Error accumulation rates of Space Shuttle IMUs

[Thorsten]

I'm currently modeling state vector management for the Flightgear Space Shuttle project. Basically the aim is to give the model the same navigational needs as the real thing, i.e. to divorce the true state vector managed by the flight dynamics from the state vector as known by GNC, such that it needs to be periodically updated by upling/GPS or augmented by star tracker or COAS data or the instruments will show wrong.

As far as I understand, the main source of the drift from reality seems to be that the propagation model is not perfect but just a model and that the IMU acceleration sensing isn't perfect, so over time input errors and model uncertainties sum to position and velocity errors.

Now I was wondering whether anyone might know how quickly the IMUs would accumulate these errors - say if I don't update my state vector for six hours and just propagate it, what's the mean uncertainty in vehicle position and velocity?

Related, does anyone know what the typcial accuracy of range and rdot are when using the Ku-antenna as a ranging radar?

Unfortunately I haven't been able to track down much so far.

 

[Urwumpe]

I only have the specification limits for the old IMUs, not the self-calibrating HAINS IMUs.

 

[Thorsten]

Could you give me an idea about the order of magnitude of those then? I suppose one can always guesstimate from there.

 

[Col_Klonk]

The problem is that our digital representation of Real values has limited accuracy, just as a more suitable analogue computer would have.

Both digital and analogue calculations have noise which has to be eliminated to improve accuracy. There are various methods to do this.. but there are other added complications such as thermal drift and time drift, not to mention a multitude of other interferences that we've yet to discover.

As small as these might be.. they add up over periods of time.

An interesting topic !
:thumbup:

Where the hell am I ??