Satellite gps

GPS Technology | Tutorial & Basics

- summary, overview or tutorial about GPS technology basics detailing its operation, what it is and how it works.

GPS, or Global Positioning System, is also sometimes called NavStar. GPS is a satellite based global navigation satellite system, GNSS that is used to provide accurate location and time information anywhere on or near the Earth.

GPS is run and maintained by the US government, although access to it has been opened up so that it is freely available worldwide when used with suitable GPS receivers.

Typically GPS is able to provide position information to within a few metres, allowing accurate positioning to be made. It is also possible to extract timing information that enables frequencies and time to be very accurately maintained. Frequency stability performance figures of systems using GPS timing are far in better than crystal or many other accurate frequency sources.

The performance and ease of use of GPS has meant that it is now an integral part of everyday life, with many portable or car-based "satnav" systems being used, as well as many mobile phones incorporating them to enable them to provide location information superimposed on the maps from the phone or satnav.

GPS basics

The basic concept behind GPS is that signals are transmitted from the satellites in space and these are received by the receivers on or near to the surface of the earth. Using timing it is possible to determine the distance from each satellite and thereby using a process of triangulation and a knowledge of the satellite positions the position on Earth can be determined.

The satellites all send timing information so the receiver knows when the message was sent. As radio signals travel at the speed of light they take a very short but finite time to travel the distance from the satellite to the receiver. The satellites also transmit information about their positions. In this way the receiver is able to calculate the distance from the satellite to the receiver. To obtain a full fix of latitude, longitude and altitude, four or more satellites are required, and when the receiver is in the clear, more than four satellites are in view all the time. A fix of just latitude and longitude can be obtained from three satellites.

GPS satellite orbits

The fully operational GPS satellite system consists of a constellation of 24 operational satellites with a few more in orbit as spares in case of the failure of one. The GPS satellites are in one of six orbits. These are in planes that are inclined at approximately 55° to the equatorial plane and there are four satellites in each orbit. This arrangement provides the earth user with a view of between five and eight satellites at any time from any point on the Earth.

Using economic ground based receivers GPS is able to provide position information to within a number of metres. The economic costs have also meant that it is now fitted to many motor vehicles, while separate GPS receivers can be bought for a few hundred pounds or dollars. As a result it is widely used by private individuals, as well as many commercial and professional users. In fact the primary use for GPS is as a military navigation system. The fact that it is used so widely is a by-product of its success.

GPS satellites

The satellites are orbiting above the Earth. Their orbits are tightly controlled because errors in their orbit will translate to errors in the final positions. The time signals are also tightly controlled. The satellites contain an atomic clock so that the time signals they transmit are very accurate. Even so these clocks will drift slightly and to overcome this, signals from Earth stations are used to correct this.

The GPS satellites themselves have a design life of ten years, but to ensure that there are no holes in service in the case of unexpected failures, spares are held in orbit and these can be brought into service at short notice.

The satellites are provide their own power through their solar panels. These extend to about 17 feet, and provide the 700 watts needed to power the satellite and its batteries when it is in sunlight. Naturally the satellite needs to remain operation when it is on the dark side of the Earth when the solar panels do not provide any power. This means that when in sunlight the solar panels need to provide additional power to charge batteries, beyond just powering the basic satellite circuitry.

GPS receivers

A large number of GPS receivers are available today. They make widespread use of digital signalling processing techniques. The transmissions from the satellites use spread spectrum technology, and the signal processors correlate the signals received to recover the data. As the signals are very weak it takes some time after the receiver is turned on to gain the first fix. This Time To First Fix (TTFF) is of importance, and in early receivers it could be as long as twelve minutes, although modern receivers use many more correlators are able to shorten this considerably.

When using a GPS receiver the receiver must be in the open. Buildings, or any structure will mask the signals and it may mean that few satellites can be seen. Thus the receivers will not operate inside buildings, and urban areas may often cause problems.

GPS Applications

The primary use for GPS is as a military navigational aid. Run by the American Department of Defense its primary role is to provide American forces with an accurate means of navigation anywhere on the globe. However its use has been opened up so that commercial and private users have access to the signals and can use the system. Accordingly it is very widely used for many commercial applications from aircraft navigation, ship navigation to surveying, and anywhere where location information is required. For private users very cost effective receivers are available these days and may be used for applications including sailing. Even many motor vehicles have them fitted now to provide SatNav systems enabling them to navigate easily without the need for additional maps.

It can be said that GPS has revolutionised global navigation since it became available. Prior to this navigation systems were comparatively localised, and did not offer anything like the same degrees of accuracy, flexibility and coverage.

GPS summary

There are a number of salient features for GPS. These are tabulated below:

Parameter Details
Number of active satellites 24
Number of satellite orbits 6
Number of satellites in each orbit 4
Orbit altitude Approx 20 200 km
Orbit relative angles 55° to each other
Satellite velocity 2.6 km/s
Main satellite transmission frequencies 1.57542 GHz (L1 signal)1.2276 GHz (L2 signal)
Signal format CDMA
Max transmitter power 50 watts
Satellite diameter Approx 5 metres with solar cells extended
Satellite weight Approx 1 tonne
Anticipated lifetime of individual satellite Approx 10 years

By Ian Poole

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Airplanes have been using GPS for many years now and many of the fundamentals may be too basic for some. But perhaps a refresher is in order:

  1. Overview
  2. How GPS Works
  3. Satellite Tracks
  4. GNSS versus GPS
  5. U.S. Requirements to use GPS
  6. ICAO Requirements to Use GPS
  7. Receiver Autonomous Integrity Monitoring (RAIM)
  8. Fault Detection and Exclusion (FDE)
  9. Availability / NOTAMS
  10. Wide Area Augmentation System (WAAS)


[FAA Instrument Handbook, pg. 7-21]

  • The Department of Defense (DOD) developed and deployed GPS as a space-based positioning, velocity, and time system. The DOD is responsible for the operation the GPS satellite constellation and constantly monitors the satellites to ensure proper operation. The GPS system permits Earth-centered coordinates to be determined and provides aircraft position referenced to the DOD World Geodetic System of 1984 (WGS-84). Satellite navigation systems are unaffected by weather and provide global navigation coverage that fully meets the civil requirements for use as the primary means of navigation in oceanic airspace and certain remote areas. Properly certified GPS equipment may be used as a supplemental means of IFR navigation for domestic en route, terminal operations, and certain IAPs. Navigational values, such as distance and bearing to a waypoint and groundspeed, are computed from the aircraft's current position (latitude and longitude) and the location of the next waypoint. Course guidance is provided as a linear deviation from the desired track of a Great Circle route between defined waypoints.
  • The space element [of GPS] consists of 24 Navstar satellites. This group of satellites is called a constellation. The satellites are in six orbital planes (with four in each plane) at about 11,000 miles above the Earth. At least five satellites are in view at all times. The GPS constellation broadcasts a pseudo-random code timing signal and data message that the aircraft equipment processes to obtain satellite position and status data. By knowing the precise location of each satellite and precisely matching timing with the atomic clocks on the satellites, the aircraft receiver/processor can accurately measure the time each signal takes to arrive at the receiver and, therefore, determine aircraft position.

How GPS Works

There is obviously much more to it than what follows, but this gives you what you need to understand how GPS has changed the way we fly airplanes. . .

The Transmitted Signal

Figure: NAVSTAR-2, from Lockheed-Martin (Public Domain)

[AFAIS Performance-Based Navigation Presentation] The U.S. Global Positioning System (Also called "Navstar") consists of 24 operational satellites (plus a few spares) of which 5 to 8 should be in view anywhere on the earth. They are at 11,000 nautical miles in altitude and complete an orbit every 12 hours.

Each Navstar satellite transmits on two frequencies:

  • L1: 1575.42 MHz — C/A and P codes
  • L2: 1227.6 MHz — P code only

[AFAIS Performance-Based Navigation Presentation] Coarse Acquisition (C/A) code is available to all users without limitations and includes

  • Ephemeris (position, altitude, speed information from the satellite)
  • Time (from the onboard atomic clock, including a time correction factor to make up for the clock's internal errors)
  • Satellite health status
  • GPS Almanac (predicted positions for entire GPS constellation, often good for months). A receiver that keeps the almanac in memory can predict from a cold start where to look for satellites, speeding acquisition times.

