GPS
AOPA has a good course on GPS usage. Garmin has a simulator that you can use to explore the features of their GPS products.
GPS has a 100′ horizontal accuracy. If augmented with WAAS it has 10′ horizontal accuracy. The WAAS capability 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
Having a panel-mounted GPS allows you to fly RITTRs (RNAV IFR terminal transition routes) that are being implemented in some crowded airspaces for VFR traffic.
GPS CDI measures distance off course, unlike VOR mode which measures degrees off course.
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.
If an alternate is required, you must have a non-GPS method of flying at least one approach at the alternate, unless you have WAAS.
Make sure you are in VOR or GPS mode as appropriate.
Make sure you are in normal or OBS mode. Approach must be activated before it can be used.
To be certified for IFR a GPS must be Panel Mounted, have an external CDI, have RAIM.
En route full scale CDI deviation is 4nm. Approach mode is .3 nm
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.