherwise

D₂₂₀₋₁ = p₁₈ 1.7891679 × 10⁻⁶ F₂₂₀ (- 0.306 - 0.44 (e₀ - 0.64))

D₂₂₁₁ = p₁₈ 1.7891679 × 10⁻⁶ (³/₂ sin²I₀) G₂₁₁

D₃₂₁₀ = p₁₉ 3.7393792 × 10⁻⁷ (¹⁵/₈ sin I₀ (1 - 2 p₁ - 3 p₁²)) G₃₁₀

D₃₂₂₂ = p₁₉ 3.7393792 × 10⁻⁷ (- ¹⁵/₈ sin I₀ (1 + 2 p₁ - 3 p₁²)) G₃₂₂

D₄₄₁₀ = 2 p₂₀ 7.3636953 × 10⁻⁹ (35 sin²I₀ F₂₂₀) G₄₁₀

D₄₄₂₂ = 2 p₂₀ 7.3636953 × 10⁻⁹ (³¹⁵/₈ sin⁴I₀) G₄₂₂

D₅₂₂₀ = p₂₁ 1.1428639 × 10⁻⁷ (³¹⁵/₃₂ sin I₀
        (sin²I₀ (1 - 2 p₁ - 5 p₁²)
        + 0.33333333 (- 2 + 4 p₁ + 6 p₁²))) G₅₂₀

D₅₂₃₂ = p₂₁ 1.1428639 × 10⁻⁷ (sin I₀
        (4.92187512 sin²I₀ (- 2 - 4 p₁ + 10 p₁²)
        + 6.56250012 (1 + p₁ - 3 p₁²))) G₅₃₂

D₅₄₂₁ = 2 p₂₁ 2.1765803 × 10⁻⁹ (⁹⁴⁵/₃₂ sin I₀
        (2 - 8 p₁ + p₁² (- 12 + 8 p₁ + 10 p₁²))) G₅₂₁

D₅₄₃₃ = 2 p₂₁ 2.1765803 × 10⁻⁹ (⁹⁴⁵/₃₂ sin I₀
        (- 2 - 8 p₁ + p₁² (12 + 8 p₁ - 10 p₁²))) G₅₃₃

Common propagation

The following values depend on the propagation time t (minutes since epoch).

Named conditions have the following meaning:

near earth: n₀" ≤ 2π / 225
low altitude near earth: near earth and p₃ < 220 / (aₑ + 1)
high altitude near earth: near earth and p₃ ≥ 220 / (aₑ + 1)
elliptic high altitude near earth: high altitude near earth and e₀ > 10⁻⁴
non-elliptic near earth: low altitude near earth or high altitude near earth and e₀ ≤ 10⁻⁴
deep space: n₀" > 2π / 225
non-Lyddane deep space: deep space and I ≥ 0.2
Lyddane deep space: deep space and I < 0.2
AFSPC Lyddane deep space: Lyddane deep space and use the same expression as the original AFSPC implementation, with an ω discontinuity at p₂₂ = 0

p₂₂ = Ω₀ + Ω̇ t + k₀ t²

p₂₃ = ω₀ + ω̇ t

I = │ I₀                    if near earth
    │ I₀ + İ t + (δIₛ + δIₗ) otherwise

Ω = │ p₂₂                      if near earth
    │ p₂₂ + (pₛ₅ + pₗ₅) / sin I if non-Lyddane deep space
    │ p₃₀ + 2π                 if Lyddane deep space and p₃₀ + π < p₂₂ rem 2π
    │ p₃₀ - 2π                 if Lyddane deep space and p₃₀ - π > p₂₂ rem 2π
    │ p₃₀                      otherwise

e = │ 10⁻⁶              if near earth and p₂₇ < 10⁻⁶
    │ p₂₇               if near earth and p₂₇ ≥ 10⁻⁶
    │ 10⁻⁶ + (δeₛ + δeₗ) if deep space and p₃₁ < 10⁻⁶
    │ p₃₁ + (δeₛ + δeₗ)  otherwise

