5.gps-and-rnav-ifr. GPS and RNAV for IFR Operations
Area Navigation (RNAV) is a method of navigation that allows an aircraft to fly any desired flight path within the coverage of station-referenced navigation aids, within the limits of self-contained system capabilities, or a combination of both. The Global Positioning System (GPS) is the most common RNAV source for IFR-equipped general aviation aircraft and is approved for en route, terminal, and approach operations when the equipment is installed and operated in accordance with the appropriate Aircraft Flight Manual Supplement (AFMS) and Advisory Circular AC 90-100A (U.S. Terminal and En Route RNAV Operations) or AC 90-105A (RNP Operations).
GPS System Overview
GPS is a U.S.-operated satellite-based navigation system consisting of three segments:
- Space segment — a constellation of at least 24 satellites in six orbital planes at roughly 10,900 NM altitude, completing an orbit every 12 hours.
- Control segment — a master control station and worldwide monitor stations that track satellites and uplink corrections.
- User segment — the GPS receiver in the aircraft.
The receiver determines position by measuring the time it takes coded signals to travel from each satellite. Receiving signals from at least four satellites allows the receiver to solve for latitude, longitude, altitude, and time. Three satellites would yield a 2D fix only.
Integrity Monitoring: RAIM, FDE, and SBAS
For IFR use, a GPS receiver must continuously verify the integrity of its position solution.
- RAIM (Receiver Autonomous Integrity Monitoring) requires a minimum of 5 satellites in view, or 4 satellites plus a barometric altimeter input (baro-aiding), to detect a satellite fault. Fault Detection and Exclusion (FDE) requires 6 satellites to identify and remove the bad satellite.
- SBAS (Satellite Based Augmentation System) — in the U.S. this is the Wide Area Augmentation System (WAAS). WAAS provides correction signals from geostationary satellites that improve accuracy to roughly 3 meters horizontal/vertical and supplies its own integrity monitoring, eliminating the need to perform a RAIM prediction prior to flight.
Non-WAAS (TSO-C129/C129a) receivers require a RAIM availability check before any IFR flight where GPS is the primary means of navigation, typically using the manufacturer's prediction tool, the FAA's Service Availability Prediction Tool (SAPT), or contacting Flight Service.
RNAV (GPS) Approach Minima Lines
A single RNAV (GPS) approach plate may publish up to four lines of minima:
- LPV (Localizer Performance with Vertical guidance) — WAAS only; provides ILS-like vertical guidance, with DA as low as 200 ft HAT.
- LNAV/VNAV — Lateral and Vertical NAV; flown with WAAS or with an approved baro-VNAV system.
- LP (Localizer Performance) — WAAS lateral-only with tighter angular sensitivity than LNAV; no vertical guidance, MDA only.
- LNAV — Lateral navigation only, non-precision, available to any IFR-approved GPS.
CDI Sensitivity
GPS CDI scaling automatically transitions with phase of flight:
- En route: ±2.0 NM full-scale deflection
- Terminal (within 30 NM of destination): ±1.0 NM
- Approach (after the FAF or within 2 NM of FAF): ramps to ±0.3 NM, with finer scaling on LPV/LP
IFR Procedures and Required Checks
Before filing or flying GPS as the sole navigation source, the pilot must:
- Verify the navigation database is current (28-day cycle). An expired database may not be used for IFR approaches.
- For non-WAAS units, perform a RAIM prediction for the destination ETA ±15 minutes (and the alternate, if required).
- Brief the approach, including the sensor (LPV/LNAV/VNAV/LNAV), the FAF, missed approach point, and missed approach holding.
- Verify CDI sensitivity changes and the proper approach mode arms (typically within 30 NM) and activates at the FAF.
Filing GPS as an Alternate
Under current AIM guidance, WAAS-equipped aircraft may flight-plan a GPS or RNAV (GPS) approach at either the destination or the alternate (but not both based on GPS alone, unless the aircraft has a fault-tolerant configuration), and may use LNAV or circling minima for alternate planning regardless of LPV availability. Non-WAAS aircraft filing a GPS approach must have an alternate served by a non-GPS approach the aircraft is equipped to fly, and that approach must be usable with the forecast weather.
Limitations
- GPS signals are line-of-sight and susceptible to interference, jamming, and ionospheric effects.
- NOTAMs must be checked for GPS outages, testing (especially military), and unmonitored WAAS LPV/LNAV/VNAV service.
- Substitution of GPS for DME, ADF, or VOR is generally permitted, but the underlying ground-based NAVAID required for the procedure must still be operational unless the procedure is titled "or GPS" or is an RNAV procedure.
Properly used, IFR GPS dramatically expands the routes and approaches available to a single-pilot IFR aircraft, but it demands rigorous database management, integrity awareness, and mode awareness from the pilot.