[AFAIS Performance-Based Navigation Presentation] P-Code provides navigation/targeting data for U.S. government users with an encryption key

  • Position data
  • Broadcast on both frequencies, allowing qualified receivers to compare both frequencies and correct for any ionospheric delays.
  • Once decrypted P-code becomes Y-code.

[FAA Instrument Handbook, pg. 7-21] The aircraft GPS receiver measures distance from a satellite using the travel time of a radio signal. Each satellite transmits a specific code, called a course/acquisition (CA) code, which contains information on the satellite's position, the GPS system time, and the health and accuracy of the transmitted data. Knowing the speed at which the signal traveled (approximately 186,000 miles per second) and the exact broadcast time, the distance traveled by the signal can be computed from the arrival time. The distance derived from this method of computing distance is called a pseudo-range because it is not a direct measurement of distance, but a measurement based on time. In addition to knowing the distance to a satellite, a receiver needs to know the satellite's exact position in space; this is know as its ephemeris. Each satellite transmits information about its exact orbital location. The GPS receiver uses this information to precisely establish the position of the satellite.

Each GPS satellite transmits these two frequencies and chances are your receiver captures the L1. There are no limits to the number of receivers since there is no interaction from these receivers back to the satellites. You will need four satellites to determine your position . . .

One satellite

Figure: One satellite, from Eddie's notes.

Each satellite sends out a signal that includes its own position and the time. The receiver can calculate the time it took the signal to travel and multiply that by the speed of the signal (the speed of light) to compute the distance. That distance ("r" in the figure) defines a sphere. The receiver could be at any point on that sphere. On the diagram it is more than just the black line, it is the entire outer shell of the sphere. (Remember: three dimensions.)

This is true in theory but hardly practical, as a very sharp reader pointed out, see Letter to Eddie, below.

Two satellites

Figure: Two satellites, from Eddie's notes.

With two satellites you have an intersection of two spheres and the receiver could be in any position along those intersecting spheres. Once again, it is more than just the black lines in the diagram, your position could be at any point inside the three-dimensional shape described by the black line.

Three satellites

Figure: Three satellites, from Eddie's notes.

With three satellites you narrow the possible location down to one of three points (the three black points).

Four satellites

Figure: Four satellites, from Eddie's notes.

With one more satellite, you have narrowed the universe of possible intersections to just one (the single black point).

Errors, of course, are possible. . .

Position Errors

[AFAIS Performance-Based Navigation Presentation] Errors are possible due to:

  • Minor disturbances in satellite orbits from gravitational variations from the sun and the moon or solar wind.
  • Ionospheric signal delays caused by water vapor in the atmosphere; this is the biggest source of signal error.
  • Slight fluctuations in the satellite atomic clocks.
  • Receiver quality (faulty clocks or internal noise).
  • Multi-path signal reflections off structures.

Any errors from even a single satellite can throw off the estimated distance computations and therefore your estimated position. The drawing makes light of a 5 second error, but at the speed of light that would be 930,000 miles, exceeding the satellite's orbit. We must obviously be talking about very small time errors.

The performance of each satellite is measured and corrected to ensure accuracy . . .

GPS Ground Stations

[AFAIS Performance-Based Navigation Presentation] There are 6 monitor stations, including the master station at Colorado Springs.

  • Collect position and timing data from satellites every 12 hours.
  • Send information to master control station.
  • Master control station computes corrections and uploads to satellites.

Some receivers are capable of greater accuracy than others, but the issue isn't as extreme as some would have you believe . . .

Positioning Services

Figure: USS Princeton launches a Harpoon missile, from US Navy (Public Domain)

[AFAIS Performance-Based Navigation Presentation]

  • Standard Positioning Service (SPS)
    • Uses C/A code – for all users
    • Single frequency (L1)
  • Precise Positioning Service (PPS)
    • Uses P-code – for military
    • Two frequencies (L1 and L2) – more accurate
    • Requires Decryption Key do use it
Selective Availability

[AFAIS Performance-Based Navigation Presentation]

  • Selective Availability was designed into the system to provide non-military or non-governmental users an intentionally limited accuracy.
  • The system was turned off in 2000 and we are told the newer satellites don't even have the capability.

As the world became more dependent on GPS they became more worried that one day the U.S. government would turn on selective availability and send airplanes into mountains. The U.S. government promises us that they've abandoned the concept entirely.

Satellite Tracks

The signal coverage is supposed to be worldwide, but the satellites do not cover the world. How can that be?

[] The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.

There are some who say you cannot get a GPS signal at either pole because they are inclined at 55° from the equator, they even have anecdotal evidence. NASA offers a website to track each satellite and it is true they never get higher than 55° but there are lots of reports of excellent GPS signals at each pole. What gives?

Figure: GPS Satellite Line of Sight, from Eddie's notes.

Each GPS satellite traces a track over the earth from 55° North to 55° South every twelve hours. At their maximum latitudes they are actually "looking down" on the poles:

Height Above Pole=10998cos55-6887=2122

Of course you have no guarantee you will have at least one satellite that high in its orbit. In order to have line of sight on the pole, a satellite would have to be at least 39° latitude:

Minimum Latitude to See Pole=arcsin(6887/10998)=39

I've not found anything in writing that tells you there will always be at least four satellites above 39° North and 39° South, but it appears so. You should have a good GPS position at either pole.

GNSS versus GPS

Figure: Toluca, Mexico RNAV (GNSS) Rwy 15, from Jeppesen FlightDeck MMTO page 12-1.

[AC 20-138D, ¶1-4.e.(2)(a)] GNSS is used internationally to indicate any satellite-based positioning system or augmentation system. The acronym 'GNSS' includes satellite constellations, such as GPS, GLONASS, Galileo, or Beidou, along with augmentation systems such as 'SBAS' and 'GBAS'; all of which provide a satellite-based positioning service.

The Global Navigation Satellite System (GNSS) includes navigation satellites and ground systems that monitor satellite signals and provide corrections and integrity messages, where needed, to support specific phases of flight. Currently, there are two navigation satellite systems in orbit: the U.S. Global Positioning Satellite (GPS) System and the Russian global navigation satellite system (GLONASS). The U.S. and Russia have offered these systems as the basis for a GNSS, free of direct user charges.

So GPS is a subset of GNSS which means all GPS approaches are GNSS but not all GNSS approaches are GPS. If the approach is marked RNAV (GNSS) you might be okay, but you have some homework to do first.

See: RNAV (GNSS) Example for a walk through of the decision making needed.

U.S. Requirements to Use GPS

General IFR Requirements

[Aeronautical Information Manual ¶1-1-19.d.]

1. Authorization to conduct any GPS operation under IFR requires that:

(a) GPS navigation equipment used must be approved in accordance with the requirements specified in Technical Standard Order (TSO) TSO-C129, or equivalent, and the installation must be done in accordance with Advisory Circular AC 20-138, Airworthiness Approval of Global Positioning System (GPS) Navigation Equipment for Use as a VFR and IFR Supplemental Navigation System, or Advisory Circular AC 20-130A, Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors, or equivalent. Equipment approved in accordance with TSO-C115a does not meet the requirements of TSO-C129. Visual flight rules (VFR) and hand-held GPS systems are not authorized for IFR navigation, instrument approaches, or as a principal instrument flight reference. During IFR operations they may be considered only an aid to situational awareness.

(b) Aircraft using GPS navigation equipment under IFR must be equipped with an approved and operational alternate means of navigation appropriate to the flight. Active monitoring of alternative navigation equipment is not required if the GPS receiver uses RAIM for integrity monitoring. Active monitoring of an alternate means of navigation is required when the RAIM capability of the GPS equipment is lost.

(c) Procedures must be established for use in the event that the loss of RAIM capability is predicted to occur. In situations where this is encountered, the flight must rely on other approved equipment, delay departure, or cancel the flight.

