GPS underwater

You went too far with your original description, and you are not able to
back your assertion.

I would think it more likely results in a thin film of water being pushed
across the surface--close to laminar flow. You gotta remember, as the water
covering that antenna is pushed aft by the airflow, that same airflow is
also pushing more water *onto* it...and yet the danged things still continue
to work. Of course, we can leave the aircraft dimension altoghether, and you
could explain why commercial trucks equipped with GPS don't experience
outages everytime they hit a rainshower, even at low speed...

So what? It occurs, and it will invariably cover GPS antennae. Yet IIRC they
have GPS based ILS that can bring aircraft down through pretty nasty
weather...

Brooks

 

Repeat after me: "GPS Will not work underwater". There is a reason that
subs use VLF to receive data (they can't send it because transmission
requires long and inefficient antennas and lots of power, both
impractical). I can honestly tell you that my previous employer (with
many underwater vehicles and big gray ships) has a great interest in
such things, and virtually unlimited money to persue them. They have not
had their needs satisfied. Your speculation and "I can use my GPS in the
house" sea stories are mental masturbation.

 

For a GPS fix, you need to get antenna above the water. At VLF
frequencies, even sea water is a sufficiently poor conductor that the
skin depth in sea water is on the order of tens of meters, so
submerged antenna has a reasonable chance of working if you use a high
power transmitter. E6 aircraft have megawatt power production on
board!

However skin depth is roughly proportional the the 1/f^.5

So going from 20 Khz to say 1500 Mhz is an increase in frequency of a
factor of 1500 x 50 or 75000. So a skin depth of meters at 20Khz, is
about 2cm at 1500Mhz.
Ever meter of water the GPS antenna is under means the loss of about
150db in signal level.! Even if the GPS system was transmitted with
megawatt ERP (which it isn't), that still wouldn't come anywhere near
overcoming the 150db/meter loss propogating through sea water.

 

Rain water isn't all that serious a problem. Rain water is a fairly
good approximation for distilled water, wihch means the conductivity
is very low. In fact conductivity is a standard test of water purity.
Very high purity water is a really poor conductor. Try electrolysing
very pure water into oxygen and hydrogen. It is such a poor conductor
that it works very badly until you add sometihng to it to make it a
bit more conductive!


Rain or even a littler rain water on the antenna doesn't really become
a problem until you get up to about Ku band, and that about 6 x the
frequency GPS uses. 2cm of saltwater will cost you about 3db.
A millimeter or two of rain just isn't a problem. 30cm of water is a
problem.

 

At low frequencies, yes. Pure water won't have the ions to transport
electrical charges.
But at microwave frequencies the situation is very different. Each
water molecule is a little dipole (the oxygen tends to pull the
electrons away from the two hydrogens and with a bond angle of
about 109 deg. you end up with a positive charge on one side and
negative on the other). In an e&m field that's varying in the
microwave frequency, these dipoles will rotate back and forth
and absorb energy from the field.
That's why water is heated so effectively in a microwave oven
and why it blocks the GPS signals. In neither case do you need
to have salt or other ionizing impurities.

It's easy to do the experiment with a plastic tray of water above
a GPS antenna (and shielding on the sides so signals can't get through
there). Using regular tap water I found that much less than 1 cm was
sufficient to block reception of all satellites. Rain, fog, clouds,
etc. in the atmosphere are not a problem since the particle size is so
small. But a significant layer of water on top of the antenna will
cause severe attenuation. Normally rain won't build up to that extent
on typical antenna designs, but if they were under a flat surface that
allows any pooling of water to occur then I'd expect reception problems.

A few millimeters of rain water in a solid sheet over the antenna is
a problem. Fortunately it's pretty easy to avoid that situation.

 

It takes about 5 mm of liquid water to attenuate the signals enough to
effectively block them. That doesn't happen with JDAMs or other ordinance.
Most antennas are located where water and ice don't build up.

Beading of water on windshields of automobiles can present a problem for
consumer level GPS receivers locate inside during a rain storm.

 

After you get your panties all unwadded, go back and read what folks were
questioning--not that GPS is not usable by a submerged submarine (unless he
has a mast erected above the surface), but instead the assertion that it
won't even work when exposed to rainfall. A bit of a difference between
those extremes, huh?

