CNS/ATM resource kit – Chapter 3: Global navigation satellite systems (GNSS)

Australians have been early adopters of satellite navigation, using it for many activities and applications including aviation. The global nature of the technology is suited to Australia’s large land mass and low population density.

Many charter operators operating under visual flight rules (VFR), especially in remote areas, use panel-mounted or portable GNSS units.

Satellite-guided tracking and approach guidance are commonplace in instrument flight rules (IFR) operations.

This chapter will look at how GNSS works, what augmentation systems are in place and how to use these systems in aircraft operations.

It’s important to remember that the use of GNSS is no substitute for thorough flight planning—you can read up on this in Chapter 7.

How does GNSS work?

GNSS units receive signals generated from constellations of satellites, and have been guiding Australian pilots for more than two decades. The first approvals for GNSS approaches were based on technical standard order (TSO) C129 in the 1990s.

GNSS satellites orbit the Earth in several included planes. The orbital planes, and the spacing of the satellites within them, are optimised to provide a wide coverage of the globe. Signals from at least four satellites are required to determine a position, one for each of the three spatial dimensions, and one for accurate time. Australia has particularly good coverage: GNSS receivers can normally ‘see’ more than eight satellites at any given time.

The satellites complete one revolution, from west to east when viewed from the Earth, about twice a day. On-board thrusters are used to correct wobbles in their orbits caused by the gravitational pull of the Sun and Moon, variation in the Earth’s gravitational field and the pounding of solar radiation.

Timing is everything in GNSS, and each satellite has up to four atomic clocks with accuracies measured in the order of thousandths of millionths of a second. Master control stations and monitoring stations around the world, track and manage the satellites, relaying critical correctional data to them.

The GNSS signals are transmitted on multiple frequencies. For example, the US GPS transmits the civil signal on the L1 frequency (1,575.42 MHz), just above the distance measuring equipment (DME) band. Military and authorised users can get more accurate measurements on the encrypted ‘L2’ frequency (1127.60 MHz).

The L5 frequency (1176.45 MHz) band is reserved for aviation safety services. It features higher power, greater bandwidth and an advanced signal design which reduces errors caused by passage of the GPS signal through the ionosphere—a layer of charged particles up to 1000 km above the Earth’s surface.

GNSS constellations

There are four major GNSS constellations:

  • the USA’s NavStar Global Positioning System (GPS)
  • the Russian Federation’s GLObal NAvigation Satellite System (GLONASS)
  • the European Union’s Galileo GNSS
  • China’s BeiDou Navigation Satellite System.

Up to now ICAO standards have been published for GPS and GLONASS.

Each system comprises a constellation of orbiting satellites supported by ground stations and aircraft receivers. These orbiting systems need to be complemented or ‘augmented’ by additional systems to produce the performance required by certain operations.

Developments in satellite technology and its use for aircraft navigation suggest that new satellite navigation systems will evolve in the future, each with unique characteristics. The four major GNSS constellations are outlined in the following table.

This table contains global navigation satellite systems
GNSS Global Positioning System BeiDou Navigation Satellite System
Operational satellites 30+ 20+
Owner/operator United States Government system operated by the Department of Defense (DOD) China National Space Administration
Service The two levels of service provided are known as the standard positioning service (SPS) and the precise positioning service (PPS). SPS is available to all users and provides horizontal positioning accuracy of 36 metres or less, with a probability of 95 per cent. PPS is more accurate than SPS, but is available only to the US military and a limited number of other authorised users. The service aims to provide global coverage with positioning, navigation and timing services, including an open and authorised service. The open service provides free location, velocity and timing data, with positioning accuracy of 10 metres, velocity accuracy of 0.2 metres/second and timing accuracy of 10 nanoseconds. The authorised service provides a more secure position, velocity, timing, communications services and level of integrity.
GNSS Galileo GLONASS
Operational satellites 11+ 24+
Owner/operator European Union Russian Federation’s MOD and managed by the Russian Space Forces
Service The constellation is planned to have a bigger footprint than GPS, with 30 satellites. Galileo’s observed dual-frequency positioning accuracy is an average 8 metres horizontal and 9 metres vertical, 95 per cent of the time. GLONASS shares the same principles of data transmission and positioning methods that are used in GPS and is also based on a constellation of orbiting satellites and a ground control segment. At peak efficiency, the standard-precision signal offers horizontal positioning accuracy within 5–10 metres and vertical positioning within 15 metres.

