Steering By The Stars
By Richard Mead.
This article appeared in Electrical Gems Issue 80, August—September 2007
Satellite navigation has come of age. Originally a technology devised for military purposes, the Global Positioning System has been continuously developed and is now accessible to any private individual who buys a GPS receiver. On land, in the air and at sea, GPS is designed to take the guesswork out of navigation and can pinpoint your position on almost any point on earth, within about 50 metres or even less. In your car, truck or van, GPS can guide you street by street, turn by turn, right to your destination. It could even save your licence.
You are putting your tools away into the van when you get the call. Its after 4pm and you are tired, dirty and dying for a beer. But the caller is persuasive. By the sound of the job, you could get at least 2 weeks work, maybe three. But the site is on the other side of the city, which means traversing the CBD. Its a year or more since you worked over that side, and the traffic was a pain in the @r$e.
You try and look it up in your street directory while you have him on the phone, and you find the street on the map. But this area is totally unfamiliar to you and a long way out of your usual patch. So in the end you just say youre too busy, no time, advise him to try someone else and let three weeks work slip through your fingers like sand.
But who can blame you? Anyone who has ever tried to navigate through dense, fast-moving city traffic using a street directory knows just how tricky that can be. Its not the fault of the street directory. As road maps go, they are about as good as they can get. Its just that the two activities, map reading and driving, cannot be done safely at the same time. So [especially if you are on your own] navigating by road map in unfamiliar territory can often be difficult to manage.
Due to the speed and volume of traffic, often its just not possible to stop in order to read the map. Even with someone navigating, unless they are very good at reading maps, recognising roads and landmarks and giving clear, unambiguous instructions at the same time, you may quite easily miss a turn, or two, or three. And then the whole exercise becomes frustrating as you try to find your way back on route in totally unfamiliar territory. Especially at night!
The other factor is time. Getting through peak hour traffic in a capital city, even if you know exactly where you are going, can take a serious chunk of time out of your day. Talk about stress! The answer? It could be GPS. With GPS you can pre-program your days itinerary, job by job, destination by destination. Then just get in the truck and go. Contractors with a fleet of vehicles can pre-program them all.
It all began with a Russian satellite.
The Global Positioning System is essentially a child of the space race, with developmental roots stretching back as far as the late 1950s. Prior to this, the hyperbolic navigation system or Loran (Long Range Navigation system) which used pulsed radio transmissions from master and slave stations, the gyroscopic compass and radar were among the navigators main tools of trade. Then, on October 4th 1957, when Russia launched Sputnik - its first satellite - into space, scientists began to wonder if the worlds first man made star could be used as an aid to navigation.
At the Massachusetts Institute of Technology, scientists at the Lincoln Laboratory found that they could track the satellites orbit by monitoring the frequency of its radio signals. The increase in radio signal strength as it approached, and the decrease in signal strength as it departed (a phenomenon known as the Doppler Shift) enabled them to pinpoint precisely the position of the satellite along its orbit. This discovery alerted the scientific world to the possibilities of satellite navigation, but it would be some years until the next development took place.
The Cold War, the catalyst that sparked development of GPS.
In the mid 1960s, the U.S. Navy deployed submarines armed with Polaris nuclear missiles. To maintain their secrecy, these submarines needed to remain submerged for months on end. Inertial navigation, the navigation system used by submarines at that time, was not entirely suitable for this type of operation as it could not maintain accuracy during the long periods when the submarines were submerged. So the Navy went in search of a system that would better fit their needs.
Once you are familiar with your GPS, you can pre-program your day's itinerary, job by job, destination by destination. Then just get in the truck and go. Contractors with a fleet of vehicles with GPS can pre-program them all.
One option was to use the Transit System of satellites. A submarine navigator could measure the Doppler Shifts of these five satellites as they traversed their polar orbits, and determine the vessels position with predictable accuracy. But the process took between 6 and 10 minutes to get a fix and it was obvious that a simpler, faster system needed to be found. Over the next seven or eight years, a variety of systems were proposed but all had their deficiencies. It was not until 1973 that the concept of GPS, as we know it, was born.
How NAVSTAR, child of the Cold War, was conceived in an empty room in The Pentagon.
In an effort to meet all of the Defence Departments requirements for a fast, reliable satellite navigation system, the projects top scientists met in an empty room in the Pentagon on the Labor Day weekend of 1973. The product of this meeting was a proposal for a system that embodied the best features of many of the earlier prototypes. As a result, the U.S. Government approved the new proposal, which was dubbed Global Positioning System/NAVSTAR.
The first GPS Block 1 satellites were launched in 1978. By 1980 a total of 6 NAVSTAR satellites were in orbit, which were sufficient for testing and development purposes. Today the NAVSTAR system consists of 25 satellites that orbit the earth every 12 hours. Their patterns of orbit are designed to ensure that every point on the planet will always be in contact with at least 4 satellites.
GPS and its use in everyday life.
Today, GPS units with the ability to navigate you by on-screen maps and spoken command, through the complexity of the worlds busiest cities, can be bought for around five hundred dollars and can be learned within a few hours by anyone with enough technological capability to use a mobile phone. Once you have experienced it, GPS navigation is one of the most technologically amazing and practical things you will ever use. GPS units are available for almost every application. From hand-held units that you can use on foot to units and systems suitable for cars and trucks, aircraft, boats and ships there is a unit and a system that is designed for the purpose. Hand held GPS capability is now available to mobile phone users. Those who purchase a PDA (Personal Digital Assistant) or a Smartphone (a PDA with a mobile phone) either have the software and maps pre-installed or (in many cases) can obtain them, in order to use their hand-held device for personal GPS navigation.
How does GPS work?
The Global Positioning System consists of 25 satellites that orbit the earth every 12 hours and continuously transmit radio signals of specific frequency and strength. These signals contain data that includes the satellites position and the exact time. Atomic clocks on board each satellite are regulated by master clocks at the U. S. Naval Observatory, and all GPS satellites are synchronised to transmit their data signals at exactly the same time.
By receiving and processing the signals from at least 4 satellites, GPS receivers can convert the data into a 3-dimensional reading and accurately calculate location, direction and speed. The receiver calculates the distance between itself and the satellites by measuring the time delay between transmission and reception. This gives the distance to each satellite, and by analysing and combining the data from at least 3 satellites, the receiver is then able to calculate its position, its direction of travel and its speed.
Getting started with GPS.
Most new GPS units fall into the plug and go category, which means that you can simply plug them in, turn them on and begin navigating. You should be aware that NAVSTAR signals can only be received while the transmitter (the satellite) and the receiver (the sat-nav unit) can see each other. So if you enter a tunnel or a building for instance or drive under a bridge, the signal will drop out until you exit again. In most cases the GPS unit will tell you if it loses signal.
Tall city buildings can also cause temporary signal loss, and underpasses and overpasses can also trick some sat-nav units, but if you know how and why it happens you can be prepared for these events and anticipate them. It causes few problems (if any) in operation, but as a novice who has tried to get the GPS to calculate my route before leaving the garage, I can advise you that it works much better once outside. Practice makes perfect.