What are the bias and spatial accelerations?

I'm reading this document to first understand the rotational dynamics of a spacecraft, and then to understand the Kalman filter that comes with it.





Problem is I don't know what bias and spatial accelerations are. I've googled these terms, but the hits I've recieved were not very good.|||A little more context would help. Bias could be attitude measuring instrument bias. Spatial could be someone's term for distinguishing translation from rotation, perhaps? Or disturbances peculiar to space such as solar wind, etc. Sorry I can't help more.