Augmentation systems

Having a way of alerting users that GNSS is underperforming is critical to the safety of the system. GNSS avionics have software to protect integrity—the measure of trust in the information supplied by the total system.

Integrity includes the ability of a system to provide timely warnings to the user when the system cannot be used for the intended operation.

Aircraft based, satellite-based and ground-based augmentation systems can ensure integrity. A number of augmentation systems can be used to improve the navigational performance provided by the GNSS constellations.

ICAO recognised augmentation systems used in Australia

This table contains ICAO recognised augmentation systems used in Australia
Type of augmentation Benefits
Aircraft-based augmentation system (ABAS) Resolves integrity deficiencies.
Satellite-based augmentation system (SBAS) Provides third-party monitoring of GNSS ranging signals and broadcasts corrections over a wide area from a communications satellite, with a moderate improvement in accuracy.
Ground-based augmentation system (GBAS) Provides third-party monitoring of GNSS ranging signals and broadcasts corrections over a local area from a ground station. GBAS provides a large improvement in accuracy and clears the way for GNSS precision approach and landing.

ABAS

Aircraft-based augmentation systems (ABAS) use on-board equipment designed to overcome performance limitations of the GNSS constellations. Current ABAS stand-alone receivers are designed to resolve integrity deficiencies. Highly integrated systems may use other aids such as inertial navigation.

The two ABAS currently in use are receiver autonomous integrity monitoring (RAIM) and the aircraft autonomous integrity monitor (AAIM).

RAIM

RAIM ensures that:

  • an erroneous ranging signal from a satellite will not adversely affect the accurate navigation of the aircraft
  • the constellation geometry is good enough to provide an accurate position—that is, the satellites are spread evenly across the sky
  • if an error is detected within the constellation, pilots are notified that they cannot rely on GNSS for navigation.

RAIM calculates the worst error that might exist in the satellite that is most difficult to detect it in. GNSS avionics compare the navigation solutions from at least six satellites with the solution using all satellites except one. If there is a substantial difference between the two solutions, it is reasonable to assume an error in one satellite.

Upon detection of an error, some avionics can continue to operate by removing the erroneous satellite from the navigation solution—this is called fault detection and exclusion (FDE). However, if a second satellite is detected with a faulty ranging signal, the avionics will notify the pilot that GNSS cannot be relied upon for navigation. If the avionics cannot remove the satellite, it has fault detection (FD) only.

All TSO-C145, TSO-C146, and TSO-C196 GNSS receivers have FDE.

Some TSO-C129 GNSS receivers have FDE, while others have FD only.

The effect of constellation geometry depends on the phase of flight. As long as the horizontal protection level (HPL)—the measure of how good the geometry is—remains less than the required navigation performance (RNP) value for the phase of flight, the operation can continue. Some examples of RNP values for different phases of flight are:

  • en route: RNP 2 (2 NM)
  • terminal: RNP 1 (1 NM)
  • approach: RNP 0.3 (0.3 NM)
  • missed approach: RNP 1 (1 NM).

If GNSS avionics cannot provide a navigation solution with RAIM, they usually have two other modes of operation:

  • 2D or 3D navigation solution without RAIM
  • dead reckoning (DR), or loss of navigation solution.
Aircraft-based augmentation system positioning with six satellites to support fault detection and exclusion
Aircraft-based augmentation system positioning with six satellites to support fault detection and exclusion

Requirements for ground-based navaids with different types of GNSS receivers (ref: AIP-GEN 1.5)

This table contains requirements for ground-based navaids with different types of GNSS receivers
  C129 receiver Other requirements C145, C146 or C196 receiver* Other requirements
Night VFR 1 Nil 1 Nil
IFR airwork and private operations 1
  • ADF or VOR
  • If alternate required, there must be ground-based en-route navigation to it. Ground-based approach navaid with a suitable approach required unless the alternate is suitable for an approach in VMC.
1 Nil
IFR RPT and charter 1

1

2

1 ADF or VOR

Nil

* or a later version

RAIM outages

RAIM outages, or holes, are times when there are too few satellites with the appropriate spacing for integrity monitoring. This can be anticipated with RAIM predictions from Airservices Australia.

Do not use your receiver’s prediction for flight planning, as it lacks some of the data forming the basis of the Airservices prediction.