(d) The GPS operation must be conducted in accordance with the FAA-approved aircraft flight manual (AFM) or flight manual supplement. Flight crew members must be thoroughly familiar with the particular GPS equipment installed in the aircraft, the receiver operation manual, and the AFM or flight manual supplement. Unlike ILS and VOR, the basic operation, receiver presentation to the pilot, and some capabilities of the equipment can vary greatly. Due to these differences, operation of different brands, or even models of the same brand, of GPS receiver under IFR should not be attempted without thorough study of the operation of that particular receiver and installation. Most receivers have a built-in simulator mode which will allow the pilot to become familiar with operation prior to attempting operation in the aircraft. Using the equipment in flight under VFR conditions prior to attempting IFR operation will allow further familiarization.

(e) Aircraft navigating by IFR approved GPS are considered to be area navigation (RNAV) aircraft and have special equipment suffixes. File the appropriate equipment suffix in accordance with TBL 5-1-2, on the ATC flight plan. If GPS avionics become inoperative, the pilot should advise ATC and amend the equipment suffix.

(f) Prior to any GPS IFR operation, the pilot must review appropriate NOTAMs and aeronautical information. (See GPS NOTAMs/Aeronautical Information.)

(g) Air carrier and commercial operators must meet the appropriate provisions of their approved operations specifications.

IFR Oceanic [Aeronautical Information Manual ¶1-1-19.e.1.]

GPS IFR operations in oceanic areas can be conducted as soon as the proper avionics systems are installed, provided all general requirements are met. A GPS installation with TSO-C129 authorization in class A1, A2, B1, B2, C1, or C2 may be used to replace one of the other approved means of long-range navigation, such as dual INS. (See TBL 1-1-5 and TBL 1-1-6.) A single GPS installation with these classes of equipment which provide RAIM for integrity monitoring may also be used on short oceanic routes which have only required one means of long-range navigation.

Domestic En Route

[Aeronautical Information Manual ¶1-1-19.e.2.] GPS domestic en route and terminal IFR operations can be conducted as soon as proper avionics systems are installed, provided all general requirements are met. The avionics necessary to receive all of the ground-based facilities appropriate for the route to the destination airport and any required alternate airport must be installed and operational. Ground-based facilities necessary for these routes must also be operational.

Terminal Area Operations

[Aeronautical Information Manual ¶1-1-19.e.3.] The GPS Approach Overlay Program is an authorization for pilots to use GPS avionics under IFR for flying designated nonprecision instrument approach procedures, except LOC, LDA, and simplified directional facility (SDF) procedures. These procedures are now identified by the name of the procedure and “or GPS” (e.g., VOR/DME or GPS RWY 15). Other previous types of overlays have either been converted to this format or replaced with stand-alone procedures. Only approaches contained in the current onboard navigation database are authorized. The navigation database may contain information about nonoverlay approach procedures that is intended to be used to enhance position orientation, generally by providing a map, while flying these approaches using conventional NAVAIDs. This approach information should not be confused with a GPS overlay approach (see the receiver operating manual, AFM, or AFM Supplement for details on how to identify these approaches in the navigation database).

[Aeronautical Information Manual ¶1-1-19.e.3.] Additionally:

  • All approach procedures to be flown must be retrievable from the current airborne navigation database
  • Prior to using a procedure or waypoint retrieved from the airborne navigation database, the pilot should verify the validity of the database.
  • Determine that the waypoints and transition names coincide with names found on the procedure chart. Do not use waypoints, which do not exactly match the spelling shown on published procedure charts.
  • Determine that the waypoints are generally logical in location, in the correct order, and that their orientation to each other is as found on the procedure chart, both laterally and vertically.

ICAO Requirements to Use GPS

[FAA Instrument Handbook, pg. 7-21] GPS may not be approved for IFR use in other countries. Prior to its use, pilots should ensure that GPS is authorized by the appropriate countries.


[ICAO Doc 9613, Attachment 2, ¶3.4 a)] Navigation data may originate from survey observations, from equipment specifications/settings or from the airspace and procedure design process. Whatever the source, the generation and the subsequent processing of the data must take account of the following: (a) all coordinate data must be referenced to the World Geodetic System — 1984 (WGS-84).

Not every country uses the same system to map coordinates. While the differences are minor for en route navigation, they can be significant on approach.

See WGS-84 for more about this.

Operational Approval

[ICAO Doc 8168 Vol 1 ¶1.2.1]: Aircraft equipped with basic GNSS receivers (either as stand-alone equipment or in a multi-sensor environment) that have been approved by the State of the Operator for departure and non-precision approach operations may use these systems to carry out RNAV procedures provided that before conducting any flight, the following criteria are met: a) the GNSS equipment is serviceable; b) the pilot has a current knowledge of how to operate the equipment so as to achieve the optimum level of navigation performance; c) satellite availability is checked to support the intended operation; d) an alternate airport with conventional navaids has been selected; and e) the procedure is retrievable from an airborne navigation database.

Navigation Database

[ICAO Doc 8168 Vol 1 ¶1.2.3]: Departure and approach waypoint information is contained in a navigation database. If the navigation database does not contain the departure or approach procedure, then the basic GNSS stand-alone receiver or FMC shall not be used for these procedures.

Receiver Autonomous Integrity Monitoring (RAIM)

[AFAIS Performance-Based Navigation Presentation] For a GPS receiver to be certified for IFR navigation, it must have RAIM or an equivalent function. RAIM is simply a computer algorithm that evaluates the integrity of the GPS signal. That means it judges whether enough satellites are in view and in a good geometry to compute a sufficiently accurate position. RAIM checked now evaluates the current satellites in view. Predictive RAIM is based solely on the Almanac. In other words, RAIM uses the Almanac data to estimate where satellites are supposed to be for the future time entered. Sometimes, the number and position of satellites may result in an accuracy good enough only for certain phases of flight, ie, en route, terminal, or approach.

  • RAIM — requires 5 satellites in view (1 extra) to provide the extra geometry needed to check the integrity of each satellite being used.
  • Predictive RAIM — Uses almanac data or NOTAMS to determine in advance if any satellites should be excluded.
  • Fault Detection and Exclusion (FDE) — With an additional satellite, an FDE system can not only detect but can automatically exclude a failed satellite. FDE is required for oceanic or remote operations.
  • Baro-Aiding — Some systems can take an altimeter input to replace 1 satellite, so that RAIM only requires 4 satellites (versus 5) and FDE only requires 5 satellites (versus 6).

[Aeronautical Information Manual ¶1-1-19.a.]

3. Receiver Autonomous Integrity Monitoring (RAIM). When GNSS equipment is not using integrity information from WAAS or LAAS, the GPS navigation receiver using RAIM provides GPS signal integrity monitoring. RAIM is necessary since delays of up to two hours can occur before an erroneous satellite transmission can be detected and corrected by the satellite control segment. The RAIM function is also referred to as fault detection. Another capability, fault exclusion, refers to the ability of the receiver to exclude a failed satellite from the position solution and is provided by some GPS receivers and by WAAS receivers.

4. The GPS receiver verifies the integrity (usability) of the signals received from the GPS constellation through receiver autonomous integrity monitoring (RAIM) to determine if a satellite is providing corrupted information. At least one satellite, in addition to those required for navigation, must be in view for the receiver to perform the RAIM function; thus, RAIM needs a minimum of 5 satellites in view, or 4 satellites and a barometric altimeter (baro-aiding) to detect an integrity anomaly. [Baro-aiding satisfies the RAIM requirement in lieu of a fifth satellite.] For receivers capable of doing so, RAIM needs 6 satellites in view (or 5 satellites with baro-aiding) to isolate the corrupt satellite signal and remove it from the navigation solution. Baro-aiding is a method of augmenting the GPS integrity solution by using a nonsatellite input source. GPS derived altitude should not be relied upon to determine aircraft altitude since the vertical error can be quite large and no integrity is provided. To ensure that baro-aiding is available, the current altimeter setting must be entered into the receiver as described in the operating manual.

5. RAIM messages vary somewhat between receivers; however, generally there are two types. One type indicates that there are not enough satellites available to provide RAIM integrity monitoring and another type indicates that the RAIM integrity monitor has detected a potential error that exceeds the limit for the current phase of flight. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position.

