Time for navigation


The broadcast scheme for time ticks from radio station WWV.

In this issue’s installment of our celestial navigation series, we cover the importance of time for getting lines of position (LOP) and multi-LOP fixes. In order to be as accurate as possible in celestial nav, you need good technique with your sextant. In addition, you need to know the time down to the second. Who do you get it from? And how do they know what time it is?

Let’s say you want to set your navigational watch, the one you keep in the sextant box. There are several ways to obtain accurate time depending on where you’re located. On land, an easy way to get time is from your computer, your smartphone or your tablet. The Windows, Macintosh, Android and iOS operating systems automatically check in with their respective time servers and update your machine to the proper time. Another option for getting time on land is to call the National Institute of Standards and Technology (NIST). A phone call to 303-499-7111 will connect you to NIST’s “time tick” service and you can get precise time. The time provided is Universal Time Coordinated (UTC), which is the same as Greenwich Mean Time (GMT), the time we use for celestial navigation.

If you’re at sea, you can use one of NIST’s two radio broadcast stations: WWV, broadcasting from Fort Collins, Colo.; or WWVH, which transmits from the island of Kauai, Hawaii. Both stations broadcast at 2.5, 5, 10 and 15 MHz. WWV also broadcasts signals at 20 MHz. The content of these radio messages is the same as the message you can receive via telephone. Each second is represented by a ticking sound, and at 52.5 seconds on WWV (45 seconds on WWVH) a voice announces, “At the tone, XX hours, XX minutes Coordinated Universal Time.” Since the U.S. West Coast can receive signals from both WWV and WWVH, NIST differentiates the stations by using a male voice to announce the time on WWV and a female voice on WWVH.

There’s something wonderful about being 1,000 miles out at sea, firing up the SSB, tuning to one of these time-tick stations, and hearing a ticking clock followed by the announcer’s voice intoning the universal time. The audio coming over the radio speaker makes it special in a way that lighted LED numbers never can.

Another way to find accurate time at sea is to get it from your GPS receiver. GPS units will provide time accurate down to the second. It is important, however, to know if your receiver is providing you with UTC or GPS time. The GPS satellite constellation is operated by the U.S. Department of Defense, and the DOD uses its own time standard for the system with a starting point of 0 hours on Jan. 6, 1980. Since that time, UTC has been repeatedly changed with the addition of leap seconds to account for the fact that the Earth’s rotation rate is slowing. For example, at the time of this writing, GPS time is 18 seconds faster than UTC. GPS satellites broadcast the time offset between GPS time and UTC, but not every GPS manufacturer uses that data to adjust the unit’s displayed time to UTC. So, to use the time from your GPS unit for celestial navigation, you should check your receiver’s manual and see if the unit makes that adjustment.

The other question, of course, is how does NIST know what time it is? The simple answer is atomic clocks. For the many people with an exaggerated fear of anything atomic or nuclear, such as nuclear power plants (which have never killed anyone in the U.S., unlike the coal, oil and natural gas accidents that have killed thousands of people), rest assured that there is nothing to fear from an atomic clock. It isn’t a miniature nuclear reactor. These clocks use cesium atoms as their pendulum. The cesium atoms are bombarded by beams of microwaves or by groups of lasers. The cesium absorbs the energy from these beams and the atoms transition from an unexcited state to an excited one. As explained on the NIST website:

“With an atomic clock, there is a natural tendency of atoms to change energy levels when they are exposed to very specific (‘resonant’) frequencies. If the correct frequency can be generated to make the atoms change, then that frequency can be counted or divided down and compared. In this case, the generated frequency is the ‘tick.’ The benefits of an atomic clock are that the resonant frequencies are natural properties (not human-made) and that they are very high frequencies, in the billions of hertz. If an atomic clock was off by 1 Hz and the frequency was 1 GHz (1 billion Hz), then it would be off by one second in 31.7 years or, roughly, 86 microseconds (0.000086 seconds) per day.”

What that boils down to is that atomic clocks are extremely accurate. Both WWV and WWVH have five atomic clocks located with their transmitters, and these clocks are routinely compared to the master atomic clock that NIST operates at its nearby facility in Boulder, Colo. GPS satellites also carry four atomic clocks on board.

As good as those atomic clocks are, scientists work to make them better. Previously, the best cesium atomic clocks were cesium beam clocks that used microwaves to push them to the transition point; now, NIST uses a cesium fountain clock excited by lasers. Getting accurate time has never been easier, ashore or at sea.