Of course anybody who has ever used GPS for DME knows about the “slant range”, we have all learned that in the PPL course … But as Achim said, the difference is completely negligible. For all special cases where the GPS distance to the DME is wrong: Check the approach plate.
Of course slant range is usually only important in the cruise, but rarely on an approach. However some IAPs start at 6000’ where using GPS as DME could get you in trouble.
A GPS waypoint that would give threshold = 0 would be the “RWY” waypoint in the GPS approach sequence.
Ted.P wrote:
A GPS waypoint that would give threshold = 0 would be the “RWY” waypoint in the GPS approach sequence.
Yes, but that may not be where the DME is, ergo there is a reason why GPS cannot substitute DME for all published approaches.
I think the point Peter was making was how to create a GPS equivalent to a DME that is adjusted to read zero at threshold.
Anyone who starts “creating a GPS equivalent to a DME” is asking for trouble. There is a reason why approaches have to be loaded in full from a database and not set up manually.
Anyone who starts “creating a GPS equivalent to a DME” is asking for trouble. There is a reason why approaches have to be loaded in full from a database and not set up manually.
Huh? GPS approaches don’t use DME.
The problem arises only with other (ILS, LOC, VOR, NDB) appraoches and these obviously don’t get “loaded”.
boscomantico wrote:
The problem arises only with other (ILS, LOC, VOR, NDB) appraoches and these obviously don’t get “loaded”.
Most of these approaches are in the Garmin database and can be loaded on the GNS, GTN, and G1000 series navigators.
Well obviously, but the loading is not essential. It’s just an extra, but not the primary thing.
There is a tendency in modern avionics to move everything towards GPS. It allows such a nice fully integrated user experience. Because of that older technology gets “mapped” onto the GPS world or there are overlays for old things. My Avidyne R9 displays the message “GPS overlays are not authorized” when I use the GPS overlay to fly to a localizer.
Obviously you can set up all units manually and not use the data from the database when you load a non-GPS approach. The database just allows the automation to do it for you and the creator of the integrated flight deck wants to make the experience seamless. Given good data is present (Jeppesen has the ball) it really makes you think you are flying something GPS… But you cannot know about the quality of the data before flying it once. So better be vigilant …
In the US the FAA seem to embrace GPS fully and uses the new capabilities as fully as possible. Others are hanging behind. Probably due to political reasons including the not-invented-here-syndrome.
it really makes you think you are flying something GPS… But you cannot know about the quality of the data before flying it once. So better be vigilant …
Sure but how many times have you found incorrect data (That would affect safety) in a GPS overlay type approach? I never have.