Gulfstream Planeview Predictive RAIM

Photo: Predictive RAIM, from Eddie's aircraft.

[G450 Aircraft Operating Manual §2B-17 ¶1.]

  • Each GNSSU has RAIM outputs for the current position and time in the form of horizontal and vertical integrity limits (HIL and VIL). To compute RAIM, the GNSSU must have a minimum of five good satellite signals. The FMS does not accept GNSSU data unless a valid RAIM figure is available.
  • The GNSSU also has a predictive RAIM function. The GNSSU supplies HIL predictions for a requested time/position and also HIL/VIL predictions for the approach area on a continuous basis (see Figure 3). The FMS can interrogate the predictive RAIM function of the GNSSU through the ARINC 429 interface. However, RAIM integrity performance requirements cannot be selected with the GNSSU.

[G450 Aircraft Operating Manual §2B-17-30]

  • The FMS uses predictive RAIM to determine the integrity levels at specific locations/times to support a non-precision approach and pilot’s flight planning activities. The GNSSUs have the following types of RAIM predictions:
    • Destination
    • Alternate waypoint
    • Approach area
  • The destination and alternate waypoint predictions are made at specific locations or they are the estimated time of arrival (ETA) when the FMS makes the request for flight planning purposes. Satellites can be manually deselected or enabled for destination and alternate waypoint predictions. The approach area RAIM prediction is an output of current RAIM projected 5 min into the future. Approach area RAIM is a continuous output that is performed without any interaction with the FMS.

The FMS is doing this check for you, the book says, 5 minutes into your future. You probably want to know if you are going to have a problem with more notice than this. Many flight planning services will tell you when you compute the flight plan if there are any known outages.

With the G450, you can also predict the future: G450 Check RAIM.

Fault Detection and Exclusion (FDE)

[FAA Order 8900.1, Vol. 4, Ch. 1, §4, ¶4-78.C.]


  1. Primary means of navigation—Navigation equipment that provides the only required means on the aircraft of satisfying the necessary levels of accuracy, integrity, and availability for a particular area, route, procedure, or operation.
  2. Class II navigation—Any en route flight operation or portion of an en route operation (irrespective of the means of navigation) which takes place outside (beyond) the designated operational service volume of ICAO standard airway navigation facilities (VOR, VOR/DME, NDB).
  3. Fault detection and exclusion (FDE)—Capability of GPS to:
    1. Detect a satellite failure which effects navigation; and
    2. Automatically exclude that satellite from the navigation solution.
  4. All operators conducting GPS primary means of Class II navigation in oceanic/remote areas under 14 CFR parts 91, 121, 125, or 135 must utilize an FAA-approved FDE prediction program for the installed GPS equipment that is capable of predicting, prior to departure, the maximum outage duration of the loss of fault exclusion, the loss of fault detection, and the loss of navigation function for flight on a specified route. The "specified route of flight" is defined by a series of waypoints (to include the route to any required alternates) with the time specified by a velocity or series of velocities. Since specific ground speeds may not be maintained, the pre-departure prediction must be performed for the range of expected ground speeds. This FDE prediction program must use the same FDE algorithm that is employed by the installed GPS equipment and must be developed using an acceptable software development methodology (e.g., RTCA/DO-178B). The FDE prediction program must provide the capability to designate manually satellites that are scheduled to be unavailable in order to perform the prediction accurately. The FDE prediction program will be evaluated as Part of the navigation system's installation approval.
  5. Any predicted satellite outages that affect the capability of GPS equipment to provide the navigation function on the specified route of flight requires that the flight be canceled, delayed, or rerouted. If the fault exclusion capability outage (exclusion of a malfunctioning satellite) exceeds the acceptable duration on the specific route of flight, the flight must be canceled, delayed, or rerouted.
  6. Prior to departure, the operator must use the FDE prediction program to demonstrate that there are no outages in the capability to navigate on the specified route of flight (the FDE prediction program determines whether the GPS constellation is robust enough to provide a navigation solution for the specified route of flight).
  7. Once navigation function is ensured (the equipment can navigate on the specified route of flight), the operator must use the FDE prediction program to demonstrate that the maximum outage of the capability of the equipment to provide fault exclusion for the specified route of flight does not exceed the acceptable duration (fault exclusion is the ability to exclude a failed satellite from the navigation solution). The acceptable duration (in minutes) is equal to the time it would take to exit the protected airspace (one-half the lateral separation minimum) assuming a 35-nautical mile (NM) per hour cross-track navigation system error growth rate when starting from the center of the route. For example, a 60-NM lateral separation minimum yields 51 minutes acceptable duration (30 NM divided by 35 NM per hour). If the fault exclusion outage exceeds the acceptable duration, the flight must be canceled, delayed, or rerouted.

This can be confusing so let's break it into a few pieces:

  • Class II navigation in oceanic/remote areas means anytime you are outside the service volume of authorized navigation aids.
  • More about this: Class I versus Class II.

  • "GPS primary means of Class II navigation" means that the only way you have of long range navigation is GPS. If you have an IRS, you have another means.
  • If your aircraft relies on GPS, and GPS only, for Class II navigation, your manufacturer should either provide or point you to a qualified FDE program you can load on a computer device to satisfy this requirement.

Availability / NOTAMS

[Aeronautical Information Manual, §1-1-18, ¶a.2.(a)] The status of GPS satellites is broadcast as part of the data message transmitted by the GPS satellites. GPS status information is also available by means of the U.S. Coast Guard navigation information service: (703) 313−5907, Internet: Additionally, satellite status is available through the Notice to Airmen (NOTAM) system.

NOTAMS are available here:

Wide Area Augmentation System (WAAS)

Satellite-Based Augmentation System (SBAS)

[AC 20-138D, ¶1-4.e.(2)(b)] The acronyms 'SBAS' and 'GBAS' are the respective international designations for satellite-based and ground-based augmentation systems complying with the International Civil Aviation Organization (ICAO) standards and recommended practices (SARPs). Several countries have implemented their own versions of 'SBAS' and 'GBAS' that have specific names and acronyms. For example, WAAS is the U.S. implementation of an 'SBAS' while EGNOS is the European implementation.

[ICAO Doc 8168 - Aircraft Operations - Vol I, chapter 2, ¶2.1.]

  • An SBAS augments core satellite constellations by providing ranging, integrity and correction information via geostationary satellites. The system comprises a network of ground reference stations that observe satellite signals, and master stations that process observed data and generate SBAS messages for uplink to the geostationary satellites, which broadcast the SBAS message to the users.
  • By providing extra ranging signals via geostationary satellites and enhanced integrity information for each navigation satellite, SBAS delivers a higher availability of service than the core satellite constellations.

These geostationary satellites are above and beyond the GPS constellation. Their positions are constantly update by reference to the ground stations and provide a high degree of accuracy.


The U.S. implementation of SBAS is WAAS. The U.S. system is compatible with the European (EGNOS) and Asia Pacific (MSAS) systems.

