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Posted By Amal

The History of GPS


You’ve just got busted by a ghost!! So, how would like it if you could have a voice activated invisible personal assistant roaming around with you at all times? Sounds good, eh? Of course with this technological invasion we’re witnessing today, you might not find it as shocking as it sounds. But the very fact that this has long existed before most of us were even born is the shocking thing. You might have, by now, guessed what I’m talking about…. or haven’t you? Well, I’m speaking of the GPS. So first things first, let’s kick this off by defining GPS.

GPS or Global Positioning System is a network of orbiting satellites that transmit accurate details of their location from fixed points in the space back to Earth by the means of GPS receivers. It was invented by Bradford ParkinsonRoger L. Easton, and Ivan A. Getting. The signals are captured by GPS receivers, such as navigation devices and are utilize to estimate the exact position, speed and time at the vehicles location.

Now, let me tell you a story about the birth of GPS. If you happen to be a fan of star wars or star trek then perhaps you will be interested in this “not-so-science-fiction-anymore” tale. Once upon a particularly terrifying time, in a far, far away land, where no humans ever existed.. No, wait a minute that was way too back in time. Humans do exist in my story so let’s forward time just a little bit.. Perhaps to 1957 long before the big fuss about Star
Trek
 
and Star Wars. On the gloomy day of October 4th in 1957, the Soviet Union launched the very first artificial satellite into the space. The only purpose behind taking such a step was to beat up the US to the space. Funny, no? The sole reason that pushed the Soviet Union to launch a freaking satellite was a space race it had against the US! Not so gloomy to the Soviet Union after all. Any who, let’s get back to the point, this satellite was the Sputnik 1 which  provided scientists with valuable information, even though it wasn’t embedded with any sensors, by tracking and studying the satellite from Earth. The satellite traveled at about 29,000 kilometers per hour taking 96.2 minutes to complete each orbit. It beamed on 20.005 and 40.002 mhz which were observed by radio operators throughout the world. The signals stayed for 21 days until the transmitter batteries ran out on 26 October 1957. Sputnik 1 burned up on 4th of January 1958, as it fell from the orbit as it reentered the Earth’s atmosphere, after traveling about 70 million km and spending three months in orbit.

Payback time was due on January 31st, 1958, when the United States successfully launched Explorer I. This satellite discovered the magnetic radiation belts around the Earth, named after principal investigator James Van Allen. The Explorer program kept its successful ongoing series of lightweight, scientifically useful space shuttles.

GPS was basically famous for its military applications and was first developed by the US Department of Defense (DoD) to assist in its global intelligence efforts at the Cold War in 1960s. Ruthless, no? But before that sailors used the GPS in order to get clear and exact navigation through oceans. The very first system called TRANSIT launched by the USA and examined by the US Navy in 1959. TRANSIT was developed to locate submarines, and started out with six satellites that gradually grew to ten. The subs usually had to keep waiting for hours to receive signals from the satellites, but the model paved the road for true GPS with regular signaling from satellites in space.

In 1967 TRANSIT was followed by the TIMATION satellite, which proved that highly accurate atomic clocks could be managed in space. In 1978 the military launched the first satellite of a proposed 24-satellite GPS system called NAVSTAR after 11 years of working on a GPS system. GPS progressed rapidly for military purposes from then on with a total of 11 satellites being launched between 1978 and 1985. Since the early beginnings of 1980s, the GPS has been available to anyone who possessed a GPS receiver. From Airlines to shipping companies to trucking firms to drivers everywhere use the GPS system to trace vehicles and follow directions of the best path in the shortest time.

However, it wasn’t until the USSR shot down a Korean passenger jet in 1983 that the Reagan Administration in the US had the motive to allow civilians to use GPS applications so that aircraft, shipping, and transportation could determine their positions and avoid entering restricted foreign territories in order to enhance navigation and air safety.

Upgrading 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.  The Air Force had planned to launch the satellite on the Space Shuttle, but changed its plans after the Challenger disaster in 1986 and used a Delta II rocket instead.

