Radar reflectors and 3G Broadband Radar

[jerrytug]

Since the FM radar judges the distance of the target by the frequency in Hz of the echo which returns to it, an active RTE would have to send a signal of the exact right frequency to paint at the correct distance on the FM radar's screen.
Correct me if I'm wrong.

 

[BelleSerene]

Since the FM radar judges the distance of the target by the frequency in Hz of the echo which returns to it, an active RTE would have to send a signal of the exact right frequency to paint at the correct distance on the FM radar's screen.
Correct me if I'm wrong.

I suppose that so long as the bandwidth of the RTE's amplifier covers the frequency sweep of the FM radar signal, it would send back an amplified version of the signal it receives. What Arida found in the unit he dismantled was an amplifier of the incoming signal, not a single-frequency transmitter triggered by a detector.

However, any delay caused by the circuitry would result in the 'reflected' signal not matching the timing of the incoming signal, so still misleading the FM radar as to its distance. I'll ask.

A further problem would be of the much lower power of FM radars simply not triggering the RTE's signal threshold to take it out of its dormant state. I'll ask.

More suspect to my ear is that reversing a polarity (of what - the supply power?!) would achieve any of this. I'll ask!

I'll report back what I find.

 

[Seajet]

Thanks all, a very interesting and informative thread.

I have a cheapo JRC LCD radar and have been thinking about sea-me versus AIS; I was unaware of this new snag with failure to respond to CW radar.

I think I'll keep to being pro-active, relying on my eyes and my radar to know what's about, with an old octahedral radar reflector hung up as a sort of nautical mistletoe for now !

Incidentally I used to work on test ranges now & again, and noted they all use octahedral reflectors on target rafts.

Then again that could be down to MOD budgetting as much as effectiveness; at one test range I helped launch weather balloons, as I'd done my proper bit preparing the aircraft onboard cameras.

The balloon launch involved tying on the radar reflector below the Sonde - so a packet of shiny gold metallic Christmas ornaments of the cheapest nastiest type was to hand, and one of these things was tied on each balloon; they were tracked up to a great height as I remember, not sure of the figure now but certainly airliner high.

 

[BelleSerene]

I spoke to John Simpson, whose company it is. He’s self-confessedly ‘not technical’ – he sells the unit which an (ex-?) colleague designed – but I learned the following.

You can put the RTE (radar target enhancer) into a 'test mode' by simply reversing the polarity of its two power wires. In this mode, it’s constantly transmitting what it receives, rather than only when it receives a signal of a certain strength. (The unit must use a polarity-sensing circuit to maintain its own power supply while working in ‘test mode’.)

As for how they respond to the variable frequency of FMCW ('broadband') transmitters, I’m going to assume that as it simply amplifies whatever it receives, the whole frequency sweep is within its receiver and transmitter bandwidth and the unit works fine.

However, John says that even if you enable the Echomax to respond to FMCW signals by wiring it as above, it’ll only respond to broadband radar up to 300-400m away because of the lower power of broadband radar transmitters. Now, commercial ships don’t use FMCW, so I needn't worry about 20 knots in the dark. Say the other boat's steaming along at 10 knots, some 18 km/hr, so it’ll cover those 300m in 60 seconds. That’s only marginally a useful radar range to protect you, so I’m not convinced it’s worth using the unit to enhance FMCW radar. There again, I could wire it from its control panel via a double-pole, double-throw switch just to reverse the polarity of the RTE in a pea-souper and leave it in low-current mode the rest of the time.

FWIW, my own conclusions are:
· Pro: At £360, the X-band active unit is cheaper than a large tri-lens reflector, and a whole lot lighter, but gives a much stronger radar signature. (It is more expensive than the £170 medium-sized tri-lens, but that has a much weaker response still.)

· Pro: it also tells you when you’re being ‘painted’, which passive units obviously don't. However, in the Channel that’s probably most of the time, and further afield ships are likely to have turned X band off in favour of their S band radars, which this unit doesn’t pick up – and I can’t afford its dual-band big sister.

· Con: I’d be invisible to a commercial vessel that was using its S band radar AND had turned its X band radar off – although I believe I shouldn’t find any of those in the Channel or southern North Sea.

· Con: it requires battery power to operate. Nothing significant, but some.

· Con: it’s invisible to pleasure craft operating ‘broadband’ FMCW radars – unless I run it in its ‘test mode’ in which case they’d only see me a close quarters

If I believe the world isn't going to broadband radar, it's worth doing. If I reckon that in five years' time lots of pleasure craft will be using broadband radars - and I'm worried about being run down by small ships as well as big ones (which I'm not sure I am) then I should stick to an affordable but effective passive reflector which'll reflect everything - to some degree.


All thoughts gratefully received.

 

[johnalison]

All thoughts gratefully received.

Thanks for that explanation. Much of your reasoning matches my previous thoughts on the subject.

My impression from my own radars is that a sailing yacht will generally show up at around 2 miles, with or without a reflector, so that in a fog I am not at any great risk from other sailing yachts going at five to six knots like me. Equally, as you indicate, I should also be visible to ships where I sail, in Channel and N Sea with my SeaMe on. It seems as though the main risk would be from a powerboat or service boat equipped with FMCW radar and travelling at speed (something that was discussed recently on the ECF). If I am using my own radar, it looks as if I might have up to eight minutes to avoid or contact a craft travelling at fifteen knots, assuming I can see it at two miles.

Clearly, the important factor is for us not to feel complacent and assume that a single peice of technology will always protect us.