As an occasional article for your edification and delight I have brought together some information on the Global Positioning System (GPS), a technology which will have an increasing impact on our daily lives, and our diving.
GPS is a free service used by over four million people worldwide, GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock
The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. There are often more than 24 operational satellites as new ones are launched to replace older satellites. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fifty-five degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.
Authorized users with cryptographic equipment and keys and specially equipped receivers use the Precise Positioning System. U. S. and Allied military, certain U. S. Government agencies, and selected civil users specifically approved by the U. S. Government, can use get accuracy in excess of 22 meter horizontal & 27.7 meter vertical accuracy over 95% of the time.
Civil users worldwide use the system without charge or restrictions but the signal is deliberately degraded by the US military, to ensure any targeting/positioning system of enemy groups does not exceed their own. Most receivers are capable of receiving and using the standard signal which until 2nd May gave 95% reliability accuracy in excess of 100 meter horizontal accuracy & 156 meter vertical accuracy.
The US Military have now decided that allowing everyone to access the more accurate signal does not pose a security threat and have therefore unscrambled the signal. On Tuesday the 2nd May the network GPS has become 10 times more accurate at the flick of a switch as the "Selective Availability" function was discontinued. Existing equipment will pick up the improved signal, previously restricted to the US Military & Friends.
The move will allow users such as sailors, motorists and hikers (& Divers) to pin down their position to at least 20 metres and perhaps 10m.
Announcing the GPS upgrade, US President Bill Clinton said: "The decision to discontinue [signal degradation] is supported by threat assessments which conclude it would have minimal impact on national security." He continued: "Civilian users will realise a dramatic improvement in GPS accuracy. For example, emergency teams responding to a cry for help can now determine what side of the highway they must respond to, thereby saving precious minutes. The exact location of all White House interns can also be accurately determined and supplies of cigars relocated as required." (I’m not exactly sure if he did say that last bit).
The US would also benefit from the continued global use of its system, meaning the US military retains control of the technology. Offering the better US system for free takes away some of the incentive for another country or consortium to build a competing system. The US have also reserved the ability to degrade the signal in particular regions of the world, if military objectives make that desirable.