the learning never stops!

Global Navigation Satellite System (GNSS)
Global Positioning Satellite (GPS) System


A lot of rules for using GPS have changed over the years and what you can and cannot do also depends on where in the world you are.

For example, can you fly the "VOR Rwy 23" to Bedford, Massachusetts using GPS? No, the U.S. overlay program that said you could a few years ago was in Advisory Circular 90-94 which was cancelled in 2009. In the U.S., the approach has to have the term "GPS" in the title. What about the "VOR DME Rwy 31" to Sibu, Malaysia? Yes, they are WGS-84 compliant and ICAO rules say you can. In the case of the U.S. approach, it would be a good idea to have the GPS up for position awareness and in Malaysia you should probably have the VOR up for back up.



Figure: GPS Constellation, from AFAIS Performance-Based Navigation Presentation.

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


Figure: GPS Position errors, from AFAIS Performance-Based Navigation Presentation.

[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


Figure: GPS Ground Stations, from AFAIS Performance-Based Navigation Presentation.

[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.


Figure: GPS Satellite Tracks, from

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 = 10998 cos 55 - 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)


Figure: GPS CDI and RAIM Scaling, from AFAIS Performance-Based Navigation Presentation.

Satellite-Based Augmentation System (SBAS)


Figure: SBASs, from AFAIS Performance-Based Navigation Presentation.

[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.



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

Revision: 20141102