[AFAIS Performance-Based Navigation Presentation]

  • WAAS is GPS - Augmented GPS
  • WAAS Ground Stations
    • 35 Stations, including 4 in Alaska, 5 in Mexico, 4 in Canada
    • Monitor GPS satellites and send data to WAAS Master Station
  • WAAS Master Station
    • Computes GPS corrections.
    • Uplinks corrections to WAAS geostationary satellites (GEOs)
  • WAAS Geostationary Satellites (GEO)
    • 2 satellites to cover North America
    • Broadcast corrections on a standard GPS frequency — L1 (1575 MHz) This correction code qualifies as another satellite.
  • Accuracy
    • Lateral Accuracy - better than GPS — More like Localizer
    • Vertical Accuracy – much better than GPS — Good enough for Vertical Guidance (glideslope)
    • LPV minima — "Localizer Performance with Vertical Guidance"
    • GPS 95% Standard GPS Actual Performance
      Horizontal 36m 2.74m
      Vertical 77m 3.89m
      WAAS 95% Standard WAAS Actual Performance
      Horizontal 16m 1.08m
      Vertical 4m 1.26m
  • Performance Monitoring
    • WAAS has built in FDE
    • No longer need to check RAIM
    • WAAS avionics provide performance levels to pilots
WAAS Overview

[AC 90-107 ¶6.b.] WAAS improves the accuracy, integrity, availability and continuity of GPS signals. Additionally, the WAAS geostationary satellites provide ranging sources to supplement the GPS signals. If there are no airworthiness limitations on other installed navigation equipment, WAAS avionics enable aircraft navigation during all phases of flight from takeoff through vertically guided approaches and guided missed approaches. WAAS avionics with an appropriate airworthiness approval can enable aircraft to fly to the LPV, LP, LNAV/VNAV and LNAV lines of minima on RNAV (GPS) approaches. One of the major improvements WAAS provides is the ability to generate glide path guidance independent of ground equipment. Temperature and pressure extremes do not affect WAAS vertical guidance unlike when baro-VNAV is used to fly to LNAV/VNAV line of minima. However, like most other navigation services, the WAAS network has service volume limits, and some airports on the fringe of WAAS coverage may experience reduced availability of WAAS vertical guidance. When a pilot selects an approach procedure, WAAS avionics display the best level of service supported by the combination of the WAAS signal-in-space, the aircraft avionics, and the selected RNAV (GPS) instrument approach.

You've got to have WAAS installed to use it. Once you've got it, life gets better.

See: Localizer Performance with Vertical Guidance (LPV) Approach for more.

Letter to Eddie

Hi Eddie,

I'm a big fan of your site. It's very thoughtful, which I enjoy. I was reading your article on GPS/GNSS, and I think your explanation of the geometry is slightly off base--though I could be wrong.You describe the possible location of the receiver when in communication with a single satellite as being a sphere--the locus of points that is a fixed distance from a point. The problem is that we don't know what time it is. We know the time at which the satellite transmitted (sent time), but the receiver doesn't know what time it is when it gets the signal (receipt time). Could be a second, could be a year. So I think with only one satellite, you could really be anywhere in the universe.With two satellites whose clocks are synchronized, the receiver knows how much closer it is to one satellite than the other based on the difference between the receipt time and the sent time from each. That locus of points is a hyperbolid.Additional satellites allow the calculation of additional hyperbolid spaces, on the intersection of which the receiver must lie.Thoughts?



Thank you for the kind words.

I think you are right. The theory depends on the receiver having an atomic clock synchronized exactly with the satellite, hardly possible. It is just my way of demonstrating why you need more than one satellite. It sounds like your math is stronger than mine. Can I add your email to the page? I could just make the changes but I think adding your email illustrates the complexity of it all.


(Geoff kindly agreed.)

Book Notes

Portions of this page can be found in the book International Flight Operations, Part II, Chapter 7.


Advisory Circular 20-138D, Positioning and Navigation Systems, 5/8/12, U.S. Department of Transportation

Aeronautical Information Manual

FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001

FAA Order 8900.1

Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

ICAO Doc 8168 - Aircraft Operations - Vol I - Flight Procedures, Appendix to Chapter 3, Procedures for Air Navigation Services, International Civil Aviation Organization, Appendix, 23/11/06

ICAO Doc 8168 - Aircraft Operations - Vol I - Flight Procedures, Procedures for Air Navigation Services, International Civil Aviation Organization, 2006

ICAO Doc 9613 - Performance Based Navigation (PBN) Manual, International Civil Aviation Organization, 2008

US Air Force Advanced Instrument School (AFAIS) Performance-Based Navigation Presentation, Oct 2009

Determining Local GPS Satellite Geometry Effects On Position Accuracy

Broadly speaking, errors in the position a GPS receiver gives you are due to two factors: the precision with which the distance to each GPS satellite is known, and the geometry of the satellites, i.e. how closely or far apart they’re spaced across the sky. Distance errors can be compensated for by using WAAS, post-processing, averaging, and other techniques, but satellite geometry is a fundamental limiting factor. The maximum position accuracy you can achieve is limited by GPS satellite geometry.

The GPS satellite geometry factor is sometimes represented by a numerical measure known as “Dilution Of Precision”, or DOP. The higher the DOP, the greater the possible error in the accuracy of your position; roughly speaking, your total error is the error due to the uncertainty in satellite distance multiplied by the DOP. Professional-grade GPS receivers often come with software that will tell you what your current DOP is, tell you what it will be at some point in the future, and even prevent you from making a measurement when it’s too high. Consumer-grade GPS models don’t usually show you the DOP but instead show a general indicator of positional uncertainty, whose definition varies among different GPS unit manufacturers.

Two makers of high-end professional-grade GPS units, Magellan and Trimble, have made available free copies of software that can calculate GPS DOP as a function of date and time. The Magellan software is available on this website as part of the GPS Toolkit 0.11, which also installs several other programs. But I won’t talk about further, because IMO it’s not as good as Trimble’s Planning Software.

Download the Planning Software installation package from this page. You should also download a copy of the most recent Ephemeris file (almanac.alm) from the link on the same page (you may have to right-click on the link, and choose “Save Link As” or “Save Target As”, depending on your browser). The Ephemeris file contains the basic orbital parameters for all the GPS satellites, and these can change over time, albeit usually slowly. You will probably want to update this file every few weeks or so.

After installing the program, start up the Planning program, and get the following:

You can maximize this to fill your screen, of course. The first step is to load in and save the most recent Ephemeris file (almanac.alm). Choose Almanac => Load, and then go to the directory with the copy of the Ephemeris file you downloaded above to load it into the program. Once loaded, choose Almanac => Save to save that most recent almanac in the program directory, so that it will be loaded in automatically the next time you start up the program.

At the top are three checkboxes. “GPS” is for the US GPS satellite system, and should always be checked. “Glonass” is the Russian GPS system, which is currently being revived, but unless you have a receiver that is Glonass-capable (unlikely), you should uncheck that box. “WAAS” is for the Wide-Area Augmentation System satellites that broadcast correction signals that most modern GPS units can use to improve accuracy, and you can check that if you like. The position of these satellites doesn’t change in the sky, and they’re not used in calculating DOP, so this is optional.

Next, you need to set the location at which you plan to use your GPS. File => Station opens up the following window:

Here you can enter and save locations for where you will be making your observations from. Under Station Name, the drop-down menu will let you choose from previously-saved station locations. To create a station that is saved in this menu, you have three choices:

1. Type in the station name into the box at the top, enter the latitude, longitude, height above sea level (meters), and the elevation cutoff (the altitude below which a satellite is considered likely to be blocked by terrain). Once done, click on Apply, and the station will be saved and added to the menu.

2. Click on the Map button, and a map of the world will come up. As you move the cursor across the map, names of cities in a database will pop up. When you have the desired city selected, double-click on it, and its position will be entered in the Station Editor; click on Apply to save it in the list.

3. You can also select a city by clicking on the City button and choosing it from the menu, then clicking Apply to save it in the list.

The station position doesn’t have to be terribly accurate; being off by 15 miles will only change the altitude of a GPS satellite by about a quarter of a degree or so.

In the area below the position, you can set the date and starting time for when you plan to use your GPS to measure positions, as well as the duration and time resolution of the calculations. The “Today” button conveniently enters the current date into these boxes. The program doesn’t change this day as the actual date changes, so you’ll have to make sure it’s set correctly every time. And you should also set your “Time Zone” correctly using the button; set it to the Standard Time Zone for your area, and check the Daylight Savings Time box if your area uses DST.

Elevation Cutoff will exclude from the calculations all satellites below that altitude in the sky; the idea here is that you’re compensating for blockage of those satellites by local obstacles and terrain. There’s a related button called Obstacles, but I’ll get to that later …

Once you’ve setup your station position, and entered the correct date and times, click on OK. You can now plot satellite positions and calculate the Dilution Of Precision (DOP) for your selected location and date.

