Agent skill

astroforcemodels-jl

Develop and maintain AstroForceModels.jl, a Julia library for astrodynamics force modeling (gravity, drag, SRP, third-body, relativity). Use when working on AstroForceModels.jl, adding new force models, or computing orbital perturbation accelerations.

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SKILL.md

AstroForceModels.jl

Julia package for modeling astrodynamics perturbation forces. Repo: HAMMERHEAD-Space/AstroForceModels.jl

Architecture

Type Hierarchy

AbstractAstroForceModel
├── AbstractPotentialBasedForce
│   └── AbstractGravityAstroModel
│       ├── GravityHarmonicsAstroModel   # Spherical harmonics (EGM96 etc.)
│       └── KeplerianGravityAstroModel   # Two-body point mass
└── AbstractNonPotentialBasedForce
    ├── DragAstroModel                   # Atmospheric drag
    ├── SRPAstroModel                    # Solar radiation pressure
    ├── ThirdBodyModel                   # Sun, Moon, planets
    └── RelativityModel                  # GR corrections

AbstractDynamicsModel
└── CentralBodyDynamicsModel{N,GT,PT}   # Gravity + N perturbations

Source Layout

src/
  AstroForceModels.jl        # Module entry, abstract types, exports
  constants.jl                # Physical constants (R_EARTH, μ_SUN, etc.)
  utils.jl                    # angle_between_vectors()
  dynamics_builder.jl         # CentralBodyDynamicsModel, build_dynamics_model
  force_models/
    drag/                     # satellite_shape_model.jl, density_calculator.jl, drag_accel.jl
    gravity/                  # utils.jl, gravity_accel.jl
    relativity/               # relativity_accel.jl
    solar_radiation_pressure/ # satellite_shape_model.jl, shadow_models.jl, srp_accel.jl
    third_body/               # celestial_body.jl, third_body_model.jl, third_body_accel.jl

Key Patterns

Unified Acceleration Interface

Every force model implements the same 4-argument signature:

julia
@inline function acceleration(
    u::AbstractVector, p::ComponentVector, t::Number, model::SomeForceModel
)::SVector{3}
    # ... compute and return acceleration in km/s^2 ...
end

Gravity models additionally implement:

  • potential(u, p, t, model) -> Number
  • potential_time_derivative(u, p, t, model) -> Number

Force Composition

julia
grav = GravityHarmonicsAstroModel(; gravity_model=..., eop_data=..., order=36, degree=36)
sun = ThirdBodyModel(; body=SunBody(), eop_data=eop_data)
moon = ThirdBodyModel(; body=MoonBody(), eop_data=eop_data)
srp = SRPAstroModel(; satellite_srp_model=CannonballFixedSRP(0.2), ...)
drag = DragAstroModel(; satellite_drag_model=CannonballFixedDrag(0.2), ...)

model = CentralBodyDynamicsModel(grav, (sun, moon, srp, drag))
accel = build_dynamics_model(u, p, t, model)  # Returns SVector{3}

sum_accelerations uses compile-time tuple recursion (first/Base.tail) for zero-overhead force summation.

Satellite Shape Models

  • Drag: CannonballFixedDrag(area_mass_ratio), StateDragModel(f)
  • SRP: CannonballFixedSRP(area_mass_ratio), StateSRPModel(f)
  • Shadow: Conical(), Cylindrical(), No_Shadow()

Atmospheric Density

compute_density() dispatches on atmosphere model type: JB2008, JR1971, MSIS2000, ExpAtmo, NoDensityModel.

Units Convention

  • Input/output: km, km/s, km/s^2 (J2000 ECI)
  • Internal SatelliteToolbox calls: convert to/from meters (* 1E3, / 1E3)

Adding a New Force Model

  1. Create struct <: AbstractNonPotentialBasedForce (or AbstractPotentialBasedForce)
  2. Implement acceleration(u, p, t, model) returning SVector{3}
  3. Use @inline, return SVector{3}, use promote_type for AD
  4. Add to CentralBodyDynamicsModel perturbation tuple
  5. Tests: correctness against reference, AD with all 5 backends, @check_allocs

Dependencies

  • ComponentArrays 0.15, Parameters 0.12, StaticArraysCore 1.4
  • SatelliteToolbox: atmospheric, gravity, celestial bodies, transformations
  • SpaceIndices 2 for space weather data

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