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Finding the time for celestial nav

May 3, 2017

There’s no question that the sextant is the star of celestial navigation. Everyone instantly recognizes it. However, there is another element that is just as important: time. For your sight reductions to be as accurate as possible, you need accurate Greenwich Mean Time (GMT). And one way to get it is to use that timepiece you carry around with you everywhere — your smartphone.

You can get apps for your phone that automatically display GMT down to the second. An example of this type of app is Zulu Time, an iPhone app written by a developer in Ashland, Ore., named Kristian Ljungkvist, whose company is called Rate of Climb Industries.

So how does Zulu Time get GMT? Here’s how Ljungkvist, a pilot who flies a Cessna 180 Skywagon, explains it: “The app queries the internal iOS time service using GMT as the time zone. iOS on the iPhone fetches the current time over cell or Wi-Fi automatically with regular intervals, which is based on (depending on the circumstance) the atomic clock at NIST or a GPS timecode at a cell tower. In testing, I’ve compared Zulu Time’s time to the NIST official time and have had good results. On the Apple Watch, Zulu Time is even more accurate (according to Apple, five times more accurate), since Apple Watch polls a set of NIST servers at more frequent intervals for the most accurate time.

“I calculate and display the time offset compared to your local time zone in the same way, using the iOS subsystem and the phone’s local time zone, comparing that to the GMT time zone time, which allows it to handle things like daylight savings automatically, etc.”

So as long as your phone is in cell coverage it is getting very accurate GMT. What about offshore? In that case, of course, the phone can’t be corrected via the cell network and its accuracy is tied to the accuracy of the phone’s internal clock circuitry. The longer the phone goes without an update, the more inaccurate phone time is likely to become. 

The solution, of course, is to fall back on other sources of accurate time available via satellite: time from GPS and from your satphone unit. So, there are ways of using high tech to help with your low-tech — and, because of that, rewarding — celestial navigation. 

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