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Old 28-07-2014, 04:22   #106
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Re: What will Happen to you when GPS goes Down

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Originally Posted by socaldmax View Post
OK, long version.

I was a Master Training Specialist in the US Navy. That's an instructor who has received the highest evaluations on the podium and has mastered curriculum development. For the first 2 yrs, I taught Operations and Maintenance on the world's most accurate RDF system, deployed on subs. I cannot tell you the accuracy, frequency coverage, speed, sensitivity or name of the system. It's all secret or above.

After getting out of the Navy, I spent 15 yrs as a Subject Matter Expert for a defense contractor, 1 of 4 in the nation. We traveled to every sub base and groomed or overhauled these systems. We also designed and installed a number of upgrades to increase accuracy, reliability or usability of the system.

Since I cannot tell you any specifics of that system, and how it worked, I can tell you this. It cost over $1.2M. Generally speaking DF accuracy increases with frequency. SHF or EHF DF accuracy is going to be orders of magnitude more accurate than MF, or even HF. Even HF or VHF accuracy is nothing to write home about.

By the time you get down to VLF, the wavelength and antenna would be far too huge (the transmitter antennas are 30 to 40 mi long) for you to deploy one on a yacht, much less get any DF accuracy out of it. Subs use a floating wire, and unless you're on something like a fishing vessel, you don't have the room to deploy and retrieve an antenna that will give you any sort of DF bearings. It's one thing to receive a signal, it's entirely different to get accurate DF bearings out of it.

Let's discuss Lojack, that's in the civilian world. It's a piece of crap relative to what the military uses, but one can learn something from it. It uses an array of 4 antennas mounted on the roof, the look like VHF or UHF frequency range. It's mounted on a vehicle that can be turned almost instantly to try to resolve errors in DF reception.

VLF is a whole different story. To duplicate that antenna array, you'd need to spread 4 antennas out 30 - 40 mi ea at 90* to each other, make some very accurate amplitude readings from each of the antennas simultaneously, then use trig to calculate the path of the incoming signal +/- 180*. Not only can the signal be coming from opposite directions simultaneously, the calculations require one to resolve the angle independent of signal strength - antennas cannot tell left from right. They can only tell if the signal is coming in weaker or stronger. Don't forget you'd have to shift the entire array a few times, maintaining 90* separation in order to get meaningful readings.

In addition:

You would need 3 VLF stations transmitting around the same time.
You would hopefully not have any changes in incoming signal strength during your measurements.
You would need to repeat the process for each transmitter.
You'd need to know which transmitter is located where.

After all of your maneuvering and calculating, your best results would still fall +/- the size of Africa, simply due to the errors inherent in receiving a signal with a wavelength that is 40 miles long.

I'm sure you have no intentions of DFing your way around the ocean using non existent VLF signals. This was just an exercise in "what if..."

I hope I gave you sufficient information to convince you it's just not worth it. You would absolutely be better off just pointing the boat based on where the sun rises and sets every day. I'm guessing that method would be at least 10x more accurate.
Thanks for the long version .
I agree with everything you have written in this post
Somehow I must have given you (and others) that I was thinking of DFing on a VLF transmitter; so my apologies if that is the case; I know this is totally unrealistic.

What I have been trying to express is the following.

The last generation of the Omega Long Rang Nav (LRN) receivers would use both the dedicated Omega transmitters and the dedicated VLF transmitters to resolve a location. As John explained, this is achieved by comparing phasing and timing of the CW of several transmitters scattered around the world. The net effect was that the receiver had at least three circular LOPs. The precision of each depended in a big way on the accuracy of the timing but once plotted, the resultant fix was within a few miles.

The LNR receiver didn't care if the signals originated from Omega transmitter or a VLF transmitter. Just like some GPS receivers can use USA or Russian satellites.

So if the some of the old VLF transmitters are still active (and I believe they are), a LNR receiver should still work. As previously posted, I accept that the coverage would be far less than average.

But if someone can point out why the above is incorrect, I would be grateful!
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Old 28-07-2014, 04:31   #107
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Re: What will Happen to you when GPS goes Down

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Fortunately, the blast site of the CMEs was not directed at Earth. Had this event occurred a week earlier when the point of eruption was Earth-facing, a potentially disastrous outcome would have unfolded.
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Old 28-07-2014, 05:32   #108
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Re: What will Happen to you when GPS goes Down

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Originally Posted by ka4wja View Post
I'm not sure if it is appropriate to wade into someone else's specific discussion....but, perhaps I can politely add some info/comment that might help clarify some things???............



AND--OR--


..............
(although my use of Omega was in the 70's, with early units....I was told that later Omega receivers actually had Lat/Lon displays that did not require dedicated Omega charts...but, I never used one of those "fancy" new Omega receivers...)
..........

