Highly accurate, available 24 hours per day and in almost all weather conditions, satellite navigation is ideal for aviation and when augmented by ground-based transmitters - as currently being tested - should eventually permit autoland operations down to Category III minima.
At present, there are 3 satellite navigation systems world-wide: GPS, GLONASS and Galileo, and ICAO have grouped all of these together under the umbrella term "Global Navigation Satellite System" (GNSS).
For the purposes of this brief, the terms GNSS and GPS may be freely inter-changed and the requirements applicable to an RNAV (GPS) procedure are the same as those applying to an RNAV (GNSS) procedure, and vice-versa.
Fully operational and the best known of the 3, the NAVSTAR Global Positioning System (GPS) is an American constellation of satellites developed and controlled by the U.S. Department of Defence.
The Russian Global Orbiting Navigation Satellite System (GLONASS) was declared operational in 1996, but fell into disrepair for a number of years and is only now being resurrected, with an on-line target of 2009.
Galileo is a European Union system with a nominal constellation of 30 satellites. Planned to be highly interoperable with GPS and using the same coordinate system, Galileo is planned to be operational in 2008. Unlike GPS, Galileo has been specifically designed with commercial aviation (and autoland operations) in mind.
A GPS receiver determines it's location using triangulation.
Using one satellite, and knowing the distance from it, the location of the receiver would be anywhere on the surface of a sphere centred about the satellite.
Given a second satellite, the receiver would know that its position was somewhere on a circle defined by the intersection of two spheres - each centred about the two satellites.
With a third satellite, the receiver then knows that it is at one of two points, formed by the intersection of three spheres. One position will be on or close to the Earth's surface and the other out in space.
For earth-based users, the radius of the earth can be used to confirm which of the two locations is correct, by rejecting the space solution.
For aerospace users, a process called Barometric-Aiding can be used for very short durations; however, this is not as accurate as using a fourth satellite.
When designing GPS, it was realised that normal ranging techniques would not work due to the distances involved and the solution used, is for the satellite and receiver to both generate the same code at precisely the same time.
On receiving the satellite's signal, the receiver compares and time-shifts its own code until it matches the satellites code; the amount of slew is the travel time of the signal. Since the receiver knows the satellites location (and that radio signals travel at the speed of light), it can then calculate its distance from the satellite.
As the receiver is only calculating distance indirectly, the result is known as a pseudorange.
However, for pseudo-ranging to work, the MMRs clock must be in perfect synchronisation with the satellites clock. Although all satellites have atomic clocks and operate in synchronisation with each other, it is both impractical and prohibitively expensive for GPS receivers to use atomic clocks and, for every microsecond (one-millionth of second) difference between the satellite clock and the receiver clock, a 300 metre error is introduced. This error is known as the clock bias.
The solution, is for the receiver to assume that its clock is in error and that the amount of clock bias must be calculated. Providing 4 satellites are available, the receiver can constantly re-compute position until the clock bias is resolved.
The only satellite navigation system in use by commercial aircraft, NAVSTAR GPS comprises three segments:
The space segment of GPS consists of a maximum constellation of 32 satellites, of which 21 are required for full system capability and a further 3 are made available as on-orbit operational spares.
To provide global coverage, the satellites operate in six orbital planes (4 satellites per plane), nominally spaced at 60 degrees apart, inclined at approximately 55 degrees to the equatorial plane and at an altitude of about 10,900 nm.
Travelling at almost 2.2 nautical miles per second, each satellite completes an orbit in just under 12 hours.
Although, to a viewer located on the surface of the earth, the satellites are constantly rising and setting, the orbital design allows between five and eight satellites to be visible at any given time, and system availability is estimated at better than 99%, even in the event of satellite failure.
Every satellite in the NAVSTAR constellation continuously transmits on the same two L band frequencies - L1 and L2.
In the same band as DME signals, extremely heavy rainfall will degrade the signals, but clouds and snow have little effect.
L1 carries two elements, a Coarse/Acquisition (C/A) ranging code used by civilian receivers (as fitted to the 757) and a Precision (P) ranging code which is only available to authorised military and scientific users. Although the C/A-code can be intentionally degraded, it is always available.
L2 only carries the P-Code and provides resistance to interference and increases system accuracy for military and selected scientific users.For the C/A ranging code, each satellite transmits a unique, 1023-bit, Pseudo-Random Number (PRN), which is repeated every millisecond.
Using a pseudo-random signal allows a receiver to differentiate between satellites, even though they all transmit on the same frequency.
In addition to the ranging code, each satellite transmits detailed information - known as the Navigation Message - on top of the ranging code.
