GPS underwater

Discussion in 'General GPS Discussion' started by ken mankoff, Dec 4, 2003.

  1. 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
     
    Matt Clonfero, Dec 6, 2003
    #21
  2. 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
     
    Steven Shelikoff, Dec 6, 2003
    #22
  3. ken mankoff

    Michael Guest


    "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
     
    Michael, Dec 6, 2003
    #23
  4. (ken mankoff) wrote:

    :Does 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.
     
    Fred J. McCall, Dec 6, 2003
    #24
  5. ken mankoff

    Guest Guest

    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!
     
    Guest, Dec 6, 2003
    #25
  6. ken mankoff

    Jim Watt Guest

    I think you don't know what you are ralking about
     
    Jim Watt, Dec 6, 2003
    #26
  7. ken mankoff

    Guest Guest

    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.
     
    Guest, Dec 7, 2003
    #27
  8. 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
     
    Mark Borgerson, Dec 7, 2003
    #28
  9. Mark Borgerson, Dec 7, 2003
    #29
  10. You mean like, um, webcams?

    Steve
     
    Steven Shelikoff, Dec 7, 2003
    #30
  11. 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
     
    Mark Borgerson, Dec 7, 2003
    #31
  12. ken mankoff

    Jim Watt Guest

    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.
     
    Jim Watt, Dec 7, 2003
    #32
  13. 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
     
    Mark Borgerson, Dec 8, 2003
    #33
  14. ken mankoff

    Michael Guest

    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
     
    Michael, Dec 8, 2003
    #34

  15. 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
     
    Mark Borgerson, Dec 8, 2003
    #35
  16. : 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,
     
    Steven James Forsberg, Dec 8, 2003
    #36
  17. ken mankoff

    Jim Watt Guest

    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.
     
    Jim Watt, Dec 8, 2003
    #37
  18. ken mankoff

    Parallax Guest


    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.
     
    Parallax, Dec 8, 2003
    #38
  19. ken mankoff

    ZZBunker Guest

    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.
     
    ZZBunker, Dec 8, 2003
    #39
  20. Ever work on the neutrino device for the same purpose?
     
    Jack Linthicum, Dec 8, 2003
    #40
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