The figure below is an example of RAIM outages (holes) across Australia. The holes move in time and space, so you need a new prediction each time you fly.

Example of RAIM outage map of Australia
Example of RAIM outage map of Australia

AAIM

Aircraft autonomous integrity monitor (AAIM) uses the redundancy of position estimates from multiple sensors, including GNSS, to provide integrity performance that is at least equivalent to RAIM. AAIM uses inertial navigation solutions as an integrity check of the GPS solution when RAIM is unavailable, but GPS positioning information continues to be valid.

SBAS

Satellite-based augmentation systems (SBAS) support wide-area or regional augmentation by using additional satellite-broadcast messages—ranging, integrity and tracking signals.

Geostationary satellites about 40,000 km above the globe are in orbits timed with the Earth’s rotation. As the name suggests, they appear stationary with respect to a point on the ground. These geostationary satellites are owned and operated independently of the GNSS constellations.

Satellite-based augmentation system transmitting integrity
Satellite-based augmentation system transmitting integrity

The SBAS system comprises:

  • a network of ground reference stations that monitor satellite signals
  • master stations that collect and process reference station data and generate SBAS messages
  • uplink stations that send the messages to geostationary satellites
  • transponders on these satellites that broadcast the SBAS messages.

By providing differential corrections, extra ranging signals via geostationary satellites and integrity information for individual constellation satellites, SBAS delivers a much higher availability of service than the core satellite constellations with RAIM alone.

The GPS signal can also be checked at monitoring stations on the ground, with the resulting corrections and integrity data sent up to geostationary satellites for transmission down to aircraft receivers.

Operational SBAS and their launch dates include:

  • FAA’s wide area augmentation system (WAAS) in 2003
  • Japanese multi-function transport satellite (MTSAT) satellite-based augmentation system (MSAS) in 2007
  • European geostationary navigation overlay service (EGNOS) in 2009
  • Indian global positioning system (GPS) aided geostationary Earth orbit (GEO) augmented navigation (GAGAN) in 2016
  • Russian Federation’s System for Differential Corrections and Monitoring (SDCM) is under development
  • the Chinese Satellite Navigation Augmentation System (SNAS) is expected to be operational by 2020.

Other SBAS are being developed in South Korea and Africa.

GBAS

Ground-based augmentation systems (GBAS) provide GPS integrity monitoring through data obtained from the ground. They also boost the accuracy of satellite navigation, clearing the way for GNSS precision approach and landing.

An airport ground station transmits locally relevant corrections, integrity data and approach data to aircraft in the terminal area in the VHF band.

Ground-based augmentation
Ground-based augmentation

A system meeting ICAO’s GBAS requirements provides two services:

  • precision approach service
  • GBAS positioning service.

The precision approach service provides deviation guidance for GNSS landing system (GLS) approaches. A GBAS installation will typically provide GNSS corrections that support precision approaches to multiple runways at a single airport.

A GBAS positioning service could provide horizontal position, velocity and time information to support area navigation (RNAV) operations in terminal areas, though no such services are currently in use.

GBAS infrastructure includes electronic equipment which can be installed in any suitable airport building, and antennas for both the data broadcast and to receive the GNSS satellite signals. The cost and flexibility of GBAS has resulted in more runway ends having electronic precision approach guidance, resulting in significant safety and efficiency benefits.

GBAS can also provide multiple approaches to the same runway end with different touchdown points (during runway threshold repairs) and different glide path angles (reduce noise under the flight path).

See Chapter 9 for information about the use of GNSS in IFR operations, and Chapter 10 for its use in VFR and night VFR.

En route radio navigation

The AIP specifies that ‘aircraft must be navigated by the most precise means of track guidance with which the aircraft is equipped and the pilot is qualified to use. The order of precision is localiser (LLZ), GNSS, VOR, then NDB’. (ENR 3.3.4.3).

Ground-based systems

Navigation by radio aids includes navigation mainly by reference to indications of bearing and distance indicated on VHF omnidirectional ranges (VOR), distance measuring equipment (DME) and automatic direction finding (ADF) equipment located on the aircraft. This information is derived from ground radio beacons (VOR, DME and non-directional beacons [NDBs]) or broadcast stations in the AM band.

Radio navigation aids and systems can be used by pilots to:

  • determine aircraft position fix solely with reference to navigation aids and systems
  • intercept tracks to and from navigation aids and systems
  • maintain tracks within specified tolerances
  • record, assess and revise timings as required recognise station passage
  • undertake instrument approaches.

