Support celestial bodies that are not the Earth

[QuantumBits]

It looks like there are a bunch of other .gfc files available for non-Earth celestial bodies that could be added pretty quickly with minimal changes.

The only thing that's preventing me from using them out-of-the-box with this package seems to be that the variable gravity_constant replaces the variable earth_gravity_constant in all of them. I think just allowing for either the existence of gravity_constant XOR earth_gravity_constant when parsing these files in here should be enough to fix this issue.

Thanks!

[ronisbr]

Hi @QuantumBits !

Sorry for the delay, I missed the notification. Perfect, I will change it to allow those cases.

[tahoe-vegas]

Hi,
I was interested in using this package to load a high-accuracy lunar gravity model. Is this possible now, or should I attempt to modify the parsing as @QuantumBits suggested?

Thanks,
Reno

[ronisbr]

Hi @tahoe-vegas !

I think we can make it work with very small changes. Can you share the file with me?

[tahoe-vegas]

The file I tried is the "JGL165P1" model here. I think I got it to work with some modifications to icgem/parse.jl:

`
# == Parse Mandatory Header Fields =====================================================

mandatory_fields = (
    :product_type,
    :modelname,
    # :earth_gravity_constant,
    :radius,
    :max_degree,
    :errors,
)

has_mandatory_fields = map(f -> haskey(keywords, f), mandatory_fields)

if !prod(has_mandatory_fields)
    missing_fields = mandatory_fields[findall(!, has_mandatory_fields)]
    error("[Invalid ICGEM file] The following mandatory fields are missing: $missing_fields.")
end

if haskey(keywords, :gravity_constant)
    gravity_constant = _parse_icgem_float(Tf, keywords[:gravity_constant])
elseif haskey(keywords, :earth_gravity_constant)
    gravity_constant = _parse_icgem_float(Tf, keywords[:earth_gravity_constant])
else
    error("[Invalid ICGEM file] earth_gravity_constant or gravity_constant missing")
end

product_type = Symbol(keywords[:product_type])
model_name   = keywords[:modelname]
max_degree   = parse(Int, keywords[:max_degree])
errors       = Symbol(keywords[:errors])

radius           = _parse_icgem_float(Tf, keywords[:radius])

`

Using the GravityModels.gravitational_acceleration function, I get results very similar to a paper I found using the same model:

Is there anything I'm missing? Seems like this package works great for non Earth celestial bodies!