Benefon, a mobile phone producer, launched the very first commercially-available GPS phone called the Benefon Esc in 1999. The GSM phone was sold mainly in Europe, but many other GPS-enabled mobile phones followed their lead. After that in 2000, the Defense Department stopped the purposeful degradation of GPS, which it integrated before the first Gulf War. GPS became ten times more precise, and all sorts of industries from fishing to forestry to freight management began utilizing it.

With the technological progress GPS receiver technology got much smaller and cheaper, private companies began integrating personal GPS into products, like the in-car navigation devices in 2000. It paved the road for phones to use cellular signal merged with GPS signal to locate the user to within feet of their actual position.

In 2005  “Block II” was launched from Cape Canaveral as the first new generation of GPS satellite. In 2010-2011 the Air Force launched two new GPS satellites, one in 2010 and one in 2011, whose purpose was to keep the constellation operable until the next generation “Block III” satellites begin launching in 2014. The new Block III satellites added an additional civilian GPS signal, and improved the performance of existing GPS service.

In 2012, the Air Force managed a constellation of 31 operational GPS satellites, in addition to three decommissioned satellites that can be reactivated upon need. The constellation was made to guarantee the availability of at least 24 GPS satellites, 95% of the time.  Today’s GPS network has around 30 active satellites in the GPS constellation. Currently, we have the US NAVSTAR , the European Union known as the Galileo positioning system, China has a local system BeiDou that might expand globally, while Russia recently restored its GLONASS system.

Today, GPS is used for countless of navigation applications, route finding for drivers, map-making, Earthquake research, climate studies, and an outdoor treasure-hunting game known as geocaching, we will cover those in details later on. But as for now let’s move on to biology class and dissect through the GPS system and see what’s inside a navigation device. Upon the first glance, a navigation device looks like nothing more than a sleek digital device, with a touch screen, however, within the exterior is a set of modern electronics that enables it to receive signals from satellites orbiting thousands of miles above the Earth and to calculate your exact position and speed on the planet.

Each component inside a navigation device has a certain purpose and each one of them is crucial to the functioning of the device. A rechargeable lithium-ion battery powers the screen and the internal electronics. There are also circuits to control the display and to respond to user through the touch-sensitive display and buttons. There are circuits too that control the data, map and route previewed as well as to produce verbal directions and camera alerts in some models. A Navigation device has an aerial inside so that it can perform its main job of locking on to the global positioning system. This receives the microwave signals from the satellites in the GPS constellation. These signals are then magnified and given to the integrated circuits that interprets 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.

How does it work?

The GPS is comprised of three parts:

  1. Space Segment: a constellation of between 24 and 32 solar-powered satellites orbiting the Earth distributed in six orbital planes inclined 55° from the equator in a Medium Earth Orbit (MEO) at about 20,200 kilometers (12,550 miles) and circling the Earth every 12 hours
  2. Control segment: Stations on Earth monitoring and maintaining the GPS satellites. A master control station and four control and monitoring stations. One is in Hawaii, the other three in remote locations as close to the equator as possible: Ascension Island in the Mid Atlantic; Kwajalein in the Pacific and Diego Garcia Atoll in the Indian Ocean. The 4 unmanned stations receive constant data from the satellites and forward it to the master control station, which ‘corrects’ the data and then sends corrected data back up to the GPS satellites.
  3. User segment: Receivers that process the navigation signals from the GPS satellites and calculate position and time such as the one found in a car.

 

Each of those satellites is in an orbit enables a receiver to detect at least four of the operational satellites. The satellites communicates microwave signals to a receiver where the built-in computer uses these signals to compute your exact distance from each of the four satellites and then triangulates your exact location on the planet to the nearest few meters based on these distances. GPS receives these signals and measure the distance to a satellite by multiplying the speed of the signal by the time it takes the signal to get there. The speed of the signal is the speed of light and the time is encoded within the signal.