The GPS satellites are in a roughly 12-hour orbit around the earth; to see what areas the orbits will cover during the day, choose Graphs => World Projection (click on the image to enlarge it):

For the satellites visible from your area, start with Graphs => Elevation:

This shows the altitude of every GPS satellite in the sky on the date you’ve selected, as a function of time of day (military time). For Graphs => Number Of Satellites:

You get a bar graph showing how many GPS satellites are visible from your location, assuming that you have an unblocked view of the sky. Graphs => Sky Plot:

This shows the trajectory across the sky of every GPS satellite during the day.

All four of the above plots are a bit busy to be of use. But Graphs => Visible Satellites => GPS combines a lot of this data into a single plot:

Each colored horizontal bar represents when a specific satellite is visible from your position. The satellite will be at its highest in the sky for a particular appearance at the time that corresponds to the center of the bar. To find out how many satellites are in the sky for a specific time, just count the number of colored bars above the time on the bottom axis of the graph.

But what you really need is the DOP, which takes into account both the number of satellites, and how widely they’re spread across the sky. There are actually 5 different types of DOP: Geometrical (including motion and 3D position), Position (3D position for a stationary observer), Horizontal (the 2D position, not including altitude), Vertical (altitude only), and Time. You can plot all of these separately using the Graphs => DOP => All Together:

All of the DOPs move up and down together with each other. But usually you’ll be the most interested in the Positional or Horizontal DOP, since you’re usually determining a static position, and latitude/longitude is a 2-dimensional horizontal parameter:

From this plot, you can see that there are several times during the day when the DOP is low, so you can achieve a lower uncertainty in your positional error. But there’s also a spike in DOP around 6 PM, so if you wanted to minimize potential position errors, you could avoid making a measurement at that time.

In addition to the graphic output, Planning Software can create text tables of satellite and DOP data, which appear in a helper application called HyperPage. HyperPage lets you print the list, or save it as an HTML file. The three options from the Lists menu are:

Intervals – A list in order of how many satellites are in the sky at one time, showing the number of satellites, the time period during which that number of satellites are in the sky, the Positional DOP during that period, and the actual satellite IDs. I find this of minimal practical use.

Elevation/Azimuth – Lists the actual position in the sky of the GPS satellites during the course of the day. Also of minimal practical use.

DOP Values – The most useful table. For every time of day, it gives you all five DOPs, plus the number of satellites in the sky. So you could look down this table, find the current time, and determine whether the DOP was low enough for you to get a high-quality measurement. Here’s an abridged sample of data from such a table:

00:00 2.51 1.28 2.16 1.23 1.78 8 0 10
00:10 3.25 1.74 2.74 1.37 2.38 7 0 9
00:20 3.30 1.77 2.78 1.37 2.43 7 0 9
00:30 3.20 1.71 2.70 1.32 2.36 7 0 9
00:40 3.01 1.59 2.55 1.26 2.22 7 0 9
00:50 2.21 1.09 1.92 1.07 1.59 8 0 10
01:00 2.25 1.11 1.95 1.05 1.65 8 0 10
01:10 2.26 1.11 1.97 1.02 1.68 8 0 10
01:20 1.72 0.75 1.54 0.84 1.30 9 0 11
01:30 1.75 0.76 1.58 0.82 1.35 9 0 11
01:40 1.76 0.75 1.60 0.81 1.38 9 0 11
01:50 2.06 0.91 1.85 0.87 1.64 8 0 10
02:00 1.81 0.75 1.65 0.82 1.43 9 0 11
02:10 1.79 0.73 1.64 0.82 1.41 9 0 11
02:20 3.22 1.53 2.83 1.25 2.54 8 0 10
02:30 2.95 1.40 2.60 1.22 2.30 8 0 10
02:40 2.09 0.91 1.89 0.99 1.61 9 0 11
02:50 1.73 0.71 1.58 0.87 1.32 8 0 10
03:00 1.70 0.70 1.55 0.88 1.28 8 0 10
03:10 2.01 0.87 1.81 1.02 1.50 8 0 10
03:20 2.32 1.06 2.07 1.15 1.71 7 0 9
03:30 2.33 1.07 2.07 1.17 1.70 7 0 9
03:40 2.30 1.06 2.04 1.18 1.67 7 0 9
03:50 2.25 1.04 1.99 1.17 1.61 7 0 9
04:00 1.85 0.79 1.68 1.11 1.25 8 0 10
04:10 1.72 0.76 1.55 0.97 1.21 8 0 10
04:20 1.80 0.81 1.61 0.96 1.29 8 0 10
04:30 1.89 0.86 1.68 0.97 1.37 8 0 10
04:40 1.98 0.92 1.76 0.98 1.46 8 0 10
04:50 1.82 0.81 1.63 0.93 1.34 9 0 11
05:00 1.61 0.70 1.45 0.87 1.16 10 0 12
05:10 1.63 0.71 1.46 0.87 1.17 10 0 12
05:20 1.63 0.72 1.46 0.88 1.16 10 0 12
05:30 2.17 1.11 1.87 1.05 1.55 9 0 11
05:40 1.85 0.92 1.60 0.97 1.27 10 0 12
05:50 1.78 0.88 1.55 0.95 1.22 10 0 12
06:00 1.71 0.83 1.50 0.93 1.17 10 0 12
06:10 1.65 0.79 1.45 0.92 1.12 10 0 12
06:20 2.98 1.57 2.53 1.44 2.08 9 0 11
06:30 1.90 0.91 1.66 1.08 1.27 10 0 12
06:40 1.94 0.93 1.70 1.08 1.32 10 0 12
06:50 1.94 0.93 1.71 1.05 1.34 10 0 12
07:00 1.91 0.94 1.67 0.93 1.38 10 0 12
07:10 1.88 0.90 1.65 0.91 1.38 11 0 13
07:20 1.63 0.76 1.45 0.83 1.19 12 0 14
07:30 1.74 0.82 1.54 0.82 1.30 12 0 14
07:40 1.71 0.80 1.51 0.79 1.29 13 0 15

… and so on.

There is, of course, a problem with this data; it doesn’t completely take into account the fact that your view of some of the satellites might be obstructed, either by small but nearby obstacles like houses, trees, or rocks, or bigger obstacles like the local terrain. As more and more satellites are blocked from view, the DOP will usually increase. Setting some angular value for the elevation cutoff can partially account for these kind of obstacles, but that’s a very coarse and inaccurate solution in most cases.

There’s not much you can do to account for nearby obstacles, but in a post tomorrow, I’ll show a way to account for terrain blockage of GPS satellite signals on the DOP.

How does GPS satellite navigation work?


by Chris Woodford. Last updated: November 1, 2017.

Let's Get Lost is the title of a 1940s jazz song, famously recorded by singer and trumpeter Chet Baker. Back then, getting lost was not just a romantic idea but still a realistic one. Today, it's almost impossible to get lost, no matter how hard you try. Whether you're haring down the freeway or scrabbling up Mount Everest, you're always in sight of satellites spinning through space that can tell you exactly where you are. Walking round with a smartphone in your pocket, you'll have ready access to a GPS (Global Positioning System) receiver that can pinpoint your position, on a good day, to just a few meters. Take a wrong turn in your car, and a determined voice—also powered by GPS—will insist you "Take the next left," "Turn right," or "Go straight ahead" until you're confidently back on track. Even riding on a bus or train, it's barely possible to get off in the wrong place. Handy display boards scroll the name of the stop you want long before you need to rise from your seat. Apart from helping us reach our destination, satellite navigation can do all kinds of other things, from tracking parcels and growing crops to finding lost children and guiding the blind. But how exactly does it work? Let's take a closer look!

Photo: Getting lost is a thing of the past thanks to mobile devices like this with built-in GPS receivers and mapping apps.

What is satellite navigation?

Satellite navigation ("satnav") means using a portable radio receiver to pick up speed-of-light signals from orbiting satellites (sometimes technically referred to as space vehicles or SVs) so you can figure out your position, speed, and local time. It's generally much more accurate than other forms of navigation, which have to contend with pesky problems like accurate timekeeping and bad weather. Because it's a broadcast system based on radio signals that reach all parts of our planet, any number of people can use it at once, anywhere they happen to be.

