Radar reflector

[LittleSister]

phrases such as 'the vast majority', 'almost all', would describe the ships using X band.

That's not the impression I got from Uricanejack's and DOMs's posts.

Yachts, tugs, workboats, fishing boats, fishfarmers, pilot vessels, bunker barges,Border Force, Harbour Patrols, RNLI, to name a few which spring to mind, will be using X, if using radar at all.

Can anyone else confirm that is the case? This is exactly the sort of thing that I want to know.

I meant it has never malfunctioned.

I have a branded radar reflector renowned for its poor performance that has never malfunctioned! (It's also at home in the shed, so I can be 100% sure of its contribution to my boat's visibility!) By the way, I'm interested to know how one could know an RTE has never malfunctioned.

I wonder whether RTEs will ever come down much in price. I understand that it's only a small market, and it needs to be decent quality, but presumably can use some fairly standard parts. There seem to be complete radars available for under £1,500, so a RTE at £450 to £700 seems relatively very expensive.

 

[Roberto]

On a short potter out the harbour last week I noticed my Active XS picking up both X and S band in the eastern inshore Solent area. It was 1.5 mile vis' max in mist.

I think a number of shore stations have S band radars which may override ships' ones; while sailing along the coast, for example near signal stations, the Active XS led for S band sometimes is fixed on, even without ships ar a reasonable distance, possibly because of the power of coastal stations spreading to secondary lobes. Surely it disappears as one sails away from th.e area

 

[Uricanejack]

If you can afford an active reflector go ahead and get one.
Personally I’m just going to stick with my traditional Reflector on my back stay in its rain catching mode. I am quite confident it will be good enough.
If and when I upgrade I will get an AIS transponder. Just my opinion it’s the the best upgrade to a traditional reflector.

If you have an X band active unit. With a traditional reflector, I would be confident an S Band RADAR properly set up and adjusted for the conditions will pick up your typical westerly centaur no problem in good, fair, miserable and even unpleasant conditions.

You can never really be sure the other vessel is keeping the best lookout regardless of what you fit. you can just hope they are. Taking Reasonable steps to ensure you will be seen by an attentive lookout.

So my idea of reasonable is a traditional reflector a good set of Nav lights and a small set of battery led emergency Nav lights JIK.
Having used both X and S band I am quite confident my boat will be sufficiently visible to an attentive watch keeper.
I keep my own look JIK I encounter an inattentive watch keeper.

 

[dom]

You can't rely on anything exactly 100%, but phrases such as 'the vast majority', 'almost all', would describe the ships using X band.
Yachts, tugs, workboats, fishing boats, fishfarmers, pilot vessels, bunker barges,Border Force, Harbour Patrols, RNLI, to name a few which spring to mind, will be using X, if using radar at all.

I disagree with your central conclusion that one only needs an X-Band RTE. In fact, I would prefer a decent passive reflector to X-RTE only and most certainly a passive plus AIS transceiver.

It is true that the vessels you cite all carry X-Band: partly for reg compliance, partly because X-Band is generally better for collision avoidance and pilotage, and of course partly because the 9-12' scanners required to achieve the necessary gain and their associated heavy power usage are a bit much for small vessels!

So what gives?

The bridge of a big ship will typically be swinging X and S scanners. The operational parameters of the two systems are quite distinct, and while X-band is often thought of as better for pilotage and collision avoidance, its signal is attenuated more by moist air than S so it becomes limited in adverse weather. Interestingly, this feature is harnessed by more sophisticated systems which compare the attenuation and scatter pattern differences between the two radar bands to resolve weather returns into rain, snow, hail, etc.

So S is better in poor weather/vis, also better in heavy seas, and having a wavelength about 3x as long will nominally have a 1.3x greater range (fourth power relationship).