Amongst other things, the Navigation Message includes details of the satellites ephemeris (it's orbital parameters), information about the satellites status and health and an atmospheric model used in error correction.
The Navigation Message also includes an almanac listing the PRN, status and location of all of the satellites.
Although a complete almanac takes over 12 minutes to download, GPS receivers are designed to store the almanac indefinitely and only update it when required.
To maintain system accuracy and integrity, 5 global monitoring stations passively track all satellites in view, relaying satellite clock and ranging data to a Master Control Station.
Using this data, the Master Control Station calculates and revises each satellite's navigation message, which is then uploaded back to the satellite. If necessary, revised messages can be uploaded every 8 hours.
The MCS is also able to adjust each satellites orbit using small command manoeuvres and completely re-locate satellites in the event of failure.
The 757s GPS navigation equipment comprises 2 GPS receiver modules located in the left and right Multi-Mode Receivers (MMRs), each with its own antenna.
After a power-on BITE check, each MMR enters a 30-second initialization mode during which the signal processor is initialised with latitude, longitude and altitude (normally derived from the IRS. Once initialised, the MMR enters the acquisition mode.
In acquisition mode, the MMR uses the last stored almanac to determine which satellites are in view, searches for them and then locks on to them.
If IRS position is unavailable, satellite acquisition can still occur, but will take slightly longer.
Likewise, the MMR can still resolve position even if the almanac is significantly out of date, but it may take up to 10 minutes to do so, whilst it downloads enough of the almanac to determine which satellites are available.
Having acquired at least 4 satellites, the MMR can calculate its position, altitude and clock bias, and enter the navigation mode.
In navigation mode, the MMR constantly updates its position, velocity, acceleration and time, and, using information from the internal almanac, automatically switches between satellites to maintain an optimal navigation solution.
The MMR is able to measure distances from more than one satellites at time, as it has multiple receiver channels.
If the MMR is unable to track at least four satellites, but has previously been in navigation mode (and has calculated clock bias), it enters a barometric altitude-aided mode.
In this mode, the MMR can still generate a navigational solution for a short period of time (using IRS altitude plus the earth's radius as the fourth range), but will revert back to the acquisition mode if a fourth satellite is not acquired within 30 seconds.
If an internal failure occurs, the MMR will stop generating data and alert the crew with a L or R GPS advisory message. The other MMR will be unaffected.
As with any other navigational system, a variety of errors can degrade GPS accuracy.
When radio waves from GPS satellites enter the earth's ionosphere and troposphere (the atmosphere), their paths can be either bent or refracted, increasing the distance travelled and, consequently, increasing the time taken for the signal to reach the receiver.
The size and shape of the ionosphere varies from day to day, between night and day and with solar conditions, and can significantly affect GPS accuracy, creating errors of up to 16 m. For satellites low on the horizon, this error can be up to 48 m.
Atmospheric effects are usually greater for satellites at less than 10 degrees above the horizon compared to those directly overhead and errors of up to 30 metres can be experienced.
For standard civilian users without access to the L2 signal, continuously-updated, mathematical models are used to simulate the ionosphere and troposphere, and are downloaded as part of the navigation message.
Although not perfect, these models can eliminate between 10 and 90% of the errors.
The satellite's atomic clock are able to maintain a very high degree of accuracy (plus or minus 1 second every 360,000 years), but they are not perfect and errors of up to 1.5 metres can occur due to clock drift.
To minimise clock drift error, the MCS includes clock drift correction data in the Navigation Message.
The design of the GPS constellation provides optimal stability and it is possible to calculate each satellite's ephemeris with high precision. Even so, variations due to the uneven density of the Earth, magnetic fields in space and solar flares, etc, can produce errors of up to 5 metres.
To maintain accuracy, the MCS makes regular updates to the ephemeris element of the Navigation Message.
GPS receivers use advanced electronic circuitry and algorithms to receive, decode and process the data sent by the satellites, but are not perfect and small errors can occur.
Multipath is the effect of the same satellite signal reaching the antenna more than once and results from signals being reflected off objects in the vicinity of the receiver. If uncorrected, multipath errors can be up to 1 metre.
For aviation antennas, GPS multipath signals from the surface of the aircraft, adjacent objects and terrain are reduced by employing a 5 degree masking angle in the antenna's design.
When initially tested, the accuracy attained using only the Coarse/Acquisition code was much better than expected and a technique called 'Selective Availability' was initially used to degrade system accuracy down to no better than 100 m.
Selective Availability can be achieved by either manipulating the satellites clock (which affects all users) or by intentionally altering part of the navigation message (usually the atmospheric model, as this only affects standard civilian users).
