Hi Sean,
a really nice idea. I did in fact try to develop something similar. Well, it was not about CELNAV but to have a game that is running on a curved surface with the same size of the earth.
Here are my findings:
1. All game engines and theire corresponding physics engines I could find use linear physics. For a game like that to work, one would need a physics engine that is using spherical terrain physics.
2. I tried to convert those physics engines to use spherical physics. The problem with that were the numbers involved: You are dealing with objecs with a huge difference in mass: Your vessel may have as little as 1000kg mass, while earth has 5,972 × 10^24 kg of mass. I was not able to fit those differences into a game physics engine, because it's hard and very inperformant to do computations on really large numbers like that. And I was simply not smart enough to come up with an abstraction model that would be fast enough to calculate for a real time game and still be accurate enough to be used for such a sophisticated use case.
3. All game engines use linear coordinate systems and assume a more or less planar gaming "surface". It was very hard to convert those to work on a huge spherical surface.
I looked at some games that introduced spherical physic/game mechanics like "Space Engineers" or "Kerbal Space Program". What those have in common is, that planets in those games are only a fraction of the size our earth has. I assume they did this because of the limitations and problems I mentioned.
To sum it up: I guess it needs someone way more capable then me, to pull this off.
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