ω = │ p₂₃ - p₂₅                                   if elliptic high altitude near earth
    │ p₂₃                                         if non-elliptic near earth
    │ p₂₃ + (pₛ₄ + pₗ₄) - cos I (pₛ₅ + pₗ₅) / sin I if non-Lyddane deep space
    │ p₂₃ + (pₛ₄ + pₗ₄) + cos I ((p₂₂ rem 2π) - Ω)
    │ - (δIₛ + δIₗ) (p₂₂ mod 2π) sin I             if AFSPC Lyddane deep space
    │ p₂₃ + (pₛ₄ + pₗ₄) + cos I ((p₂₂ rem 2π) - Ω)
    │ - (δIₛ + δIₗ) (p₂₂ rem 2π) sin I             otherwise

M = │ p₂₆ + n₀" (k₁ t² + k₈ t³ + t⁴ (k₉ + t k₁₀) if high altitude near earth
    │ p₂₄ + n₀" k₁ t²                            if low altitude near earth
    │ p₂₉ + (δMₛ + δMₗ) + n₀" k₁ t²               otherwise


a = │ a₀" (1 - C₁ t - D₂ t² - D₃ t³ - D₄ t⁴)² if high altitude near earth
    │ a₀" (1 - C₁ t)²                         if low altitude near earth
    │ p₂₈ (1 - C₁ t)²                         otherwise

n = kₑ / a³ᐟ²

p₃₂ = │ k₂           if near earth
      │   1 J₃
      │ - - -- sin I othewise
      │   2 J₂

p₃₃ = │ k₃        if near earth
      │ 1 - cos²I othewise

p₃₄ = │ k₄          if near earth
      │ 7 cos²I - 1 otherwise

p₃₅ = │ k₅                       if near earth
      │   1 J₃       3 + 5 cos I
      │ - - -- sin I ----------- if deep space and |1 + cos I| > 1.5 × 10⁻¹²
      │   4 J₂        1 + cos I
      │   1 J₃       3 + 5 cos I
      │ - - -- sin I ----------- otherwise
      │   4 J₂       1.5 × 10⁻¹²

p₃₆ = │ k₆          if near earth
      │ 3 cos²I - 1 otherwise

p₃₇ = 1 / (a (1 - e²))

aₓₙ = e cos ω

aᵧₙ = e sin ω + p₃₇ p₃₂

p₃₈ = M + ω + p₃₇ p₃₅ aₓₙ rem 2π

(E + ω)₀ = p₃₈

            p₃₈ - aᵧₙ cos (E + ω)ᵢ + aₓₙ sin (E + ω)ᵢ - (E + ω)ᵢ
Δ(E + ω)ᵢ = ---------------------------------------------------
                  1 - cos (E + ω)ᵢ aₓₙ - sin (E + ω)ᵢ aᵧₙ

(E + ω)ᵢ₊₁ = (E + ω)ᵢ + Δ(E + ω)ᵢ|[-0.95, 0.95]

E + ω = │ (E + ω)₁₀ if ∀ j ∈ [0, 9], Δ(E + ω)ⱼ ≥ 10⁻¹²
        │ (E + ω)ⱼ  otherwise, with j the smallest integer | Δ(E + ω)ⱼ < 10⁻¹²

p₃₉ = aₓₙ² + aᵧₙ²

pₗ = a (1 - p₃₉)

p₄₀ = aₓₙ sin(E + ω) - aᵧₙ cos(E + ω)

r = a (1 - (aₓₙ cos(E + ω) + aᵧₙ sin(E + ω)))

ṙ = a¹ᐟ² p₄₀ / r

β = (1 - p₃₉)¹ᐟ²

p₄₁ = p₄₀ / (1 + β)

p₄₂ = a / r (sin(E + ω) - aᵧₙ - aₓₙ p₄₁)

p₄₃ = a / r (cos(E + ω) - aₓₙ + aᵧₙ p₄₁)

