SGP4 is a small C++ library which is bundled in python-sgp4 currently under review [1].
The source code is available from
https://celestrak.org/publications/AIAA/2006-6753/ and
in the FAQ it says:
"""
Are there any Licenses required to use the SGP4 code?
There is no license associated with the code and you may use it for any purpose—personal
or commercial—as you wish. We ask only that you include citations in your documentation
and source code to show the source of the code and provide links to the main page, to
facilitate communications regarding any questions on the theory or source code.
"""
Is there a license that fits? It seems some sort of Public Domain with attribution...
Mattia
[1]
https://bugzilla.redhat.com/show_bug.cgi?id=2156932