Broadband radar - living up to expectations?

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

  1. apex1

    apex1 Guest

    A clear yes. But.........additional..thats important.
    And the range is only 1 mile, after that the conventional Radar is better.

    Regards
    Richard
     
  2. marshmat
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    marshmat Senior Member

    A notable problem in early versions of the Navico FMCW system was interference from pulse radars- if a powerful pulse on the same band hit the receiver, it'd show a solid radial spike at that bearing. Apparently this is filtered out in the latest version of the software.
    Interference between different RF and microwave devices will continue to become a bigger and bigger problem as the number and diversity of transmitting devices grows. Using low-power devices such as FMCW where possible may help keep things from getting impossibly cluttered.
    Also note that FMCW will not trigger racons.
     
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  3. tspeer
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    tspeer Senior Member

    A frequency modulated continuous wave (FM-CW) radar transmits a chirp instead of pulses. The delay between transmission and return is detected by comparing where the same frequency occurs in time for the return vs the transmitted signal.

    The peak radiated power is much lower than a pulsed radar, but that doesn't mean a corresponding reduction in range. The energy from a continuous wave return is integrated over a much longer period, so the ability to detect the return is much better than the peak radiated power would suggest. It would be better to compare the pulsed radar and the FM-CW radars on the basis of their average powers, rather than their peak powers.

    The pulse repetition frequency of a pulsed radar is limited by the need to allow time for an unambiguous return from a target at long range. If the return pulse comes back after the next pulse is transmitted, then it's not clear if it is a return of the first pulse from a target at long range vs. the return from the second pulse from a close-in target. So there's a limit to the average power that can be obtained from a given peak power and maximum range. The FM-CW radar's chirp would have a similar restriction with regard to how long the chirp can be before it repeats, but there's no impact to the average power.

    There's also a minimum resolution due to the pulse width. The wider the pulse width, the more the target gets smeared out because the return from the leading edge of the pulse comes back from land farther away at the same time as the return from the trailing edge of the pulse comes back from land a little closer in. So good range resolution also reduces the average power from a pulsed radar of a given peak power.

    These design aspects all add up to require much higher peak power from a pulsed radar than FM-CW for the same range performance.


    The magazine reviews I've read online typically have pictures like this that show the difference in resolution between FM-CW and pulsed radars at close range:
    [​IMG]

    [​IMG]

    All the reviews say the maximum range is less than 4Kw pulsed radar, and the ability to detect weather is not as good. And it can't detect radar beacons at all. However, I'm interested in a radar for navigating narrow channels between islands in fog, seeing shipping in time to get out of the way, and especially avoiding other boats - stuff I can't get from a chart plotter and AIS (like the pair of fishing boats charging through the fog that almost ran down a buddy of mine). For my coastal cruising, I can get weather reports so detecting a frontal system at a distance is not so important.

    Does anyone on the forum have actual experience with Navico's FM-CW radar instead of speculation?
     
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  4. Fanie
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    Fanie Fanie

    No experience, but I think it's going to be a lot like echo sounders/fish finders/ depth sounders. You're going to have to use the thing a bit to figure out how it displays what. It's not going to show a picture of the coca cola tin 100m head of you :D or flash the red block on screen saying 'Mayday ! Female skipper approaching'. Would be nice though ;)

    I would like one for the same reason, short range indication of whatever could be in the area.
     
  5. CDK
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    CDK retired engineer

    That's what the Navico brochure says, but I have serious doubts.
    The burst from a pulse radar has an amplitude several orders of magnitude larger (1000 V or more) and the echo is received by an exactly tuned receiver during the silence between pulses. A typical microwave receiver needs only 10 uV to accurately distinguish a signal from the noise, so minute amounts of return signal can provide a stable picture.

    Broadband radar emits only 100 milliwatts (says Navico). Imagine how much of that is reflected by an object 10 miles away: almost nothing and that means the device (and you) cannot see it because it is lost in the noise.

    Because this new technology needs no high voltages, magnetron and waveguides, it can be produced very cheap. It would not surprise me if within a few years prices will drop to the level of a fish-finder, so every boater can afford to have his own "radar".
     
  6. TeddyDiver
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    TeddyDiver Gollywobbler

    I don't see the point comparing pulse radar and FMCV.. If you wan't to find a ship or weather front 15nm away you get a Furuno or to find a bouyo or runaway dinghy in limited visibility in 1/2nm range you get a Navico.. I believe also that most of the latter have been installed to boats not having a radar before and would othervise have been pretty much without and stayed that way onwards..
     
