RADAR

[Jabberwocky]

I am an Air Traffic Engineer by trade so I understand how RADAR etc works, I was just interested how ship/boat board Radars deal with different situations. I am new to the world of boating so made some presumptions that may or may not be correct, the 1st one is AIS, Is that of the same family of device as IFF/SSR?

How does Primary radar cope with moving targets if the boat is moving? would it not then decide everything is moving?

How is Anaprop dealt with as I used to work at an airport by the sea and it caused no end of problems? I would imagine that this is sorted out with the clutter map but would still cause problems with moving targets.

How do you stop getting interference from other ships? fixed radars use different frequencies so as not to affect surrounding radars. However on a boat that is moving among other boats that is an impractical solution.

 

[KellysEye]

>How does Primary radar cope with moving targets if the boat is moving? would it not then decide everything is moving?

Tbe boat is in the centre of the radar area and we set the display to the bow at 180 degrees up. Thus we appear not to be moving. Obviously you need to be aware that something coming towards you could be you overtaking something.

>How is Anaprop dealt with as I used to work at an airport by the sea and it caused no end of problems? I would imagine that this is sorted out with the clutter map but would still cause problems with moving targets.

There is a clutter control but you have to use it carefully because it can remove small targets such as yachts.

>How do you stop getting interference from other ships? fixed radars use different frequencies so as not to affect surrounding radars. However on a boat that is moving among other boats that is an impractical solution.

I've never seen interference from other yachts, I assume there is a filter to take it out. Ships use a different frequency so aren't relevant.

 

[Seajet]

I am an Air Traffic Engineer by trade so I understand how RADAR etc works, I was just interested how ship/boat board Radars deal with different situations. I am new to the world of boating so made some presumptions that may or may not be correct, the 1st one is AIS, Is that of the same family of device as IFF/SSR?

How does Primary radar cope with moving targets if the boat is moving? would it not then decide everything is moving?

How is Anaprop dealt with as I used to work at an airport by the sea and it caused no end of problems? I would imagine that this is sorted out with the clutter map but would still cause problems with moving targets.

How do you stop getting interference from other ships? fixed radars use different frequencies so as not to affect surrounding radars. However on a boat that is moving among other boats that is an impractical solution.

I have a JRC radar on my boat, and am used to aircraft systems; I think the simple answer to your very good questions is that boat radar is much more simplistic, and re movement just shows a picture relevant to the boat; AIS is indeed like modern IFF, and I suspect will come a lot more to the fore in future as a 'squawk'.

 

[GrahamM376]

AIS can be a stand alone system with its own display or can be displayed on some radars, as can navigational charts. It's not the same as IFF/secondary as used airside.

Moving targets present no problem to the radar as all targets are relative to your current position whether you're moving or not.

I've personally never had a problem with anaprop, remember that small boat radars are typically only around 2kw. Rain and squall lines do show up tough.

Interference from other radars not a problem.

I see Seajet's reply is different to mine. I see AIS as different to secondary/IFF as it does not respond to radar interrogation, it transmits/receives in its own right but can be displayed on some radars.

 

[nigel1]

Not sure what IFF/SSR is, but AIS is an automated identification system based on VHF and GPS.
With a type A Transponder, a vessel will transmit name, ID number, course, speed, heading, position, and navigation status.
The AIS will also receive same data from other vessels. Note, this is for Class A Transponder, there are Class B for small vessels, less info sent, and receivers only


How does Primary radar cope with moving targets if the boat is moving? would it not then decide everything is moving

Use relative motion vectors to calculate true vectors

How is Anaprop dealt with as I used to work at an airport by the sea and it caused no end of problems? I would imagine that this is sorted out with the clutter map but would still cause problems with moving targets.

No idea what anaprop is, but marine radars have Sea Clutter controls (de-clutters the radar returns closer to the scanner, and Rain Clutter controls which de-clutters across the whole range.
You can also fine tune the returns by altering pulse length, and modern marine radars are video based so the picture can be enhanced, such as echo averaging or echo stretching.

