Signal Propagation on sea water path

[orcus]

Hi
Is the propagation of a signal on a sea water path faster than that on a all ground path.
I know that conductivity causes this but what is the equation or theory that explains this effect on speed.
All literature I have got explains propagation INSIDE the medium (in water, in air).
My application frequency is 100 KHz.
Thanks

 

[VEBill]

The fastest it gets is C (in a vacuum, sans sea water).

 

[MikeHalloran]

That's a pretty low frequency. You need older books.



Mike Halloran
Pembroke Pines, FL, USA

 

[orcus]

Thanks for the reply.
ofcourse C is the max.
But in my application time is very critical.
a few hundred miles (390 miles) of sea water path as compared to ground path causes a 1 microsecond timing difference
and this is throwing me off in my further calculations.
Does anyone know any papers and stuff.
There is a Milingtons method for calculating this stuff but I cannot find that paper (1949 IEE).

 

[Comcokid]

I think you should be able to find plenty of research articles on the subject. Loran-C operates at 100 kHz and there were tons of research done on Loran-C propagation and accuracy in the 50's, 60's, 70's, and 80's. There was (is?) a technical organization for Loran called "The Wild Goose Association" through whose publications and meetings many technical papers were presented.

 

[orcus]

You are correct. I am doing research in Loran. Hence all the timing problems.
I think I will have to dig more into the Loran papers as there is no other good solid source of info.

 

[VEBill]

1us is 300m. I'm curious how you can be sure that the path length is the same over 390 miles?

 

[orcus]

These are some numbers I have come across in a paper and they mention that it is an all sea water path.

I have found a paper "Geographic information system Loran C coverage modeling".

I think this paper has few equations which may answer the question (I am not sure).

these equations discuss conductivity, path impedance, SNR and TOA (time of arrival varaince).

If anyone is interested you can check it out at

http://www.colorado.edu/geography/gcraft/notes/gisloran/gisloran.html

 

[BrianR]

A paper that might be of interest is: "Radio Spectrum Utilisation", Joint Technical Advisory Committee (IEEE & EIA), IEEE New York; 1964. While I don't have a copy I see an extract (page 105 from the reference above) in a text book that suggests there might be useful info there. This extract discusses phase velocity of propogation in LF signals vs altitude.

It is possible that the library here could have one in the archives, but I see in this extract that the phase velocity reduces with altitude and frequency and changes with path conductivity (seawater sigma = 5.0 mhos/m, e = 80), so I expect the solution to your issue is going to be quite complex. I see that skywave propogation is also a factor.
Might be time to bring out the Kalman filter.

 

[VEBill]

If the velocity of propagation changes with altitude, then the path taken by the wave would tend to curve up or down (probably very slightly down, if any, because if it went up then we wouldn't hear it).

Years ago I was involved in a project to grab the phase of a MW groundwave signal before the skywave arrived. One could clearly see on the scope the clean and steady waveform up to the point when the skywave arrived and made a mess of it.

I hope that orcus drops by again with the answer once he finds it - interesting topic.

 

[Higgler]

I would expect the propagation speed would depend on the distance the rf travels.
Seems like the rf would travel thru the water on long distance over the horizon trips and be 9x slower than in air. On shorter trips, travelling thru air would seem correct. Hence the rf travel speed must transition from standard lightish speed to 9x slower as distance changes from short to long. Throw in the ground travel in shallow water areas and the answer is even more convoluted.

In the reference by orcus above, sea water is 80 dielectric constant and land is 15 dielectric constant. Hence the rf over very long distances is probably 4 to 9 times slower than air travel. No way it's faster.

kch

dielectric constant of sea water reference.

http://www.tdx.cesca.es/TESIS_UPC/AVAILABLE/TDX-0104105-083419//01Cgp01de02.pdf

 

[BrianR]

The reference I quoted gives V/c-1 against range and altitude. The value goes more -ve as both increase, suggesting propogation is faster at altitude. Looking back I can see my original reply could suggest the opposite - not so.

 

[VEBill]

"...propogation is faster at altitude..."

That fits with the vacuum of space being the fastest.

OP seems to have been asking (backwards) if the speed over land is slower than over the sea. I've never heard about the ~adjacent~ media affecting the speed of light.

 

[BrianR]

I guess this is splitting hairs but it's not really adjacent as part of the propogation occurs in that media too. Eg the skin depth of seawater at 100KHz is 0.8m while over rich agricultural land it is 15m and over mountainous terrain it's typically 50m.

 

[orcus]

Hi
I too was under the impression that adjacent media does not affect the speed of light. But apparently there is a very very small change in speed (Loran needs a heck of a timing accuracy and 1us corressponds to 300m)

Brian I think 100 Khz is still a high frequency to propagate through the seawater. (skin depth is 0.8m) and I am looking at distances in excess of 100 miles. Hence I do not agree on
"it's not really adjacent as part of the propogation occurs in that media too "

I would definitely recommend to read through those few equations in that paper

http://www.colorado.edu/geography/gcraft/notes/gisloran/gisloran.html

which discusses the variance in time of arrival. I am still lost in those equations cause I am not sure if they are talking abt adjacent media.

This is definitely a interesting topic and I am getting a good experience with you guys.

 

[Comcokid]

I was trying to refresh my memory as to the published accuracy of Loran when I tripped across this article:

http://www.eurofix.tudelft.nl/pdf/ion97.pdf

It mentions an "Additional Secondary phase Factor (ASF)" as being the term applied to the propagation difference arising from soil and water conductivity differences. Some references are given. Maybe this "ASF" phenomenon is what you are looking for.

 

[orcus]

Hi
Yes the ASF tables correspond to variations in conductivity.

the time required for a loran groundwave signal to propagate
comprises of 3 factors
1) primary factor using c = 3e8 m/s. (distance/speed)
2) secondary phase factor (SF), corrects for signal propagation delays over seawater compared to propagation through the atmosphere. (this is what I am looking for)
3)ASFs correct for propagation delay over a mixed land/seawater path compared to an all-seawater path.

These factors show that adjacent medium does affect the speed of light.

 

[VEBill]

Tons of info:

http://www.google.com/search?q=additional+secondary+phase+factor+ASF

Interesting topic.

 

[BrianR]

Orcus, my reference for " the skin depth of seawater at 100KHz is 0.8m ", is Reference Data For Radio Engineers, 6th Ed, (Howard Sams) pp 28-3-28-5.

 

[orcus]

Thanks a lot guys
I think a lot more research is needed
If I get any solution I will definitely
post it.