RADAR

john_morris_uk said:
It only happens when another radar happens to be transmitting nearby and on EXACTLY the same frequency AND its radar antenna happens to be pointing more or less at yours at the same time.
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The significant British invention with respect to Radar was that it could be made to work without having an incredibly precise control over the emitted frequency. The Germans were trying to control the frequency precisely (and did pretty well actually), but the British method was to live with the frequency variation by using an auto-correlation method - i.e. calibrating the received signals vs the transmitted signals instead of trying to directly interpret them.
 
michaelchapman said:
The significant British invention with respect to Radar was that it could be made to work without having an incredibly precise control over the emitted frequency. The Germans were trying to control the frequency precisely (and did pretty well actually), but the British method was to live with the frequency variation by using an auto-correlation method - i.e. calibrating the received signals vs the transmitted signals instead of trying to directly interpret them.
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Or just using one timing unit for both, so any drift is in transmit and receive.
 
One thing I haven't seen mentioned (forgive me if it was and I missed it) is that most marine radars are plumbed in to GPS and a compass - so it knows the heading and vector of the base station.

This is necessary to compute CPA etc
 
I am talking about MARPA CPA with a radar 'target' not an AIS one.

If you have both, it is interesting to compare the two. I think the AIS one will be the more accurate as (I think) the target's course is taken fom its AIS data whereas radar has to try to work it out by the movement of the target relative to the base station.

My MARpa takes quite some time to settle down and give a consistant reading and often it doesn't seem to be that accurate.
 
davidej said:
One thing I haven't seen mentioned (forgive me if it was and I missed it) is that most marine radars are plumbed in to GPS and a compass - so it knows the heading and vector of the base station.

This is necessary to compute CPA etc
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CPA etc is a rel vel problem which needs no GPS data. Or was my training in the 60s which was pre GPS ( yes some of us can remember life before GPS)
completely wrong.
 
Jabberwocky said:
what is MARPA CPA
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MARPA is Mini ARPA, ie a cut-down version of >>>
ARPA is Automatic Radar Plotting Aid, which is a software device built into ship radars that performs most of the functions that can be achieved by manual plotting.
Manual Plotting involves recording the relative position of a contact over time, in order to calculate various useful pieces of information including its >>>
CPA is Closest Point of Approach -- the closest you are likely to get to the target if neither of you alters course or speed.
MARPA/ARPA/manual plotting will also give you the target's course, speed, time to CPA and allows you too ask for predicted figures if you alter course or speed.

Manual plotting will do all those (but more slowly), but can also tell you whether the target will pass ahead or aster, and allows you to calculate course/speed to intercept etc.
 
timbartlett said:
MARPA is Mini ARPA, ie a cut-down version of >>>
ARPA is Automatic Radar Plotting Aid, which is a software device built into ship radars that performs most of the functions that can be achieved by manual plotting.
Manual Plotting involves recording the relative position of a contact over time, in order to calculate various useful pieces of information including its >>>
CPA is Closest Point of Approach -- the closest you are likely to get to the target if neither of you alters course or speed.
MARPA/ARPA/manual plotting will also give you the target's course, speed, time to CPA and allows you too ask for predicted figures if you alter course or speed.

Manual plotting will do all those (but more slowly), but can also tell you whether the target will pass ahead or aster, and allows you to calculate course/speed to intercept etc.
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Whilst I agree that Marpa systems can do most things for you I do suggest that those who use radar learn manual plotting both for collision avoidance and for blind pilotage. Having learmt the basics them one can get the maximum benefits from having automated plotting aids and avoid being sucked into some of the more basic traps. Radar is wonderful but it does need undestanding to keep you safe.
 
Jabberwocky said:
Ref the Power/voltage/amp question as someone has already said it is due to 2kw peak power, but the amount of time the radar transmits is very small compared to how long it "listens" Receives which uses a lot less power. So the average power use is small.

Anaprop, it came to me in the middle of the night some of the classroom stuff. Inversions layers etc. Sporadic E etc etc.
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The main reason there's so little hassle with anaprop is that it's a low power radar with 25 degree vertical beam, so doesn't tend to get near most of the conditions that cause anaprop. I would imagine the digital sets may well process out anything above the horizon as well.

