Global Positioning System (GPS): Definition, Function, Types, Use

Global Positioning System (GPS) is a U.S.-owned space-based radio-navigation system that provides positioning, navigation, and timing services worldwide. Consisting of a constellation of satellites broadcasting navigation signals and aided by ground facilities, GPS offers reliable and accurate navigation sources for air, sea, and land applications.

Expert behind this article

Jim Goodrich

Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.

What is GPS in aviation?

The Global Positioning System (GPS) is a satellite-based radio navigation system which consists of a constellation of satellites and a network of ground stations. The Global Positioning System (GPS) is a satellite based hyperbolic navigation system owned by the United States Space Force and operated by Mission Delta 31. It is a space based positioning, velocity and time system that provides a precise global navigation service. In aviation this utility supplies the aircraft with positioning, navigation, and timing (PNT) data in Cartesian earth centered fixed coordinates specified in the World Geodetic System 1984 (WGS 84). In GPS aircraft tracking, the receiver on board calculates three dimensional position and time after it locks onto the signal of at least three satellites and processes the pseudo random code timing signal and data message.

How is GPS used in aviation?

GPS use in modern flight begins with the constellation: twenty-four GPS satellites, each carrying atomic clocks and transmitting a course/acquisition code, circle Earth so at least five are always visible. A receiver in the flight deck listens to four of these transmitters, measures the pseudo-random-code arrival times, solves for latitude, longitude, altitude and precise time, and applies digital filtering techniques like Kalman filter to smooth the result. With WAAS, the space-based augmentation broadcast improves this solution from 100 m (328 ft) to about 7 m (23 ft), giving LPV approaches that provide vertical guidance similar to CAT I ILS while still being officially non-precision approaches.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

From take-off to touchdown, the same receiver drives the navigation, orientation and IFR. Almost every large airport worldwide has RNAV departures and arrivals coded as latitude/longitude waypoints. The aircraft flies the computed track and altitude, with no need for ground aids. Under IFR, the system supplies continuous position, groundspeed and time to the flight-management computer, letting the crew fly arrivals and approaches without external beacons. Even VFR pilots use any navigation source under Part 91. By comparing present position with stored sequences, the GPS unit derives the aircraft's orientation, letting a gyroscope-free attitude reference be inferred from its position history. Thus, one receiver converts radio travel-time into the four-dimensional path that aviation relies on.

How does GPS navigation work in aviation?

GPS navigation works as each GPS receiver collects ephemeris data from all the satellites in view. GPS satellite constellation includes at least 24 satellites. The 24 satellite constellation is designed to ensure at least five satellites are always visible to a user worldwide. Each satellite transmits its position and time at regular intervals. The receiver uses the time difference between the time of signal reception and the broadcast time to compute the distance, or range, from the receiver to the satellite. The receiver uses pseudo-range measurements. GPS distance is measured indirectly with time and the receiver corrects for any timing errors. The receiver uses four satellites to compute latitude, longitude, altitude, and time. Taking a measurement from a fourth satellite avoids the need for an atomic clock. Each GPS receiver uses these signals to calculate its three-dimensional location (latitude, longitude, and altitude) and the current time.

GPS satellites transmit signals that travel at 186,000 miles per second (299,337.6 kilometers per second). GPS satellites circulate the earth twice daily in 12-hour circular orbits about 20,200 kilometers (12,551 miles) above the surface. Each satellite has an onboard atomic clock and the GPS uses these atomic clocks. GPS satellites broadcast ephemeris data and almanac data to transmit messages on the L-band. GPS satellites broadcast range codes C/A and P(Y). The GPS receiver uses time difference to compute pseudo-range.

