Global Positioning System technology has become integral to modern aviation, providing precise navigation and timing data that enhance flight safety and efficiency. Within the aviation sector, several distinct types of GPS systems are employed, each suited to specific operational requirements and aircraft categories. The types of GPS in aviation are outlined below.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
1. Localizer Performance with Vertical Guidance (LPV)
Localizer Performance with Vertical Guidance (LPV) is the highest-precision GPS approach that needs only a Wide Area Augmentation System (WAAS) receiver and no specialized crew training. It is classed as an Approach with Vertical Guidance (APV) to distinguish it from both precision (PA) and non-precision (NPA) procedures. LPV delivers lateral course and vertical glidepath deviation information comparable to a Category I Instrument Landing System, but derives its signals from SBAS satellites instead of ground antennas. A continuous descent to a Decision Altitude as low as 200 feet (61 meters) above touchdown with visibility down to one-half mile (805 meters) is therefore possible at airports where installing an ILS is impractical or uneconomical. Angular sensitivity increases as the aircraft nears the runway, providing stable guidance that supports business-aviation, air-ambulance and regional-scheduled operations in almost all weather.
Jim GoodrichPilot, Airplane Broker and Founder of Tsunami Air
2. Vertical Navigation (VNAV)
VNAV is vertical navigation and it is the vertical part of the navigation flight profile. From Takeoff through Climb, Cruise, Descent, Approach and Missed Approach phases, VNAV provides flight-control steering and thrust guidance along the computed vertical path. The system begins at the runway waypoint-constraint altitude and then follows a fixed vertical angle upstream. It selects a profile angle that will place the top-of-descent (TOD) point a very small distance in front of the aircraft, so the pilot has time to configure for descent. To maintain that fixed angle, the aircraft must descend at a higher vertical speed if groundspeed increases. Most systems default to a three-degree profile, yet allow the pilot to select angles between zero and six degrees. Path segments are formed by altitude-constrained waypoints whose slope and vertical speed are displayed on the FMS VNAV page.
Guidance commands for the autopilot, flight director and autothrottle are computed so the airplane keeps the desired height for every position. Vertical guidance comes from an internally generated WAAS-enhanced glideslope or, in non-satellite systems, from Barometric Vertical Navigation. Baro VNAV uses barometric altitude information from the aircraft's pitot-static system and air-data computer, and satisfies the Receiver Autonomous Integrity Monitoring requirement in lieu of a fifth satellite and consequently, GNSS waypoints are not always required. Because Baro VNAV is affected by temperature and altimeter errors, the current altimeter setting must be entered into the receiver. Less-integrated installations advise the crew of the vertical path but have no autothrottle capability, whereas the FM/ATM suite couples with VNAV to protect against impending overspeed conditions by sacrificing path if necessary.
When the approach features no vertical guidance, Garmin G1000-based systems will allow VNAV to the fix preceding the Final Approach Fix (FAF) but not beyond. If the approach has vertical guidance (ILS, LPV, LNAV with advisory vertical guidance) VNAV is permitted up to the fix before the FAF. The approach plate depicts the Vertical Descent Angle and FAF-crossing altitude as part of VNAV information, providing glide-path information during the instrument approach.
3. Lateral Navigation (LNAV)
LNAV simply stands for Lateral Navigation. LNAV approaches are a type of GPS approach that provides only lateral guidance. Lateral guidance is azimuth navigation; lateral guidance is without vertical navigation. LNAV approaches are non-precision approaches that provide only horizontal guidance. Every RNAV(GPS) approach will have an LNAV line at a minimum because that is the basic capability using GPS to navigate a course to the runway. LNAV is the original GPS approach standard, and it is used with a non-WAAS IFR GPS receiver - WAAS equipment is not mandatory for LNAV, although WAAS equipment can be utilized.
LNAV approaches have a lateral full-scale instrument limit of 0.3 NM, and lateral sensitivity changes from 1 NM to 0.3 NM approaching the final approach waypoint. The same 0.3 NM limit applies throughout the final segment. Because LNAV lacks vertical guidance, LNAV approaches are flown to a Minimum Descent Altitude (MDA), using an MDA like any non-precision approach. Consequently, LNAV approaches have the highest minimums because there is no vertical guidance.
The obstacle clearance surface for LNAV starts as a trapezoidal 2-mile (3.2 km) surface at the FAF and gradually narrows to 1 mile (1.6 km) at the runway threshold. Pilots flying LNAV approaches follow the lateral flight path programmed into the Flight Management Computer, and LNAV is also the name of an autopilot lateral mode that follows that path. For angular presentation near the runway, full deflection to either side is 350 ft (106.7 m). The CDI has 5 dots on each side, so each dot at the threshold equals 70 ft (21.3 m).
4. Localizer Performance (LP)
Localizer Performance (LP) is the WAAS GPS equivalent of a Localizer (LOC) approach, providing a lateral-only procedure similar in accuracy to a localizer. LP approaches are non-precision approaches with WAAS lateral guidance and no vertical guidance. They use angular lateral guidance with sensitivity increasing as the aircraft approaches the runway, resulting in smaller position errors than LNAV approaches.
LP approaches require a WAAS receiver and are flown like conventional non-precision approaches using a Minimum Descent Altitude (MDA) of 300 feet (91.4 meters) above the runway. The FAA publishes LP minima at locations where obstacles or terrain prevent vertically guided procedures. LP approaches are the rarest type of WAAS approach and cannot be downgraded to LNAV approaches.
