GPS underwater

No, you need to raise a mast. High frequency radio waves (VHF and above)
don't penetrate seawater at all. You can still have a GPS set on a
submarine, but you need to raise a mast to receive a signal.

Aetherem Vincere
Matt

 

The Navy's ELF transmitters operate at in the 40-80Hz. That's right,
Hz, not kHz or MHz. There are 2 transmitting antennas, one is 28 miles
long and the other is 56 miles. These are very low data rate messages
and usually tell the sub to come to a shallow depth to recieve a VLF,
HF, or satellite message.

They also operate fixed VLF stations in the 14-60kHz range. This is
capable of sending data at 50 baud to subs.

The Navy also uses Tacamo aircraft to talk to submerged subs. It trails
a very long VLF antenna and keeps it verticle over a fixed point by
flying in tight circles. The plane carries two antennas, one is almost
1 mile long and the other is 5 (five) miles long.

Steve

 

"The most promising approach is
the use of lasers at about 454 or 488nm (blue and blue green)."

Where did you get that from? I find it extremely hard to believe.
I had tried sending my lasers through liquids and the attenuation is huge
(unless you have an exact match for the color.)

Thanks,
Michael

 

(ken mankoff) wrote:

oes GPS work underwater? If yes, to what depth? What errors are
:introduced?
:
: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".
:
:Any ideas?

Think about the power and frequency of the signals. GPS doesn't
penetrate FOLIAGE particularly well. Submarines use SINS and update
it via GPS when they raise an antenna to copy traffic.

 

I would have to say that you're wrong. Also, I have one of those
impossible video transmitters around here somewhere in a parts box. I
used to play with it between the garage and the house and it worked
fine. I'll admit that I was off a bit on the freq's for the boat but
that wasn't my area. I -did- have to learn to use a sextant. grin!

 

I think you don't know what you are ralking about

 

Something I was too polite to tell you in those words but yes, you
need to check on it before you spout again. It's also off topic for
this group.

 

Clean seawater (as found out in the middle of the oceans, has
very little 'color' What you see near the shore is usually
due to plankton and particles from river runoff.

Pure water has a minimum in it's absorbtion spectrum near
between 400 and 500 nm. There is a plot at

http://omlc.ogi.edu/spectra/water/gif/segelstein81.gif

454 and 488nm happen to be the wavelengths of a laser
(an argon laser, IIRC) that has significant output power.

When sending lasers throug liguids, a lot of the loss
comes at the air/water interface. This is particularly
true when sending a laser beam up or down through the
sea surface. When the beam strikes the sides of
waves, big or small, you can lose a lot more energy
to reflection than you do with a beam normal to a
nice, flat surface.

Mark Borgerson

 

'Video' these days doesn't necessarily mean 30FPS NTSC. There are
lots of folks offering equipment to transmit low frame-rate video
over 56K modems (or less)

http://iraf.noao.edu/iraf/web/ADASS/adass_proc/adass_95/percivalj/percivalj.html

Those systems could well work with a 300KHz carrier.

Mark Borgerson

 

You mean like, um, webcams?

Steve

 

Similar in resolution and frame rates, but designed to work with
standard serial ports. You don't see them too much any more
since USB webcams got so cheap. They used to show up
in the back pages of the hobby electronics mags quite often
5 or 6 years ago.

Mark Borgerson

 

But not on LF.

Video means what it says, SSTV is not video.

a 300Khz WIDE carrier maybe with a FSK modem. But that
bandwidth is simply not available on LF.

The modulation systems used in a V90 modem are not
suitable for radio use.

http://www.rsgb.org/bandplans/bandplans.pdf

V e r y s l o w telegraphy, The sort that Governments use
to communicate with submarines underwater.

 

Well, AvalonRF doesn't seem to think so.

http://www.avalonrf.com/literature/AvalonRF_ST_Presentation.pdf

Among other things on the web site is "A long-range 1Mbit/Sec VLF/SW
video link from caves and canyons (BW of 12.5KHz).

Most of the others I found only offered about 300 bits per second
over LF.

I don't think the original respondent had one of these sitting
out in the garage, though!

Mark Borgerson

 

I am still very confused how this could work.
If the beam is collimated, then how can it 'hit' the sub.
If it is NOT, then how can it have enough power.

I would love to read a technical paper on this.

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 am still very confused how this could work.
: If the beam is collimated, then how can it 'hit' the sub.
: If it is NOT, then how can it have enough power.

Therein lies the rub. Basically, the narrow beamwidth of the laser
signal requires that the location of the submarine be known with considerable
precision, and that the satellite/aircraft be able to precisely aim. This
is one reason the navy was not too hot on the concept back in the 80s.
However, times change and technology is a lot better these days. The navy
is putting considerable R&D effort into the advanced development of the
submarine laser communications satellite.
Of course, there are all sorts of ways to approach the problem.
For example, it is possible (at least on a test basis) for mast-up
submarines to use laser to communicate back with central facilities by
lasing a "non-optimal" receiving satellite. Basically, what that means is
that some satellite (esp. multi/hyperspectral) can detect laser illumination.
Depending on a lot of details, you can illuminate the satellite with a
laser beam modulated so that it can be detected and 'decoded' at the
satellite processing facility.
As stated, most systems require excellent knowledge of the submarine
location for a spacebased laser to keep the submarine within beamwidth.
For a mast-up submarine, this is not so hard, it can communicate its location
either on a bi-directional laser link or using duplex communication, for
example the submarine uses EHF to transmit its location, the laser satellite
is then pointed. If the submarine is maneuvering underwater, however, it
becomes much harder to determine where the thing is and communicate that
to the laser satellite.
Laser has two real big advantages. The first is that it is very
directional and therefore hard to detect or intercept if you are not within
the mainbeam. Super low probability of intercept. Secondly, the extremely
high frequency can give incredible data rates. The Navy is aiming at
something like 40 gigabits per second.
Another important point to keep in mind is that it isn't just
submarines that are underwater. Deployed sensor and mine fields, for example,
can have their locations tagged and communicate with lasers. A deployed
accoustic array, for example, could use laser to forward its entire raw
collection back to shorebased supercomputers. Cool stuff.
:I I would love to read a technical paper on this.
I
For more of an operational analysis type of paper, there is a somewhat
dated but informative master's thesis from the Naval Postgraduate School,
Lt. Thomas McGuinness "investigation of FBM submarine communications using
a candidate submarine laser system", MS 1984.

regards,

 

Neither do I

But if he has, it would be a good product to market, anything that
can cram more TV channels onto a satellite transponder is a very
valuable commodity.

1mbs claimed on any phone line is interesting, but having spent
a few years in the computer business, I believe things when I see
them work. Even some of the demo's are rigged.

We have got some nice 2mbs links running over copper phone
circuits, but not within the POTS passband which is a lot less than
half the 12khz quoted for the RF device.

 

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.

 

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.

 

Ever work on the neutrino device for the same purpose?