GPS in aviation uses trilateration. Trilateration determines position by knowing distances from at least three known points. When the signal from the first satellite reaches the receiver, any point on the surface of the sphere is a possible location of the receiver. When the signal from the second satellite reaches the receiver it overlaps with the first sphere and narrows down the possible locations of the receiver to a circle. When a sphere of signal from the third satellite reaches the receiver, the overlap narrows the possibilities of the receiver's location to just two points. The receiver uses four satellites and the fourth satellite measurement eliminates the remaining ambiguity and allows the receiver to avoid the need for an atomic clock. A GPS receiver adjusts the receiver clock until all ranges cross the same point.

GPS receivers select the best satellites automatically and use data from satellites above the mask angle. GPS receiver matches each satellite's CA code with an identical copy of the code contained in the receiver's database. GPS navigation provides continuous accurate three-dimensional coverage. GPS navigation allows dispatchers to plan flights on more precise routes that use latitude/longitude waypoints and GPS airways. GPS navigation is used in airplanes for Visual Flight Rules (VFR) operations and for IFR operations. GPS navigation allows for point-to-point navigation. GPS receivers can store a sequence of waypoints and provide navigation information to each waypoint in the sequence; when you reach one waypoint, the GPS automatically switches to the next. Conventional approaches rely on ground-based radio navigation, while GPS relies on an adequate number of satellites within view.

What are the segments of GPS in aviation?

The GPS comprises three segments that together allow the system to function: space segment, control segment, and user segment. The space segment consists of 24-32 satellites that fly in circular orbits at an altitude of 10,900 nautical miles with a period of 12 hours. The orbits are tilted 55° to the earth's equator to guarantee full global coverage and polar service. Each satellite transmits a Course/Acquisition (CA) code, a pseudo-random code timing signal, and a data message containing the satellite's ephemeris. The control segment is ground based monitoring composed of global network of ground facilities: a master control station at Schriever Air Force Base in Colorado, an alternate master control station, five monitor stations located at Cape Canaveral, Hawaii, Ascension Island, Diego Garcia Atoll and Kwajalein Island, plus additional sites in Argentina, Bahrain, United Kingdom, Ecuador, Washington DC and Australia. These monitor stations are very precise GPS receivers installed at known locations, collecting navigation signals, range & carrier measurements and atmospheric data. Three ground antennas send commands, navigation data uploads and processor program loads to the satellites, collect telemetry, communicate via S-band and perform S-band ranging.

The control segment predicts the behavior of each satellite's orbit and clock, ensures that GPS satellite orbits and clocks remain within acceptable limits, performs satellite maintenance and anomaly resolution, repositions satellites to maintain optimal constellation, and uses AEP & LADO systems to control operational and non-operational satellites. The user segment is aircraft carrying antennas and receiver-processors that provide positioning, velocity and precise timing to the pilot. Receiver-processors match each satellite's CA code with an identical copy stored in the receiver database, use pseudo-range to compute the aircraft's three-dimensional position and time, and rely on trilateration to determine aircraft position by receiving signals from four or more satellites above the mask angle, using at least 11 satellites at a time for a worldwide fix.

What are the GPS modes of operation in aviation?

A GPS has three modes. During flight, a GPS receiver automatically switches between three sensitivity modes also known as Required Navigation Performance (RNP): en-route, terminal, and approach. En-route mode keeps things efficient during cruise with a tolerance of 2/5 NM ( 0.74 km), terminal mode adds precision when approaching or departing airports by tightening tolerance to 1 NM (1.85 km) when the aircraft is within 30 miles (48.28 km) of the destination, and approach mode ensures the highest accuracy - 0.3 NM (0.56 km) - when the aircraft nears a predetermined GPS waypoint on an approach procedure. If a RAIM alarm is active, the receiver will continue to navigate in en-route mode but will not operate in the more demanding approach mode until the limitation is resolved.