So, to sum up....
- I hope I didn't step on anybody's toes...
AND
- I hope my "technical explanation" cuts the mustard without being condescending....



Fair winds...

John
s/v Annie Laurie
Hey John, welcome to the light hearted "discussion" - the more the merrier and my toes are fine.

I never used any of those early LNR receivers; all the ones I repaired (&used) had fancy lat/lon displays

But I do recall they received RF from VLF transmitters and used that (as well as Omega transmitters) to resolve a position.

Perhaps they needed at least one Omega signal but I seem to recall they would work with VLF transmitters only - but prolly wrong

And perhaps the VLF transmitters no longer transmit in the same format
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Old 28-07-2014, 06:27   #109
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Re: What will Happen to you when GPS goes Down

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Originally Posted by Wotname View Post
Thanks for the long version .
I agree with everything you have written in this post
Somehow I must have given you (and others) that I was thinking of DFing on a VLF transmitter; so my apologies if that is the case; I know this is totally unrealistic.

What I have been trying to express is the following.

The last generation of the Omega Long Rang Nav (LRN) receivers would use both the dedicated Omega transmitters and the dedicated VLF transmitters to resolve a location. As John explained, this is achieved by comparing phasing and timing of the CW of several transmitters scattered around the world. The net effect was that the receiver had at least three circular LOPs. The precision of each depended in a big way on the accuracy of the timing but once plotted, the resultant fix was within a few miles.

The LNR receiver didn't care if the signals originated from Omega transmitter or a VLF transmitter. Just like some GPS receivers can use USA or Russian satellites.

So if the some of the old VLF transmitters are still active (and I believe they are), a LNR receiver should still work. As previously posted, I accept that the coverage would be far less than average.

But if someone can point out why the above is incorrect, I would be grateful!
Wotname,

Like John I don't wish to bore with facts what is a great light hearted deviation away from threads about washing diapers in the galley sink.

Many governments around the world are steadily working to provide their coastlines with some other system (e.g eLoran) as a backup to GPS. There are many reasons such as North Korea, Middle East troubles and genuine concern over solar flares. Nearly all late model satellites have a solar flare protection system. NASA and ESA have 24/7 monitoring of the sun so they know when to engage the spacecraft protection systems. But one overriding concern the US has is a rogue state launching a nuclear EMP weapon. One high altitude burst could knock out satellite comms without destroying any satellites. That seems more likely to many military minds than a solar event.

VLF stations are indeed operating today. VLF is a group of frequencies from 10kHz to 30kHz used primarily for surface ship and submarine communications. The largest network is operated by the US Navy with world wide coverage. Many other countries operate VLF systems such as Japan, Italy, UK, France, Korea, Norway, Turkey, Russia, China, Israel and others. Pretty much anywhere on the ocean you should be able to receive VLF with a very small loop antenna. There are online instructions available that require nothing more than wire, styrofoam, fiberglass and resin. If you have a VLF receiver you can do a bit of RDF once you get close (few hundreds of NM) to every continent except Antarctica. One of the biggest VLF transmitters in the world is on the NWC of Oz. There is an active online amateur community that plays around with VLF reception.

But there is a problem using your old Omega receiver. It relied on CW transmissions from VLF. Because the word rate can be 4 times faster using what is known as MSK nearly all VLF stations have stopped transmitting CW. Newer VLF receivers that the military use can decode MSK and in fact can generate a surprisingly good position fix from VLF transmissions. The antennas are quite small contrary to the assertions of another poster. They are about the size of a typical recreational radar antenna.

So your idea is not far fetched and the military are doing it right now. Even so, my advice would be to worry about keeping the water outside, steering, sails, keel, safety gear, food and water before spending mental energy on loss of all GPS.

Be safe out there.

Dan
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Old 28-07-2014, 07:23   #110
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Re: What will Happen to you when GPS goes Down

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None of this is going to matter much once the alien space fleet shows up!


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They'll have to use celestial navigation to find us.

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Old 28-07-2014, 07:41   #111
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Re: What will Happen to you when GPS goes Down

Canibul, although we have a chartplotter and a handheld gps we still plot on paper as well. On longer runs we record positions , compass headings etc in a log. We also note compass headings required to reach our intended destinations and alternates. and yes we posses a sextant and one of us knows how to use it.
We practice our DR skills just to keep in practice in case of trouble with electronics.
Besides it's kind of fun to match our markings on the paper charts to the positions given by the GPS.
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Old 28-07-2014, 09:42   #112
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Re: What will Happen to you when GPS goes Down

I learnt CN 30 years ago, I had a wonderful platform, a VLCC that had the new fangled Sat Nav. With practice and instruction from the the 1st Mate, I got tiny cocked hats and I reckon with a proformer I could still get it sorted (a classic book at the time was by Mary Blewitt).
The point that I want to make is that the noon shot is so simple, think I could still teach that to most anyone in 5 minutes, That with a morning and afternoon running fix (Of the sun) would give you so much info, and dead easy, the rest of day would have to be by DR.

