Odd O'blography

GPS Relativity

Einstein's theory of relativity suggests that time is not a constant. Rather, it depends on both perception and location. Time runs slower for an object perceived to have a high velocity relative to the observer. Time also runs slower for an object located in a strong gravitational field. These strange concepts are merely academic for those traveling together on a small blue planet at the same velocity and subject to the same gravity. Clocks anywhere on earth match to an extremely close degree of accuracy, but they do not match clocks in orbit or traveling in outer space.

The predictions of relativity became measurable with the advent of the space age, and they became a practical consideration when the Global Positioning System was designed. GPS satellites carry an atomic clock which is synchronized with a similar clock on earth. The high velocity of the clock in orbit causes it to run slow relative to the clock on earth. But the clock on earth is subject to a stronger gravitational field, so it runs slower than the clock in orbit. These effects cancel each other to some degree, but they don't have the same magnitude.

The original GPS clock software was designed with two different correction factors, one for velocity and another for gravity. Some scientists were still skeptical of relativity, so these correction factors were made optional. When the system went into operation, both factors were found to be necessary to achieve the desired results. Just as Einstein predicted.