satellitetoolbox-gravity

name	satellitetoolbox-gravity
description	SatelliteToolboxGravityModels.jl for spherical harmonic gravity field computation. Use when loading gravity models (EGM96, EGM2008), computing gravitational acceleration or potential, or working with ICGEM gravity field files.

SatelliteToolboxGravityModels.jl

Spherical harmonic gravity field modeling. Repo: JuliaSpace/SatelliteToolboxGravityModels.jl

Loading a Model

using SatelliteToolboxGravityModels

# Load from ICGEM file (downloads and caches automatically)
model = GravityModels.load(IcgemFile, fetch_icgem_file(:EGM96))
model = GravityModels.load(IcgemFile, fetch_icgem_file(:EGM2008))

Computing Accelerations

All positions must be in ITRF (body-fixed frame) in meters.

# Gravitational acceleration (mass-attraction only) -> SVector{3} [m/s²]
a = GravityModels.gravitational_acceleration(model, r_itrf; max_degree=70, max_order=70)

# Gravity acceleration (includes centrifugal) -> SVector{3} [m/s²]
g = GravityModels.gravity_acceleration(model, r_itrf; max_degree=70)

# Gravitational potential -> scalar [J/kg]
U = GravityModels.gravitational_potential(model, r_itrf; max_degree=70)

# Field derivative in spherical coords -> (dU/dr, dU/dϕ, dU/dλ)
fd = GravityModels.gravitational_field_derivative(model, r_itrf; max_degree=70)

Pre-allocated Legendre Buffers (for hot-path performance)

P  = LowerTriangularStorage{RowMajor, Float64}(max_degree + 1)
dP = LowerTriangularStorage{RowMajor, Float64}(max_degree + 1)
a  = GravityModels.gravitational_acceleration(model, r; max_degree=70, P=P, dP=dP)

Model Queries

GravityModels.gravity_constant(model)       # μ [m³/s²]
GravityModels.radius(model)                 # Reference radius [m]
GravityModels.maximum_degree(model)         # Max available degree
GravityModels.coefficients(model, n, m)     # (Clm, Slm) at degree n, order m
GravityModels.coefficient_norm(model)       # :full, :schmidt, or :unnormalized

Key Conventions

• Positions in ITRF, meters. Returns in m/s².
• max_degree and max_order keywords truncate the expansion
• Time-varying coefficients supported (pass time argument as seconds since J2000 or DateTime)
• AD support via ForwardDiff/Zygote package extension
• Uses LowerTriangularStorage{RowMajor} from SatelliteToolboxBase for efficient coefficient storage