Despite earlier detractors, Null point on RDF, may cost a few extra miles, so what, under the original OP scenario, all resources would be required.
Plotting sheets would be handy on a vessel, that doesn't have a fulls set of paper charts. Cheap plastic sextants can be had (before the calamity), so if you do have a concern about solar flares taking down the GPS while you are crossing an ocean, its not out of the question to arrange the contingency.
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Old 28-07-2014, 10:40   #113
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Re: What will Happen to you when GPS goes Down

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Wotname,

Like John I don't wish to bore with facts what is a great light hearted deviation away from threads about washing diapers in the galley sink.

Many governments around the world are steadily working to provide their coastlines with some other system (e.g eLoran) as a backup to GPS. There are many reasons such as North Korea, Middle East troubles and genuine concern over solar flares. Nearly all late model satellites have a solar flare protection system. NASA and ESA have 24/7 monitoring of the sun so they know when to engage the spacecraft protection systems. But one overriding concern the US has is a rogue state launching a nuclear EMP weapon. One high altitude burst could knock out satellite comms without destroying any satellites. That seems more likely to many military minds than a solar event.

VLF stations are indeed operating today. VLF is a group of frequencies from 10kHz to 30kHz used primarily for surface ship and submarine communications. The largest network is operated by the US Navy with world wide coverage. Many other countries operate VLF systems such as Japan, Italy, UK, France, Korea, Norway, Turkey, Russia, China, Israel and others. Pretty much anywhere on the ocean you should be able to receive VLF with a very small loop antenna. There are online instructions available that require nothing more than wire, styrofoam, fiberglass and resin. If you have a VLF receiver you can do a bit of RDF once you get close (few hundreds of NM) to every continent except Antarctica. One of the biggest VLF transmitters in the world is on the NWC of Oz. There is an active online amateur community that plays around with VLF reception.

But there is a problem using your old Omega receiver. It relied on CW transmissions from VLF. Because the word rate can be 4 times faster using what is known as MSK nearly all VLF stations have stopped transmitting CW. Newer VLF receivers that the military use can decode MSK and in fact can generate a surprisingly good position fix from VLF transmissions. The antennas are quite small contrary to the assertions of another poster. They are about the size of a typical recreational radar antenna.

So your idea is not far fetched and the military are doing it right now. Even so, my advice would be to worry about keeping the water outside, steering, sails, keel, safety gear, food and water before spending mental energy on loss of all GPS.

Be safe out there.

Dan
Dan,

Let me clarify a little about my experiences with VLF - in my 10 yrs on US Navy subs, I think we caught 5 or 6 VLF transmissions, just as an exercise to make sure that the equipment worked. Due to the excruciatingly slow data rate, it was my assumption that nobody in their right mind, anywhere in the world, would be using VLF to transmit data, much less anything time critical or important to their naval forces.

But the fact is, you're right, there are still a quite a few VLF stations, many of them transmitting 24/7.

Yes, you can use crossed loops to DF a signal, but there is a resulting loss in signal strength, the more compact you make the antenna vs actual wavelength. This also affects DF bearing accuracy because you're trying to resolve incoming bearing ambiguity and receive a signal with a wavelength of 30 or 40 mi with something that is - 18"? 20"? 36"? You're only getting a very small portion of the entire wavelength.

Do you have any links to these accurate VLF RDF receivers?

How accurate do you think these VLF RDF receivers are?

How accurate do you feel is good enough for a 3 point fix?


After working with the Navy's best RDF receiver for 25 yrs I can tell you this - VLF is the absolute LAST frequency band I would even consider for RDF. All other transmitters would have to be shut down before I got to that point.

My preferred method for a quickie setup on an average yacht would be using a Yagi antenna to DF any known shore HF station above 8 Mhz. Bearing accuracy would still be almost useless, at least +/- 5*, probably more like +/- 7 or 8*. Plot that out over a few thousand miles and you're somewhere inside of that 1200 mi circle. LOL

The best analogy I can think of off the top of my head is the human's ability to determine the direction of audio frequencies.

Let's say you walk into a room with a blindfold on and all you hear is bass. You cannot tell where the bass is coming from with any accuracy. You can tell if it's from in front of you or behind you, but that's only 180* of resolution, you can't point directly at the speaker.

Repeat the exercise with a higher frequency instrument, like cymbals, coming out of the speaker. You'll be able to point in the direction of the speaker, probably right at it. That's the difference in resolving relative bearings to a source, the higher frequencies are much more directional, the lower frequencies almost seem omni-directional. That is a desirable trait for communicating with subs globally, which is why they spent the money building huge VLF transmitters. It's not a desirable trait when trying to DF the signal.

