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Old 28-01-2011, 08:07   #46
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No GPS? Then perhaps a navigation course is a good idea? A celestial fix is not used to tell you where you are? How is that?
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Old 28-01-2011, 08:14   #47
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Southbound.... Southbound... Westbound... Hang on... follow that contrail... its heading north....
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Old 28-01-2011, 08:47   #48
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Both my wife and I can do CN and pilotage. I'm under 50 and my wife's 37. My son, 9, has a Davis Mk 2 and I intend to give him his first lessons this year. I consider it notable that every CN instructor I've had has been over 70 years old. Haven't met an amateur radio guy under 60, come to think of it, either.

Putting aside the debate over accuracy and the point that GPS can be and has been "turned off" for reasons that are either known or remain a mystery, the stars, sun and moon cannot be turned off. The means of navigation in CN, while not accessible 24/7, is autonomous to the boat. No outside human sources are required. This is, for me, the big difference. I actively determine my position, and I do not passively receive it. Am I as "accurate"? No, unless the GPS is being jammed. Of course, I am accurate enough on the open ocean, and if I can see the shore, I can rotate the sextant and can, with the proper charts, take remarkably good bearings to obtain a fix.

Less discussed is the esoteric side of non-GPS navigation. You must sense wind, sun and water in order to refine your DR. You have to look around with CN. The compass twitches...why? It's all much more hands-on, or kinesthetic, as some would say. Looking at a moving plot with changing lat/lon numbers on a small monitor? Not so much.

Lastly, the dirty secret of cruising is that in fair weather with the windvane working well and all critical boat tasks solved...well, it can get a little dull on deck. A half-hour of CN is no worse, and considerably more useful in unexpected circumstances like a GPS "brownout", than doing Sudoku, and practice does improve one's technique.
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Old 28-01-2011, 09:04   #49
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No GPS? Then perhaps a navigation course is a good idea? A celestial fix is not used to tell you where you are? How is that?
An accurate celestial fix tells you approximatly where you were when you did the sextant sight. Normally one takes three sights and your position is somewhere in the center. It takes awhile to do the math and when you're finished, you're nolonger there.
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Old 28-01-2011, 09:06   #50
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The means of navigation in CN, while not accessible 24/7, is autonomous to the boat. No outside human sources are required. This is, for me, the big difference.
Well except for a new almanac each year and occasional time updates.
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Old 28-01-2011, 09:25   #51
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Hardly differs from resetting the chart datum or buying batteries, really, does it? A regulated/rated chronometer solves even the time issue, and is a reason to visit the Prime Meridian at noon on June or December 21 ... ehhehhhehhehh...

In truth, if you want some very dull, extensive math to do, there is the "lunar" method: Lunar distance (navigation) - Wikipedia, the free encyclopedia

It works, allegedly, and means you can chuck the watch, or rather, get a fix with a suspect one, or use the lunar method to confirm that the watch is off. In the words of Joshua Slocum, a very late user of this method, "the work of the lunarian, though seldom practised in these days of chronometers, is beautifully edifying, and there is nothing in the realm of navigation that lifts one’s heart up more in adoration."
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Old 28-01-2011, 10:02   #52
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Here is another story on the possible outages on GPS testing in the Southeast. It offers the same basic info as the link posted by the OP, but without the government conspiracy innuendos. GPS Tracking News

Just from doing a search, these tests are not uncommon. Of course, there appear to be several stories out there that are geared for the ultra-paranoid, one even suggesting government GPS "tests" and related conspiracy are involved with the recent bird deaths.

In the event the system is down for say an hour at a time, would it be that hard to just maintain a heading? If the system was out for days, I can see that would be a different story. Just asking.
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Old 28-01-2011, 10:04   #53
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I don't know what you searched, but there are notices in both "Notices to Airmen" (Notams) put out by the FAA and in the "Local Notice to Mariners" (LNM) put out by the USCG. These are official US government publications. If this wasn't true these documents would not contain such advisories. If I was sailing within 150 miles of the published position I would plan on temporary GPS outages during the periods indicated.
Look on page 4 (near the bottom) of the current USCG District 7 LNM (LNM 04/11, 25 January 2011, http://www.navcen.uscg.gov/pdf/lnms/lnm07042011.pdf) for the referenced notice.

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Old 28-01-2011, 10:08   #54
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Well except for a new almanac each year and occasional time updates.
LOL..... you mean my 1972 'Cards' and Ocean Passage Charts from an RN lifeboat are no good anymore.... shoot...
And I thought angles and seasons were pretty constant... ahh well... back to the 5-30pm Air France contrail just East of the Azores.....
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Old 28-01-2011, 10:46   #55
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In the event the system is down for say an hour at a time, would it be that hard to just maintain a heading?
That would work fine on the open ocean. If you're trying to run an inlet at night or in fog you probably can't afford to remain on the same heading for an hour.

