JD-EE,
That might need a little clarification.
While gps is primarily a system to calculate position, it can also be used to provide fairly accurate heading and ground speed data, in conjunction with appropriate software.
For example, if gps position is sampled at a constant, known rate, it only needs a bit of trig to calculate heading and for ground speed, a bit of arithmetic using sample rate (time) and change in position + scale accordingly. Not exactly rocket science.
Of course, ground speed != air speed, but in a typical flight, with small changes in wind force and direction, there would be a measurable relationship between the two, which could be used as an added data input to the various system filters. It may be "poor quality data", but still usable within it's limitations...
Regards,
Chris