GPS underwater

[Ned Forrester]

|> GPS will not work under water.
|> Water will block the signal very effectivly, maybe a few centimiters would
|> be fine, but I doubt it.

I once measured the ability of a particular receiver/antenna
combination to receive GPS signals through a layer of seawater. I was
able to get reception through about 1mm of water, but at 3mm there
was no signal. This was with a commercial pre-amplified antenna.

Only one data point, I know, but centimeters or inches is unlikely.

--
NOTE: to reply, remove all punctuation from email name field

Ned Forrester [email protected] 508-289-2226
Applied Ocean Physics and Engineering Dept.
Oceanographic Systems Lab http://adcp.whoi.edu/
Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

 

[Michael]

So would I! I haven't really kept up with submarine laser
comms since I left the ocean optics field many years ago. I suppose
that much of the data on the actual success of such communications
is still classified.

Even a collimated laser beam spreads a bit over a hundred
kilometers. You can easily design optics to give the surface
footprint whatever size you want.

The rule of thumb when designing optical instruments was
that you could get reasonable signal-to-noise ratios
from 1x10-12 Watts of light energy. I'm not sure if
that is still a sensible value for comms lasers, though.

When the laser beam hits the ocean surface, it is scattered
and diffracted through a fairly large volume. It then
starts to make a big difference how far your reciever
is from the surface, how well the incoming beam is aimed, and
the aperture of your receiver.

One possible scenario is that the sub shoots a beam toward
the surface, which appears as a 'hot spot' to the satellite.
The satellite then knows where to aim it's own laser and
puts a very concentrated comm beam on that spot.

It's late Sunday night and the math of the path losses
is beyond me at this time. Suffice it to say that it
probably takes honkin' big lasers and subs that
are pretty close to the surface. It's probably
diminished in importance since a lot of sub ops
have probably transitioned to brown water ops
where even honkin' big lasers don't help very much.

Since airborne lasers are commonly used to measure
water depths near shore, you may be able to get
some more information on path losses by
looking up 'laser bathymetry'.


Mark Borgerson

I don't think they want to show up as a 'hot spot' Too many eyes in the
sky!


Michael

 

[Michael]

Fred J. McCall <[email protected]> wrote in message


Once worked on a project to communicate with deep subs involving
Extremely Low Frequencies (ELF) and the data rate was limited to a few
characters/sec. The transmitter on land was to be a huge array miles
in length. There was a proposal to make such an array in MI
(something to do with the properties of the rock there) and another
propoisal to send the signal using long distance power lines as the
antenna. The sub would tow a very long wire to receive and could not
transmit. These frequencies are so low thta the entire earth
sometimes acts as a resonant cavity with the ground forming one
conductor and the ionisphere as the other.
Another place where GPS does now work is in cave exploring. There is
no way that the GPS signal can penetrate the earth to most caves so
mapping is done the hard old fashioned way, compass, tape measure and
inclinometer. Furthermore, there is generally no way to find cave
passage from the surface (except entrances)so exploration is needed to
find passage.

Wasn't there a 'Project Sanguine' or something like that?

 

[Michael]

[email protected] (Parallax) wrote in message

GPS works in caves since I've used it in them.
As with all things military, you shouldn't blame
your equipment problems on science. Just because
NAZI Army GPS receivers don't work in caves, doesn't
mean *nobody's* GPS receivers works in caves.

Since GPS has nothing to do with
ground-penetrating *RADAR*, which it what science
cave dork geologists do. So they are usually
simply advised to brush up their SONAR handbooks,
and let people with non-neanderthal computers
do the RADAR.

This I would love to see!
What brand of GPS did you use in this cave? How far below the surface were
you?

Michael

 

[Mark Borgerson]

I don't think they want to show up as a 'hot spot' Too many eyes in the
sky!

Hmmm, perhaps the 'hot spot' is actually a very brief, coded pulse
that would only be detected by an observer with the proper correlation
key.

Or the sub could respond with a coded pulse to a coded raster scan
from the satellite that covers a very wide area. The response would
be buried in the backscatter from the interrogating beam.

The satellite would probably have to have a very wide aperture
receiver to pull the signal out of the noise.


There are lots of other facets of low-probability-of-intercept
comms that could be applied.


In any case, if anyone else were using it, I suspect that
NSA would find a way to intercept it! ;-)



Mark Borgerson

 

[Jim Watt]

What brand of GPS did you use in this cave? How far below the surface were
you?

The secret is matching the GPS receiver to the special foil helmet
needed to deflect the yobba radiation from the cave walls.

 

[Steven Shelikoff]

I don't think they want to show up as a 'hot spot' Too many eyes in the
sky!

They already show up to the eyes in the sky if the eye is using radar to
find the wake of a moving sub. Even underwater, subs create a wake
that's findable on the surface as a very small bulge in average sea
level in a well defined shape.

