GPS has a 100′ horizontal accuracy. If augmented with WAAS it has 10′ horizontal accuracy. The WAAS capability also allows approaches with vertical guidance—LPV and LNAV/VNAV.
- LPV—Localizer performance with vertical guidance. ILS-like guidance. Minimums as low as with ILS 200ft and 1/2 mile vis
- LNAV/VNAV—Lateral Navigation/Vertical Navigation. Designed for use by WAAS equiped aircraft. Minimums are higher then LPV.
- LNAV—Normal minimums
Terms to know:
- DTK—Desired track, desired magnetic course between waypoints
- TRK—Track, current magnetic course relative to the ground
- BRG—Bearing, compass position from the present position of the aircraft to a waypoint
- XTK—Cross track, distance the aircraft is off the DTK, to left or right
- DIS—Distance, distance from the aircraft’s present position to the next waypoint
GPS CDI measures distance off course, unlike VOR mode which measures degrees off course.
AIM 1−1−17. Global Positioning System (GPS)
5. GPS Instrument Approach Procedures (3) When an approach has been loaded in the navigation system, GPS receivers will give an “arm” annunciation 30 NM straight line distance from the airport/heliport reference point. Pilots should arm the approach mode at this time if not already armed (some receivers arm automatically). Without arming, the receiver will not change from en route CDI and RAIM sensitivity of ±5 NM either side of centerline to ±1 NM terminal sensitivity. Where the IAWP is inside this 30 mile point, a CDI sensitivity change will occur once the approach mode is armed and the aircraft is inside 30 NM. Where the IAWP is beyond 30 NM from the airport/heliport reference point and the approach is armed, the CDI sensitivity will not change until the aircraft is within 30 miles of the airport/heliport reference point. Feeder route obstacle clearance is predicated on the receiver being in terminal (±1 NM) CDI sensitivity and RAIM within 30 NM of the airport/heliport reference point; therefore, the receiver should always be armed (if required) not later than the 30 NM annunciation.
(5) When within 2 NM of the Final Approach Waypoint (FAWP) with the approach mode armed, the approach mode will switch to active, which results in RAIM and CDI changing to approach sensitivity. Beginning 2 NM prior to the FAWP, the full scale CDI sensitivity will smoothly change from ±1 NM to ±0.3 NM at the FAWP. As sensitivity changes from ±1 NM to ±0.3 NM approaching the FAWP, with the CDI not centered, the corresponding increase in CDI displacement may give the impression that the aircraft is moving further away from the intended course even though it is on an acceptable intercept heading. Referencing the digital track displacement information (cross track error), if it is available in the approach mode, may help the pilot remain position oriented in this situation. Being established on the final approach course prior to the beginning of the sensitivity change at 2 NM will help prevent problems in interpreting the CDI display during ramp down. Therefore, request- ing or accepting vectors which will cause the aircraft to intercept the final approach course within 2 NM of the FAWP is not recommended.
Within 30 nm of the destination airport, but not yet within 2.0 nm of the final approach fix (FAF), the GPS receiver is operating in Terminal Mode with the CDI depicting a total course width of 2.0 nm. GPS switches to approach mode 2nm before the FAF. Full CDI deflection becomes .3 nm. In most cases GPS sensitivity in approach mode stays the same regardless of distance to the waypoint. However, when a WAAS-certified receiver is used to fly an LPV approach, once past the FAF the CDI needles will behave like localizer/glideslope needles, becoming more sensitive as the aircraft proceeds down the final approach course.
The OBS display when using GPS navigation differs from the OBS display when navigating using a VOR or localizer. In VLOC mode, the CDI needle sensitivity increases as an aircraft flies closer to the station because the CDI displays the angular deviation from the course. If the aircraft remains the same distance from the course, CDI needle deflection will increase as the aircraft gets closer to the VOR. In GPS mode the CDI needle deflection displays the distance from the course. If the aircraft remains the same distance from the course, CDI needle deflection will remain the same as the aircraft gets closer to the waypoint.
WAAS-enabled GPS units will begin to display angular deviation when the aircraft approaches the FAF of an LPV approach.
To summarize: En route full scale CDI deviation is 5nm. Terminal is 1 nm. Approach mode is .3 nm
9) Pilots should pay particular attention to the exact operation of their GPS receivers for performing holding patterns and in the case of overlay approaches, operations such as procedure turns. These procedures may require manual intervention by the pilot to stop the sequencing of waypoints by the receiver and to resume automatic GPS navigation sequencing once the maneuver is complete.
On the Garmin 430 for example, press the OBS key before entering the holding pattern, before executing a course reversal, before proceeding to the first waypoint in the missed approach procedure.