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Old 28-01-2011, 10:53   #56
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That would work fine on the open ocean. If you're trying to run an inlet at night or in fog you probably can't afford to remain on the same heading for an hour.

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Thats where 4 other tools come into play... Eye's, ear's, depth and chart...
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Old 28-01-2011, 11:58   #57
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In my very humble opinion, and I apologize if I am stating the obvious, but, using GPS fixes and updating your pos on a paper chart every couple of hours seems to be the simplest means to work through this problem area and a good practice at any time. Looking at your plotted course I think you will be able to easily detect when the GPS signal has either been completely jammed or has been "fuzzed" a bit. The intentionally erroneous GPS fixes should fall well outside your plotted course. At that point DR using your last few reliable fixes and monitor your GPS while you cross this relatively small area, until it returns reliably to your plotted DR position and/or course. A few hours of accurate GPS fixes and you’re more than likely out of the affected area.

I would venture to say that most satellites do have "reset" switches on them. As most of us will remember the days of Win 3.1, 95, and 98. Rebooting to fix a glitch was a common practice. With NT and XP that problem became almost a thing of the past but even still that is still the only solution to a frozen machine. Satellites have computer systems not much different than the one your using right now. Add in a bit of redundancy and your pretty much using the same systems. Every once in a while those computers require an unplanned reboot to fix a glitch in programming. The risk is absolutely there that it won't restart and fingers are frequently crossed when doing a reboot, but just like your home pc, sometimes it is the last resort to return a malfunctioning system to functioning. The last one I remember, that got some media attention, was an unplanned reboot by NASA of one of the Mars rovers onboard computer systems.

Most modern communications satellites have multiple "transponders" which allow one satellite to broadcast multiple data streams to the Earth and to receive multiple data streams from the Earth. Some if not all of these transponders are intentionally very directional because of their low transmit power and the focus of the transponders "beam". A good example of this ability to "spotlight" or shine a beam from a satellite onto a particular area of the Earth is satellite tv. If you own a sat tv system you may have experienced a sports blackout for a particular game being played in your area. The area your are living in is not beamed that game, your area is blocked simply by not broadcast that particular data through the transponder that covers your area. That is why you living in Dallas can watch a Rockets game while I living in Houston cannot, all I see is a blank screen while on the very same channel you are. Directv and Dishnetowrk can control where the satellites signal lands on the Earth and what it contains when it does.

Now apply that beam technology to a satellite broadcasting a GPS signal. Add that the signal does not have to be blocked, you just change the timing on the signal. The signal is still broadcast but your GPS receiver interprets the timing and gives you a fix based on timing that is wrong! Maybe not even in the same hemisphere!

Of course there are satellites that broadcast blindly across whatever area they within line of sight of.

I know that we all understand why this is happening, but just in case. Even though we as US citizens own those satellites and the signals they produce, they are primarily a military asset and therefore controlled by the military and for the best of reasons. First, they guide our troops and weapons systems. Second, there are people out there that wish to do us harm, those very same people are using our own GPS signals to guide their weapons and determine where those weapons should detonate. Other countries, not necessarily friendly to us, use our GPS signals to guide their weapons to targets, such as our Navy's ships, our industrial facilities and our cities. You should read our cities as including your home, the homes of your family and friends. Of personal importance to me, I have two sons and a son-n-law in the US military, I don't want the people who are trying to kill them to be able to use our own technology against my boys. So, if the military wants to play with GPS signals and learn how to effectively deflect those weapons...tell me how to help.
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Old 28-01-2011, 12:37   #58
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Back in the day, we would venture 30-40 miles off the Sarasota coast in a small boat.... fish (troll) 6 or 8 hours, and come back to the same inlet.... Didn't have anything more than a compass, depth finder and a chart... and didn't look at the chart all that often. I have no aspiration to make any passage in excess of 24 hours.... I think I'll be fine if they turn it off entirely! It is troubling though when the Bermuda Triangle gets things all a-spinning, but What-the-heck, I always wanted to meet my Grandfather who got lost flying a plane in the "Big One"
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Old 28-01-2011, 12:52   #59
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An accurate celestial fix tells you approximatly where you were when you did the sextant sight. Normally one takes three sights and your position is somewhere in the center. It takes awhile to do the math and when you're finished, you're no longer there.
That much is true. Where you are has come down to a few meters. Where you were in the old days, used to be where you were within a few miles. I was doing celestial nav before GPS. If your boat is traveling five knots and it takes you half an hour to reduce your sights, then you have moved 2.5 miles. If a typical celestial fix is only good for 3 miles or so depending on the atmospheric conditions, sea conditions and the height of eye then by the old standards, you are still where you were, somewhere within a circle of estimated accuracy if you are going slow enough.

How accurate is "there"? GPS will not put you "there" by millimeter survey standards plus the latency caused by the time it takes to process and display the fix.

It's just an exercise in "there" and what's considered good enough.
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Old 28-01-2011, 13:19   #60
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Since this testing started already, has anyone had any problems?
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