For IFR operation, the GPS offers two pilot-selectable sequencing modes: Leg and OBS. Bendix/King labels them ‘LEG’ and ‘OBS’, while Garmin labels them ‘auto’ and ‘hold’. In Leg mode, the unit autosequences from one waypoint to the next. OBS mode duplicates standard VOR function and is useful at holding patterns, procedure turns, and missed approaches. Annunciators labeled GPS SEQ: AUTO/HOLD indicate which mode is active, and the center of the HSI will clearly display the words ‘en route’, ‘terminal’, or ‘approach’ to keep the pilot informed.

What are the types of GPS in aviation?

The types of GPS in aviation are listed below.

• WAAS (Wide Area Augmentation System) GPS: This is the highest standard for general aviation GPS. WAAS improves accuracy, integrity, and availability, allowing GPS to be used as a primary navigation source from takeoff through approach.
• RAIM (Receiver Autonomous Integrity Monitoring) GPS: These units are certified for IFR (Instrument Flight Rules) navigation, provided they have TSO (Technical Standard Order) certification and can monitor signal integrity.
• Non-RAIM/VFR GPS: These systems are not legal for IFR flight and are primarily used for situational awareness or VFR (Visual Flight Rules) navigation.
• GBAS (Ground Based Augmentation System) GPS: Alternatively known as LAAS (Local Area Augmentation System), this ground-based system offers extremely high accuracy (less than 1 meter) for precision approaches, departure procedures, and terminal operations.
• LPV (Localizer Performance with Vertical Guidance): It is the most precise GPS approach, requiring WAAS. It provides lateral and vertical guidance, with sensitivity increasing closer to the runway.
• LNAV/VNAV (Lateral Navigation/Vertical Navigation): It provides both horizontal and vertical guidance, often using barometric-aided GPS (Baro-VNAV).
• LP (Localizer Performance): It is a WAAS-enabled approach providing high lateral precision without vertical guidance.
• LNAV (Lateral Navigation): It is a basic GPS approach providing only lateral guidance, similar to a VOR or Localizer approach.

GLONASS is developed and operated by the Russian Federation, Galileo is developed and operated by the European Union, and BeiDou is developed and operated by China, while the United States GPS uses satellites in medium earth orbit that carry atomic clocks and transmit a Course/Acquisition (CA) code, ephemeris, and a pseudo-random code timing signal that travels at 186,000 miles per second (299,337.6 kilometers per second).

What is the difference between GPS and GNSS in aviation?

The differences between GPS and GNSS in aviation are given in the table below.

GNSS is an umbrella term that encompasses GPS plus other satellite systems like Galileo, GLONASS, and BeiDou. GPS is one type of GNSS constellation, while GNSS refers to any satellite-based navigation system that provides autonomous geo-spatial positioning with global coverage.

A GNSS receiver can use signals from any positioning satellite, not just the ones in the GPS system. Because GNSS gives the receiver more satellites to work with, it offers better accuracy, redundancy, and reliability than relying on GPS alone. If part of the GPS satellite constellation fails, the receiver will fall back on GLONASS, BeiDou, or Galileo, so outages are less likely.

By using multiple constellations, GNSS provides broader coverage, reduces blind spots, and improves geometry, making dropouts rare. Under ideal conditions, GNSS can position an aircraft to within 1-2 meters (3.28-6.56 feet), whereas stand-alone GPS accuracy is generally within ten meters (32.8 feet). There is no fundamental difference between a GPS antenna and a GNSS antenna; the same antenna feeds both systems, so the hardware change is invisible to the flight crew.

In practical terms, GPS and GNSS are often used interchangeably on the flight deck, and the interface still says ‘GPS’ even when it is tracking Galileo, BeiDou, or GLONASS. The key point for aviators is that multi-GNSS performs better than GPS alone, delivering more robust coverage and more reliable positioning without any extra pilot action.

What are the GPS requirements in aviation?

GPS requirements in aviation include that GPS equipment must be approved under TSO-C129, TSO-C196, TSO-C145, or TSO-C146 and installed per Advisory Circular AC 20-138. For IFR approach/departure the aircraft must hold a TSO-C145, TSO-C146, TSO-C196, or TSO-C129 receiver in Class A1, B1, B3, C1, or C3. The FAA requires flight crew members to be thoroughly familiar with the particular GPS equipment installed in the aircraft.