The best-known satnav system, the Navstar Global Positioning System (GPS), uses about 24 active satellites (including backups). Day and night, 365 days a year, they whiz round Earth once every 12 hours on orbital planes inclined at 55 degrees to the equator. Wherever you are, you're usually in sight of at least half a dozen of them, but you need signals from only three or four to determine your position to an accuracy of just a few meters.

GPS was kick-started by the US military in 1973 and its satellites are designed to last about 7.5 years, but the latest generation typically survive about 10–12 years. In total, around 60 Navstar satellites have been launched altogether, in several separate groups called blocks, though many of them have now retired; at the time of writing, the last Navstar launch was satellite IIF-9 on March 25, 2015.

Photo: A NAVSTAR GPS satellite pictured during construction on Earth in 1981. You can get an idea how big the satellite is from the engineer pictured some distance beneath it. Picture courtesy of US Department of Defense.

GPS has three major components, technically known as "segments": there's one part in space, one part on the ground, and one part in your pocket. The 24 satellites form what's known as the "space segment" of GPS, but the system also relies on an intricate ground-control network of antennas, monitors, and control stations (the "control segment"), centered on a Master Control Station (MCS) at Shriever Air Force Base in Colorado, USA (with a backup at Vandenberg Air Force Base in California). Apart from the space and control segments, the other essential part of satellite navigation is the "user segment"—an electronic receiver you hold in your hand or carry in your vehicle.


Finding your position using satellite signals is a hi-tech version of an age-old navigator's trick that goes by the name triangulation. Suppose you're walking through the woods, on completely flat ground, but you don't know where you are. If you can see a landmark through the trees (maybe a distant hill), and you can guess how far away it is, you can look at a map and figure out that you must be somewhere on a circle whose radius (distance from the hill) is the distance you've guessed. One landmark alone can't narrow your position any more than this. But what if you suddenly see a second landmark in another direction. Now you can repeat the process: you must be a certain distance from that object too, somewhere on a second circle. Put these two bits of information together and you know you must be somewhere where the two circles meet—one of either two places on the ground. With a third landmark, you can narrow your position to a single point. And that's the essence of simple triangulation (you'll find a longer introduction at Compass Dude). Triangulation works with line-of-sight and a bit of guesswork, with a compass and a map, and with fancier methods like radio signals, and radar. And it also works, in a more sophisticated way, using space satellites.

Photo: Ferdinand Magellan (1480–1521) sailed the globe with great skill and ingenuity, proving that the "flat Earth" was, in fact, more or less spherical. It's tempting to imagine how much easier Magellan's life would have been with satellite navigation, but that gets the logic of things the wrong way round. Without Magellan's insight, we wouldn't have satellite navigation technology at all: to build it and get it working, we had to know that we lived on a round Earth to begin with! Public domain engraving courtesy of US Library of Congress.


With satellite navigation, your navigational "landmarks" are space satellites whizzing through the sky above your head. Because they're about 20,000km (12,600 miles) away, well beyond Earth's atmosphere, and because they're constantly moving (not stationary, like Earth-bound landmarks), finding your position from them is a bit more tricky. If you pick up a signal from one satellite and you know it's 20,000km away, you must be somewhere on a sphere (not a circle) of radius 20,000km, centered on that satellite. With two signals, from two different satellites, you must be somewhere where two spheres meet (somewhere in a circle of overlap). Three signals puts you at one of two points on that circle—and that's usually enough to figure out where you are, because one of the points might be up in the air or in the middle of the ocean. But with four signals, you know your position precisely. Finding your location this way is called trilateration.

How GPS works

Photo: An artist's impression of the 24 NAVSTAR satellites in orbit around Earth. Picture courtesy of US Department of Defense.

Satellite navigation systems all work in broadly the same way. There are three parts: the network of satellites, a control station somewhere on Earth that manages the satellites, and the receiving device you carry with you.

Each satellite is constantly beaming out a radio-wave signal toward Earth. The receiver "listens out" for these signals and, if it can pick up signals from three or four different satellites, it can figure out your precise location (including your altitude).

How does that work? The satellites stay in known positions and the signals travel at the speed of light. Each signal includes information about the satellite it came from and a time-stamp that says when it left the satellite. Since the signals are radio waves, they must travel at the speed of light. By noting when each signal arrives, the receiver can figure out how long it took to travel and how far it has come—in other words, how far it is from the sending satellite. With three or four signals, the receiver can figure out exactly where it is on Earth.

Where in the world are you?

  1. If your satellite receiver picks up a signal from the yellow satellite, you must be somewhere on the yellow sphere.
  2. If you're also picking up signals from the blue and red satellites, you must be at the black dot where the signals from the three satellites meet.
  3. You need a signal from a minimum of three satellites to fix your position this way (and four satellites if you want to find your altitude as well). Since there are many more GPS satellites, there's more chance you'll be able to locate yourself wherever on Earth you happen to be.

How do satnavs calculate distance from time?

Suppose you're carrying a GPS-enabled cellphone or satnav in your car. How does it know the exact distance to the three or four satellites it uses to compute your position? Every satellite constantly beams out signals that are, in effect, time-stamped records of its position at that time. Since they're carried by radio waves, the signals must be traveling at the speed of light (300,000km or 186,000 miles per second). Theoretically, then, if a receiver picks up the signals some time later, and has a clock of its own, it knows how long the signals have taken to get from the satellite, and how far they've traveled (because distance = speed × time). That sounds like a nice, simple solution, but it introduces two further problems.

First, how long does the signal take to travel? Haven't we just swapped one problem for another (time for distance)? The solution to this involves a hi-tech version of "synchronizing watches": each satellite carries four extremely precise atomic clocks (two cesium and two rubidium, typically accurate to something like one second in 100,000 years), while the receivers (which have less accurate clocks of their own) receive their signals and compensate for the time it takes for them to travel down from space. That means each receiver can figure out how long each signal has taken to reach it and therefore how far it's traveled.

Second, although radio waves do indeed travel at the speed of light, they only do so in a vacuum (in completely empty space). Radio signals beaming down to us from space satellites aren't traveling through empty space but through Earth's atmosphere, including the ionosphere (the upper region of Earth's atmosphere, containing charged particles, which help radio waves to travel) and the troposphere (the turbulent, uncharged region of the atmosphere, where weather happens, which extends about 50km or 30 miles above Earth's surface). The ionosphere and troposphere distort and delay satellite signals in quite complex ways, for quite different reasons that we won't go into here, and GPS receivers have to compensate to ensure they can make accurate measurements of distance.

Are military and civilian GPS any different?

Photo: Satellite-guided missiles and drones use the military-grade PPS version of GPS, which is theoretically more accurate than civilian GPS. Photo by Nicholas Messina courtesy of US Navy.

GPS was originally conceived as a military invention that would give US forces an advantage over other nations, but its inventors soon realized the system would be just as useful to civilians. The only trouble was, if civilians (or rival forces) could pick up the same signals, where would that leave their military advantage? For that reason, they developed two different "flavors" of GPS: a highly accurate military-grade, known as Precise Positioning Service (PPS), and a somewhat degraded civilian version called Standard Positioning Service (SPS). While PPS-enabled receivers could originally locate things to an accuracy of about 22m meters (72ft), SPS receivers were deliberately made about five times less accurate (to within the length of a football field, or about 100m) using a tweak called Selective Availability (SA). That was switched off by order of US President Bill Clinton in May 2000, greatly improving accuracy for civilian users, which is largely why GPS has taken off so readily ever since. Even civilian SPS receivers are now officially accurate to within "13 meters (95 percent) horizontally and 22 meters (95 percent) vertically", though a variety of different errors (caused by the atmosphere, obstructions blocking line of sight to satellites, signal reflections, atmospheric delays, and so on) can compound to make them very much less accurate at times.

Theoretically, military and civilian GPS could be as accurate as one another if we didn't have to worry about them traveling through Earth's atmosphere. According to the official website "The accuracy of the GPS signal in space is actually the same for both the civilian GPS service (SPS) and the military GPS service (PPS)." In practice, while SPS signals are broadcast using only one frequency, PPS uses two. Comparing the two frequencies allows military grade GPS receivers to calculate precise corrections for radio delays and distortions caused by transmission through the atmosphere, and that still gives military GPS an edge over civilian systems. In time, civilian GPS will become increasingly accurate, especially as more satellites (and more different satellite systems) are added, but it's likely that military systems will always have an advantage, for one reason or another.

