GPS

History of GPS

Originally designed for military and intelligence applications at the height of the Cold War in the 1960s, with inspiration coming from the launch of the Soviet spacecraft Sputnik in 1957, the global positioning system (GPS) – is a network of satellites that orbit the earth at fixed points above the planet and beam down signals to anyone on earth with a GPS receiver. These signals carry a time code and geographical data point that allows the user to pinpoint their exact position, speed and time anywhere on the planet.

GPS imageTransit was the first satellite system launched by the USA and tested by the US Navy in 1960. Just five satellites orbiting the earth allowed ships to fix their position on the seas once every hour. In 1967 Transit was succeeded by the Timation satellite, which demonstrated that highly accurate atomic clocks could be operated in space. GPS developed quickly for military purposes thereafter with a total of 11 “Block” satellites being launched between 1978 and 1985.

However, it wasn’t until the USSR shot down a Korean passenger jet – flight 007 – in 1983 that the Reagan Administration in the US had the incentive to open up GPS for civilian applications so that aircraft, shipping, and transport the world over could fix their positions and avoid straying into restricted foreign territory.

GPS imageUpgrading the GPS was delayed by NASA space shuttle SS Challenger disaster in 1986 and it was not until 1989 that the first Block II satellites were launched. By the summer of 1993, the US launched their 24th Navstar satellite into orbit, which completed the modern GPS constellation of satellites – a network of 24 – familiar now as the Global Positioning System, or GPS. 21 of the constellation of satellites were active at any one time; the other 3 satellites were spares; in 1995 it was declared fully operational. Today’s GPS network has around 30 active satellites in the GPS constellation.

Today, GPS is used for dozens of navigation applications, route finding for drivers, map-making, earthquake research, climate studies, and an outdoor treasure-hunting game known as geocaching.

Applications of GPS

Although originally designed for military and intelligence applications at the height of the Cold War in the 1960s, in the 1980s GPS was released for use in civilian applications. Today, millions of users rely on satellite navigation for finding their way from A to B and a whole lot more besides.

GPS imageThe most obvious application for GPS is satellite navigation in vehicles, aircraft and ships. It allows anyone with a GPS receiver to pinpoint their speed and position on land, air or sea, with incredible accuracy. Drivers can use in-vehicle portable navigation devices to follow a route, find detours around traffic problems and with additional software receive traffic alerts and warnings on safety camera locations.

GPS is also available for other uses: hikers and ramblers can use GPS receivers to ensure they are following their chosen route and to mark rendezvous points along the way. While gamers can take part in geocaching, a kind of treasure hunt for the digital age, which uses precise GPS signals to help the players track down a hidden stash.

GPS imageThe emergency services, for instance, can use GPS not only to find their way to an incident quicker than ever before but also to pinpoint the location of accidents and allow follow-up staff to find the scene quickly. This is particularly useful for search and rescue teams at sea and in extreme weather conditions on land where time can be a matter of life or death.

Scientists and engineers also have applications for GPS receivers, in scientific experiments, and in monitoring geological activity such as earth tremors, earthquakes and volcanic rumblings. They can use strategically positioned GPS devices to assist them in tracking climate change and other phenomena. Fundamentally, GPS can now be used to produce very accurate maps.

How does GPS work?

There are three parts to a GPS system: a constellation of between 24 and 32 solar-powered satellites orbiting the earth in orbits at an altitude of approximately 20000 kilometers, a master control station and four control and monitoring stations (on Hawaii, Ascension Islands, Diego Garcia and Kawajale) and GPS receivers such as the one in a car.

Each of the satellites is in an orbit that allows a receiver to detect at least four of the operational satellites. The satellites send out microwave signals to a receiver where the built-in computer uses these signals to work out your precise distance from each of the four satellites and then triangulates your exact position on the planet to the nearest few meters based on these distances.

