Broadband radar - living up to expectations?

Discussion in 'OnBoard Electronics & Controls' started by expedition, Jul 27, 2010.

  1. BTPost
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    BTPost Junior Member

    Well actually Microwave are very similar to light and radio waves. All three are Electromagnetic Radiation, just of different wavelengths. They all are subject to the Laws of Physics, especially the Inverse Square Law. When it comes to Marine Radars, there are NOT any commercial Marine Radars, on the market today, that have a Power Density, 5 feet in front of the antenna, enough to cause any ill effects to human physiology. I am talking "Commercial Marine Radars" specifically, NOT Military Radars here, as Military Radars are a "Cat of a Different Stripe". Your much more likely to be hurt, falling down, after getting hit in the head by the rotating SlotLine Radar Antenna, than by radiation coming from the Antenna Face. This has not ALWAYS been true, Back in the Old Days, some of the 40 & 80 Kw xBand Decca Ship Radars, could give your eyes a tingle, but solidstate receivers have made a 20 Kw xBand Marine Radar, about the biggest thing in the world.
     
  2. Fishton
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    Fishton Junior Member

    CDK,

    A very interesting read indeed. What is your take on Broadband Sonar?
     
  3. CDK
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    CDK retired engineer

    Fishton, I quickly glanced at the Lowrance site, where I found the same appalling language as with the original topic. They must have hired a guy from l'Oreal or another cosmetics company.

    It is not broadband nor is it sonar.
    The proper terminology would be dual frequency fish finder, but that is far less appealing and nothing new.

    Over the years it has been established that with a higher frequency you can see deeper, obtain a smaller aperture angle and use a smaller, cheaper transducer. There is also less interference from other vibration sources.
    With a lower frequency you can get more detail from less sharply defined reflections like temperature gradients, albeit with a shorter range, so the more expensive fish finders have a dual frequency transducer.

    Lowrance now wants you to buy an additional device, placed between the fish finder and the transducer and claims it uses an impossible 83 Khz with a 200 Khz transducer. The proper technical term is mismatch!

    It has about the same credibility as l'Oreal's telescopic eyelash extending mascara.
     
  4. Fishton
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    Fishton Junior Member

    Thanks for the reply CDK. Your posts certainly make one think.

    Here is a screenshot taken from my HDS with a 200kHz standard skimmer transducer. The 83kHz certainly seems to have a larger coverage area of the 200kHz (longer arches / echoes).

    [​IMG]
     
  5. CDK
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    CDK retired engineer

    Nice pictures!

    All a depth sounder (ANY depth sounder) does is send a short ultrasonic burst to the bottom, then record the echos. What you see on the screen is how the firmware interprets the received signals, based on signal strength and waveform. Some devices show you real fishes instead of colorful arches.

    Most of the time it does not resemble the real situation. The last echo received is always the one from the bottom, so anything beyond that is displayed as solid brown, even if the bottom is just a thin sheet of rock with a large cave underneath. Any object that seems to be floating is interpreted as fish, even a pipe sticking out the mud.

    When anchoring in deep water, my North Star fish finder thinks I am surrounded by fishes because where the cone gets wider than a boat's length, it wrongly interprets echos from the anchor chain.

    For a more realistic picture of the bottom, an array of transducers would be necessary. Then you do not need a timebase but can construct an image based on the signal strength of each transducer. It becomes true sonar then, but has quite a different price tag.

    One last word about the transducer frequency.
    The active element is a thin layer of a piezoelectric material on a brass disc with the proper dimensions to almost resonate on a certain frequency band. I use the word almost because the vibrations are dampened by the material surrounding it.
    A 200 Khz transducer is also able to work with 100 Khz or 50 Khz, but has less sensitivity there. At 83 Khz it would be practically useless.
     
  6. Fishton
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    Fishton Junior Member

    I know I'm becoming a bit of a pain CDK, but I find this all very interesting. Please have a look at this screenshot of the same tree, taken at the same speed and bearing, only now with the Airmar P66 (dual frequency 50/200kHz) transducer connected to the HDS Broadband.