Selective Availability was discontinued by Presidential Decree in May 2000
GPS signals are weak and it is relatively easy for another source (such as solar flares or ground based transmitters) to make acquiring and tracking the signals either difficult or impossible.
Fortunately, both intentional and unintentional interference is easy to detect and locate, and for aviation purposes, aircraft will only be affected by ground-based sources of interference when in line of sight of the transmitter.
With Selective Availability OFF, as is normal, the dominant error is ionospheric delay, followed by satellite clock and ephemeris errors. When mathematically summed, these amount to an ranging error of 5.3 meters for 95% of the time.
To allow for worst case errors (with Selective Availability ON), the accepted ranging error is 33.3 metres (100 feet).
Just as with conventional DME/DME fixing, unsuitable satellite geometry will degrade the accuracy of a fix and, for satellite navigation, this effect is declared as a numerical factor (normally between 1 and 3) and is called the Dilution of Precision (DOP).
In satellite navigation, the closer overhead to the receiver the satellites are, the greater the Dilution of Precision, and best results are obtained with one satellite overhead and the other 3 satellites equally spaced around the horizon and approximately 30 degrees above the horizon.
Since the ephemeris of each satellite is stored in the almanac, the receiver can select those satellites offering the best geometry and minimise the Dilution of Precision.
As satellite visibility can be affected by large buildings, terrain masking and by the way an aircraft banks and pitches, the DOP will be constantly changing.
Overall GPS accuracy is the product of the system errors (ephemeris, atmospheric and clock errors, etc) multiplied by the Dilution of Position (satellite geometry) and is normally expressed in metres.
In aviation, the overall system accuracy is expressed in nautical miles and, for the Pegasus FMC, is displayed as the Actual Navigation Performance.
Using the above data, it can be seen that ANP will normally be relatively constant between 0.05 and 0.1 nautical miles for all flight conditions.
The one significant deficiency of GPS is its integrity - the inability of the system to provide timely warnings in the event of failure.
Satellites are not continuously monitored and up to 8 hours can elapse before a failure is detected by the MCS and included in the navigation message.
To overcome this deficiency, Receiver Autonomous Integrity Monitoring (RAIM) has been developed and is a mandatory feature of GPS receivers used in commercial aviation.
Simply speaking, by using an additional - fifth - satellite to that required for a basic navigation solution, the receiver can determine if a satellite is transmitting erroneous information and, if necessary, stop producing a navigation solution.
By using a method called Fault Detection and Exclusion (FDE) and a further - sixth - satellite, the receiver can perform further analysis to identify and exclude the faulty satellite, thus allowing GPS navigation to continue.
Should the receiver be unable to resolve its position, it will stop providing data to the FMS which, in turn, will revert to using the next-best navigation solution (normally the RADIO position) and its associated ANP.
If this new ANP exceeds the RNP, an UNABLE RNP alert will be generated.
Provided by the associated on-side ADC, the FMC incorporates barometric altitude into the navigation solution.
Whilst barometric-aiding cannot be used as part of the navigation solution for more than 30 seconds, it can be used continuously as a pseudo-satellite, improving Fault Detection and RAIM, and minimising those occasions when an aircraft would be without RAIM.
To ensure that the speed with which an alarm is generated is appropriate to the stage of flight, different phases of flight use different Integrity Monitoring Alarm Limits (IMAL) prior to an alert being generated.
| Phase of Flight | IMAL (nm) | Time to Alarm (sec) |
|---|---|---|
| En-route | 2.0 | 30 |
| Terminal (Within 30nm of ARP) |
1.0 | 10 |
| Approach | 0.3 | 10 |
| Missed Approach | 1.0 | 10 |
A World Geodetic System (WGS) is a means of describing the size and shape of the earth, and how coordinates are referenced with respect to specific points on the Earth.
Ever since the first (flat-earth) world geodetic system, a succession of more accurate geodetic systems have been developed for mapping, aviation and military purposes.
For a number of years, the World Geodetic System 1972 (WGS-72) and North American Datum 1927 (NAD-27) were used in aviation but, due to its greater accuracy, WGS-84 is now recognised as a better system and, significantly, is the one used by GPS and adopted by ICAO.
Although the difference between datum's can be less than 100 feet and a number of systems are compatible, differences can exceed two nautical miles and recent surveys of non-WGS-84 approaches have highlighted final approach track definition errors as great as 800 metres.
For those countries that do not publish data in WGS-84 compatible coordinates, navigation accuracy is limited and although enroute operations will be unaffected, approaches using GPS as a primary means of navigation are prohibited.
Note: Jeppesen maintains a list of those countries that use WGS-84 and the list includes all TCX destinations.
Geoff Middleton