Global Positioning System
This pages discussed the generic GPS support in FlightGear, not any specific real-world device or operating procedure. Nevertheless, many similarities should exist. It is assumed the reader is familiar with radio navigation terminology and basic instrument flying procedures.
Basic Information
The GPS computes position, direction and velocity information independently of other instruments in FlightGear - for example the computed altitude is not related to any altimeter installed on the aircraft. GPS computed values are subject to different errors and precision as compared with other instruments.
Useful information above includes true groundspeed (not airpseed) and ground track (different from heading), i.e the path the aircraft is taking over the ground, and the direction of movement.
Active Waypoint
The GPS tracks an active waypoint, which can be an airport, VOR, NDB, fix or user-defined in various ways. Information about the active waypoint is provided - bearing, range, time to reach the waypoint, and so on.
Modes
OBS
OBS mode makes the GPS behave like a nav-radio tuned to the active waypoint (which doesn't have to be a VOR). You select a desired radial, and the GPS will generate to/from-flags and a course deviation output, exactly like a VOR receiver. Since there's no radio reception involved, this works for NDBs or fixes too, and there is no issue with stations being out of range (or out of service).
The OBS radial can be adjusted via a dial at the bottom of the default GPS dialog - note the dial is only shown when in OBS mode, and hidden in DTO and LEG modes, to avoid confusion. Due to PLIB limitations, also note that the 0-degree radial is at the bottom, and the 180-degree radial is at the top - i.e the dial is upside down.
LEG
LEG mode is mostly used with an active route (defined in the route manager). In LEG mode, the active waypoint is the next waypoint on the current route, and the GPS calculates course deviation to fly the line between the previous waypoint and the active waypoint.
Upon reaching the active waypoint, the GPS automatically switches ('sequences') to the next waypoint, if one exists.
DTO
This mode is used to fly directly to a waypoint. When DTO mode is entered, the course deviation will be centred, and a bearing calculated to fly directly to the requested, active waypoint.
Upon reaching the waypoint, the GPS will continue flying the inbound bearing, *unless* the active waypoint is part of an active route. In this case, the GPS will automatically switch to LEG mode, and resume waypoint sequencing.
Finding Waypoints
Waypoints can be selected in the following ways:
- searching by ident or name. Enter the full, or first few letters of a waypoint name, and search. The closest matching waypoint will be displayed, with successive results available, sorted by distance to the current GPS position.
- selecting the nearest waypoint (of a particular type). The closest ten waypoints can be loaded, and searched through
- loading waypoints from the active route (flight-plan)
The GPS code automatically selects a reference navid, if one can be found nearby, and provides its location, range, bearing and frequency. This feature allows the pilot to cross-check the GPS indicated position against the reference navid.
Common Procedures
Executing a ATC direct clearance
Commonly, ATC will clear an inbound or outbound aircraft to a waypoint in their area. Particularly for departures, various waypoints on a procedure (SID) may be skipped. In this case, simply load the active WPT, search forwards to the cleared waypoint, and press 'DTO'. The GPS will fly directly to the waypoint, and skip over any waypoints as necessary before resuming LEG mode.
Missed Approaches
The current GPS code lacks approach support, but it's still valuable to set the missed approach point as the active waypoint. Upon going missed, execute a direct-to, and you'll have one less thing to worry about when going missed.