  7. powerabout
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    powerabout Senior Member

    it does happen that you get 2 radars on the same frequency and yes they do interfere with each other.
    Its just the law of averages and build of the magnetron that means they end up on slightly different frequencies
    The typical IMO approved ones are from roughly 25-74KW
    Your leaking 1kw microwave oven at home is more dangerous
     
  8. powerabout
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    powerabout Senior Member

    I ddi read somewhere when the broadband was coming out that you would be able to get one that is just a USB device for your laptop because of the reasons as stated by CDK
     
  9. powerabout
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    powerabout Senior Member

    You can get a S with a 6' antenna but no it will not see the rain it will see through it which is what they are for
     
  10. Fanie
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    Fanie Fanie

    The TX RX frequencies may not be a problem from multiple units. It would be very easy for a unit to scan before TX - if it gets a reading, measure the frequency(s) and TX it's own on a different frequency. Offsetting a frequency could be a matter of switching a filter network in and out. The TX could also possible contain an ID, and more than 1 unit in an area could skip several TX RX and you won't notice, so they may well take turns operating in the same area.
     
  11. CDK
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    CDK retired engineer

    Turn your cellphone into a powerful broadband radar!

    Today, anchored in a quiet bay, instead of finishing reading John Grisham's "The Appeal", I mused upon the merits of "broadband radar".

    I reread the Navico documentation and saw they are a marriage of Northstar, Simrad and Brunswick. Simrad I know nothing about, Northstar made the fish finder I cannot use because the sun's radiation turns the display dark brown and also my VHF set that refuses to communicate with a Garmin GPS. And Brunswick I am quite familiar with: they made jukeboxes I listened to, bowling alleys and now market old fashioned gas engines as if they were invented yesterday. Not a trustworthy nest!

    Two quotes from the document:
    "the most significant radar breakthrough since 1940" and this one
    "Over five years of R&D went into achieving this revolutionary advance in marine navigation"
    Long ago I was in the advertising business and wrote a lot of such crap.

    And then all of a sudden I saw it all crisp and clear: they are selling us a $50 cellphone for $1600. The cellphone has all the basic ingredients - memory, processor, receiver, transmitter, signal level detector - to build a "broadband radar", and costs virtually nothing. Just add a plastic dome under which you hide a piece of printed circuit board turned around with a small electric motor, and overwrite the phone's firmware with a far simpler program.

    Less than 15 minutes later I invented a new radar breakthrough: the cell phone digital broadband radar! I didn't need 5 years, just 15 minutes.

    I'll explain, but this is getting a bit technical, so bear with me:

    Your cellphone is a powerful transmitter/receiver and can instantly switch between these functions. While you are talking and listening, it communicates with the fixed antenna in very short bursts: most of the time it keeps quiet while other phone customers are served.

    To make the phone aware of it's surroundings, it needs a simple software program that counts from 1 to 256 and transmits the binary code for these number, which are only 8 bits wide and can be sent very quickly. Than the phone switches to reception for a short time.
    When standing in the middle of a desert, the phone receives nothing, but if you came by car and parked it somewhere, some of the energy is reflected back to the phone and will be received. Because the phone knows when a certain code was sent, it can calculate the distance to the object with great precision.

    The phone can tell you the distance, but not the direction the reflected number came from. To make that possible, just like Navico did, you need to add a rotating antenna - a cheap piece of circuit board - and a salad bowl to protect it from rain etc. With a modern 1800 Mhz phone even a butter cup is large enough. Add one sensor to mark the beginning of each revolution and you're done.

    Navigo uses a frequency sweep instead of a digital code, but that isn't clever because a fixed frequency is much more effective, especially for the receiver, that can be much finer tuned making it more sensitive. My cell phone radar uses 256 steps; to fully use the accuracy a display with 512x512 dots is required. That means that for a row range application, one dot can be one ft. I assume that to be fully adequate.

    I do not expect anyone to undertake such an experiment tomorrow, just wanted to illustrate how simple it is to make a "radar breakthrough".
    Some company will pick up the idea, maybe apply for a patent and try to sell it to you.

    And if the price is under $300, I encourage you to buy one.
     
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  12. Fanie
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    Fanie Fanie

    It's a bit more involved than that, but you about nailed it there. The price is probably $300, it's those between you and the manufacturer doubling the price each time.
     
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  13. CDK
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    CDK retired engineer

    I know I cut a few corners but writing it was a lot of fun.

    When I was young, irresponsible and in the air force, they let me play with a tracking radar on an open air exhibition. I quickly discovered that pointing the dish to a visitor with a camera and pressing the transmit/lock button instantly triggered flashbulbs as far as 500 yards away. I remember one guy with Philips Flashcubes in his shirt pocket: we needed a fire extinguisher to save his ***.

    That's the kind of energy you need to see distant targets on a radar screen.
     
  14. Fanie
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    Fanie Fanie

    And make them flash as far as 500 yards eh. Right.

    Remember, the idea is to detect things (without setting them alight :D) and the broadband MCD (modified cellphone device :rolleyes:) is ok except for the price.
     

  15. apex1

    apex1 Guest

    Was my first comment not clear enough? Yes.

    And install it in addition to your conventional unit, because above one mile the "broadband" does not see much.

    And I am sure Thorwald knows that he cannot see the rain, but through it. His comment was clear on that.

    Regards
    Richard
     
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