Global Positioning System (GPS)

How does GPS work and what can it do for airports and air carriers as navigation aid? This article and the next will provide the answer to these questions.

Most people now have a GPS device of one type or another: A vehicle GPS device to help navigate on the roads, a handheld device for hiking or a marine model to consistently locate your favourite fishing hole. The applications for GPS are almost unlimited and, like most technologies, new applications are always being developed.

Before discussing aviation applications, a short lesson on how GPS works. One way to determine a position is through triangulation. By having three or more bearings from a three or more points and finding the intersection of these, a three dimensional position can be determined. That is the easy part but how is this accomplished with satellites?

There are currently 31 GPS satellites orbiting the earth. Each satellite transmits a signal that is decoded by a receiver. Part of the message sent by the satellites is a timing signal and information about the orbit of the satellite. The timing signal is used by the receiver to determine how long it took for the signal travel from the satellite to the receiver. By knowing that radio waves travel at the speed of light (300,000 km per second) the distance to the satellite can be determined. If the receiver determines that the signal took a one thousandth of a second, the satellite is 300 km from the receiver.

The location of the satellite at any point in time is provided by the almanac information which is also broadcast to the receiver. The almanac is a description of where the satellites are in space at any point in time. So, by knowing where the satellite is and how far the receiver is from it, a bearing can be determined. With three satellite bearings, the position of the receiver can be determined in three dimensions (latitude/longitude/altitude).

As you can imagine there is a lot of computation going on to decode and calculate with this information then provide it on a display. Fortunately, the introduction of GPS combined with the increased capabilities of computer chips has made this technology available to a wide community and at an incredibly affordable cost.

GPS was developed by the United States military for its own navigation purposes. The US government has kindly permitted worldwide civilian access to this invaluable utility at no charge. As a result, numerous applications have been developed of which aviation applications is one.

Aviation is prime candidate to apply this new method of navigation. Aircraft are completely dependent upon navigation and the presence of a worldwide navigation system that can provide this vital information is a huge benefit, particularly when it is free! GPS is now used in all phases of flight – departure, climb, enroute, descent, arrival, approach and landing. It is also used on the ground by aircraft to follow taxiways particularly during low visibility conditions.

GPS is of particular benefit during the approach and landing phase of flight. In order to transition from the enroute phase to ultimately land the aircraft, an instrument approach is required. This means that every airport that wants to have all weather capability needs an instrument approach procedure. With conventional navigation aids, it also means the capital and operating cost of an NDB, VOR, DME or ILS. This could be in the hundreds of thousands to over a million dollars in capital costs and tens of thousands of dollars per year in operating costs. With GPS, the same (and in many cases, better) capability is available without any capital or operating costs of a navigation device as the GPS signal is available for free, compliments of the US Government.

GPS is available worldwide, 24/7. With GPS, the monitoring and maintenance of the navigation system is performed by the US Air Force. The US Government has guaranteed a 10 year advance notice before denying civil access ndash; this is unlikely to ever happen as GPS is now woven into the technological fabric of the US and the world, much like the internet is. Moreover, other satellite navigation systems are either in use or under development that would compliment GPS. The Russians have GLONASS, the Europeans are developing Galileo, and the Chinese are deploying the Beidou satellite navigation system.

GPS has proven to be a highly accurate, reliable and dependable navigation system. Refinements to satellite navigation have been developed specifically for aircraft navigation and instrument approach operations with the aim to provide precision approaches to all airports in North America.