🚀 go-pugleaf

In <1992Nov16.194042.10071@erg.sri.com> ple@erg.sri.com (Paul Evans) writes:

>My understanding is that what is usually referred to as
>the state vector is really two vectors, a position vector and a
>velocity vector.  
This is more properly thought of as a single, six-dimensional vector.
Sometimes, it may be a list of orbital elements or even a quaternion
in some applications.

>Are the spacecraft onboard computers continually updating the state
>vector?  I could see where this might be a possibility for the shuttle
>in earth orbit, but what about the Apollo missions?  I can't imagine
>continually solving that kind of n-body problem with the limited
>onboard computing resources available at that time.

The STS computers are indeed continually updating the state vector. This
just consists of reading the data from the INS accelerometers and tweaking
the values. It's a fairly straightforward process to get an orbit out
of a state vector.

On Apollo, you could safely assume the spacecraft had zero mass compared
to the Earth-Moon system. You were thus reduced to a two-body problem 
with a zero-mass object migrating between them. Actually, since the motions
of Earth and Moon could be computed ahead of time and stored in a table,
it was even simpler than that.