Satellitetoolbox Gravity

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