I can't say whether this effect is actually being used to find subs
though.

Steve

 

[Parallax]

My experience with GPS in caves was in Climax Cave in South GA which
is about 120 ft below ground surface. Most of the earth above is wet
soil underlain by wet limestone. I used one of those Etrex units. I
have been told by cave mappers in N. AL that GPS doesnt work there
either.

Am familiar with Sanguine.

I now make x-ray optics but when I want to get a physicists attention,
I casually mention that I am working on neutrino optics (which I have
given some thought to).

 

[bambi]

Does GPS work underwater? If yes, to what depth? What errors are
introduced?

It does not. The closest thing to GPS underwater is a floating antenna
with a cable going down to the diver below. There has also been some
work using LBL and USBL accoustic navigation and correlation to GPS, but
it's all kind of flakey right now.

My knowledge in this domain is based on hollywood, but I assume "yes"
because submarines use satellites. But they often surface or raise
scope implying the answer is "no".

This is right. We WISH it would work.

 

[Stan Gosnell]

It does not. The closest thing to GPS underwater is a floating antenna
with a cable going down to the diver below. There has also been some
work using LBL and USBL accoustic navigation and correlation to GPS,
but it's all kind of flakey right now.

Even the water in your hand will block GPS signals. Putting your hand
over a GPS antenna completely stops it from receiving anything.
Actually, even a rather thin film will do it, such as the rain on your
car windshield.

 

[Jack Erbes]

Even the water in your hand will block GPS signals. Putting your hand
over a GPS antenna completely stops it from receiving anything.
Actually, even a rather thin film will do it, such as the rain on your
car windshield.

You've not done any research on this have you? Immersion under water
will severely limit signal reception but I think you're overstating the
effect for other situations.

Receiver sensitivity will vary from one model to another and the antenna
type is a key factor in performance. Here is a good discussion of the
effect of different antenna types:
http://www.gpsinformation.org/joe/gpsantennaspecs.htm

My Magellan 330M will acquire 3 to 5 satellites and a 3D fix (in 3
minutes or less) in the basement of my single story home (wood frame,
composition shingle roof).

In my living room, not near a window or with any line of sight to a sky
view, it will get a 3D fix much more quickly with 6 to 10 satellites
visible.

It works well almost anywhere in a fibreglas boat.

It will maintain a fix laying on the transmission console, the
dashboard, back seat, or rear cargo area in my car (a Dodge Durango).
The fix may drop to 2D or be lost in an area with tall buildings, in
tunnels, or on the lower level of multi-level freeways.

It maintains a 3D fix, with an occasional lapse to 2D, strapped to the
cargo rack on my ATV in heavy woods around my home. If I stop for a
minute or so in a heavily wooded area with no sky view, it may drop to
2D and once or twice Ozi-CE (I am normally using the 330M as a NMEA
input for Ozi-CE on a iPAQ 3630) has reported no fix for a short period
of time.

The optimal receiving position for the 330M is when it is held vertical
or near vertical (the typical position as you hold it and look at it).
I seldom use mine in that position, and the results above in mobile use
are with it laying more or less horizontally either face up or down.

Jack

 

[Peter]

You've not done any research on this have you? Immersion under water
will severely limit signal reception but I think you're overstating the
effect for other situations.

My GPS receivers also have no problem getting signals inside my house
or in most heavily wooded areas, but I agree with Stan that just
wrapping my hand around the antennas will effectively block the
signals due to the water content.

 

[meil]

Even the water in your hand will block GPS signals. Putting your hand

You've not done any research on this have you? Immersion under water will
severely limit signal reception but I think you're overstating the effect
for other situations.

I've used my Garmin Geko 201 for swimming and splashing or some drops will
not affect reception much but just putting it even 1/2 an inch under water
results in no signal whatsoever (and this after having a full 100% WAAS
reception of all 12 sats.
GPS signals are so low that they will not penetrate even a thin layer of
H2O, aluminum foil (any metal) ...
I guess your research was not very deep either

(the Geko is pretty good for swimming though, I tuck it behind my goggle
strap and get a very nice trip log, only for outdoor swimming off-course
)

 

[Stan Gosnell]

[email protected]:

You've not done any research on this have you? Immersion under water
will severely limit signal reception but I think you're overstating the
effect for other situations.

Yes, I have.

My Magellan 330M will acquire 3 to 5 satellites and a 3D fix (in 3
minutes or less) in the basement of my single story home (wood frame,
composition shingle roof).

Of course. I never said dry wood would stop the signals.

In my living room, not near a window or with any line of sight to a sky
view, it will get a 3D fix much more quickly with 6 to 10 satellites
visible.

It works well almost anywhere in a fibreglas boat.

What does that have to do with water? The water is underneath the boat,
hopefully, not above it.

All your examples have nothing to do with the subject of the discussion,
which was water.

