15.gps-navigation-rnav. GPS and Area Navigation (RNAV)
Area Navigation (RNAV) is a method of navigation that permits aircraft operation on any desired flight path within the coverage of ground- or space-based navigation aids, or within the limits of the capability of self-contained systems, or a combination of these. RNAV frees the pilot from the constraint of flying directly to and from ground-based navigation facilities (VORs, NDBs), allowing direct routing between any two waypoints defined by latitude/longitude coordinates.
The Global Positioning System (GPS) is a satellite-based radio navigation system operated by the U.S. Department of Defense and is the most common RNAV source in general aviation. GPS provides three-dimensional position, velocity, and precise time information to suitably equipped users anywhere on or near the surface of the Earth.
GPS System Architecture
GPS consists of three segments:
- Space segment — A constellation of at least 24 satellites (typically 30+) in six orbital planes at approximately 10,900 NM altitude. Each satellite circles the Earth twice per day.
- Control segment — A master control station, monitor stations, and ground antennas that track the satellites, update orbital data (ephemeris), and synchronize atomic clocks.
- User segment — The GPS receiver in the aircraft, which decodes satellite signals to compute position.
How GPS Determines Position
A GPS receiver measures the time required for a signal to travel from each satellite to the receiver. Multiplying that time by the speed of light yields a pseudorange (distance) to that satellite. With pseudoranges from at least four satellites, the receiver can solve for latitude, longitude, altitude, and time. Three satellites would suffice geometrically, but a fourth is required because the receiver clock is not as accurate as the satellites' atomic clocks; the fourth equation resolves clock bias.
Position accuracy depends on satellite geometry, expressed as Dilution of Precision (DOP). Satellites widely spread across the sky give better geometry (lower DOP) than satellites bunched together.
Integrity Monitoring
GPS signals can degrade or fail. To ensure the receiver is providing usable data, panel-mount IFR-approved units use RAIM (Receiver Autonomous Integrity Monitoring). RAIM compares the redundant pseudorange measurements from multiple satellites to detect a faulty signal:
- 5 satellites are required to detect a faulty signal.
- 6 satellites are required to detect AND exclude (FDE) a faulty signal.
For IFR operations, pilots must check RAIM availability for the route and approach (typically through the unit, an FBO, or 1-800-WX-BRIEF) when required.
WAAS (Wide Area Augmentation System) improves GPS accuracy and integrity using a network of ground reference stations and geostationary satellites that broadcast correction data. WAAS receivers achieve accuracy of about 3 meters or better and do not require external RAIM checks for departure or destination because integrity is monitored through WAAS itself. WAAS enables vertically guided approaches such as LPV (Localizer Performance with Vertical guidance) with minimums as low as 200 feet HAT, comparable to an ILS.
Waypoints and Flight Plans
GPS navigation is built around waypoints, which include:
- VORs, NDBs, and intersections from the database
- Airports and runways
- User-defined waypoints (entered by lat/long or radial/distance)
A flight plan is a sequence of waypoints. The receiver provides desired track (DTK), bearing to the next waypoint (BRG), distance, ground speed (GS), and estimated time en route (ETE). Cross-track error (XTE) shows lateral deviation from the programmed course.
IFR Use of GPS
To use GPS for IFR navigation:
- The unit must be approved under TSO-C129/C145/C146 and installed per AC 20-138.
- The aircraft must have current navigation database for IFR operations (28-day cycle).
- The pilot must be familiar with the specific unit (button-pushing proficiency is required).
- A non-WAAS GPS used for an approach requires a current RAIM prediction.
- An alternate airport using a non-WAAS GPS approach as the only approach must be checked carefully; with WAAS, alternate planning rules are simplified per the AIM.
Advantages and Limitations
Advantages:
- Direct point-to-point routing saves time and fuel.
- Three-dimensional position with worldwide coverage.
- Enables LNAV, LNAV/VNAV, and LPV approaches at airports without ILS.
Limitations:
- Susceptible to interference, jamming, and rare signal outages.
- Database currency is essential — outdated data may contain errors.
- Pilot workload during reprogramming in flight (heads-down time).
- Position accuracy depends on satellite geometry and number of satellites tracked.
Example
A pilot flies direct from KAPA to KASE using GPS. The unit displays DTK 252°, distance 92 NM, GS 135 kt, ETE 0:41, XTE 0.2 NM left. By centering the CDI and matching track to DTK, the pilot maintains the great-circle course to the destination — something impractical using only ground-based VORs.
GPS has become the backbone of modern navigation, but pilots must still understand its principles, monitor its performance, and be prepared to revert to conventional navigation if the system fails.