What is gps instruments?

GPS stands for Global Positioning System, and is a technology by which you may fix your position rather accurately, anywhere on the globe.

It relies on a constellation of 24 satellites (plus a few spares) that emit accurate clock 'ticks' that may be used to give you (via your handheld machine) an accurate position. Of great value to mariners, backpackers and so on.

Soon another equivalent system may be up and running, called Galileo, and the Russians have another system called Glonass. All systems operate similarly.