Brooks

 

Now *that* is an explanation that makes much more sense.

Brooks

 

I have been told - but can't find a way to verify experimentally, that dry
leaf canopy blocks less signal that moist canopy. It is the moisture in the
leaves that most effectively blocks the GPS signal.

Of course, the tree trunks themselves are full of water as well. Most
species when just cut may be as high as 70% water.

 

I've got two, they both work nicely indoors, one has
been up for 2 years without a break. Would not
take it in the bath though.

 

I walk in the woods a lot with my GPS often under dense foliage.
Signal bars tend to get a bit lower.
Didn't notice any difference between pine and normal leaves.

However, during rainfall I have almost no reception at all in the woods, due
to the water on the leaves.

 

The point I left out in my post was that when I used the gps in the

Most car windshields I've ever seen are angled or vertical, no large water
accumulation is possible. I'm not sure about snow on the roof but you should
know that 10" of snow is equal 1 inch of water so it's not like you're under
10" of water and also you do have clear of snow windows? I hope you have
enough food supplies to last you that long otherwise send us your GPS location
and we will send rescue team?

 

For many years accurate underwater positioning has been done using
various types of sonar equipment. When coupled to GPS through
computers and precise motion sensors on board vessels - usually using
RTK GPS these days -underwater positions can be derived in real time
to a few centimeters. This technology is routinely used in many
construction, oilfield and hydrographic survey applications. It is
certainly not all kind of flakey - you just don't appear to know
anything about it.

 

ofcourse gps works underwater !
but not the standard types !

 

Read the thread and my post again. Slowly and for content.

I said "Immersion under water will severely limit signal reception but
I think you're overstating the effect for other situations." I have a
good feel for the signal levels that GPS antennas need to work and they
will work when wet and even when under a very shallow layer of water.

So you're wrong when you say "GPS will not work underwater." "Severely
limit" is a more accurate description of what happens than is "will not
work".

I said that in response to a post that said that the presence of even a
thin film of water, such as the rain on your car windshield would render
an antenna useless. As far as the other situations I described, all of
those have been experienced with water on them (rain, etc.) at times and
that did not shut down the GPS reception.

I neglected to mention rain in my post, maybe the post would have not
drawn criticism for being off topic if I had spelled that out in my post.

As far as your ramblings about your former employment, which contributed
nothing to the point under discussion, are you a former member of the
U.S. Navy? Or did you just work for them? If so, former is a good
thing because I think highly of the Navy. I'm retired Navy (1964-1990).
I spent 26 years as a Collection Branch Cryptologic Technician, and
have more than a little knowledge about receiving signals.

You are wrong again. Sea stories always start out "When I was in the
Navy... or This is a no shitter..." My remarks went to the topic under
discussion.

Do you get excited when you get a chance to talk dirty in public?

Welcome to my kill file. <plonk>

Jack

 

Make and model number please!

 

I believe you are right about car windows, not much water there.

The house windows are clear of snow, the overhang on the house keep
those clear. The Magellan quad helix type antenna does a good job on
receiving the birds that are nearer the horizon but I don't think I'm
seeing any of them through windows. Especially in the basement which
only has small windows.

Thanks for the gem of knowledge on the snow/water ratio, the wife and I
were wondering what the approximate conversion was.

We're in great shape. Lots of wood for the stove, lot of chow, plenty
of beer and snacks. I've only been a Mainer for about 4 years so I
still like snow. Walked around the block during the blizzard the other
day just to get a feel for it. Not bad if you're dressed right, high
stepping in snow is good exercise.

Jack

 

See: http://www.underwater-gps.com/

 

This is certainly not true from the standpoint of GPS reception. Water
that is dispersed such as in snow or rain is not nearly as much of a
problem as water in its normal liquid form. For reference a GPS can
receive a signal to a depth of about a meter of snow. Rain does not
materially effect reception at all. Rain in clouds effects accuracy due
to propogation delays but otherwise does not effect reception.

Dale

 

Repeat after me: "GPS Will not work underwater".

I suggest you try it, take your GPS, get a good lock and dunk it in the
water, see how good your reception is ... that is right, no reception at
all.
I swim with my GPS (strapped to the back of my head), splashes etc do not
have an effect but immersion of only a fraction of an inch loses all
connection.