Global Positioning System

The Global Positioning System (GPS) is at present the only fully functional Global Navigation Satellite System (GNSS). More than two dozen GPS satellites are in medium Earth orbit, transmitting signals allowing GPS receivers to determine the receiver's location, speed and direction.

Since the first investigational satellite was launched in 1978, GPS has become an indispensable aid to navigation around the world, and an essential tool for map-making and land surveying. GPS also provides a precise time reference used in many applications including scientific study of earthquakes, and synchronization of telecommunications networks.

Developed by the United States Department of Defense, it is authoritatively named NAVSTAR GPS (NAVigation Satellite Timing And Ranging Global Positioning System). The satellite constellation is managed by the United States Air Force 50th Space Wing. The cost of maintaining the system is around US$750 million per year,[1] including the replacement of aging satellites, and research and development. Despite this fact, GPS is free for civilian use as a public good.

Mean sea level pressure

Mean sea level pressure is the pressure at sea level or the station pressure condensed to sea level assuming an isothermal layer at the station temperature. This is the pressure usually given in weather reports on radio, television, and newspapers. When barometers in the home are set to match the local weather reports, they measure pressure condensed to sea level, not the actual local atmospheric pressure. The reduction to sea level means that the usual range of fluctuations in pressure is the same for everyone. The pressures which are measured high pressure or low pressure do not depend on geographical location. This makes isobars on a weather map meaningful and useful tool. The altimeter setting in aviation, set either QNH or QFE, is another atmospheric pressure reduced to sea level, but the method of making this reduction differs slightly.

QNH barometric altimeter setting which will cause the altimeter to read airfield elevation when on the airfield. In ISA temperature conditions the altimeter will read altitude above mean sea level in the vicinity of the airfield.QFE barometric altimeter setting which will cause an altimeter to read zero when at the reference datum of a particular airfield. In ISA temperature conditions the altimeter will read height above the datum in the vicinity of the airfield.