If you're ATC or Ground Eng then there's one huge difference between AIS and IFF/SSR. You can pretty much rely on IFF/SSR data, especially the big boys. AIS data is often rubbish - I've seen headings 180 degrees out, ships at anchor doing 15 knots and and ships berthed up doing the same. More worryingly I've also being passed by a ship doing 4 knts when we were making 7 or 8. There's absolutely no guarantee of coverage with AIS - trawlers especially will just switch the thing off..

Yep, transmit time, PRF's, antennae speed etc. mean it's now pretty unlikely to get whited out by another set. If you see Broadband Radar (from Lowrance/Simrad/B&G) it's FMCW by the way.

Have a quick read of one of the Radar Handbooks for sea stabilisation, etc. Sadly not that many seem to use true sea stabilisation. Remember you need to be looking at a/c systems to get a comparison in the Aviation world, not Ground stuff. In general, from what I've seen the majority of boats have a radar that performs way in excess of it inputs. A/c don't tend to pitch around and definitely don't have the constant changes in heading that a small boat in a seaway has. Their relatively slow speed means that GPS course-derived heading is not a very reliable source either. Throw in a usually flakey speed through the water from a paddlewheel device and any radar will struggle really,
 
michaelchapman said:
The significant British invention with respect to Radar was that it could be made to work without having an incredibly precise control over the emitted frequency. The Germans were trying to control the frequency precisely (and did pretty well actually), but the British method was to live with the frequency variation by using an auto-correlation method - i.e. calibrating the received signals vs the transmitted signals instead of trying to directly interpret them.
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Which system was this - its not a technique I'd heard of being used on early radars
 
billcowan said:
AIS is not at all a substitute for radar, ais is very comforting as it puts descriptions and names to the blobs, or at least some of them. but quite large fishing boats and fish farms don't seam use it much. If your coastal and get 3g, you can get a very good AIS app on your phone

Related question for the boffins, my radar is 2Kw working off of 12v that would correspond to 166 amps! how does it achieve this using a 10 amp fuse?
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2kW when transmitting. What percentage is the pulse length to the repetition time?

E.g, if your pulse is 1 us, and it fires 1000 times per second, the average power would be 2W.
 
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davidej said:
I am talking about MARPA CPA with a radar 'target' not an AIS one.

If you have both, it is interesting to compare the two. I think the AIS one will be the more accurate as (I think) the target's course is taken fom its AIS data whereas radar has to try to work it out by the movement of the target relative to the base station.

My MARpa takes quite some time to settle down and give a consistant reading and often it doesn't seem to be that accurate.
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The AIS one will be as accurate as the GPS was on the target the last time it transmitted.

MARPA is a running average system. Even if it's performing to the full ARPA spec, is still have a north up plotting sheet out.
 
michaelchapman said:
The significant British invention with respect to Radar was that it could be made to work without having an incredibly precise control over the emitted frequency. The Germans were trying to control the frequency precisely (and did pretty well actually), but the British method was to live with the frequency variation by using an auto-correlation method - i.e. calibrating the received signals vs the transmitted signals instead of trying to directly interpret them.
Click to expand...
Well - sort of. Actually they use the transmit magnetron as part of the LO for the receive system - hence the absolute frequency doesn't matter.

However the point I was making is that interference is rare because a whole variety of varying factors have to occur at the same time for the interference to show (unless the other radar is VERY close and powerful in which case other factors such as blocking of the receiver may come into play.)

However this is thread drift and its a long time since I worked on radar design and signal analysis.
 
st599 said:
The AIS one will be as accurate as the GPS was on the target the last time it transmitted.

MARPA is a running average system. Even if it's performing to the full ARPA spec, is still have a north up plotting sheet out.
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In reality both Arpa and AIS info is as much driven by own ship data as target data. In almost any seaway own ship dat may well be good averaged over a period of time but as seen by the Arpa or AIS it is very unstable giving less stable answers. As with many systems one needs to apply some CDF to the output
 
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