There currently are 24 operational satellites as part of the GPS satellite constellation, that orbit the earth with a orbital radius of ~16,000 miles (from the center of the earth). The satellite orbits are spaced out in 6 orbital planes (orbital plane is the plane surface formed by the path traced by the orbit of the satellite – it is nearly circular). The six orbital planes are separated by an angle of 60 degrees. Refer to the constellation figure below. Four to six satellites occupy each orbital plane.
In addition to the 24 operational satellites, there currently are 6 satellites in orbit that are maintained as spares, should any of the 24 operational satellites malfunction. The six spare satellites are kept operational for users to receive signals from them just like from the 24 satellites. However, the U.S. Government does not guarantee their availability at all times.
Remember that the GPS satellites are being launched since 1974. Satellites like everything else have limited lifespan. The GPS satellites have been designed to provide a lifespan of ~8 years. New satellites are being launched to space at regular intervals of time to make up for satellites that have lived out their lifespan.
When the satellites are launched into space, they are inserted to pre-calculated positions in their orbital planes. The pre-calculations ascertain that a GPS receiver located anywhere on earth and at all times, can receive signals from at least four satellites. This is required because earth blocks the GPS signals from passing through it. For a GPS receiver to be able to receive a signal from a satellite, the straight line-of-sight line should not have any obstructions in between. Since the satellites are flying around the earth, not all satellites are visible to a GPS receiver located somewhere on earth. Thus, though there are 30 operational satellites, not all of them can be visible at a location on earth, at any given time.
For a more comprehensive knowledge of GPS today, refer to websites such as www.gadgetknowledge.com/gps1.html
Steve Kinder
http://www.articlesbase.com/gps-articles/gps-satellite-constellation-85046.html
#1 by Average Joe on November 3rd, 2009
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How does the Iridium satellite constellation work?
#2 by doug_donaghue on November 3rd, 2009
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Rather well, actually. Contact is established through an uplink and the system scans for the intended receiver. Then the nearest satellite to the receiver is used for downlink contact and a 2-way is established.
Doug
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#3 by poornakumar b on November 3rd, 2009
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While it lasted it worked very well.
There were 11 polar low-earth orbits; in each 7 satellites were placed at equal intervals, so all of them moved at the same inter-satellite ditances. At poles, where orbits overlapped, it was designed so that each orbit is separated in height (by distributing each of the 11having perigees at different, equally distributed latitudes). Thus, there were 77; and the elemnt ’77′ in periodic table is ‘Iridium’; that is why the name. But later, it was modified with 6 satellites each in the 11 orbits.
Then the particular satellite that happens to be overhead of the transmitting radio, picks up the signal (in ‘packets’), then efficiently convey it by relaying from satellite to satellite till the satellite covering the geographical location of intended recipient radio. Because they are all in LEO (low-Earth Orbit about 400km above groung), the latency (the time taken by the electricalmagnetic signals, at the speed of light; almost following the great circle distance between those two points) is not noticeable, unlike the Geo-stationary satellites placed at 33,000 km disatnce (electromagnetic signals cover it in 0.11 seconds, one way) if it is directly above, that is not the usual case. Two points diametrically opposite on earth ( antipodes) are at a distance of 12,550 km that is the maximum.
It needed lot of highly sophisticated software (no ‘bugs’ can be tolerated) running into reams of paper (hardware to boot, too) and cost of launching all those satellites in precise orbits just to say ‘Papa! how are you?’. Iridium satellites sense international borders and switch as enter a non-member countries. During ‘Kargil’ war, the TV channels in India got ‘real’ time coverage from the near zone of war front from ‘Iridium’; the satellites switched off in Pakistan, a non-member of Iridium.
How much did it cost, paying all those high-paid dozens of software programmers, electronics & communication experts and workers! Its way too expensive and could not sustain. The consumer wanted all these at affordable (cheap) cost that we have now!
After it declared ‘bankrupt’, it looks like American armed forces acquired it very cheaply.
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