How do you stop getting interference from other ships? fixed radars use different frequencies so as not to affect surrounding radars. However on a boat that is moving among other boats that is an impractical solution.[/QUOTE]
No much of an issue, most radar frequencies are slightly different. Marine radars are fitted with IF rejection controls (fixed, normally a choice of 3)



Your average radar on a yacht is going to be pretty basic, as with most things, the more you pay, the more gizmo's you get, although alot of gizmo's on modern radar sets seem to be an expensive luxury and of not much use

 

[maxi77]

AIS is nothing like IFF which is as you know a secondary radar. AIS is a vhf radio based system Large ships and some yachts have transmitters which transmit a collection of fixed and variable data, including name, calsign, mmsi number position heading and speed, and quite a lot more. Try google. Vessels with recievers can have either a stand alone display or overrlay the data on a plotter or radar screen. Probably better on the plotter as due to different time bases and position data the AIS may not overlay it's radar equivalent, unlike IFF.

 

[Seajet]

AIS is an active transponder, responding when interrogated by a signal, rather like Phase 4 IFF; if it's a 'secondary radar' how come it's not an active sensor as in spotting things then ?!

 

[nimbusgb]

AIS is an active transponder, responding when interrogated by a signal, rather like Phase 4 IFF; if it's a 'secondary radar' how come it's not an active sensor as in spotting things then ?!

AIS is not an interrogated system. It uses self-organised time-division multiple-access (SOTDMA). AIS will transmit regardless of, or rather, even with the lack of, external signals.

 

[Seajet]

AIS is not an interrogated system. It uses self-organised time-division multiple-access (SOTDMA). AIS will transmit regardless of, or rather, even with the lack of, external signals.

Nimbus,

agreed, sorry I was having an off moment thinking about other systems; however the notion of AIS being a 'secondary radar' seems a touch optimistic, relying on everyone advertising their presence...

 

[maxi77]

AIS is an active transponder, responding when interrogated by a signal, rather like Phase 4 IFF; if it's a 'secondary radar' how come it's not an active sensor as in spotting things then ?!

I do not believe AIS has an 'active transponder' My AIS rx does not ask for info it sits there dumbly waitting for other ships to transmit to it. This is one of the reasons not all the data is transmitted on every pulse, and in busy conditions the update rate cam be slow.

"Vessel mounted AIS transceiver (transmit and receive) which operates using self-organised time-division multiple-access (SOTDMA). SOTDMA requires a transceiver to maintain a constantly updated slot map in its memory such that it has prior knowledge of slots which are available for it to transmit. SOTDMA transceivers will then pre-announce their transmission, effectively reserving their transmit slot. SOTDMA transmissions are therefore prioritised within the AIS system. This is achieved through 2 receivers in continuous operation. Class A's must have an integrated display, transmit at 12 W, interface capability with multiple ship systems, and offer a sophisticated selection of features and functions. Default transmit rate is every few seconds. AIS Class A type compliant devices receive all types of AIS messages."

IFF signals are tied to the main radar echo and will always sit on them. AIS is but a download of data given a geographic position from GPS data, so will sit at that position irrespective of any radar echo on the same display. For this reason one needs to take care when using radar with an AIS overlay to confirm which radar echo belongs to which AIS info.

Mind you the advantage of AIS over IFF is that first it can be accessed with a low cost Rx and no radar, and that it can carry much more information

 

[timbartlett]

[/quote]

I am an Air Traffic Engineer by trade so I understand how RADAR etc works, I was just interested how ship/boat board Radars deal with different situations. I am new to the world of boating so made some presumptions that may or may not be correct, the 1st one is AIS, Is that of the same family of device as IFF/SSR?

