GPS Sensitivity?

Source: Wally Moran

Subscriber question:

"Sometimes when making a GPS approach, as I approach the final fix, the GPS seems to get more sensitive. Is GPS like a VOR in that it is more sensitive near the station?"
- Peter T.

Wally:

"Sometimes it can look like that but it is not like VOR in that manner, GPS deviation is linear. For example one dot deviation is the same distance off course at one mile from the waypoint as it is at 100 miles.

When making a GPS approach, at 2 nautical miles from the final fix the GPS unit switches from terminal mode to approach mode. When this occurs, the CDI full scale sensitivity changes smoothly from 1 nautical mile to .3 nautical mile.

As this sensitivity change occurs approaching the final fix, if the CDI is not centered, the corresponding increase in CDI displacement may give the impression that the aircraft is actually moving away from the course when actually it may be on a perfectly good intercept heading.

Once this transition is complete the full scale displacement remains at .3 nautical miles all the way to the missed approach point. This is required to give us the precision necessary to be able to make safe GPS approaches.

The GPS also changes full scale displacement values as it switches from enroute to terminal mode. So if you happen to have a CDI deviation during this change, you might also see a change in displacement. Of course, pilots who keep the CDI needle centered all the time never get to experience this problem."