In fact, signals from just three satellites are needed to carry out this trilateration process; the calculation of your position on earth based on your distance from three satellites. The signal from the fourth satellite is redundant and is used to confirm the results of the initial calculation. If the position calculated from distances to satellites “A-B-C” do not match the calculation based on “A-B-D” then other combinations are tested until a consistent result is obtained.

The process of measuring the distance from satellite to GPS receiver is based on timed signals. For example, at 16h45m precisely, the satellite may begin broadcasting its signal. The GPS receiver will also begin running the same random sequence at 16h45m local time, but does not broadcast the sequence. When the receiver picks up the signal from the different satellites, there will be a time lag, because the microwaves take a fraction of a second to travel from the satellite to the receiver. The time lag is easily converted into distance to each satellite. The slight difference between signals from each satellite is then used to calculate the receiver’s position.

How Rock Mars navigation device works?

A satellite navigation device, such as the Rock Mars navigation Moov uses the global positioning system (GPS) to pinpoint exactly where you are on the planet. It detects signals from at least three satellites in a constellation of around 30 constantly orbiting the earth.GPS imageUsing the data transmitted at the speed of light by these satellites the Moov’s internal electronics convert the signals into distances between the device and each of these satellites. It then reckons its precise position using trilateration, which is essentially a mathematical formula programmed into the navigation device.The same calculations not only provide the position, but also lock that on to a position on the device’s built-in maps; it then calculates the speed and direction you are travelling.

 

GPS imageThe Navigation device uses the information obtained from the GPS to overlay your exact position on to the map for the area in which you are travelling.The maps used in Rock Mars navigationdevices are regularly updated and can be added at any time.

Software built into the Navigation device allows you to set a destination and will guide you along the best route. The device can also read data from TMC (Traffic Message Channel) and alert you to traffic problems on major roads and optionally give you a revised route.

Rock Mars Navigation devices also have built in information on safety cameras. The location of safety cameras is constantly changing so updates can be downloaded regularly. Some models will warn drivers when they are approaching a safety cameras based on your direction and speed.

How does GPS mapping software work?

The global positioning system (GPS) is a network of satellites that orbit the earth and send a signal to GPS receivers and Navigation devices, giving them the precise location, speed, and if you are in an aircraft or up a mountain, altitude.

To be really useful for navigation, GPS mapping software has to be kept up to date. Every year, some 5 percent of our roads are changed in some way. New bypasses are built, lanes are added to existing roads, speed limits are changed, one-way systems are introduced, and traffic signals are changed. Your Navigation device needs mapping software that is regularly updated.

This is the most obvious advantage of a digital mapping device, such as a Navigation device. Long gone are the days of relying on a traditional road atlas that is almost obsolete in some areas before you get into your vehicle.

Digital mapping companies are working constantly to update the GPS mapping software and to make it available to navigation device users as quickly and efficiently as possible. Digital maps come with street level detail to help you to find your way from A to B as conveniently as possible.

GPS satellites do not provide the maps they simply provide the position lock that is then overlaid on the maps by your GPS receiver. All navigation devices come with preloaded maps, but to make sure you have the latest maps for your navigation device, you have to install updated GPS mapping software regularly.

Map updates are available in two forms: either purchase an expansion card from a retailer or go online and download the updates to your computer ready for installing next time you connect your navigation device.

How GPS receivers can help you?

Consumer GPS receivers are used mainly for navigation and route planning. By locking on to a constellation of satellites orbiting the earth, the receiver can pinpoint your exact position on the planet, calculate the speed and direction in which you are travelling, whether you are in a vehicle, a boat, or walking, and if you are in an aircraft it will tell you your altitude too.

Drivers – Most drivers, whether driving for business or pleasure can benefit from the satellite navigation made possible by a GPS receiver. They can find their exact location and follow a route from A to B provided by the navigation device.

GPS imageSafety Cameras – GPS based navigation devices have made spotting permanent safety cameras on the roads that much easier so that drivers can make sure they stay within the roads speed limit.