Similar objective, different principle. AIS is an automated messaging system, in which AIS-equipped vessels automatically transmit data about themselves at intervals that vary but are generally in the order of a few seconds. AIS A messages are generally transmitted by ships, and contain lots of information, AIS B messages are generally transmitted by small craft, contain less information, at wider intervals, and are given lower priority. Many yachts are fitted with receive-only AIS, which does what it says.

Having received the information, it can then be superimposed on any of a wide range of displays -- standard yottie practice isto overlay it on the chart plotter, commercial practice inclines more towards overlaying it on the radar, but there are also dedicated displays

How does Primary radar cope with moving targets if the boat is moving? would it not then decide everything is moving?

Sort of. It depends which display mode you are using (Typically Head up, North up, or True Motion) Head up is the simplest to explain, because the PPI display shows each contact on the correct range and bearing relative to the boat's heading -- so yes, if you are moving, you see relative motion on the screen -- stationary objects appear as contacts moving parallel to the heading marker. And when you turn, the whole picture turns.

North up stabilises the picture, but it is still relative to the boat. True motion moves the picture so that stationary targets appear as stationary contacts -- but it needs a big screen, and is seldom very effective

How is Anaprop dealt with as I used to work at an airport by the sea and it caused no end of problems? I would imagine that this is sorted out with the clutter map but would still cause problems with moving targets.

If you mean anomalous propagation cause by weather, it isn't a problem -- probably because marine radar is generally operating at quite short ranges -- typically <24miles

How do you stop getting interference from other ships? fixed radars use different frequencies so as not to affect surrounding radars. However on a boat that is moving among other boats that is an impractical solution.

Various methods. Ships and boats all use 3cm radar at roughly 9400MHz (ships also use 10cm radar, but that's not a problem) but the key thing is "roughly": manufacturing tolerances mean that the chances of finding anyone transmitting on exactly the same frequency as you are remote, and having them transmitting towards you while your antenna is pointing towards them is even more remote! Nowadays, almost all radars use at least some degree of digital processing -- one part of the process removes interference. On someradars you can switch the InterferenceRejection process off -- but there's no point, and on most modern radars it's permanently on.

If you mean "clutter" (eg echoes from waves and clouds) there are controls that will minimise the effect of both. They need to be used with caution, though, because -- despite increasingly sophisticated processing -- the sea clutter control cannot100% distinguish between waves and buoys or boats, so it is easy to delete small contacts along with the waves!

 

[john_morris_uk]

Just to add - it is possible very very occasionally to get interference from other radars - you get great big meaningless patterns and its all very obvious.

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.

So many things have to line up that it doesn't happen very often, but you used to see it on commercial radars occasionally in heavy shipping density. I haven't seen it on modern digital processed radars - but I haven't been on the bridge of a ship for extended periods for a couple of years.

 

[Jabberwocky]

So AIS is like IFF/SSR as in it sends craft type information. However it "squawks" when it wants to instead of when interrogated by another secondary radar. Plus it give its information including long/lat so it is close to Mode S.

Thinking about the moving targets problems, I was thinking about static objects not being shown, however on a boat you want to know that the light house etc is there.

From memory Anaprop (Anomalous Propagation) over water is caused by polarization phase shift. However it has been a few years since I did RADAR theory in that much detail. However if relatively low power is transmitter the range is limited so the phase shift would be less prominent. However in a strange way I would imagine there would be more problems with STAR (Second Time Around Returns) how is that combated? is it by stagger or frequency hopping?

Cheers for you help guys

 

[john_morris_uk]

So AIS is like IFF/SSR as in it sends craft type information. However it "squawks" when it wants to instead of when interrogated by another secondary radar. Plus it give its information including long/lat so it is close to Mode S.

Thinking about the moving targets problems, I was thinking about static objects not being shown, however on a boat you want to know that the light house etc is there.

From memory Anaprop (Anomalous Propagation) over water is caused by polarization phase shift. However it has been a few years since I did RADAR theory in that much detail. However if relatively low power is transmitter the range is limited so the phase shift would be less prominent. However in a strange way I would imagine there would be more problems with STAR (Second Time Around Returns) how is that combated? is it by stagger or frequency hopping?

