The Global Positioning System (GPS), is a constellation of 24 satellites – three of which are backup satellites – which orbit the Earth twice a day at an altitude of about 12,000 miles. Twenty-four hours a day, these satellites continuously broadcast high-frequency radio signals containing position and time data, enabling anyone with a GPS receiver to determine their location anywhere on Earth. The GPS network is run by the US Department of Defense and free-to-use by anyone with a GPS receiver or handheld GPS systems.
The GPS satellites are referred to as NAVSTAR satellites. The first was launched way back in early 1978. Each satellite weighs approximately 2,000 pounds and is about 17 feet across with the solar panels extended. Transmitter power is 50 watts or less, with satellites transmitting on three different frequencies. Each satellite is expected to last approximately 10 years and replacements are constantly being built and launched into orbit.
Every point on Earth can be identified by a pair of co-ordinates, which represent the exact spot where a horizontal line (latitude) crosses a vertical line (longitude). The basis of GPS technology is precise time and position information. Using atomic clocks accurate to within one second every 30 years and location data, each satellite continuously broadcasts the time and its position. GPS works on the principle of triangulation. By knowing its distance from three or more satellites, the receiver can calculate its position by solving a set of equations. Information from three satellites is needed to calculate longitude and latitude at a known elevation; four satellites are needed to include altitude as well.
Although run by the US Department of Defense, GPS has wide commercial applicability. Linked to a vehicle, it becomes a tool of navigation. Within the context of a co-ordinate system, it is an instrument of surveying. With a cellular phone or transceiver, it becomes a method of tracking vehicles or people. With a digitised map base, it provides an all-electronic chart. For weapons guidance, it is unsurpassed.
Until 1 May 2000, GPS offered two levels of service. The civilian typically provided a level of accuracy of around 100 yards, depending on the number of satellites available and the geometry of those satellites. Accuracy could be improved to fifteen feet or better through a process known as Differential GPS (DGPS), which employs a second receiver placed at a known location – and accessible from a number of sources on a free subscription basis – to compute corrections to the GPS satellite measurements. The service used by the military was capable of pin-point accuracy.
In fact, the reason for the two different service levels was a consequence of artificial errors having been introduced into the satellite signals so as to intentionally give the military a far more accurate system than civilians. Bill Clinton, President of the USA, ordered the military to stop scrambling the satellite signals used by civilians GPS users from midnight, 1 May 2000. The military would be allowed to scramble signals on a regional basis should it need to do so for national security reasons.
While this move led to an increase consumer confidence in the technology, the fact remained that GPS had several major shortcomings:
- A mediocre and varying position accuracy (sometimes to only several dozen meters), depending on place and time
- Regions at high latitudes, crossed by many aviation routes, do not have dependable coverage. Signal penetration in dense areas and town centers is unreliable. Furthermore, the predominantly military character of GPS means there is always a risk of civil users being cut off in the event of a crisis.
A consequence of these issues was that the European Union (EU), in close cooperation with the European Space Agency (ESA), decided to develop a system of its own that met what it deemed the necessary criteria for accuracy, reliability and security
- GSM Technology
- GPRS Technology
- 3G Technology
- WAP Technology
- Bluetooth technology
- GPS Technology
- IEEE 802.11b
- IEEE 802.11a
- IEEE 802.11g
- WiFi Access
- WiMAX Technology
- IEEE 802.11n