Traffic Jams – Some GPS receivers and navigation devices are able to access traffic news announcements and alert drivers to problems on the road ahead and help them by re-route their journey to avoid traffic jams and accidents.

Entertainment – Your Navigation device can even help you plan your day if it has POIs (points of interest) such as restaurants, gas stations, emergency assistance, hotels and more, embedded in the mapping software.

GPS imageOutdoors – Hikers, cross-country runners, tourists, and others can use a GPS receiver to pinpoint their location and find their way from landmark to checkpoint and safely back home. A GPS can be used to mark a particular spot on the map so that you can return to that exact location later.

Gaming – Outdoor gamers can even use a GPS receiver to take part in treasure hunts for the digital age known as geocaching. The organizers hide a cache of “treasure” at a secret location and then provide clues to its whereabouts that rely on your use of a GPS receiver to find the bounty.

What’s inside a navigation device?

From the outside a navigation device looks like nothing more than a sleek digital device, with a touch screen, however, within the shell is a host of modern electronics that allows it to pick up signals from satellites orbiting thousands of miles above the earth and to calculate your precise position and speed on the planet.Each component inside a navigation device has a specific purpose and each is essential to the functioning of the device.

The rechargeable lithium-ion battery provides the power for the screen and the internal electronics. There are also circuits to control the display and to respond to user interaction via the touch-sensitive display and buttons. There are circuits too that control the information, map and route displayed as well as to produce spoken directions and camera alerts in some models. Some Navigation devices, such as the Mio Moov 580 even have Bluetooth capability.In order to carry out its main job of locking on to the global positioning system (GPS), a Navigation device has an aerial inside. This receives the microwave signals from the satellites in the GPS constellation. These signals are then amplified and fed to the integrated circuits that analyze the signals and calculate your position. The circuitry uses a system known as trilateration, which is the 3D equivalent of trilateration on a map. The trilateration process depends on the GPS device being able to determine the distance to the satellites by timing the signals using its inbuilt clock. The clock itself is an electronic circuit known as an oscillator.

What signal does GPS use?

There are currently between 27 and 32 global positions system (GPS) satellites in orbit around the earth. Of these, three act as backups. Each satellite transmits a regular GPS signal that is carried by radio waves in the microwave part of the electromagnetic spectrum.Each GPS satellite continuously broadcasts a navigation message at 50 bits per second on the microwave carrier frequency of approx 1600 MHz .FM radio, for comparison, is broadcast at between 87.5 and 108.0 MHz and Wi-Fi networks operate at around 5000 MHz and 2400 MHz More precisely, all satellites broadcast at 1575.42 MHz (this is the L1 signal) and 1227.6 MHz (the L2 signal).The GPS signal gives the precise “time-of-week” according to the satellite’s onboard atomic clock, the GPS week number and a health report for the satellite so that it can be discounted if faulty.Each transmission lasts 30 seconds and carries 1500 bits of encrypted data. This small amount of data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. GPS receivers know the PRN codes for each satellite and so can not only decode the signal but distinguish between different satellites.

 

What is trilateration?

A GPS receiver uses trilateration (a more complex version of triangulation) to determine its position on the surface of the earth by timing signals from three satellites in the Global Positioning System. The GPS is a network of satellites that orbit the earth and send a signal to GPS receivers providing precise details of the receiver’s location, the time of day, and the speed the device is moving in relation to the three satellites.Each satellite in the GPS constellation sends out periodic signals along with a time signal. These are received by GPS devices, which then calculate the distance between the device and each satellite based on the delay between the time the signal was sent and the time when it was received. The signals travel at the speed of light, but there is a delay because the satellites are at an altitude of tens of thousands of kilometers above the earth.Once a GPS device has distances for at least three satellites, it can perform the trilateration calculations. Trilateration works in a similar way to pinpointing your position on a map knowing the precise distance from three different landmarks using a pair of compasses. Where the three circles centered on each of the landmarks overlap is your location given the radius of each circle is your distance from each landmark.