Cheers for you help guys

If I remember correctly, on big ship radars second time round return problems are solved by altering the pulse repetition frequency. On warships if you are in the radar transmitter room you can sometimes hear the different squeals of the pulse rate modulators as the thing changes repetition rate.

Small craft radars are fairly simple affairs and you take your chances with the other problems you outline. They are not usually significant problems!

As the radars are pulse only - the fact that a buoy is moving relative to you is irrelevant. The buoy still gives an echo and if you plot it you find that its doing exactly your speed etc. Its the ones that aren't doing your reciprical speed that are actually moving...

 

[timbartlett]

Thinking about the moving targets problems, I was thinking about static objects not being shown, however on a boat you want to know that the light house etc is there.

Marine radar will show everything that produces a strong enough echo, whether it is moving or not. The problem is that when you are moving, things that are stationary appear (on the screen) to be moving.

It's not as bad as it sounds. We live our entire lives dealing with relative motion -- we know that as we drive along a motorway, the bridge that appears to be rushing towards us is, in fact, perfectly still, and the car that appears to be reversing slowly towards us is more likely to be moving forwards, but slightly slower than we are! It's just the same on a marine radar.

However in a strange way I would imagine there would be more problems with STAR (Second Time Around Returns) how is that combated? is it by stagger or frequency hopping?

Neither. It's by using very long pulse repetition intervals. Typical PRI for a small marine radar is in the order of 1millisecond, so a target would have to be more than 150km away in order to produce a second-trace echo. A small-boat radar simply isn't powerful enough to create a discernible echo from anything at that range -- and if it were, it would almost certainly be a very transient effect.

 

[billcowan]

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?

 

[timbartlett]

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?

If you look in the back of the manual you will find that the specifications give you the Pulse length and Pulse Repetition Frequency (PRF). The pulse length is typically in the order of 1 microsecond or less, while the PRF is typically in the order of about 1000 pulses per second. So you'll see that it is only transmitting for about 0.1% of the time. So although the peak power may be 2kW, the average transmitted power will probably be <2W.

Most of the power is probably used to light the display!

 

[William_H]

SSR /IFF

Just a bit more explanation of this aviation system. Surveillance (primary) radar receives returns from anything in the sky as a picture of returns similar to ship radar.
SSR is another "radar"with an antenna mounted on the primary radar antenna so always scanning the same narrow area as the primary radar. Working on a much lower frequency the SSR signal is picked up by the aircraft transponder and an answering signal is sent back. Very much like an active radar reflector except this is just for the interogating radar signal not the primary radar signal. The reply contains ID information, altitude, a specific squark signal and can even carry a code for radio failure or hijack. On the scanner screen this info is attached to the primary radar return so that a blob return can be identified. Also extraneous returns can be disregarded. This technology started in WW2 as Identification Friend or Foe AIS on the other hand is a natural outcome of Sat Nav technology. olewill

 

[maxi77]

From memory Anaprop (Anomalous Propagation) over water is caused by polarization phase shift. However it has been a few years since I did RADAR theory in that much detail. However if relatively low power is transmitter the range is limited so the phase shift would be less prominent. However in a strange way I would imagine there would be more problems with STAR (Second Time Around Returns) how is that combated? is it by stagger or frequency hopping?

Cheers for you help guys

From my memory anaprop is caused by atmospheric ducting which refracts the radar signal in such a way more of the energy is concentrated into the near surface area and that energy is refracted again to follow the re of the earth. This can result in the doubling or in extreme cases tripling of ranges. It is rather weird to pick up a target at near maximum range, track it all the way in but never see it visually then have it jump back out to max range and then track in again this time becoming visual at the expected range.

Anaprop can be easily predicted from met data and there were programs to do this a good few years ago.

 

[Jabberwocky]

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.