Timing \u2014 the unglamorous yet essential T in PNT (positioning, navigation and timing) \u2014 has been called \u201cthe invisible utility.\u201d In fact, it\u2019s been a long time since we last put a GNSS-timing receiver on the cover. (Partly that\u2019s because, like with simulators, it\u2019s hard to come up with a visually compelling image that conveys the role of such a device.)<\/p>\n
From St. Augustine (\u201cWhat, then, is time? If no one asks me, I know what it is. If I wish to explain it to him who asks, I do not know.\u201d) to theoretical physicist Carlo Rovelli (who argues that time is \u201cpart of a complicated geometry woven together with the geometry of space\u201d), time is both one of the greatest mysteries of nature and one of our most practical concerns. For satellite navigation, time is both essential to its functioning and a fabulous by-product. As David Wells and Alfred Kleusberg wrote in the first \u201cInnovation\u201d column, in the first issue of this magazine, \u201cOne of the byproducts of getting an SPS [Standard Positioning Service] position fix is that a clock in the user\u2019s receiver is automatically synchronized to clocks in the GPS satellites to an accuracy of one ten-millionth of a second. Therefore, any GPS receiver is a very accurate time distribution device.\u201d (\u201cGPS: A Multipurpose System,\u201d January-February 1990.)<\/p>\n
As Richard Langley wrote in another early \u201cInnovation\u201d column, \u201cThanks to minute energy changes in individual atoms of cesium and rubidium, humankind possesses the ability to synchronize clocks anywhere in the world to better than 10 nanoseconds. But given this amazing ability to measure time, we still don\u2019t know what time actually is.\u201d (\u201cTime, Clocks, and GPS,\u201d NovemberDecember 1991.)<\/p>\n