GPS operation & limits

GPS made cross-country navigation almost effortless — and "almost" is doing a lot of work in that sentence. Knowing how it finds you, and the handful of ways it can quietly stop being right, is the difference between a tool and a trap.

How GPS knows where you are

• A constellation of at least 24 satellites orbits roughly 12,000 NM up, each broadcasting its own position and a very precise time.
• Your receiver measures how long each signal took to arrive, turns that into a distance, and trilaterates: three satellites fix you in two dimensions, and a fourth resolves altitude and corrects the receiver's clock. More satellites in view means a better fix.
• The receiver only listens — it transmits nothing — which is why every airplane in the sky can use the same satellites at once. And because the fix depends on satellite geometry, a receiver seeing satellites spread across the sky gets a sharper fix than one seeing them bunched in a single quadrant.

Civilian GPS is accurate to a few meters today because Selective Availability — the deliberate accuracy degradation the military once applied — was switched off in 2000.

RAIM and WAAS: knowing the fix is trustworthy

A position that's wrong but looks confident is worse than no position at all, so the system checks itself:

• RAIM (Receiver Autonomous Integrity Monitoring) is the receiver auditing its own work. It needs 5 satellites to detect a faulty signal and 6 to detect and exclude the bad one and keep navigating. If RAIM can't guarantee integrity, it warns you — and not every portable unit even has true RAIM.
• WAAS (Wide Area Augmentation System) adds ground stations that broadcast correction data, sharpening accuracy and integrity enough to fly LPV approaches down to near-ILS minimums. For VFR, WAAS mostly buys you a more trustworthy position.

Not all GPS is equal

The unit on your panel and the app on your tablet are not the same thing in the eyes of the regulations:

• A panel-mounted, TSO-certified GPS with a current database can be approved for IFR navigation and approaches.
• A handheld receiver or a tablet app is a VFR situational-awareness aid only.

For the checkride, the distinction that matters is simple: a VFR GPS is a wonderful supplement, but it is not a legal substitute for the navigation equipment the regulations require, and you cannot use it to fly an instrument procedure.

The limitations that bite

• Database currency — the navigation database runs on a 28-day cycle. An expired database isn't just a stale waypoint; it can mislead you about an airport, a frequency, or a procedure that changed.
• GPS altitude is geometrically weak — far less accurate than its horizontal position. Never use it as your primary altimeter.
• Interference and jamming are real — the military runs GPS testing NOTAMs regularly — and the signal is line-of-sight from space. Usually fine; not magic.

In day-to-day VFR flying the GPS earns its keep with features worth knowing: "direct-to" for an instant course to any airport or waypoint, a "nearest" page that lists the closest airports (invaluable in an emergency), and a moving map that makes situational awareness almost effortless. The catch is that those features are so good they tempt you to stop looking outside and stop cross-checking — which is precisely the failure mode R2 is about.

When integrity is lost the receiver should tell you. Recognizing that annunciation and reverting to another source is the subject of R1.