[time-nuts] GPS 18 behavior

[Jim Lux]

On 7/23/13 4:51 AM, Scott McGrath wrote:
> Thanks Jim
>
> Was not aware that <10 Ghz signals could penetrate so deeply.  I work for a enterprise wifi company on the RF side and one of our key challenges is signal attenuation/distortion by building materials
>
> Any pointers to papers on this?

there's tons of papers (and Master's theses) on propagation of 2.45 and 
5.8 GHz signals in buildings.  It seems that every grad student goes out 
and hooks up a VNA to some antennas.

As for other frequencies, there's some papers out there on propagation 
in various building materials and substances.  I put a list of some 
useful references (there's others, at the end of theis email)

from a modeling standpoint:
Target & Propagation Models for the FINDER Radar
Authors: 	Vaughn Cable; Jet Propulsion Laboratory, California Institute 
of Technology, United States		
  	James Lux; Jet Propulsion Laboratory, California Institute of 
Technology, United States		
  	Salman Haque; Jet Propulsion Laboratory, California Institute of 
Technology, United States		

Was presented at URSI a couple weeks ago.
T
he attenuation is quite high compared to free space, but the dominant 
factor in the propagation is not the resistive losses, but is the 
multiple changes in dielectric media: lots of reflections, and many 
scatter energy in directions other than where you want it to go (e.g. 
sideways).

It's like shining a laser pointer into milk.

So, I'm not surprised you have trouble with high speed digital comm. 
There's two things biting you. One is just the signal strength issue: A 
few dB/meter adds up quick.  The other is the delay spread in the path, 
which will limit your symbol rate, and getting to a more time-nuts-y area.

Our models (and measurements) show that in dense rubble with smallish 
chunks (few cm to tens of cm scale), the delay spread is on the order of 
10-20% of the propagation time.  On a 10 meter path (about 50 
nanoseconds), the delay spread is on the order of 5-10 nanoseconds. 
Clearly, you're not going be sending 100 megasymbols/second.  802.11 
sends 250 ksymbols/second, so it doesn't have a big problem with ISI, 
but it does get bitten by the frequency selective fading of these paths: 
and that fading varies dramatically on the scale of a wavelength: move 
5-10 cm and a completely different set of OFDM channels are alive.



>
> I did know that /tvb was using seismic sensors but moles are small you would need a high resolution seismic grid to accurately place them
>

Actually, you don't need that high resolution to do location. You just 
need good timing resolution The challenge would be the uncertainty in 
propagation velocity in the medium. It's like acoustic location of 
gunshots, which works pretty well in free space, but not so well in an 
echoey environment.

Now if he were looking for diamond toothed rock boring gophers deep in a 
granite pluton, where the medium is more uniform, it would work better.




  Here's some possibilities.
There's tons of others..

J. A. Boan, “Radio Propagation in Fire Environments”, University of 
Adelaide, Australia, 2009.

C. Dissanayake, M. N. Halgamuge, K. Ramamohanarao, B. Moran, P. Farrell, 
“The Signal Propagation Effects on IEEE 802.15.4 Radio Link in Fire 
Environment”, 5th International Conference on Information and Automation 
for Sustainability, 17-19 December 2010

G. L. Charvat, L. C. Kempel, E. J. Rothwell, C. M. Coleman, E. L. 
Mokole, “A Through -Dielectric Radar Imaging System”, IEEE Transactions 
on Antennas and Propagation, Vol. 38, No. 8, August 2010

M. C. Dobson, F. T. Ulaby, M. T. Hallikainen, H. A. El-Rayes, “Microwave 
Dielectric Behavior of Wet Soil – Part II: Dielectric Mixing Models”, 
IEEE Transactions on Geoscience and Remote Sensing, Vol. GE-23, No. 1, 
January 1985

M. T. Hallikainen, F. T. Ulaby, M. C. Dobson, M. A. El-Rayes, L-K Wu, 
“Microwave Dielectric Behavior of Wet Soil – Part 1: Empirical Models 
and Experimental Observations”, IEEE Transactions on Geoscience and 
Remote Sensing, Vol. GE-23, No. 1, January 1985


W. A. Wensink, “Dielectric Properties of Wet Soils in the Frequency 
Range 1-3000 MHz”, Geophysical Prospecting, Vol. 41, Issue 6, 27 April 2006