When the USSR launched Sputnik in 1957, American scientists discovered they could locate its position by monitoring changes in its radio frequency as it moved. The military realized submarines could likewise track satellite signals to orient them after months under water. In 1973, the military came up with an alternative navigation system, the same Global Positioning System in use today. Each GPS unit receives digital satellite transmissions that it uses to determine its location, based on the distance from the satellites.
For years, the government restricted non-military uses of the Global Positioning System. Developed in the 1970s, the system moved into the civilian world in the 1980s, but the military blurred the signals so that 100-foot accuracy was the best pilots or other users could hope for. In the 21st century, that's changed: Your GPS aviation unit has 10 times the accuracy of its 20th century ancestor.
The satellite transmissions include the satellite's position and the exact time of transmission. The time it takes to reach your GPS shows how far your plane is from the satellites. To ensure accurate timing, each GPS satellite carries four atomic clocks. Knowing the time and source location of three satellite signals gives your unit enough information to triangulate your plane's location, but only if your GPS has an atomic clock of comparable accuracy. Adding a signal from a fourth satellite lets the unit get by with an ordinary timepiece.
The key to GPS's accuracy is the tremendous precision of atomic clocks. Atomic-clock technology places atoms in an oscillating field, causing the field to resonate at a frequency that matches the magnetic resonance of the atoms. By using the resonance as the basis for the clock's movement, rather than a quartz crystal or a set of gears, the clocks attain unparalleled accuracy. Scientists have developed several technologies for atomic clocks, but most of the GPS timepieces run on cesium atoms.
Although the military no longer degrades GPS signals to civilians, it reserves the right to do so at any time. Even without that factor, GPS signals sometimes suffer slight errors due to signals bouncing off obstructions or slowing down in the atmosphere. In 2003, the government launched the Wide Area Augmentation System to provide greater accuracy for aviation GPS. The added information from the WAAS satellites and ground stations makes aviation GPS units accurate to within 1 to 2 meters horizontally and 2 to 3 meters vertically.
- Photo Credit Jupiterimages/Creatas/Getty Images