 

[Kevin Brooks]

Even the water in your hand will block GPS signals. Putting your hand
over a GPS antenna completely stops it from receiving anything.
Actually, even a rather thin film will do it, such as the rain on your
car windshield.

I don't believe that. If that were the case, then oodles of DoD equipment
would be largely tits-up in every rain squall--including JDAM, etc. Not to
mention how aircraft operate at high altitude with attendant icing and still
manage to get GPS data...?

Brooks

 

[Peter]

I don't believe that.

It's easy enough to check the attenuation due to the slightly salty
water in your hand - when I tightly close my fist around the antenna
of any of my GPS receivers (or my external antenna) the signal
strength (SNR) bars quickly drop to zero. I haven't noticed the
problem with rain on the windshield, but then I normally use the
wipers to prevent much of a water sheet from forming.

If that were the case, then oodles of DoD equipment
would be largely tits-up in every rain squall--including JDAM, etc.

It's easy enough to avoid with proper physical design of the antenna
so no water layer builds up on top of it.

Not to
mention how aircraft operate at high altitude with attendant icing and still
manage to get GPS data...?

Icing is a problem at low altitudes (below ~18kft), not high where
the air contains less moisture. And I'd hope that planes operating
in icing conditions would be equipped with anti-icing systems.
Otherwise they're likely to have far more serious problems than GPS
reception, like loss of lift.

 

[Kevin Brooks]

It's easy enough to check the attenuation due to the slightly salty
water in your hand - when I tightly close my fist around the antenna
of any of my GPS receivers (or my external antenna) the signal
strength (SNR) bars quickly drop to zero. I haven't noticed the
problem with rain on the windshield, but then I normally use the
wipers to prevent much of a water sheet from forming.


It's easy enough to avoid with proper physical design of the antenna
so no water layer builds up on top of it.

I never heard anybody complain that their handheld GPS-in-a-green-package
went automatically tits up when it rained.

Icing is a problem at low altitudes (below ~18kft), not high where
the air contains less moisture. And I'd hope that planes operating
in icing conditions would be equipped with anti-icing systems.
Otherwise they're likely to have far more serious problems than GPS
reception, like loss of lift.

Anti-icing systems are usually limited to the wings and, IIRC, the tail.

Brooks

 

[Peter]

I never heard anybody complain that their handheld GPS-in-a-green-package
went automatically tits up when it rained.

Enough rain to form a reasonable *sheet* of water on a handheld is quite
a downpour. Any reasonable person would try to get a bit of shelter
from the rain or at least hold the unit up so the water flows off the
screen and antenna areas (otherwise the screen would be illegible
anyway).

I just did a test with my external GPS antenna and handheld. Had good
solid reception on 7 satellites with an empty jar lid over the antenna.
I then put a 2-3 mm layer of water in the jar lid and all the signal
locks went away completely. That's about as thin a layer as I can
measure with what I've got available right here, but presumably even
thinner layers would still substantially reduce signal strength and lead
to poor performance.

Anti-icing systems are usually limited to the wings and, IIRC, the tail.

Specifically to leading edges which are the most susceptible to icing
from supercooled droplets impinging on them. The antenna is unlikely to
get that much icing. The other major factor is that the microwave
absorption of ice is quite different from that of liquid water.
Microwaves are absorbed because they make the whole water molecule
rotate (the side with the oxygen is negatively charged and the side
with the two hydrogens is positive - in an alternating electric field
this causes the molecule to rotate back and forth and absorb energy
from the field). In ice, the motion of the molecules is much more
contrained in the crystal structure and therefore less microwave
energy is absorbed. So it's only the small liquid water component of
any ice layer that strongly absorbs the signals.

 

[Jack Erbes]

Stan Gosnell wrote:

What does that have to do with water? The water is underneath the boat,
hopefully, not above it.

All your examples have nothing to do with the subject of the discussion,
which was water.

I was replying to your statement that "Actually, even a rather thin film
will do it, such as the rain on your car windshield."

The point I left out in my post was that when I used the gps in the
house, car, boat, etc., it was not kept from working by rain (wet roofs,
windows, etc.). The presence of a film of water it not as nefarious as
you imply, I thought you over stated the effect.

We have 10-12" of snow on the roof now and my 330M just worked as it
does typically. I know that snow is not the same as water so I'll keep
an eye on it as it thaws and see how it goes. Don't hold your breath
though, I'll get back to you in April or May hopefully.

Jack

 

[Stan Gosnell]

I don't believe that. If that were the case, then oodles of DoD
equipment would be largely tits-up in every rain squall--including
JDAM, etc. Not to mention how aircraft operate at high altitude with
attendant icing and still manage to get GPS data...?

So don't believe it. I don't care.

Rain on aircraft in flight isn't a problem, because the airstream blows
it off. Ice isn't quite the same as liquid water, and seldom occurs at
high altitudes. Icing normally occurs below 10,000 ft.