Global Positioning System
Previously, we talked about your position on Earth. In order to locate your position on a
map, you must know your latitude and your longitude. But you need several instruments
to measure latitude and longitude. What if you could do the same thing with only one
instrument? Satellites can also help you locate your position on the Earth’s surface (Figure
2.40).
Figure 2.40: There are 24 satellites in the US Global Positioning System. ( 2 )By 1993, the United States military had launched 24 satellites to help soldiers locate their
positions on battlefields. This system of satellites was called the Global Positioning System
(GPS). Later, the United States government allowed the public to use this system. Here’s
how it works.
You must have a GPS receiver to use the system (Figure A2.41). You can buy many of
these in stores. The GPS receiver detects radio signals from nearby GPS satellites. There
are precise clocks on each satellite and in the receiver. The receiver measures the time for
radio signals from satellite to reach it. The receiver uses the time and the speed of radio
signals to calculate the distance between the receiver and the satellite. The receiver does this
with at least four different satellites to locate its position on the Earth’s surface (Figure B
2.41). GPS receivers are now being built into many items, such as cell phones and cars.