In the GPS version, the calculations are carried out in three-dimensions with an imaginary set of 3D compasses so that your location is where three spheres of radius given by the distance to each of three satellites overlap. If the GPS device can see a fourth satellite, then the measurements can be double-checked.

The calculation process happens very quickly, allowing the GPS device to pinpoint its location, altitude (if it is in an aircraft), speed and direction.

The transmissions are timed to begin precisely on the minute and the half minute as indicated by the satellite’s atomic clock. The first part of the GPS signal tells the receiver the relationship between the satellite’s clock and GPS time. The next chunk of data gives the receiver the satellite’s precise orbit information.

GPS accuracy and error sources

The Global Positioning System (GPS) can provide your location, altitude, and speed with near-pinpoint accuracy, but the system has intrinsic error sources that have to be taken into account when a receiver reads the GPS signals from the constellation of satellites in orbit.The main GPS error source is due to inaccurate time-keeping by the receiver’s clock. Microwave radio signals travelling at the speed of light from at least three satellites are used by the receiver’s built-in computer to calculate its position, altitude and velocity.Tiny discrepancies between the GPS receiver’s onboard clock and GPS time, which synchronizes the whole global positioning system, mean distances calculated can drift. There are two solutions to this problem. The first would be to use an atomic clock in each receiver costing $100,000. The second is to use some clever mathematical trickery to account for the time-keeping error based on how the signals from three or more satellite signals are detected by the receiver, which essentially allows the receiver to reset its clock. The latter is the less expensive solution used by Navigation device manufacturers.

There is also an intrinsic error source in GPS associated with the way the system works. GPS receivers analyze three signals from satellites in the system and work out how long it has taken each signal to reach them. This allows them to carry out a trilateration calculation to pinpoint the exact location of the receiver. The signals are transmitted by the satellites at a specific rate.

Unfortunately, the electronic detector in standard GPS devices is accurate to just 1 percent of a bit time. This is approximately 10 billionths of a second (10 nanoseconds). Given that the GPS microwave signals travel at the speed of light, this equates to an error of about 3 meters. So standard GPS cannot determine position to greater than 3-metre accuracy. More sophisticated GPS receivers used by the military are ten times more accurate to 300 millimeters.

Other errors arise because of atmospheric disturbances that distort the signals before they reach a receiver. Reflections from buildings and other large, solid objects can lead to GPS accuracy problems too. There may also be problems with the time-keeping accuracy and the data onboard a particular satellite. These accuracy problems are circumvented by GPS receivers which endeavor to lock on to more than three satellites to get consistent data.

GPS Glossary

GPS – Global Positioning System, a satellite based navigational aid available to the public

GPS receiver – a device that can lock on to signals from navigational satellites and use them to provide location, altitude and velocity to the user

Navigation device – Satellite Navigation device, a type of GPS receiver used for routing and mapping

Touch screen – the LCD display on Navigation device devices that responds to fingertip pressure to change settings

GPS time – the standardized time to which GPS satellites are synchronized, it is not tied to the rotation of the earth so there are no “leap seconds” in GPS time

GPS constellation – the group of approximately 30 GPS satellites orbiting the earth that broadcast microwave signal to be received by GPS receivers

Oscillator – an electronic circuit built into a GPS receiver that provides timing for the microwave signals from the satellites

Microwave signal – a high-energy radio signal that carries a burst of data from the GPS satellites at the speed of light to receivers on earth

Nanosecond – one billionth of a second

Geocaching( aka GPS treasure hunt) – an outdoor game for the digital age in which someone “hides” the treasure or “geocache” and the gamers, “geocachers” use their GPS receiver to try to find it

POIs – Points of interest, restaurants, gas stations, emergency assistance, hotels and more, often embedded in the Navigation device mapping software

Traffic Pack – optional add-on for Navigation devices that taps into traffic news and provides incident alerts and optionally gives you a new route

MP3 player – audio compression format offered in some navigation devices that allows users to listen to music, audio books and podcasts on the go