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GravityModels API

This document describes the API required for a gravity model. The user can add new models by overloading the functions listed here.

Structure

All models require a structure with supertype AbstractGravityModel{T<:Number, NT}, where T is the type of the coefficients in the model and NT is the norm type.

API Functions

function coefficients(model::AbstractGravityModel{T, NT}, degree::Int, order::Int, time::DataTime = DateTime("2000-01-01")) where {T<:Number, NT} -> T, T

This function must return the coefficients Clm and Slm of the gravity model for the specified degree, order, and time. If the latter argument is omitted, the J2000.0 epoch is used. Hence:

coefficients(model, 10, 8, DateTime("2023-06-19"))

must return a Tuple{T, T} with the Clm and Slm, respectively, for the degree 10, order 8, and computed at day 2023-06-19.

Note If the model has constant coefficients, the function must still accept the positional argument time, but it will be neglected. The package already defines the function without the time for the sake of usage simplification.

function coefficient_norm(model::AbstractGravityModel{T, NT}) where {T<:Number, NT} -> Symbol

This function must return the normalization we must use in the spherical harmonics when computing the Legendre associated functions. The accepted values are:

  • :full: Use full normalization.
  • :schmidt: Use Schmidt quasi-normalization.
  • :unnormalized: Do not perform normalization.
function gravity_constant(model::AbstractGravityModel{T, NT}) where {T<:Number, NT} -> T

This function must return the gravity constant [m³ / s²] for the gravity model.

function load(::Type{T}, args...; kwargs...) where T<:AbstractGravityModel -> T

This function must return gravity model structure, which is loaded using the arguments args... and keywords kwargs....

function maximum_degree(model::AbstractGravityModel) -> Int

This function must return the maximum degree of the gravity model.

function radius(model::AbstractGravityModel{T, NT}) where {T<:Number, NT} -> T

This function must return the radius [m] for the gravity model.