In order to find longitude, latitude, and altitude, four satellites are needed. If a measurement is taken using just one satellite, then all that is known is that the receiver is on the surface of a sphere with radius equal to the distance to the satellite. If two satellites are used, then the receiver must be on the surface of both spheres which is the intersection of the two spheres or the perimeter of a circle. If a third satellite is used, then the location of the user is narrowed down to the two points where the three spheres intersect. Three measurements are enough for land receivers since the lower of the two points would be selected. But when in the air or space, four satellites are needed: the intersection of all four spheres will be the receiver’s location. When more than four satellites are used, greater accuracy can be achieved.

Now on to the geometry class, our lesson for today is: SPACE!

GPS has 24 artificial satellites that orbit the Earth at a distance of 12,600 miles/ 20,300 kilometers, issuing radio signals. The pattern of their orbit is set up in way that a GPS receiver anywhere on the Earth’s surface is always visible therefore receiving signals from at least 4 satellites. From these 4 satellite readings your GPS can work out your location through ‘Trilateration’. It is essentially the same idea as triangulation, but without using angles. In case you’re not familiar with triangulation, here’s a brief explanation: it is a process by which the location of a radio transmitter can be determined by measuring either the radial distance, or the direction, of the received signal from two or three different points. Explaining trilateration in 3-D space is somewhat tricky, so let’s start with 2-D trilateration.

2-D Trilateration:
 2D means that it has a reliable fix from 3 satellites. This provides position but not altitude. 3D on the other hand indicates a fix with 4 satellites, and position and altitude data is valid. One thing to remember, even though the GPS screen may be showing 4 or more satellites being received, the data from one or more of them may have too many received errors, or for other reasons the information from some satellites may be considered unreliable. For example if a particular satellites data is being received after a reflection, the derived position may be unrealistic. In that example it may result in an unrealistic altitude or it may not agree with triangulation of other received satellites.

The actual position is at any instant derived from at most 4 satellites. The algorithms are always looking at and comparing all satellites in view to constantly use the best group of satellites for the best and most reliable position data. The actual triangulation may change satellites every few seconds as you drive, as the signals fade and bounce. The receiver also always tries to use the satellites that give the best geometric accuracy. If the included angle between satellites is small, then the triangulation has a higher error due to geometric calculation accuracies. This error is known in GPS as Dilution of Precision (DOP). This is one of the main factors in the indicated accuracy number shown on the GPS screen. This is another consideration in which satellites the receiver chooses from all available.
3-D Trilateration

Basically, three-dimensional trilateration isn’t much different from two-dimensional trilateration, but it’s a little trickier to visualize, instead of a series of circles, you get a series of spheres. If you know you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a huge, imaginary sphere with a 10-mile radius. If you also know you are 15 miles from satellite B, you can overlap the first sphere with another, larger sphere. The spheres intersect in a perfect circle. If you know the distance to a third satellite, you get a third sphere, which intersects with this circle at two points. The Earth itself can act as a fourth sphere only one of the two possible points will actually be on the surface of the planet, so you can eliminate the one in space. Receivers generally look to four or more satellites, however, to improve accuracy and provide precise altitude information.

In order to make this simple calculation, then, the GPS receiver has to know two things:

  • The location of at least three satellites above you
  • The distance between you and each of those satellites

The GPS receiver figures both of these things out by analyzing high-frequency, low-power radio signals from the GPS satellites. Better units have multiple receivers, so they can pick up signals from several satellites simultaneously.

GPS has become a crucial part of our society, impacting our lives in many ways. The vast range of applications of this technology has an impact on many aspects of society.  With many affordable models to satisfy everyone’s preferences and budgets, now is the ideal time to learn more about GPS technologies. I guarantee that one day we will take all of this for granted just like we do now with the Internet and smart phones. The secret is to sink in, without debilitating yourself with the vast amount of choices in the GPS market, and enjoy some truly astonishing technology.

 


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