From ground- to satellite-based navigation

Airservices Australia has implemented the Navigation Rationalisation Project (NRP) in conjunction with CASA’s Civil Aviation Order (CAO) 20.18. This requires that all aircraft operating under IFR must be equipped with a TSO-C129, C145, C146 or C196 GNSS receiver.

The GNSS mandate of 4 February 2016 enabled Airservices Australia to implement the NRP. The project involved decommissioning about 180 ground-based aids, including NDBs, VORs and DMEs.

This means that GNSS is now the primary means of navigation for all IFR aircraft. The backup navigation network (BNN) serves as a contingency in the case of failure in the GNSS constellation or in the aircraft receiver. However, because of the limited number and wide geographical spacing of remaining navaids, the BNN alone may not be capable of sustaining navigation services to flight-planned destinations.

Finding position using GNSS

Getting a fix

GPS satellites broadcast two codes—the coarse/acquisition (C/A) code which is unique to the satellite and the navigation data message.

The codes contain information the receiver needs to determine latitude, longitude and altitude, and to synchronise its quartz clock with GPS time used through the GPS system. The information includes almanac data—the predicted orbital parameters of the satellites beamed up to each satellite from the ground stations—and the more accurate ‘ephemeris’ tracking data from each satellite.

The C/A code is transmitted in binary form—a series of zeros and ones—and is superimposed on the carrier wave through a method called phase modulation.

The GNSS receiver computes its distance from a satellite from the time it takes the signal from each satellite, travelling at 300,000 km/sec—the speed of light—to reach it. The computer deduces the value for time from the degree to which the pattern of zeros and ones in the C/A code is out of sync with the same pattern retrieved from its own memory and replayed at the same time.

The distance to the receiver is the product of velocity (300,000 km/sec), and time, and the unit’s computer plugs these values into equations, which it solves simultaneously to get the navigation solution.

GPS navigation solution
GPS navigation solution

The GPS unit displays the coordinates as latitude and longitude, or as bearing and distance information relative to a known point. Current approvals for the use of GPS equipment in IFR operations require GPS-derived data to be in the WGS-84 coordinate system, or worldwide geodetic datum standard 84.

A GPS unit display
A GPS unit display

Getting the timing right

The principles underlying GNSS are simple, but the system is very complex in practice. The main problem is timing errors.

The main source of error is the delay in the transmission as the signal passes through the ionosphere. The waves are slowed down as they pass through this electrical whirlpool of ions—atoms stripped of their outer electrons by solar radiation. The rate to which they are slowed down depends on the thickness of the ionosphere, which changes continuously and cannot be predicted by the avionics.

Water vapour in the atmosphere also slows the signal down. And sometimes the satellites’ atomic clocks go haywire, while the receivers’ quartz crystal clocks always carry significant uncertainties.

Multipath error, caused when obstacles near the GPS receiver reflect the radio waves, could throw the navigation solution out by as much as 10 metres.

Another error was, until 2000, deliberately introduced into the system. A legacy of the Cold War, selective availability (SA), which skewed the satellite clock and ephemeris data, was designed to prevent hostile forces using the publicly-available GPS system against the US.

This is important for aircraft fitted with C129 receivers, because these receivers assume SA is still switched on. This limits the availability, but not the accuracy, of GNSS. The newer C145/C146/C196 units, however, check to see if SA is off, or assume it is.

It takes data from four satellites—and four equations—to get position coordinates. The fourth satellite is needed to obtain the timing error, or user clock bias, in the receiver clock.

GPS measures time with an accuracy in the order of a few tens of thousandths of millionths of a second, and for this reason is used as a timepiece in fields ranging from telecommunications, through physics experiments, to electricity generation. The Australian radar systems rely on GPS for a precise readout of time, critical to integrating radar displays when tracking aircraft within multiple radar coverage.

GNSS performance: accuracy, availability, integrity and continuity

GNSS performance may be measured in a number of ways. While accuracy is the most obvious quality of a navigation system, other measures, such as system availability, data integrity and continuity of service, are also important.

How accurate is GPS?

