Longitude: what if the GPS satellite is out of commission?

sextant
Originally uploaded by dn

GPS, the Global Positioning System, works well when all the satellites are functioning. There are many reasons why a satellite might be out of service, however. How do ships and airplanes find their longitude when the GPS is malfunctioning?

Sextant and chronometer.

A sextant, says Wikipedia.org, is an instrument generally used to measure the altitude of a celestial object above the horizon. Making this measurement is known as sighting the object, shooting the object, or taking a sight. The angle, and the time when it was measured, can be used to calculate a position line on a nautical or aeronautical chart. A common use of the sextant is to sight the sun at noon to find one's latitude.

A chronometer is a watch tested and certified to meet certain precision standards. The marine chronometer was invented by John Harrison in 1730. It enabled accurate marine navigation. For the next 250 years, an accurate chronometer was essential to any kind of marine or air navigation until the implementation of global satellite navigation at the end of the 20th century.