GPS satellite signals

Navstar satellites constantly broadcast the two different flavors of GPS, PPS and SPS, on two different radio frequencies (carrier waves) known as L1 (1575.42MHz) and L2 (1227.6MHz). L1 carries the civilian SPS code signal (also known as the C/A code or Coarse Acquisition code), which is relatively short and broadcast about 1000 times a second, and what's known as the navigation data message, which includes the date and time, satellite orbit details, and other essential data. L2 carries the military PPS code, also known as P-code (Precision code), which is very long and precise and takes an entire week to transmit. It's encrypted to form what's known as the Y-code, partly so that only authorized users can access it, and partly (because encryption is a form of signing things to confirm they're authentic) to help prevent things like "spoofing" (where third parties broadcast fake, disruptive signals purporting to be from GPS satellites). Military-grade GPS receivers pick up both frequencies, and compare them to correct for the effects of the ionosphere. Civilian receivers pick up only one frequency and have to use mathematical models to correct for the ionosphere instead.

Applications of satellite navigation

Most of us use satellite navigation for driving to places we've never been before—but that's a relatively trivial application. Once you can pinpoint your precise position on Earth, much more interesting things become possible. Roll time forward a few decades to the point where all cars have onboard satnav and can drive themselves automatically. Theoretically, if a car knows where it is at all times, and can transmit that information to some sort of centralized monitoring system, we could solve problems like urban congestion, finding parking places, and even auto theft at a stroke. If every car knows its location, and knows where nearby cars are too, highway driving could become both faster and safer; it will no longer rely on the vigilance of error-prone human drivers, too easily confused by tiredness and bad weather, so cars will be able to travel at much higher densities. The same goes for airplanes, where GPS is finally set to become an integral part of air traffic control—gradually reducing our historic overdependence on radar—over the next decade.

Photo: Many tractors, combine harvesters, and crop dusters are now equipped with GPS.

And it's not just cars and planes that will benefit from pinpoint precision. For emergency services and search and rescue workers, navigating to remote, sometimes uncharted locations, in a hurry, makes all the difference between life and death. Farmers have been using GPS systems in tractors, combines, and crop-dusters to map, plant, manage, and harvest their crops with efficiency and precision. According to an industry body called the GPS Alliance, high-precision satellite navigation boosted US crop yields by almost $20 billion from 2007 to 2010 and is now used in 95 percent of crop dusting. Meanwhile, farm animals, pets, and rare wildlife are easier than ever to track using GPS-enabled collars and backpacks. Blind people, traditionally guided by seeing-eye dogs or the elbows of friends and family, can finally gain true independence equipped with talking handheld GPS systems, such as Trekker Breeze, that can announce street names or read spoken directions from A to B. Needless to say, a system conceived by the military still enjoys many military applications, from guiding so-called "smart bombs" to their targets with pinpoint accuracy to helping troops navigate through unfamiliar terrain. GPS is as standard a part of modern military equipment as maps and compasses were 100 years ago.

Rival satellite navigation systems

In the United States, GPS is universally used as a synonym for any and every kind of satellite navigation; in other countries, such as the UK, "satnav" is a more familiar generic term. In fact, GPS is only one of several global satnav systems. The Soviet Union launched a rival system called GLONASS in 1982 (also using 24 satellites) and Russia continues to operate it today. Europe has been slowly building its own, more accurate 30-satellite system called Galileo, which is expected to be completed around 2020, and China is developing a global system known as Compass. The preferred umbrella term for world-spanning satnav systems is GNSS (Global Navigation Satellite Systems). Apart from the four big global systems, there are also a few smaller regional rivals, including China's BeiDou and India's IRNSS.

Although a given satellite receiver is typically designed to use only one of the global systems, there's no reason why it can't use signals from two or more at once. Theoretically, combining signals from GPS, GLONASS, and Galileo could give satnav devices something like a 10-fold increase in precision, especially in urban areas where tall buildings can block or distort signals, reducing the accuracy of any one system used alone. Using multiple systems also promises to make satellite navigation much faster: if more satellites are "in view," the so-called Time-to-First-Fix (TTFF)—the initial delay before your satnav locks onto satellites, downloads the data it needs, and is ready to start calculating your position—is reduced. Since TTFF typically varies from about 30 seconds to several minutes, it makes a big difference to casual GPS users (and is one of the first features people compare when they look at buying a new satnav receiver).

Challenges and issues

Knowing the absolute position of anything, anytime, anywhere brings obvious benefits in a globalized world that relies on swift, safe, and reliable transportation. But it raises issues too. If civilian transportation systems are designed to rely on satellite systems provided by the US or Russian military, doesn't that make us too vulnerable to the sudden twists of international politics, especially in times of war? Although the US military no longer routinely degrades the quality of GPS signals, and announced in September 2007 that it would be removing Selective Availability altogether from future versions of GPS satellites, currently it can still nobble the system anytime it pleases. Could a future world of driverless cars, hyper-efficient parcel shipping, and automated air-traffic control be plunged into chaos purely at the whim of the superpowers? The European Galileo project is entirely a civilian system, which should eliminate possible military interference in time. But for the moment, it remains a concern.

Fast-disappearing privacy is the flipside of the same coin. If your car and your cellphone are both equipped with satnav, and you're always using one or the other (or both), your movements can be tracked at all times. That raises obvious privacy issues, especially in repressive states. But every new technology brings its pros and cons, from internal combustion engines to submachine guns, and nuclear power plants to antibiotics. Progress involves making a tradeoff between benefits and costs, in the hope of doing things better than we ever could before. Satellite navigation is no different, swapping safe and unreliable navigation for efficient and effective transportation, albeit at a cost in privacy and (for the time being) continued dependence on military infrastructure.

Find out more

On other sites


  • Superaccurate GPS Coming to Smartphones in 2018 by Samuel K. Moore. IEEE Spectrum, October 23, 2017. The next generation of smartphone GPS will be accurate to about 30cm (1ft).
  • Protecting GPS From Spoofers Is Critical to the Future of Navigation by Mark L. Psiaki and Todd E. Humphreys. IEEE Spectrum, July 29, 2016. It's difficult but not impossible to spoof GPS signals, potentially sending ships and boats on course to disaster. What technical methods are there to protect against spoofers?
  • What Stand-Alone GPS Devices Do That Smartphones Can't by Eric A. Taub. The New York Times, July 15, 2015. There are still good reasons to own a standalone GPS device, although now smartphones have bigger screens the gap between the two kinds of devices is closing fast.
  • Russian Global Navigation System, GLONASS, Falling Short by James Oberg, IEEE Spectrum, February 1, 2008. Can Russia's alternative ever hope to compete with GPS?
  • End of Flight Delays? FAA's GPS Fix Could Bust Sky Gridlock by Barbara S. Peterson, Popular Mechanics, July 19, 2007. An introduction to the use of GPS in air traffic control.


  • The Global Positioning System: A Shared National Asset by Aeronautics and Space Engineering Board, National Research Council. National Academies Press, 1995. A technical report evaluating the success of GPS and making recommendations for its future development as a joint civilian and military system.

Technical references


  • US Patent 5,663,734: GPS receiver and method for processing GPS signals by Norman F. Krasner, Precision Tracking, Inc. September 2, 1997. A detailed technical description of how a typical GPS receiver works.
  • US Patent 5,841,396: GPS receiver utilizing a communication link by Norman F. Krasner, Snaptrack, Inc. Another of Krasner's patents, covering assisted GPS.
  • US Patent 5,841,396: Locating a mobile station using a plurality of wireless networks and applications therefor by Charles L. Karr, Tracbeam LLC. Oct 4, 2005. Another patent describing "assisted GPS" that combines GPS and wireless networks.

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Woodford, Chris. (2007/2015) Satellite navigation. Retrieved from [Accessed (Insert date here)]

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