Time Signals

Old Jul 17th 2011, 3:36 pm
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Default Re: Time Signals

Originally Posted by Fred James
There was a "work around" for some time called "differential GPS. That worked by a fixed land station with an exactly known location, receiving the GPS position, comparing it to the actual position and then transmitting the "error" by radio to the special DGPS receiver which corrected their received signal using the broadcast error hence getting the true position.

It only lasted for a couple of years before the US turned off the wobble and the DGPS receivers became redundant.

There are special techniques for averaging out the received position to get a more accurate position.

You mention LORAN. I never tried that but in the 80's I had a small yacht version of the Decca Navigator - like the system the fishermen used and that was very good - better than the alternative "Satnav" system that existed at the same time.

Interesting memories!
it had to be good to get a fishing boat into harbour in fog - and it did thousands of times
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Old Jul 17th 2011, 3:47 pm
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Default Re: Time Signals

Originally Posted by Domino
it had to be good to get a fishing boat into harbour in fog - and it did thousands of times
It was pretty good for cross Channel yacht racing in fog - especially if no one else had it! I got a few cups for that!

GPS on yachts is far better than a Tom Tom - they interface with the autopilot and you can set a complicated route and let it do all the work.

One day cars will be like that!

And no - none of this is off topic as they all rely on time signals!
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Old Jul 17th 2011, 3:51 pm
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Default Re: Time Signals

Originally Posted by Domino
all the ships I was on used LORAN but thats a radio based system based on the principle of the time difference between the receipt of signals from a pair of radio transmitters.
Don't forget Omega, Decca and the original Sat Navs that only gave a position roughly every 90 minutes.

Being old I can use a sextant but never had a very accurate timepiece
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Old Jul 17th 2011, 3:59 pm
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Default Re: Time Signals

I don't remember Omega but Decca gave a continuous position which is why it was so much better than Satnav.
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Old Jul 17th 2011, 4:35 pm
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Default Re: Time Signals

Originally Posted by Fred James
I don't remember Omega but Decca gave a continuous position which is why it was so much better than Satnav.
I agree Decca was better than Satnav, but Decca only covered a small percentage of the Globe.

http://en.wikipedia.org/wiki/Decca_Navigator_System

Omega was a long range radio positioning but was more widely used in aviation.
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Old Jul 17th 2011, 4:54 pm
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Default Re: Time Signals

The odd thing with Decca was that because the signal had fixed errors you could buy special "Decca" Admiralty charts with lots of overprinted squiggly lines on them to give you a more accurate position.

I seem to remember that they were a tad expensive! But then the Decca Yacht Navigator was a snip at £2000 - in 1980!
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Old Jul 17th 2011, 5:17 pm
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Default Re: Time Signals

Omega was a US proprietary system operated by their coastguards in the early 1970's in certain "convenient" places in\around US countries of influence.
As they operated at much low frequencies than those used by other systems the transmission was nearly all ground wave, no sky wave, so when when it was pensioned off (about in the mid-late 90's) some were converted to "other use" - such as submarine communications.
As they were all ground and no sky the frequency was also very accurate at long distances
Remember the "Luxembourg effect" where it faded in and out ? That was caused by the sky wave nulling out the ground wave
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Old Jul 17th 2011, 5:18 pm
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Default Re: Time Signals

Originally Posted by Fred James
The odd thing with Decca was that because the signal had fixed errors you could buy special "Decca" Admiralty charts with lots of overprinted squiggly lines on them to give you a more accurate position.

I seem to remember that they were a tad expensive! But then the Decca Yacht Navigator was a snip at £2000 - in 1980!
but someone had to pay to keep those white "warships" out at sea carrying out surveys !!
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