Figure: GPS Constellation, from AFAIS Performance-Based Navigation Presentation.
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.
What about an approach that references GNSS and not GPS? Well, it depends. If your airplane lists it as a viable approach, if the country is WGS-84 compliant, and if the country's rules allow you to, then probably. It is a confusing world out there.
What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.
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:
[FAA Instrument Handbook, pg. 7-21]
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. . .
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:
[AFAIS Performance-Based Navigation Presentation] Coarse Acquisition (C/A) code is available to all users without limitations and includes
[AFAIS Performance-Based Navigation Presentation] P-Code provides navigation/targeting data for U.S. government users with an encryption key
[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 . . .
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.
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.
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).
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. . .
Figure: GPS Position errors, from AFAIS Performance-Based Navigation Presentation.
[AFAIS Performance-Based Navigation Presentation] Errors are possible due to:
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 . . .
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.
Some receivers are capable of greater accuracy than others, but the issue isn't as extreme as some would have you believe . . .
Figure: USS Princeton launches a Harpoon missile, from US Navy (Public Domain)
[AFAIS Performance-Based Navigation Presentation]
[AFAIS Performance-Based Navigation Presentation]
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.
The signal coverage is supposed to be worldwide, but the satellites do not cover the world. How can that be?
Figure: GPS Ground Tracks, from http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html.
[http://www.colorado.edu/geography/gcraft/notes/gps/gps_f.html] 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 http://science.nasa.gov/iSat/iSAT-text-only/.
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:
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:
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.
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: Procedures & Techniques / RNAV (GNSS) Example for a walk through of the decision making needed.
[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.
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.
[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.
[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:
[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 International Operations / WGS-84 for more about this.
[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.
[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.
[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.
[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.
Photo: Predictive RAIM, from Eddie's aircraft.
[G450 Aircraft Operating Manual §2B-17 ¶1.]
[G450 Aircraft Operating Manual §2B-17-30]
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 Systems / FMS: Check RAIM.
[FAA Order 8900.1, Vol. 4, Ch. 1, §4, ¶4-78.C.]
This can be confusing so let's break it into a few pieces:
[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: http://www.navcen.uscg.gov/?Do=constellationStatus. Additionally, satellite status is available through the Notice to Airmen (NOTAM) system.
NOTAMS are available here: https://pilotweb.nas.faa.gov/PilotWeb/.
Figure: GPS CDI and RAIM Scaling, from AFAIS Performance-Based Navigation Presentation.
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.]
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]
|GPS 95% Standard||GPS Actual Performance|
|WAAS 95% Standard||WAAS Actual Performance|
[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: Procedures & Techniques / Localizer Performance with Vertical Guidance (LPV) Approach for more.
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.)
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
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