It is impossible to put a single figure on the accuracy of GPS as it depends on several constantly changing factors, many of which affect the ionosphere—the largest single source of error. Common causes of reduced accuracy include:

  • Ephemeris software calculates the position of planets and their satellites, asteroids or comets. Although the satellite orbits are extremely stable and predictable, they can be disturbed by gravitational effects of the Earth and Moon, and the pressure of solar radiation. This can generate errors of up to 3 metres.
  • Clock (timing) errors due to inaccuracies in both the satellite and receiver clocks, as well as relativity effects, can result in position errors of up to 3 metres.
  • Receiver—pseudo-random noise codes are at a lower level than the receiver ambient noise, due to the low signal strength of GNSS transmissions. This results in a fuzzy correlation of receiver code to the satellite code, and produces some uncertainty in the relationship of one code to another. The position error that results from this effect is about 1.5 metres.
  • Ionosphere error is one of the most significant—up to 10 metres—with pseudo-range calculations from the passage of the satellite signal through the Earth’s ionosphere. It varies depending on the time of day, solar activity and a range of other factors. Ionospheric delays can be predicted and an average correction applied to the GPS position, although there will still be some errors introduced by this phenomenon.
  • Multipath produces an error in the pseudo-range measurement of up to 3 metres, resulting from the reflection and refraction of the satellite signal by objects such as buildings and terrain near the receiver. Distortion of FM radio signals is an example of multipath effect. Because GNSS is a three-dimensional navigation system, the errors do not all lie along a line and therefore should not be added arithmetically. Total system range error is calculated by the root-sum-square method, where the total is the square root of the sum of the squares of the individual errors.
  • Geometric dilution of precision (GDOP) is an effect that degrades the accuracy of a position fix because of the number and relative geometry of satellites in view at the time of calculation. The value given is the factor by which the system range errors are multiplied to give a total system error. Position dilution of precision (PDOP) is a subset of GDOP that affects latitude, longitude and altitude. Many GPS receivers are able to provide an estimate of PDOP, which can be up to 3 metres.

ICAO standards and recommended practices (SARPS) specify the accuracy requirements for various phases of flight. Current technology can use the GNSS constellations to meet the IFR accuracy requirements for oceanic and domestic en route use, as well as for terminal area and non-precision (dive) approaches.

Precision (glide) approaches require some form of GNSS augmentation to overcome the known limitations of the constellation systems.

Availability

Availability is defined as the percentage of time the services of a navigation system are accessible. It’s a function of both the physical characteristics of the environment and the technical capabilities of the transmitter facilities.

GNSS availability is the system’s capacity to provide the number of satellites required for position fixing within the specified coverage area. Theoretically, at least three satellites need to be in view to determine a two-dimensional (2D) position. In practice, four are required to establish an accurate three-dimensional (3D) position.

As mentioned on page 35, selective availability (SA) was, until 2000, used by the US Department of Defense to limit the accuracy of GPS to other than approved users. It artificially created a significant clock or ephemeris error. Many early GPS receivers were ‘hard-wired’ for SA in the expectation that civil use would need to assume that SA was active.

For receivers that cannot take advantage of SA being discontinued, average receiver autonomous integrity monitoring (RAIM)—fault detection (FD)—availability is 99.7 per cent for non-precision approach operations for a 24-satellite GPS constellation.

By contrast, receivers that can take advantage of SA having been discontinued have 99.99 per cent availability of RAIM (FD) for non-precision approaches. These percentages will vary depending on which satellites are out of service at any given time. Currently, the US maintains the constellation in a 27-satellite configuration, further improving availability.

Integrity

Integrity is the ability of a system to provide timely warnings to the user when the equipment is unreliable for navigation purposes. The concept of integrity includes both a failure to alarm and a false alarm.

In Australia, conventional ground-based navigation aids incorporate monitoring equipment at the ground site. Should the equipment detect an out-of-tolerance condition, the transmitter is shut down, and the user alerted by means of a flag or loss of aural identification.

GNSS integrity relates to the trust that can be placed in the accuracy of the information supplied by the total system. This includes the ability of the system to notify the pilot if a satellite is transmitting erroneous signals.

Individual GNSS satellites are not continuously monitored, and several hours can elapse between the onset of a failure and its detection and correction. Without some additional integrity monitoring, a clock or ephemeris error, for example, can have a significant effect on any navigation system using that satellite.

RAIM is the most common form of integrity monitoring and is discussed in more detail earlier in this chapter. Many non-aviation and non-TSO GPS receivers do not monitor integrity and will continue to display a navigation solution based on erroneous data.