          p₄₂
u = tan⁻¹ ---
          p₄₃

p₄₄ = 2 p₄₃ p₄₂

p₄₅ = 1 - 2 p₄₂²

p₄₆ = (¹/₂ J₂ / pₗ) / pₗ

rₖ = r (1 - ³/₂ p₄₆ β p₃₆) + ¹/₂ (¹/₂ J₂ / pₗ) p₃₃ p₄₅

uₖ = u - ¹/₄ p₄₆ p₃₄ p₄₄

Ωₖ = Ω + ³/₂ p₄₆ cos I p₄₄

Iₖ = I + ³/₂ p₄₆ cos I sin I p₄₅

ṙₖ = ṙ + n (¹/₂ J₂ / pₗ) p₃₃ / kₑ

rḟₖ = pₗ¹ᐟ² / r + n (¹/₂ J₂ / pₗ) (p₃₃ p₄₅ + ³/₂ p₃₆) / kₑ

u₀ = - sin Ωₖ cos Iₖ sin uₖ + cos Ωₖ cos uₖ

u₁ = cos Ωₖ cos Iₖ sin uₖ + sin Ωₖ cos uₖ

u₂ = sin Iₖ sin uₖ

r₀ = rₖ u₀ aₑ

r₁ = rₖ u₁ aₑ

r₂ = rₖ u₂ aₑ

ṙ₀ = (ṙₖ u₀ + rḟₖ (- sin Ωₖ cos Iₖ cos uₖ - cos Ωₖ sin uₖ)) aₑ kₑ / 60

ṙ₁ = (ṙₖ u₁ + rḟₖ (cos Ωₖ cos Iₖ cos uₖ - sin Ωₖ sin uₖ)) aₑ kₑ / 60

ṙ₂ = (ṙₖ u₂ + rḟₖ (sin Iₖ cos uₖ)) aₑ kₑ / 60

Near earth propagation

Defined only if n₀" > 2π / 225 (near earth).

p₂₄ = M₀ + Ṁ t

p₂₇ = | e₀ - (C₄ t + C₅ (sin p₂₆ - k₇)) if high altitude
      | e₀ - C₄ t                       otherwise

High altitude near earth propagation

Defined only if n₀" > 2π / 225 (near earth) and p₃ ≥ 220 / (aₑ + 1) (high altitude).

elliptic means e₀ > 10⁻⁴.

p₂₅ = k₁₃ ((1 + η cos p₂₄)³ - k₁₁) + k₁₂ t

p₂₆ = │ p₂₄ + p₂₅ if elliptic
      │ p₂₄       otherwise

Deep space propagation

Defined only if n₀" ≤ 2π / 225 (deep space).

p₂₈ = │ (kₑ / (nⱼ + ṅⱼ (t - tⱼ) + ¹/₂ n̈ⱼ (t - tⱼ)²))²ᐟ³ if geosynchronous or Molniya
      │ a₀"                                            otherwise

p₂₉ = │ λⱼ + λ̇ⱼ (t - tⱼ) + ¹/₂ ṅᵢ (t - tⱼ)² - p₂₂ - p₂₃ + θ if geosynchronous
      │ λⱼ + λ̇ⱼ (t - tⱼ) + ¹/₂ ṅᵢ (t - tⱼ)² - 2 p₂₂ + 2 θ   if Molniya
      │ M₀ + Ṁ t                                            otherwise

j is │ the largest positive integer | tⱼ ≤ t  if t > 0
     │ the smallest negative integer | tⱼ ≥ t if t < 0
     │ 0                                      otherwise

p₃₁ = e₀ + ė t - C₄ t

Third body propagation

Defined only if n₀" ≤ 2π / 225 (deep space).

The following variables are evaluated for two third bodies, the Sun (solar perturbations s) and the Moon (lunar perturbations l). Variables specific to the third body are annotated with x. In other sections, x is either s or l.

Mₓ = Mₓ₀ + nₓ t

fₓ = Mₓ + 2 eₓ sin Mₓ

Fₓ₂ = ¹/₂ sin²fₓ - ¹/₄

Fₓ₃ = - ¹/₂ sin fₓ cos fₓ

δeₓ = kₓ₀ Fₓ₂ + kₓ₁ Fₓ₃

δIₓ = kₓ₂ Fₓ₂ + kₓ₃ Fₓ₃

δMₓ = kₓ₄ Fₓ₂ + kₓ₅ Fₓ₃ + kₓ₆ sin fₓ

pₓ₄ = kₓ₇ Fₓ₂ + kₓ₈ Fₓ₃ + kₓ₉ sin fₓ

pₓ₅ = kₓ₁₀ Fₓ₂ + kₓ₁₁ Fₓ₃

Resonant deep space propagation

Defined only if n₀" ≤ 2π / 225 (deep space) and either:

