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Lodesman · 11-12-2008, 05:21

An estimated correction is transmitted in the GPS signal - depending on your GPS unit, this could be applied to give a displayed time that is accurate within a second of UTC. I doubt anyone's celestial nav skills are so great (or so poor) as to make a difference from that 1-second error.

**Ex-Calif** · 11-12-2008, 06:00

Dumb guy mode. If a GPS keeps time within 250 nanoseconds and does not update for leap seconds, but we know how many of these have been missed, wouldn't it make sense to just add the leap seconds to GPS time and call it within 250 non-seconds?

Gord's caveat regarding serial updates notwithstanding.

Viking Sailor · 11-12-2008, 06:15

Quote:

Originally Posted by Ex-Calif

Dumb guy mode. If a GPS keeps time within 250 nanoseconds and does not update for leap seconds, but we know how many of these have been missed, wouldn't it make sense to just add the leap seconds to GPS time and call it within 250 non-seconds?

Gord's caveat regarding serial updates notwithstanding.

This is exactly what is done to display ~UTC on a GPS receiver. However, it can take minutes for the receiver to receive that part of the data message that contains the leap second count. It looks like there will be a leap second added at the end of this year. BTW, one second is about 1/4 of a nm.

slomotion · 11-12-2008, 08:45

More dumb guy mode: If you have GPS and it's working, why would you bother with celestial navigation?

You can use SNTP to automatically set your computer to atomic time from the Naval Observatory or other time server every time you connect to the Internet. Portable computers aren't otherwise particularly good time keepers but if recently updated, they can provide a good source for setting a digital watch.

Lodesman · 11-12-2008, 09:10

Quote:

Originally Posted by Viking Sailor

one second is about 1/4 of a nm.

Wouldn't that be a maximum value - E or W azimuth, taken at the Equator?

Randy · 11-12-2008, 09:10

I keep going back to my Navy days as a reference and example. For all the sophistication and resources the Navy has and they gladly use all of these available tools, they still operate in the failsafe mode. Every ship in the Navy is taking a minimum of 3 celestial fixes a day and it is these celestial fixes that are hand delivered to the captain. Electronics, while they are reliable - FAIL. You have to be able to be capable of total self reliance NO GPS.
It's unbelievable to me how sailors are setting themselves up for failure.

**Casco Bay** · 11-12-2008, 13:19

Suunto X10 Military GPS Watch

I agree about cel nav, and also am looking for a good watch. A friend of mine is in the Marine Corps, and uses the above watch to help with Land Navigation, although as mentioned by Randy about the Navy, the Marines still must use dead reckoning or cel nav as the primary technique.

Little Otter · 11-12-2008, 20:13

We have Land Nav for ROTC and we just use a compass and count our steps for the approx. distance.

**GordMay** · 12-12-2008, 02:19

Quote:

Originally Posted by Viking Sailor

BTW, one second is about 1/4 of a nm.

Disclaimer: I don’t “do” celestial navigation.
Since one nautical mile corresponds to an arc of one minute on the surface of earth, and the earth goes one complete revolution (360 deg) in 24 hours, each hour corresponds to 15 degrees̊ (15 x 60 = 900 minutes) of longitude, or 900 Nautical miles.
Hence, each minute corresponds to 15 nm, and each second to 0.25 nm.

**Pblais** · 12-12-2008, 04:28

Quote:

More dumb guy mode: If you have GPS and it's working, why would you bother with celestial navigation?

I think the question was too technical

I have to admit when the GPS is working I use it. If I am some place where I can visually verify the location with something on land I tend to try and verify from that too. Neither celestial nor GPS can tell you where you are supposed to be.

bruce in oz · 12-12-2008, 05:39

250 nanoseconds, 14seconds? by the time you have written down the fix, its wrong using that timeframe. where i've seen most folk come unstuck is when they are not using utc or dont change the tiemzone relative to their apparent location. i've supported defence forces (technically) with their megabucks hardware and noted that the clock in my digital camera was as accurate. but then again i had a french sheila onboard for a week or two a few years back, she would always fart at 0900 and demand i make her coffee, defence forces cost less....

counsel1956 · 19-12-2008, 20:42

You can buy a WWV receiving watch on ebay for $6.95. Error less than one second. Mine always matches my Garmin output. When I'm reducing the watch error is zero.

Randy · 20-12-2008, 10:09

Counsel,
Yes, I've got one too. My concern with it is whether or not it'd pick up a signal reliably in the nether reaches as I believe it's receiver only works on one band.

counsel1956 · 20-12-2008, 21:28

From Wikipedia: WWV is the callsign of NIST's shortwave radio station located in Fort Collins, Colorado. WWV's main function is the continuous dissemination of official U.S. Government time signals. The station broadcasts simultaneously on five distinct frequencies: 2.5 MHz, 5 MHz, 10 MHz, 15 MHz and 20 MHz.

I don't know if the watch receives on more than one frequency but it worked from the East coast to Bermuda and back. Depending on conditions and range 20 meters is usually pretty reliable. Any HF signal is vulnerable to solar anomalies.

counsel