    Here are the P66 specs:- http://airmartechnology.com/uploads/brochures/p66.pdf


    [​IMG]
     
  7. CDK
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    CDK retired engineer

    Airmar uses less swollen language, more down to earth. I prefer that.

    At 50 Khz the cone is much wider, so there is more echo. It is not certain that the object passes exactly under the boat, the picture always suggests it is straight below, but that needs not be the case.
    You would have to make several runs and compare the 200 Khz pictures to determine the exact location.At that point, both parts of the screen must show more or less the same image ( the 200 Khz with sharper contours, 50 Khz with more shades).
     
  8. jonr
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    jonr Senior Member

    Digital signal processing allows one make use of signals far below the noise level - which is why FMCW can reach some distance with such low power levels. But perhaps passive radar (using all the RF signals already floating around) will make all current radar obsolete.

    Isn't a big advantage the life in hours?
     
  9. BTPost
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    BTPost Junior Member

    Actually, CDK, You have that backwards. Higher Frequency= Shorter WaveLength, Higher Resolution, Tighter Beam Cone, Less Depth....

    Lower Frequency= Longer WaveLength, Lower Resolution, Broader Beam Cone, Deeper Depth....

    This is why all the shallow Fishfinders are 200Khz, and the Commercial King and Tanner Crabbers use 28 Khz to see down 200-400 Fathoms.

    Most Side-Scanning Sonars are 100 Khz or higher, as they need higher frequencies to get better resolution on submerged objects, but only down 300-400 feet.
     
  10. Fishton
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    Fishton Junior Member

    This is the 800kHz DownScan screenshot.

    [​IMG]
     
  11. CDK
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    CDK retired engineer

    I think range is more a matter of transmission power than frequency. My old low frequency Seafarer Mk 3 had trouble finding the bottom at 30 meters, most 200 Khz fish finders can see a hard bottom at 200 m. and need only a fraction of the power.

    The 28 Khz units I am not familiar with. Sounds like an older, steam powered generation, but I may be wrong.
     
  12. BTPost
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    BTPost Junior Member

    The Fishton Post just before yours shows the kind of resolution that very High Ultrasonic frequencies (400+ Khz) can produce in shallow waters. (>150 Ft) Most Hydrologic Survey Sounders, used by NOAA, run in the 100-150 Khz with 20 Degree Transducer Cones, and can give good readings down to 100 fathoms, even with muddy bottoms. Deeper than that NOAA will go to 50 Khz, down to 500 Fathoms, and 28 Khz for the really deep stuff. (below 500 Fathoms) The folks at Honeywell Marine Systems built most of the NOAA stuff, and Ross Laboratories did much of the pioneering work in the late 40's and early 50's.
     
  13. jonr
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    jonr Senior Member

    I must say, that 800 khz picture is amazing. Reminds me that I'd love to be a treasure hunter.
     
  14. TeddyDiver
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    TeddyDiver Gollywobbler

    :D
    http://forum.treasurenet.com/index.php
    Here's a 100years old wreck of a freighter. bottom lies in ~50m..
     

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  15. cthippo
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    cthippo Senior Member

    [​IMG]

    Screenshot from one of the Humminbird side scan units. I desperately want one of these, but they don't seem to sell the side scan ones as stand alone units, all of them I have seen include a GPS. On the plus side, they work over ethernet so I can just feed the data to my computer.

    Getting back to Radars, one technology that I wish they would adopt from the military is frequency agility. The modern military radars can detect interference (jamming) and automatically switch frequencies to a clear channel.

    Another thing I would like to see is ARPA data integrated into the AIS and chartplotter information. That way you could see on one display all the targets, whether AIS equipped or not instead of seeing the AIS ones on one screen and trying to figure out where the ones from the radar are in relation.

    And while I'm dreaming...
     
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