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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » Time ticks by radio: USCG cel nav question

   
Author Topic: Time ticks by radio: USCG cel nav question
JimG


 - posted January 28, 2006 01:06 AM      Profile for JimG           Edit/Delete Post 
Starting on page 89 in the USCG Cel Nav Notes questions 02579 thru 02591 they talk about time ticks using a signal from Montevideo, Uraguay. I have no idea what they are talking about. What would be a good source to read up on this stuff?
From: Steamboat Sp, Co.
David Burch


 - posted February 02, 2006 07:37 PM      Profile for David Burch           Edit/Delete Post 
Time keeping is covered in Bowditch, Chapter 18, but you have to go back to about the 1977 edition (the green ones) to get much detail related to using it in Cel Nav.

A better reference is Chapter 2 (Radio Time Signals) of Publication 117, Radio Aids to Navigation. I have added a copy of that here for quick reference. This is from the 2001 edition.

You can also see the latest version at

http://pollux.nss.nima.mil/pubs/pubs_j_rna_sections.html?rid=8555

but they no longer let us download the full book. We have a copy on our Bowditch Plus 2-cd set of nav references.

Also, both the 2001 and 2006 editions no longer have Uraguay time signals, so the test room must use an older version. I will try to look into that... if they still in fact even ask these questions. Some things have changed on recent tests. These days you get accurate GMT (UT) from your GPS, so there is much less call for this onetime crucial navigation service.

From: Starpath, Seattle, WA
David Burch


 - posted February 02, 2006 08:22 PM      Profile for David Burch           Edit/Delete Post 
In the note above, i mention that the full book cannot be downloaded, however i should point out that sometimes it is available. This changes with time. If you go there, you will see they have a place for the link, just no link.

This is an important reference to have onboard for ocean sailing. It is available in navigation supply stores. Several commercial companies reprint the government editions and sell them, though this is one you would want to be sure you have the latest edition of.

From: Starpath, Seattle, WA
David Burch


 - posted February 02, 2006 08:57 PM      Profile for David Burch           Edit/Delete Post 
Here is a sample problem of this type so we can tell if we even care where the signal comes from (short answer, we do not.)

02580 You take a time tick using the 2000 GMT signal from Montevideo,
Uruguay. You hear a 10 second dash, a 5 second silent period, and then six
dots. At the sixth dot your comparing watch reads 07h 58m 18s. When compared
to the chronometer, the watch reads 07h 59m 56s, and the chronometer reads
08h 00m 02s. What is the chronometer error?

(A) 1m 42s slow (B) 1m 36s slow, (C) 0m 02s fast, (D) 1m 38s fast

ANS: B

Solution:
They are reading their watch at the end of 6 dots, so this must be marking the reference time. And that is consistent with article 200D in the chapter 2 we provided in the link above. This is the "New International System" so we do not really care who broadcasted it. We know that at the end of the 6 dots, it was precisely 20h 00m 00s GMT.

Next step is, we do not know what time zone they are in or checking, but we have to assume that 08h on the ship's clock must be about 20h GMT. This is a trick part of USCG test questions that applies to all areas of cel nav issues. In USCG test problems, the ship is always keeping GMT on their "chronometer" (called CT or chronometer time) using a 12h watch face without ever telling whether or not it is am or pm -- this is their way of supporting navigation schools. So we do not know where they are, but 0800 CT (if it had no errors in it) is the same as 2000 GMT. Our job is to find out from this observation what the error in the CT is.

We know the comparing watch is slow by 08h 00m 00s - 07h 58m 18s = 01m 42s. so when they compared the comparing watch to the Chronometer that watch was slow by 1m 42 sec. that is, at the comparing time (07h 59m 56s) the actual GMT was 07h 59m 56s + 01m 42s = 07h 60m 98s = 08h 00m 98s = 08h 01m 38s. At that time the CT read 08h 00m 02s, so the CT is slow by 01m 36s.

Tedious, and never a way we would check a watch these days, but still straight forward once you see the system. And the punch line here is we do not care that the signal came from Montevideo. It could have come from any station using the 6 dot international system.

From: Starpath, Seattle, WA
David Burch


 - posted February 02, 2006 09:28 PM      Profile for David Burch           Edit/Delete Post 
by the way, our oldest edition of Pub 117 in the school is 1993-94 and it also does not have Montevideo as a source of time ticks, but that station did at that time provide other nav data by radio, that it no longer does now.
From: Starpath, Seattle, WA
David Burch


 - posted February 02, 2006 09:36 PM      Profile for David Burch           Edit/Delete Post 
Note this general observation about this problem that applies to almost all of the USCG cel nav questions and to most of their nav problem questions (which i believe were originally made up by students at the USCG academy).

The questions themselves are not "tricky," regardless of some claims to that effect by unhappy candidates. But the answers are not random, and that in itself might be called tricky, or even unfair. The wrong answers offered to most USCG cel nav questions are what you would get if you make a specific mistake.

What that means to the test taker is, you cannot feel particularly confident if you see your exact answer on the list. You should still check your work from the beginning on each question if you have the time.

From: Starpath, Seattle, WA
David Burch


 - posted March 11, 2006 08:00 PM      Profile for David Burch           Edit/Delete Post 
There is a list of current stations that broadcast time signals given on this page UTC time notes.

I am not sure it is correct or exhaustive. I would also check Pub 117 as outlined above if looking for these.

From: Starpath, Seattle, WA


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