GPS is a navigational marvel offering unprecedented capabilities while at the same time too high user expectations are not always balanced with its limitations. GPS is easy to use and provides simple results. However, the technology needed and the software used to generate those results is far from simple.
GPS publications are frequent and diverse. Opinions settle based on the $200 GPS receiver used for all kind of private applications from hiking to sailplanes. The industry push to sell GPS to the world is tremendous. Some believe GPS will become the sole means of navigation, replacing all other navigation systems, others believe a military controlled system should not be used at all.
This description may exceed the need to know level for the airline pilot. However, to objectively understand this navigational marvel, it’s worth the while to dig a bit deeper. Most technological and mathematical deliberations are evaded and the contents match training requirements laid down by JAR- FCL 1 paragraph 062 06 05 00.
Those with an interest in GPS beyond this description are advised to surf the internet. Many information sources can be found through the available web search tools and every aspect of GPS is covered, Marine-, Land-, Air-, Geodesy-, Military-, Civil-, are only some of the unlimited area’s of application.
This description has been sequenced as follows:
− A summary or primer page provides a quick overview of the grounds covered.
− Followed by the most relevant knowledge.
− Topped by background and more detailed information with an eye on the future.
Suggestions for future updates are invited.
NOTE: GPS is the familiar abbreviation used. However, ICAO nomenclature for a Satellite navigation system in general is GNSS (Global Navigation Satellite System) of which GPS is considered a subset together with several other systems and augmentations.
NOTE:The Russian equivalent of GPS is known as GLONASS. Presently there are no plans to implement GLONASS in western civil airliners. Although at some point in the future it may be considered to supplement GPS, GLONASS will not be discussed as part of this description.
2. Summary or Primer on GPS
USA militaries launched 24+ satellites as a global navigation system.
The constellation of 24 evenly spaced satellites circle the earth each 12 hours at 20 000 km Ground control stations monitor and control all satellites.
Precise digital data regarding orbits and time is up-linked to each satellite and stored.
Each satellite broadcasts this data on ca. 1575 MHz to all users.
Each satellite is recognized by its unique and precisely timed acquisition code modulation. Superimposed on this code modulation is the detailed orbit and time digital data.
A user receiver detects the extremely weak signals from all satellites in view.
The receiver also generates a synthetic duplicate of the known acquisition code modulation. Received code modulations are compared against the, time shifting, generated code signal. Once two code signals correlate that satellites signal is filtered out of all the signals received. The receiver now decodes and stores the precise orbit and time data for that satellite.
The time it took for the satellite signal to reach the receiver is proportional to the distance. The receiver needs to measure the distance to 3 satellites to triangulate its 3D position.
A 4th satellite is needed to synchronize the receiver clock to satellite (GPS) time.
From the orbit/time data received, the receiver computes the precise satellite X, Y, Z position. From the distances measured the receiver computes precise receiver X, Y, Z position.
The receiver converts X, Y, Z, into to geodetic latitude, longitude, height.
The geodetic reference system used is WGS84.
The output of the GPS receiver is LATITUDE, LONGITUDE, HEIGHT, UTC TIME.
Velocity can be derived from the rate of change of position.
The generic civil position accuracy achieved is 100 meters (95%).
To initially deliver a completed navigation solution the receiver may require up to 15 minutes. This is mainly because it takes 12.5 minutes to receive one complete data set from a satellite. Initialisation with approximate time and position will reduce this time drastically.
Once locked on the required minimum of 4 satellites the output is update once per second. GPS navigation has low integrity and will not alert the user immediately for faults and errors. Augmentation techniques have been developed to overcome this problem.
The satellite signals are extremely weak and can easily be shielded by obstacles.
The very weak signals on a single frequency can also easily be jammed or interfered with. Multi-channel aviation receivers detect signals from up to 12 satellites simultaneously.
The receiver corrects for Ionosphere propagation errors, using data from the satellite. Receiving 5 or more satellites the receiver can autonomously detect satellite errors (RAIM). GPS receivers with RAIM are certified for primary and supplemental use in aviation.
Several augmentations may improve accuracy, integrity, availability and continuity.
DGPS uses Differential corrections up-linked from a reference receiver at a known location. Geo-stationary satellites may down link integrity information to users over a wide area. Pseudolites mimic additional local satellites from the ground up and improve availability.
GPS operates fully automatic with hardly any pilot controls involved.
IRS PPOS may initialise GPS for faster acquisition or to coast through periods of signal loss. GPS and augmentations may develop into use for all phases of flight in the long term.
GPS precision approach to CATI limits is at the horizon and CATIII limits are predicted. Ultimately GPS & augmentations may replace some of the legacy navigation systems in use. GPS cost $12,000,000,000 to build $500,000,000/yr to maintain and a $200 receiver to use. For the time being use of the GPS signals is free of charge.
New aircraft presently have GPS as basic fit for use in the following applications:
− Primary navigation sensor input for the Flight Management System (FMS)
− Position sensor input for the Enhanced Ground Proximity Warning System (EGPWS)
− UTC time reference (or auto sync) for the aircraft time clocks.
The GPS receiver card is located inside the Multi-Mode Receiver (MMR), the antenna on top.
Above brief descriptions are expanded in subsequent chapters.