Continuity

Continuity is the probability that the performance of a system, comprising all elements needed to maintain an aircraft’s position within a defined area), will be maintained from the beginning to the end of an operation.

How many GNSS satellites does your aircraft receiver need to ‘see’ for various operations?

This table contains how many GNSS satellites does aircraft receiver need to see forvarious operations
Number of satellites Type of navigation Comments
1-3 Nil Not sufficient for navigation
4 3D Position solution but no integrity monitoring
5 3D + RAIM Can detect faulty satellite data (integrity) and will stop providing navigation solution
6 or more 3D + fault detection and exclusion >Capable of detecting and excluding faulty satellite data, and continuing to supply valid navigation solution. (TSO 145, 146 and 196 receivers only)

Navigation databases

RNP approaches require the use of a valid and current database.

The data on the GNSS approach extracted from the database includes other parameters for the approach, not just the waypoint positions. This information is used by the receiver to alter the course deviation indicator (CDI) scaling and change the RAIM protection limits.

The approaches are coded as a series of waypoints which the receiver can retrieve and automatically sequence during an approach. Included with the waypoint coordinates in the database is information about the waypoint type.

This information includes whether the waypoint is a fly-over point, or a fly-by point, and whether it is an initial, intermediate, final or missed approach point.

Under the requirements of CAO 20.91:

  • the database must be valid for the current aeronautical information regulation and control (AIRAC) cycle
  • all terminal routes—standard instrument departures (SIDs), standard terminal arrival routes (STARs) and approaches—must be loaded from the database and may not be modified by the pilot except as provided for in the CAO.

Key points

  • The four major GNSS constellations are the USA’s NavStar Global Positioning System (GPS), the Russian Federation’s GLObal NAvigation Satellite System (GLONASS), European Union’s Galileo GNSS and China’s BeiDou navigation satellite system.
  • GNSS antennas on aircraft pick up signals generated from constellations of satellites. It is expected that about 120 satellites will be available once all four major systems are fully deployed by 2020.
  • GNSS uses the difference in the time of travel of radio waves from at least four satellites to fix the position of the receiver and get an accurate value for time.
  • Aviation GNSS units have software to protect integrity—the measure of trust you can place in the information supplied by the total system. Integrity includes the ability of a system to provide timely warnings to the user when the system must not be used for the intended operation.
  • RAIM outages, or holes, are times when there are too few satellites with the appropriate spacing for integrity monitoring. The holes move in time and space, so you need a new prediction from Airservices Australia each time you fly.
  • GNSS is now the primary means of navigation for all instrument flight rules aircraft, and is supported by the backup navigation network (BNN).

Resources

  • Airservices Australia (2016). Navigation Rationalisation Project Retrieved April 2017.
  • Beidou Navigation Satellite System (2016). Service target Retrieved April 2017.
  • CASA (2006). Civil Aviation Advisory Publication 179A-1(1). Navigation using Global Navigation Satellite Systems (GNSS) Canberra.
  • CASA (2006). Overview. Global Navigation Satellite Systems Canberra.
  • CASA (2014). Performance-based Navigation Canberra.
  • CASA (2016). Performance-based Navigation in Australian airspace Retrieved April 2017.
  • European GNSS Service Centre (2016). Constellation Information Retrieved April 2017.
  • European Space Agency (2014). Beidou General Introduction Retrieved April 2017.
  • Federal Aviation Administration (2014). Global SBAS Status Retrieved April 2017.
  • Federal Space Agency Information-Analytical Centre (2016). GLONASS Constellation Status Retrieved April 2017.
  • Li, Zhang, Ren, Fritche, Wickert and Schuh (2015). Precise positioning with current multi-constellation Global Navigation Satellite Systems: GPS, GLONASS, Galileo and BeiDou Retrieved April 2017.
  • National Coordination Office for Space-Based Positioning, Navigation, and Timing (2015). New Civil Signals Retrieved April 2017.
  • National Geospatial-Intelligence Agency (2015). Current GPS Satellite Data, April. Retrieved April 2017.
  • Skybray (2016). Non-precision approach Retrieved April 2017.
  • Skybray (2016). Precision approach Retrieved from April 2017.
Online version available at: https://www.casa.gov.au//search-centre/safety-kits/cnsatm-resource-kit/cnsatm-resource-kit-chapter-3-global-navigation-satellite-systems-gnss
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