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#1
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"Steve Lusardi" wrote in
: The restriction at close range is Pulse width and receiver turn on time. A RADAR mile is 6.36 micro seconds. If you want to see a target 100 yards in front, the RADAR set must transmit a pulse and turn on the receiver to catch the echo in less than .31 micro seconds. That's a very tall order with a magnetron, as they are not gated. They operate by dumping high voltage on the cathode, which rings the hell out of the cavity. They turn off when the cavity decides it no longer is excited and the receiver can not turn on until there is no more energy being emitted from the magnetron. This is becoming a very big issue in Europe at the moment. There now is a new commercial regulation as of Jan. '06 specifically pointed at canal traffic that stipulates that all new RADAR sets work at 50 meters. For exactly the reason you mentioned in your post. Now that's tough to do. Steve Before the water in the dome rots the hell out of the Raymarine radar on Lionheart, that little sucker can see the 4th boat down our dock on the 1/8 mile range! It even plots the dock correctly from our 20' antenna on the mizzen. Pulse width must be picoseconds. I don't think it ever gets very wide to keep resolution high and current drain low. Hell, the scanner cable to the RL70CRC display where it gets its power from has very small, long power conductors and most of the power has got to be heating up the maggie filaments. I had a helluva time explaining to some captains why a 2KW radar didn't draw more than 2KW off their batteries. Some of them were afraid to turn 'em on without the engine charging all that power!...(c; AIS to the rescue! Need shore fixed stations with all up-to-date obstruction data coming out of them.... |
#2
posted to rec.boats.cruising
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On Fri, 06 Jan 2006 00:34:49 -0500, Larry wrote:
AIS to the rescue! Need shore fixed stations with all up-to-date obstruction data coming out of them.... And that will tell you about the 16 ft Boston Whaler fishing in the fog bank right in front of you? |
#3
posted to rec.boats.cruising
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Wayne.B wrote in
: And that will tell you about the 16 ft Boston Whaler fishing in the fog bank right in front of you? No, and neither will the radar scanner at 55 ft as some suggest to get long range. Boston Whalers with little metal are hard to detect. Of course, if we were to make $99 AIS transponders MANDATORY, problem solved. |
#4
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On Fri, 06 Jan 2006 23:38:05 -0500, Larry wrote:
No, and neither will the radar scanner at 55 ft as some suggest to get long range. Boston Whalers with little metal are hard to detect. ====================================== We have no problem picking up small boats with the scanner at 24 ft. It is unlikely that mandatory AIS will ever become a reality for boats under 30 ft or so, perhaps even larger. |
#5
posted to rec.boats.cruising
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No, and neither will the radar scanner at 55 ft as some suggest to get long
range. Boston Whalers with little metal are hard to detect. Wayne.B wrote: We have no problem picking up small boats with the scanner at 24 ft. It is unlikely that mandatory AIS will ever become a reality for boats under 30 ft or so, perhaps even larger. And if it is made mandatory for pleasure boats, how many people will still not have it, or forget to turn it on, or leave it broken? DSK |
#6
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i am reminded of the guy with an early handheld plotting GPS walking around
in ten foot circles saying, "look, it tracks me exactly".....pre SA! I am a relative newcomer to radar and use a digital RADAR-PC setup and during the day i can imagine i am seeing all manner of things that show up on my screen...if it is flat calm and the gain is high enough and the sewa clutter is OFF....but at night in the fog healed 20Deg that dot that appears for one scan...then mis and then two scans again then gone will have you staring at the screen instead of looking ahead to see if it is a contact or a wave or a ghost reflection from your rigging or even the bouy at 90degrees from the blip. What about sidelobe reflections which are again reflected and recieved...they are only interpretable after the fact...not a priori..? they are lower in intensity and can look like a small contact in any place. The receiver knows only when it switched from transmit to receive [time] and the radial angle of the antenae at that time so a reflected signal appears only to have been recieved from a distance equivelent to the total pathlength and in a direction in a straight line perp to the face of the antenae at that moment of capture...a ghost image. Then when you get a circular series of large contacts you may well wonder what semi circular beast is ahead of you.... read a book about the propagation of radar microwaves and see all the ways a blip can mislead you and thank God the guy you almost mowed over didn;t have radar and was keeping a lookout. Radar assisted collision are a significant reality. Real life radar is a tool that must be interpreted and i am finding out it takes a LOT of interpretation and experience to be able to rely on it more so than your eyes. The mainbang is suppressed so you don;t see the big donut around your boat extending for 200ft on a 1/2 mile range...if you are at 1/8th mile you might see a target at 50ft but only if the mainbang is not supressed and the gain turned way down and the sea clutter way up to exponentially deminish the gain applied to close returns. As for styrofoam cups....the intensity of an electromagnetic wave falls off in a cubic [3rd power] manner relative to distance and the reflected wave similarly diminishes but the part reflected is only that portion perfectly perpendicular to the antenae...as it dips and turns on a weaving mast even less of it is oriented in a 'perfect' manner. The intensity of the emmitted electromagnetic field recieived by the antenna is so small it is a marvel that modern electronics can even discriminate it from the background noise. Now the clincher....what portion on the emitted signal would a round styrofoam cup reflect from half amile away? hint, styrofoam is not a reflector of electromagnetic energy..is it an insulator and absorbs microwave energy. the only reflection would be from moisture in a thin lhorizontal line...perpendicular to the antena and the relfected signal is likely a billionth of the emmitted signal at best. Granted there are galenium arsenide semiconductor equiped ultra low noise receivers that could discriminate that SNR but at a few thousand dollars in the hands of a relatively untrained operator the pleasure boat operators radar.....it makes for good bench racing stories but little more. AND...if you really are detecting the water on a birds wings i suggest you tune and adjust the radar to pick up and discriminate larger targets...else they will be lost in the clutter In the process of ruining a 'story' i hope to have saved someones life by stimulating you to really learn what a radar can and can't do...repeatably. Quod erat...you know the rest of the story. rick "DSK" wrote in message ... No, and neither will the radar scanner at 55 ft as some suggest to get long range. Boston Whalers with little metal are hard to detect. Wayne.B wrote: We have no problem picking up small boats with the scanner at 24 ft. It is unlikely that mandatory AIS will ever become a reality for boats under 30 ft or so, perhaps even larger. And if it is made mandatory for pleasure boats, how many people will still not have it, or forget to turn it on, or leave it broken? DSK |
#7
posted to rec.boats.cruising
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Wayne.B wrote:
On Fri, 06 Jan 2006 23:38:05 -0500, Larry wrote: No, and neither will the radar scanner at 55 ft as some suggest to get long range. Boston Whalers with little metal are hard to detect. ====================================== We have no problem picking up small boats with the scanner at 24 ft. It is unlikely that mandatory AIS will ever become a reality for boats under 30 ft or so, perhaps even larger. It won't be mandatory for everything. Kayaks, 14' alu boats, logs. Radar is still better. Gaz |
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