We now have a viable docking code, and for those who have looked a bit into the manual, there's some of the relative position now displayed on SPEC 33. You can approach and dock ISS, and then undock and leave (there's no real simulation of ISS done when outside of the bubble around the Shuttle yet - it's just loaded into the same orbit when the scenario is selected). I've also started to make the radar ranging via the Ku-antenna usable.
The rest of SPEC 33 is... interesting. Basically it deals a lot with the state vector of the orbiter. It's basically continuously computed via the signal of the IMUs, but this is an error-prone procedure, so it needs to be updated regularly - which is what uplink information, star trackers and COAS are for.
I've started to (optionally) divorce navigation information of the Shuttle from reality to model this better. In this concept, the instruments will show what GNC thinks is the position of the vehicle and the AP tracking and pointing routines will be based on this (wrong) state vector, whereas JSBSIm computes the real state vector.
So it might be that the instruments show you zero meters to ISS whereas in reality you're three meters distant. This can be improved by improving the propagated IMU state vector via the measured and filtered data, e.g. from the ranging radar, which will have a much lower error.
The ultimate and cool goal would be to e.g. implement the COAS attitude fixing procedure correctly - you point the Shuttle at a known star in the sky, center the star in a HUD aiming pattern, select the ID of the star in the avionics and then press attitude reference button when it's really centered. You do this for two stars, and the inertial attitude of the orbiter is fixed. I think if I get this to work, it'd be an incredibly cool feature.