Making a separate reply, but if you are interested in mapping approach paths, Mode C and Mode S (non-ADS-B) data on the 1090 band may still have interest for you. It will be difficult to correlate, however Mode-S contains the ICAO address and altitude info (even at its most basic without GPS), which you could at least map to Flight IDs and the filed flight plan, giving you a basic approach path in the X and Y direction, which you can add semi accurate (100 or 25 foot resolution) information in the Z direction. Even harder to correlate, Mode C data could be used for non-correlatable, but still trackable info to watch a plane land (gather the data, if a Mode-C target went from in-air (>N feet above airport MSL corrected for pressure altitude) to on-ground (<N feet above airport MSL correct for pressure altitude), you could still get approach information for almost any aircraft, not just ADS-B aircraft. Note: Mode C data has 100 foot resolution, and is pretty noisy.
That is ultimately the goal. Having a ubiquitous metric for evaluating pilot performance when subject to different airspace/approaches/airport environments. Right now, I would be happy with reliable interpolation of spacial data. The broadcast packets are temporally inconsistent and the speed/altitude/heading are spread over more than one packet. So a simple interpolation only goes so far. I expect to have to write code to intelligently interpolate based on what aircraft can actually do. So no 3g turns in a transport airplane in the traffic pattern, etc. Once I have a functional model with the "tidy" ADS-B data, then the challenge will be making the process robust enough for mode S/C. It's a plan subject to change...