Radar Query

[Mbase]

Does running the radar on a boat make the boat more visible to other boat's radar?
in other words does using radar onboard your own boat effectively act as an active target enhancer?

 

[jlavery]

Does running the radar on a boat make the boat more visible to other boat's radar?
in other words does using radar onboard your own boat effectively act as an active target enhancer?

No - radar only works on reflected (or retransmitted - see target enhancers below) pulses from its own transmissions.

Only a reflector or active target enhancer will improve the return from your vessel to another vessel's radar.

 

[Mbase]

Thank you. "own transmissions". Got it, and that's answered the question perfectly.
I presume "own transmissions" are coded in some way to only be recognizable by the sending radar?
So, active radar technology like Echomax, recognizes a radar signal, then boosts the return in the same format that the sending radar will see as it's own returning signal?

 

[srm]

As above.

However, should two radars be transmitting on exactly the same frequency a basic set, without signal processing or filtering, could show interesting interference patterns on its screen, but with no indication of the source of the interference.

 

[Sandy]

No - radar only works on reflected (or retransmitted - see target enhancers below) pulses from its own transmissions.

Only a reflector or active target enhancer will improve the return from your vessel to another vessel's radar.

I'd be interested to know how RADAR knows its own transmissions?

 

[jlavery]

I'd be interested to know how RADAR knows its own transmissions?

I'm referring to the fact that the unit only processes returns which arrive back from the focussed transmission, in the period the antenna is still pointing at the target.

I assume that this means active target enhancers must respond with the right frequency fast enough for their signal to arrive while the transmitting RADAR is pointing at it.

 

[srm]

I'd be interested to know how RADAR knows its own transmissions?

I'm referring to the fact that the unit only processes returns which arrive back from the focussed transmission, in the period the antenna is still pointing at the target.

A radar set can display any received signals that are exactly on its Tx frequency and will display them as if received from the direction that the scanner is pointing. This is how it is able to display target enhancers and SART transmissions that Tx on a wide frequency. Although the main Tx/ Rx direction is at 90 degrees to the scanner's antenna face it is not a sharp focused beam but has a number of side lobes as well. The shorter the scanner antenna relative to the frequency the wider the beam.

If you are close to a strong reflector, such as a big ship or oil rig, the set is likely to display side lobe returns that will make the target appear to stretch around part or even all of the display. It can also give false or "ghost" returns where a returning pulse is reflected back to the scanner off part of the vessel's structure.

The interference I mentioned in post #4 (above ) usually shows spirally patterns on the display, with the pattern being a function of the pulse repetition rates and scanner speeds of both radars that are transmitting on identical frequencies.

However, modern radars have signal processing and filtering software that aims to clean up the display by only showing consistent returns. This is why small vessels need very effective radar reflectors or enhancers to ensure that they are actually displayed on a ship's system at sufficient range for the ship to observe and take appropriate action. Without consistent returns they will be filtered out as 'noise'.

Edit: For comparison with yacht radars a SOLAS compliant x-band radar will have a six foot long antenna to give the required narrow beam width to discriminate between separate targets.

 

[jlavery]

Thank you - that makes sense!

I use (or practice using) my radar quite bit, so that I'm familiar and confident with it when I really need it.

I'm often surprised by how many yachts (which I know are there from AIS) don't show up on the radar.

 

[Sandy]

A radar set can display any received signals that are exactly on its Tx frequency and will display them as if received from the direction that the scanner is pointing. This is how it is able to display target enhancers and SART transmissions that Tx on a wide frequency. Although the main Tx/ Rx direction is at 90 degrees to the scanner's antenna face it is not a sharp focused beam but has a number of side lobes as well. The shorter the scanner antenna relative to the frequency the wider the beam.

Thanks, that was my understand although I did the theory a very, very long time ago hence the question.

 

[srm]

Thanks, that was my understand although I did the theory a very, very long time ago hence the question.

Its quite a while since I was teaching the theory and running simulator courses so its good to know that I remembered some of it.

 

[st599]

Modern "HD" or "Broadband" radars use a constant frequency sweep (Frequency Modulated Continuous Wave) and measure the returned frequency to accurately measure distance. They no longer pulse. This gives you much better accuracy close in (not waiting for the pulse to finish and the switch from tx to Rx)

But they have the same issue, they can't distinguish side lobe returns, distant echoes from previous sweeps etc.

 

[srm]

Modern "HD" or "Broadband" radars use a constant frequency sweep (Frequency Modulated Continuous Wave) and measure the returned frequency to accurately measure distance. They no longer pulse. This gives you much better accuracy close in (not waiting for the pulse to finish and the switch from tx to Rx)

But they have the same issue, they can't distinguish side lobe returns, distant echoes from previous sweeps etc.

I have not had the chance to use one, but the specs look impressive.

However, not using radar pulses means they will not trigger a response from a SART, RACON, or other transponders/target enhancers so there is some loss of ability in terms of identifying SAR targets and nav aids. Nav aids are using AIS so RACONs may well be going the way of RDF beacons. The improved detection performance and growing use of AIS transponders reduces the reliance on target enhancers, so the remaining real loss is not being able to recognise a SART. Tough on those in a liferaft if the only vessels withing RADAR range are using Broadband RADAR.

 

[Sargunes1903]

No, it doesn't make it more visible.