Global Positioning System(GPS)
The Global Positioning System (GPS) is a space- based global reliable location and time information in all weather and at all times and at all times and anywhere on or near the Earth where there is an unobstructed line of sight to four or more GPS satellite.
History:
The GPS System was created and realized by the American Department of Defense (DOD) and was originally based on and run with 24 satellites.
The design of GPS is based partly on similar ground-based radio navigation system such as Loran and the Decca Navigator which was developed in 1940s and used during world war II.Friedwardt winterberg in 1956 propsed a test which was named as general relativity which uses accurate atomic clocks placed in orbit in artificial satellite.The first satellite navigation system used by United States navy named as Transit ,was first successfully tested in 1960.
It used a constellation of five satellites and could provide a navigational fix approximately once per hour. Nowadays, about 30 active satellites orbit the earth in a distance of 20200 km. GPS satellites transmit signals which enable the exact location of a GPS receiver, if it is positioned on the surface of the earth, in the earth atmosphere or in a low orbit. GPS is being used in aviation, nautical navigation and for the orientation ashore. Further it is used in land surveying and other applications where the determination of the exact position is required. The GPS signal can be used without a fee by any person in posession of a GPS receiver. The only prerequisite is an unobstructed view of the satellites (or rather of the sky). The correct name of the system is NAVSTAR (Navigation System for Timing and Ranging), but commonly it is referred to as GPS (Global Positioning System).
Structure:
The GPS consists of three parts:
- Space Segment
- Control Segment
- User Segement
The Air Force of United States developes, maintaines,Operates the Space and Control Segments.
GPS receiver uses to calculate its three-dimensional location (latitude, longitude, and altitude) plus the current time.
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The Space Segment is composed of 24 to 32 satellites in medium earth orbit and also includes the bosters required to launch them into orbit.
The control segment is composed of a master control station, an alternate master control station.
The user segment is composed of hundreds of thousands of U.S. and allied military users of the secure GPS Precise Positioning Service, and tens of millions of civil, commercial, and scientific users of the Standard Positioning Service .
GPS Working:
GPS is arguably one of the most important inventions of our time, and has so many different applications that many technologies and ways of working are continually being improved in order to make the most of it.
GPS Signals:
In order for GPS to work, a network of satellites was placed into orbit around planet Earth, each broadcasting a specific signal, much like a normal radio signal. This signal can be received by a low cost, low technology aerial, even though the signal is very weak.
Rather than carrying an actual radio or television program, the signals that are broadcast by the satellites carry data that is passed from the aerial, decoded and used by to the GPS software.
The information is specific enough that the GPS software can identify the satellite, it’s location in space, and calculate the time that the signal took to travel from the satellite to the GPS receiver
Using different signals from different satellites, the GPS software is able to calculate the position of the receiver. The principle is very similar to that which is used in orienteering – if you can identify three places on your map, take a bearing to where they are, and draw three lines on the map, then you will find out where you are on the map.The lines will intersect, and, depending on the accuracy of the bearings, the triangle that they form where they intersect will approximate your position, within a margin of error.
GPS software performs a similar kind of exercise, using the known positions of the satellites in space, and measuring the time that the signal has taken to travel from the satellite to Earth.
Time Correction:
The satellites each contain atomic clocks which are extremely accurate, and certainly accurate with respect to each other, we can assume that most of the problem lies with the clock inside the GPS unit itself.
Keeping the cost of the technology down to a minimum is a key part of the success of any consumer device, and it is simply not possible to fit each GPS unit with an atomic clock costing tens of thousands of dollars. Luckily, in creating the system, the designers designed GPS to work whether the receiver’s clock is accurate or not.
There are a few solutions. However the solution that was chosen uses a fourth satellite to provide a cross check in the trilateration process. Since trilateration from three signals should pinpoint the location exactly, adding a fourth will move that location; that is, it will not intersect with the calculated location.
This indicates to the GPS software that there is a discrepancy, and so it performs an additional calculation to find a value that it can use to adjust all the signals so that the four lines intersect.
Usually, this is as simple as subtracting a second (for example) from each of the calculated travel times of the signals. Thus, the GPS software can also update its’ own internal clock; and means that not only do we have an accurate positioning device, but also an atomic clock in the palm of our hands.
Applications:
The Global Positioning System, while originally a military project, is considered a dual-use technology, meaning it has significant applications for both the military and the civilian industry.
Military:
The military appliactions for GPS are:
- Navigation: GPS allows soldiers to find objectives in the dark and to coordinate the movement of troops and supplies.
- Target Tracking: Various military weapons systems use GPS to track potential ground and air targets before they are flagged as hostile.Military aircraft, particularly those used in air to ground roles uses GPS to find targets (for example, gun camera video from AH-1 COBRAS IN I raq show GPS coordiantes.
The GPS satellites also carry a set of nuclear detonation detectors consisting of an optical sensor (Y-sensor), an X-ray sensor, a dosimeter, and an electromagnetic pulse (EMP) sensor (W-sensor) which form a major portion of the United States Nuclear Detonation Detection System.
Civilian:
Civilian applications benefit from GPS signals, using one or more of three basic components of the GPS: absolute location, relative movement, and time transfer.
- Absolute location
- Relative movement
- Time transfer
Atmospheric Effect:
Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earths atmosphere
Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the path through the atmosphere is longer . Once the receiver’s approximate location is known, a mathematical model can be used to estimate and compensate for these errors.
Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the trophosphere This effect both is more localized and changes more quickly than ionospheric effects, and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects.
Changes in receiver altitude also change the amount of delay, due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct, either by applying a function regression or correlating margin of atmospheric error to ambient pressure using a barometric altimeter.
Satellites Frequencies:
L1 (1575.42 MHZ)
L2 (1227.60 MHZ)
L3 (1381.60 MHZ)
L4 (1379.913 MHZ)
L5 (1176.45 MHZ)
USES:
GPS has been used to pinpoint any ship or submarine on the ocean, and to measure Mount Everest. GPS receivers have been miniaturized to just a few integrated circuits, becoming very economical. Today, GPS is finding its way into cars, boats, planes, construction equipment, movie making gear, farm machinery and even laptop computers.
Written by AYESHA NAWAZ