Non-WAAS IFR GPS receivers must provide Receiver Autonomous Integrity Monitoring. TSO-C129 requires all IFR-approved receivers to have RAIM. RAIM needs at least five satellites to detect a bad satellite and six satellites to exclude it when the receiver incorporates a fault-detection-and-exclusion algorithm; baro-aiding satisfies the RAIM requirement in lieu of a fifth satellite. En-route sensitivity mode accuracy must stay within 2 or 5 nautical miles of full-scale deflection, whereas approach sensitivity mode accuracy is 0.3 nautical miles of full-scale deflection. A RAIM check is performed at least two miles before the final approach waypoint, and FDE prediction is required for oceanic or remote operation where GPS is the primary navigation source.

Although GPS augmented by WAAS and LAAS can meet navigation requirements in the National Airspace System, operators conducting IFR operations under 14 CFR 121.349, 125.203, 129.17, and 135.65 must still retain a non-GPS navigation capability like DME/DME, IRU, VOR, or ILS. Approach/departure procedures must be retrievable from the current airborne navigation database, and the onboard navigation data must be current and appropriate for the region of intended operation. If GPS avionics fail, pilots must advise ATC as required by FAR 91.187 and amend the equipment suffix. ICAO Standards and Recommended Practices for GNSS spell out performance and availability requirements - ranging from 99 % to 99.999 % - for each phase of flight.

Do all planes have a GPS?

Nearly every modern commercial aircraft carries GPS, while older or smaller airframes and military planes rely on alternate navigation aids, but the presence of dual receivers and WAAS ensures that the system works wherever the airplane flies. In today's commercial cockpits, GPS is included in more than 80 percent of the U.S. fleet. Airplanes like airliners and military transport aircraft use not one but two independent GPS receivers so that a single failure never leaves the crew without guidance. The signal itself is unbroken aloft because GPS satellites must be directly visible to the device, and at altitude there is always an unobstructed view of the constellation. Most small planes in the general aviation fleet do not have built-in GPS, and many regional jets still operate under rules that let CRJ 700s dispatch after loss of all GPS units, but with tighter operational restrictions. Advanced receivers equipped with the Wide-Area Augmentation System - WAAS gives the most accurate GPS location information, delivering LPV performance accurate to less than one meter horizontally - are standard on new transports, and some airports LIKE Savonlinna only have GPS approaches, making the receiver mandatory for arrival.

What are the reasons for a GPS to not work in an airplane?

There are several reasons a GPS may not work. Dreamliner (787) windows contain a conductive material that prevents the GPS signals from getting in, turning the fuselage into an unfortunate GPS shielding bubble. Even when a seat-back receiver peers through ordinary glass, the device needs to see four data streams to get a three-dimensional position, and at 27k ft the thin air offers no improvement. Solar flares generate charged particles that interfere with communication between satellites and receivers, while intentional jamming can also overpower the already-weakened link. Because the aircraft is traveling at 450-650 mph (724-1046 km/h), a handset cannot stay fixed on any external reference long enough for the data to mean anything. GPS signals are disabled for testing frequently in targeted bubbles with a radius of about 400 miles (643.738 km), so over some regions the constellation itself is briefly incomplete.

Why is the aircraft GPS not positioned?