The brain is a super computer and does an amazing job of DFing incoming audio, we're not even aware that we do it. If our ears were 20ft apart, we could do even better at DFing lower frequencies. That's how important it is for the distance for the antenna elements be as large as physically possible to increase bearing accuracy.

But like many have pointed out, there are a lot more things to worry about besides GPS going out. It would take a huge solar event to wipe out the current constellation of 31 GPS satellites, everything ground based would probably get wiped out as well.

I'm more concerned with how to keep my beer and cocktails cold without running a generator...
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Old 28-07-2014, 10:45   #114
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Re: What will Happen to you when GPS goes Down

What about calling the uscg on your iridium and, with their permission, engaging your EPIRB? Both iridium and satsat are less susceptible to failure than gps and the uscg, via satsar, should be able to give you your position within 12miles.


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Old 28-07-2014, 12:41   #115
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Re: What will Happen to you when GPS goes Down

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What about calling the uscg on your iridium and, with their permission, engaging your EPIRB? Both iridium and satsat are less susceptible to failure than gps and the uscg, via satsar, should be able to give you your position within 12miles.

John Konrad
No, no, try to keep up! In these end of days scenarios, every single electronics circuit on board has been melted. There is no iridium or epirb to turn to.

It is only a matter of time, and worthy of MUCH thoughtful preparation.

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Old 28-07-2014, 13:45   #116
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Re: What will Happen to you when GPS goes Down

In that case, I would steer AWAY from the big mushroom clouds.
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Old 28-07-2014, 14:10   #117
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Re: What will Happen to you when GPS goes Down

Doesn't take a nuke - as stated above, there are imminent solar flares capable of melting all electronic devices. Of course, apparently it will also set your boat on fire, so navigation will not seem very important at this time.

And we haven't even started talking about asteroid impacts! Who has enough hubris to go to sea without first preparing for that contingency?

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Old 28-07-2014, 14:36   #118
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Re: What will Happen to you when GPS goes Down

That's it. I'm dumping all my electronically triggered fire extinguishers and getting longer snorkels.

And more duct tape.
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Old 28-07-2014, 15:08   #119
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Re: What will Happen to you when GPS goes Down

DISCLAIMER:

I don't think there is any serious point to this discussion as it pertains to cruising. If that is your goal then skip on past this post.

Quote:
Originally Posted by socaldmax View Post
Dan,

Let me clarify a little about my experiences with VLF - in my 10 yrs on US Navy subs, I think we caught 5 or 6 VLF transmissions, just as an exercise to make sure that the equipment worked. Due to the excruciatingly slow data rate, it was my assumption that nobody in their right mind, anywhere in the world, would be using VLF to transmit data, much less anything time critical or important to their naval forces.

But the fact is, you're right, there are still a quite a few VLF stations, many of them transmitting 24/7.
VLF is used in peacetime for non time critical data such as messages from family members, news from home and other mundane things. It is used by several countries as a secure data channel to surface ships and land based installations too. Typical data rates per channel are 50 bits per second with 2 or 4 channels per station. In a 24 hour day they can send a little over 4 megabytes of data per channel or about 16 megabytes per station. Since it is broadcast there is no good way for unfriendlies to know the intended recipient's location unlike most other terrestrial radio communication methods such as HF.

In war time they send compressed messages. It doesn't take a lot of time to say "Proceed to coordinates x,y,z and await launch codes."

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Originally Posted by socaldmax View Post
Do you have any links to these accurate VLF RDF receivers?

How accurate do you think these VLF RDF receivers are?

How accurate do you feel is good enough for a 3 point fix?
Most of this work is confidential so I don't think there are any published specifications. You could try googling for fun though.

Introduction to VLF | Stanford VLF Group

Quote:
Originally Posted by socaldmax View Post
After working with the Navy's best RDF receiver for 25 yrs I can tell you this - VLF is the absolute LAST frequency band I would even consider for RDF. All other transmitters would have to be shut down before I got to that point.
Most VLF position sensing systems don't use RDF. They try to measure distance to more than one transmitter and then triangulate a fix.

In the silly scenario of the OP though one only needs to know a single bearing to find land. The bearing of a big VLF transmitter is pretty easy to identify without a sophisticated antenna. A loop will do nicely. If the world is ending you don't need to know where you are. You just need a heading. Call signs of some "big" VLF transmitters:

NAA - Maine (Wikipedia page for this station needs some work)
NLK - Washington (the state not the city)
NPM - Hawaii
NWC - Australia
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Old 28-07-2014, 15:46   #120
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Re: What will Happen to you when GPS goes Down

Just call sea tow. /
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