GPS & Modern Navigation

GPS basics

• Receives signals from a constellation of satellites. Triangulates 3D position and time. Modern receivers track GPS + GLONASS + Galileo for accuracy and redundancy.
• Position accuracy in normal operations: typically ~3-10 meters horizontal, slightly worse vertical.
• Updates several times per second. Smoother than VOR/DME but not instantaneous.

WAAS — Wide Area Augmentation System

• Ground stations correct GPS errors and broadcast corrections via geostationary satellite.
• Improves accuracy to ~1-2 meters horizontal, much better vertical.
• Required for LPV approaches (Localizer Performance with Vertical guidance) — these provide ILS-like precision approach minimums down to 200 ft DA.

RAIM — Receiver Autonomous Integrity Monitoring

• The receiver continuously checks GPS signal integrity by comparing redundant satellite measurements.
• Required for non-WAAS GPS approaches. WAAS-equipped receivers don't need RAIM because WAAS provides equivalent integrity.
• Check RAIM availability before flight via fuel-route or destination-approach RAIM prediction (free FAA tools, often integrated into flight planning apps).
• If RAIM is unavailable for your approach time, plan an alternate.

GPS approach modes

Sensitivity tightens as you approach the airport, providing finer course guidance:

• En route — ±5 NM CDI full-scale deflection. Coarse guidance for cross-country.
• Terminal — ±1 NM. Within ~30 NM of the destination airport.
• Approach — ±0.3 NM (LNAV) or finer (LPV/LP). Activates when you sequence the approach.
• LPV / LNAV+V / LP — vertical guidance modes, with sensitivity scaled to runway-based path angles.

Always have a backup nav source — check the GPS warning flags during the scan, especially before relying on it for the approach. A failed GPS during an approach without an alternate is one of the most uncomfortable IFR positions.

Transponder — Mode A, C, S

• Mode A: Reports the squawk code (4-digit identifier).
• Mode C: Reports squawk + pressure altitude (encoded altitude).
• Mode S: Reports squawk + altitude + a unique aircraft identifier; supports ADS-B Out and TCAS.

Most modern IFR aircraft are Mode S equipped. Transponders are tested every 24 calendar months per 14 CFR § 91.413.

ADS-B Out — required where

ADS-B Out is required in the contiguous US per 14 CFR § 91.225 in:

• Class A, B, and C airspace
• Class E airspace at or above 10,000 ft MSL (excluding airspace at and below 2,500 ft AGL)
• Within 30 NM of a Class B primary airport (the "Mode C veil")
• Above the ceiling and within the lateral boundaries of Class B/C airspace up to 10,000 ft
• Class E over the Gulf of Mexico at or above 3,000 ft MSL within 12 NM of the US coast

ADS-B Out broadcasts position, velocity, and identity to other aircraft and ground stations. Replaces secondary radar in the airspace where it's required.

ADS-B In — receive traffic and weather

Optional but cheap. ADS-B In receivers display:

• TIS-B — Traffic Information Service Broadcast. Other aircraft positions on a moving map.
• FIS-B — Flight Information Service Broadcast. METAR, TAF, NEXRAD radar mosaic, NOTAMs, AIRMETs/SIGMETs.

Caveat: FIS-B weather data is delayed by 5-15 minutes. Don't use it for tactical thunderstorm avoidance — only strategic. Same with traffic — TIS-B doesn't catch all aircraft (only those participating in the system).

Marker Beacons

Largely legacy. Most modern ILS approaches use DME or GPS in lieu of markers. Where they still exist:

• Outer Marker (OM): 4-7 NM from threshold. Low-pitched continuous dashes (2/sec). Purple/blue light.
• Middle Marker (MM): ~3,500 ft from threshold. Alternating dots and dashes (95/min). Amber light.
• Inner Marker (IM): Used on Cat II/III ILS only. High-pitched dots (6/sec). White light.

Marker receiver "TEST" position usually only tests the bulb, not the receiver — verify reception in flight by listening for the audio.