The aircraft GPS is not positioned as the position is always slightly behind because receiving signals and processing them takes time. The screen shows where you were, not where you are. If the antenna sits where part of the airframe blocks the view of satellites, or if the location was chosen for convenience rather than a clear sky, satellite geometry weakens and the fix degrades. Without RAIM capability the receiver can neither guarantee accuracy nor warn the crew, so an erroneous position will appear while the cockpit still believes it is valid. Jamming saturates the front-end with noise. Spoofing goes farther by transmitting a look-alike signal that tricks the receiver into placing the aircraft at an incorrect position and/or time, and spoofing is always a deliberate act. Over conflict zones like the Black Sea off the northern coast of Turkey, military jamming makes GPS and ADS-B unreliable, so controllers fall back on MLAT and pilots must radio position reports while the autopilot couples to IRS, which is much more accurate over short times, distances, and rapid changes in attitude and speed.

What are the benefits of GPS in aviation?

GPS benefits include that it provides real-time, three-dimensional position and time with 95% accuracy. GPS-enabled aircraft can fly continuous climb and descent operations, which reduces fuel consumption, noise, and carbon emissions. These operations shorten flight paths and flight times, lowering operating costs for airspace users and service providers. GPS allows aircraft to follow direct, latitude/longitude waypoints and flexible, user-preferred route structures, easing congestion and increasing capacity. GNSS benefits aviation by enabling aircraft to fly the most fuel-efficient routes from departure to destination, creating a smaller carbon footprint. GPS reduces separation minimums, resulting in increased arrival and departure rates and system capacity.

GPS improves safety by providing reliable and accurate navigation, refined ground and cockpit situational awareness, and support for Terrain Awareness and Warning Systems. GBAS yields extremely high accuracy, availability, and integrity necessary for Category I, II, and III precision approaches, adding vertical guidance where it did not exist. WAAS improves GPS accuracy from 100 meters (328 feet) to approximately 7 meters (23 feet), further boosting approach performance. GPS supports ADS-B, which opens up more capacity in already crowded skies by decreasing required vertical and horizontal separation distances between aircraft. ADS-B enables integration of real-time weather, pilot reports, and 3D airspace information, allowing pilots to make more informed and safer decisions. GPS reduces maintenance and operation of unnecessary ground-based systems, yielding further economies for each state while increasing overall benefits to the entire region. GPS provides continuous all-weather coverage and accurate time synchronization, enabling digital data links to controllers and trajectory-based operations.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What are the limitations of GPS in aviation?

Existing GPS systems have four major limitations: lack of accuracy for pivotal phases of flight, lack of integrity, lack of availability and continuity of service, and no control by an international civil body. Receiver errors, satellite errors, atmospheric errors, and environmental errors are possible sources of errors that degrade computed user position. Without a current database, the moving map display offers erroneous information around critical airspace areas, and VFR waypoints are rejected for IFR routing purposes. GPS interference, jamming, and spoofing cause loss of navigation, spurious TAWS alerts, disagreement in navigation position, and false location or time information. Active monitoring of alternate navigation equipment like VOR, DME/DME, or IRU is required when RAIM is predicted to be unavailable.

What is the history of GPS in aviation?

Global Positioning System was initiated as a joint civil/military technical program in 1973. GPS has its origins in the Sputnik era, when scientists were able to track the satellite with shifts in its radio signal known as the Doppler Effect. The project combined ideas from several predecessors including classified engineering design studies from the 1960s. In 1978 the first experimental Block-I GPS satellite was launched; Navstar 1 lifted off on 22 February 1978. By December 1993 GPS achieved initial operational capability with a full constellation of 24 satellites available and providing the Standard Positioning Service.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

The Korean Air Lines Flight 007 crash led to the 1983 announcement to make GPS available for civilian use. In aviation, this civil opening meant airlines would begin evaluating satellite navigation. GPS was first used in war in the 1991 Persian Gulf War, demonstrating precise positioning to a global audience. The first GPS unit certified for IFR operations was approved on 16 February 1994, marking the moment when airlines could legally replace ground-based navigation aids with satellite signals from 20,200 km (12,551 mi) circular orbits inclined at 55 degrees. Operated today by Mission Delta 31, GPS evolved over time and is now owned by the United States Space Force, providing continuous position, velocity and time data to every Boeing and Airbus cockpit.