Originally Posted by
APMR
GPS NPAs had so much promise so it is a great shame they have turned out this way. But, all is not lost - the deficiencies can be rectified.
I am a professional pilot (not GPS NPA endorsed) and also a software developer. Back in 1992, I had to write a GPS based navigation system for a survey company. The survey "runs" consisted of up to a dozen waypoints that were roughly aligned.
Before the software even flew, it was obvious to all of us that the crew, once established on the run, were not interested in the intermediate waypoints - they only wanted to know the distance to the final waypoint.
So, I altered the software to the effect that the intermediate waypoints were effectively hidden. And, if the track from one segment to another changed too much (requiring a significant turn), extra waypoints could be inserted on either side of the transition so as to "smooth out" the change from segment to segment.
The crew had only an across-track display and a distance to run (to the final waypoint of the run). If we had wanted to, we could have easily added turn guidance info for those moments when transitioning from one segment to another.
All very easy. The end result, for the pilot, was one apparent segment that would occasionally change track slightly. We could have flown perfect figure 8's if we had wanted - all as one apparent segment.
So, don't let anyone tell you that the GPS NPAs we have today must work the way they do.
If I had designed them, the pilot would see only 2 waypoints - the IAF and the MAP. Upon activating the approach, the GPS would guide him to the IAF. After passing the IAF, the guidance would then be to the MAP, via several (invisible) intermediate points, with the displayed distance being the track miles to the MAP. The FAF would just be a published distance from the MAP.
To avoid stepped descents and limiting altitudes, vertical guidance (via a glideslope like indication) would be provided and the descent path would be a constant angle. Tricky terrain around the airport would be accommodated via curving approaches. For altimeter check purposes, the altitude the aircraft should be at would be continuously updated and displayed.
Flying one would feel like flying an ILS that changes heading occasionally.
interesting comments.
however i'll tell you that RNAV (GNSS) [aka GPS NPAs] do have to be designed the way they are currently.
whatever programming you have done was not done for any of the current certified RNAV receivers [GPS or otherwise].
as i've stated in other posts today, the US military gave a spec to which manufacturers created their receivers. the procedures are then just following the MINIMUM spec available as TSO129C. ICAO spelt out how that is done and your regulator will insist how that is implemented.
if there had been input in the late '70s when the spec was created, maybe things would have been different. hindsight is wonderful!