0.0034906585 < n₀" < 0.0052359877 (geosynchronous)
8.26 × 10⁻³ ≤ n₀" ≤ 9.24 × 10⁻³ and e₀ ≥ 0.5 (Molniya)

θ = θ₀ + 4.37526908801129966 × 10⁻³ t rem 2π

Δt = │ |Δt|  if t > 0
     │ -|Δt| if t < 0
     │ 0     otherwise

λ̇ᵢ = nᵢ + λ̇₀

ṅᵢ = │ 𝛿ᵣ₁ sin(λᵢ - λ₃₁) + 𝛿ᵣ₂ sin(2 (λᵢ - λ₂₂)) + 𝛿ᵣ₃ sin(3 (λᵢ - λ₃₃)) if geosynchronous
     │ Σ₍ₗₘₚₖ₎ Dₗₘₚₖ sin((l - 2 p) ωᵢ + m / 2 λᵢ - Gₗₘ)                    otherwise

n̈ᵢ = │ (𝛿ᵣ₁ cos(λᵢ - λ₃₁) + 𝛿ᵣ₂ cos(2 (λᵢ - λ₂₂)) + 𝛿ᵣ₃ cos(3 (λᵢ - λ₃₃))) λ̇ᵢ if geosynchronous
     │ (Σ₍ₗₘₚₖ₎ m / 2 Dₗₘₚₖ cos((l - 2 p) ωᵢ + m / 2 λᵢ - Gₗₘ)) λ̇ᵢ               otherwise

(l, m, p, k) ∈ {(2, 2, 0, -1), (2, 2, 1, 1), (3, 2, 1, 0),
    (3, 2, 2, 2), (4, 4, 1, 0), (4, 4, 2, 2), (5, 2, 2, 0),
    (5, 2, 3, 2), (5, 4, 2, 1), (5, 4, 3, 3)}

tᵢ₊₁ = tᵢ + Δt

nᵢ₊₁ = nᵢ + ṅᵢ Δt + n̈ᵢ (Δt² / 2)

λᵢ₊₁ = λᵢ + λ̇ᵢ Δt + ṅᵢ (Δt² / 2)

Lyddane deep space propagation

Defined only if n₀" ≤ 2π / 225 (deep space) and I < 0.2 (Lyddane).

            sin I sin p₂₂ + (pₛ₅ + pₗ₅) cos p₂₂ + (δIₛ + δIₗ) cos I sin p₂₂
p₃₀ = tan⁻¹ -------------------------------------------------------------
            sin I cos p₂₂ - (pₛ₅ + pₗ₅) sin p₂₂ + (δIₛ + δIₗ) cos I cos p₂₂

References

[1] David A. Vallado, Paul Crawford, R. S. Hujsak and T. S. Kelso, "Revisiting Spacetrack Report #3", presented at the AIAA/AAS Astrodynamics Specialist Conference, Keystone, CO, 2006 August 21–24, https://doi.org/10.2514/6.2006-6753

[2] F. R. Hoots, P. W. Schumacher Jr. and R. A. Glover, "History of Analytical Orbit Modeling in the U. S. Space Surveillance System", Journal of Guidance, Control, and Dynamics, 27(2), 174–185, 2004, https://doi.org/10.2514/1.9161

[3] F. R. Hoots and R. L. Roehrich, "Spacetrack Report No. 3: Models for propagation of NORAD element sets", U.S. Air Force Aerospace Defense Command, Colorado Springs, CO, 1980, https://www.celestrak.com/NORAD/documentation/

[4] R. S. Hujsak, "A Restricted Four Body Solution for Resonating Satellites Without Drag", Project SPACETRACK, Rept. 1, U.S. Air Force Aerospace Defense Command, Colorado Springs, CO, Nov. 1979, https://doi.org/10.21236/ada081263

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benches		benches
examples		examples
src		src
tests		tests
.gitignore		.gitignore
Cargo.lock		Cargo.lock
Cargo.toml		Cargo.toml
LICENSE		LICENSE
README.md		README.md
test_cases.rs		test_cases.rs
test_cases.toml		test_cases.toml

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