Global Positioning System

Exactly five years ago, Bill Clinton announced the use of satellite navigation systems for the Global Positioning System began to be used by civilians, before the GPS is a proprietary technology that is used only in the military. GPS consists of three parts, the part that regulates a constellation of space satellites, the controls that monitor and adjust the orbit and transmit data, and user as the user of GPS. GPS was designed and controlled by the U.S. Department of Defense that can now be enjoyed by publik. GPS as we know it like a mobile phone or PDA is a GPS receiver. The first time I held GPS is about mid-2001 for the purposes of employment, because the brand was bought by the office more and more relatively cheap prices. GPS receiver works to receive signals from GPS satellites. This is pretty much the satellite orbiting in Medium Earth Orbit (MEO), at an altitude of 1.400km (Low Earth Orbit – LEO) up to 36.000km (geostationary Earth Orbit – GEO). Collection of GPS satellites known as GPS satellite constellation, currently has more than 24 satellites orbiting above the earth in 6 orbital plane field (not just orbit parallel to the equator).

GPS satellites first launched in 1978 and is orbiting at an altitude of 20.200km and the cutting edge (all 52 GPS satellites) was launched 6 November last year. GPS satellites are not geostationary, fixed position relative to the equator, orbiting one field with the rotation of the earth-like satellites we use for internet communications and television broadcasts, namn orbit twice in one day (probably the same as the linear speed of the linear speed of a geostationary satellite, but due to a lower orbit could encircle the earth twice in 24 hours, compared with a linear speed of 8km per second LEO satellites, MEO satellite linear speed of course higher).

The entire GPS satellite orbits as he broadcast the signal to earth. Signal that is sent is the atomic time epoch, the satellite coordinates, inclination, orbital velocity and so are useful for the GPS receiver in determining the precise position. A GPS receiver can determine the position coordinates of latitude / longitude and elevation above sea level appropriately when receiving signals from four GPS satellites, if only three signals are obtained then the elevation would not be accurate.

Each satellite is broadcasting signal which is distinguished by the CDMA code and differences in the time period at the same frequency. 4 GPS receiver calculates the satellite signal obtained with an internal database that has been standardized available. Elevation could be determined due to the calculation of delay each signal from each satellite, the delay is then regarded as the distance (range), known then as the pseudorange. When the GPS receiver receives satellite signals 4 then at that position the GPS receiver into slices of four lines of satellite signals, hence the positioning precision of GPS receivers can be determined, namely the coordinates of latitude, longitude and elevation. Reference time on the GPS receiver does not have to be accurate, sufficient degree of accuracy in the short term (eg enough in a matter of minutes) because for the next time reference will be obtained from the satellite signal, which is an atomic time, atomic clock.

GPS positioning were not spared from the error factors, such errors can arise from:

Reference time, for accuracy at least 4 satellite signals must be obtained.
Ionosphere, weather conditions affecting delaynya time signal reception, as the correction of other satellites transmit signals at different frequencies, as a comparative calculation to achieve precision.
Multipath, GPS receiver not only receives signals from satellites but could have been from reflection, from other devices on the mainland and so on. Transmit GPS signals at L1 frequency (1575, 42MHz), L2 (1227, 60MHz) are used for correction due to the ionosphere, L3 (1381, 05MHz), L4 (1841, 40MHz) and L5 (1176, 45MHz).
Selective Availability, originally the civilian could not catch any GPS signal, but after Bill Clinton opened the right to use the GPS then the civilians could get more precision GPS positioning.

In the future mobile phone device may be equipped with GPS and built mass, so that users and operators can track the position. One disadvantage is the basic GPS receiver must be in open space. It may also someday GSM and CDMA cellular base stations could serve as a relay satellite GPS, so it indoors or in the basement of the position can be determined precisely.

Posted in: GPS unit