Back to the bridge. Good practice requires the OOW to be watching the two, radars, typically with the S on a longer range. Moreover, for optimum results the OOW should be constantly fiddling with his/her X-Band's gain, tuning (AFC), rain clutter (FTC), and sea clutter (STC). In practice he/she often starts to primarily rely on the S-Band at times the X-Band becomes compromised.

So the yacht equipped with only an X-RTE is invisible on the ship's S-Band and is now 100% reliant on said OOW glancing across to / switching-to / overlaying his X-Band and thinking WTF is that big RTE return on my screen! Worse, as he does this, he may have tightened the range on the X-Band to a level that he's now hard up against the maneuvering parameters of his ship.

This is now the last place a small yacht wants to be!

Fortunately, it can remain entirely hypothetical if one transmits on AIS and/or is visible on S

 

[lw395]

I disagree with your central conclusion that one only needs an X-Band RTE. In fact, I would prefer a decent passive reflector to X-RTE only and most certainly a passive plus AIS transceiver.

It is true that the vessels you cite all carry X-Band: partly for reg compliance, partly because X-Band is generally better for collision avoidance and pilotage, and of course partly because the 9-12' scanners required to achieve the necessary gain and their associated heavy power usage are a bit much for small vessels!

So what gives?

The bridge of a big ship will typically be swinging X and S scanners. The operational parameters of the two systems are quite distinct, and while X-band is often thought of as better for pilotage and collision avoidance, its signal is attenuated more by moist air than S so it becomes limited in adverse weather. Interestingly, this feature is harnessed by more sophisticated systems which compare the attenuation and scatter pattern differences between the two radar bands to resolve weather returns into rain, snow, hail, etc.

So S is better in poor weather/vis, also better in heavy seas, and having a wavelength about 3x as long will nominally have a 1.3x greater range (fourth power relationship).

Back to the bridge. Good practice requires the OOW to be watching the two, radars, typically with the S on a longer range. Moreover, for optimum results the OOW should be constantly fiddling with his/her X-Band's gain, tuning (AFC), rain clutter (FTC), and sea clutter (STC). In practice he/she often starts to primarily rely on the S-Band at times the X-Band becomes compromised.

So the yacht equipped with only an X-RTE is invisible on the ship's S-Band and is now 100% reliant on said OOW glancing across to / switching-to / overlaying his X-Band and thinking WTF is that big RTE return on my screen! Worse, as he does this, he may have tightened the range on the X-Band to a level that he's now hard up against the maneuvering parameters of his ship.

This is now the last place a small yacht wants to be!

Fortunately, it can remain entirely hypothetical if one transmits on AIS and/or is visible on S

I think this is probably deeply flawed.
A yacht is quite likely to disappear into a hole close to a big ship on S band. (Read the whole Qinetiq report!)
Yachts very, very rarely get hit by big ships.
When they do, it's usually either a total FUBAR on the part of the ship (IOW ferry recently?) or the main problem is the yacht being unpredictable or altering course wrongly.
It's often the smaller 'ships' which are more of a problem to avoid.

 

[lw395]

...

I wonder whether RTEs will ever come down much in price. I understand that it's only a small market, and it needs to be decent quality, but presumably can use some fairly standard parts. There seem to be complete radars available for under £1,500, so a RTE at £450 to £700 seems relatively very expensive.

We sketched out a design for an RTE using under a tenner's worth of components.
But, it's a transmitter so the approval costs are quite serious.
It's also possibly not desirable for too many yachts to be transmitting in an uncontrolled fashion.

 

[Uricanejack]

I think this is probably deeply flawed.
A yacht is quite likely to disappear into a hole close to a big ship on S band. (Read the whole Qinetiq report!)
Yachts very, very rarely get hit by big ships.
When they do, it's usually either a total FUBAR on the part of the ship (IOW ferry recently?) or the main problem is the yacht being unpredictable or altering course wrongly.
It's often the smaller 'ships' which are more of a problem to avoid.

What hole would that be?

A RADAR does have a minimum range, they physical cannot measure a distance less than a pulse length. Pulse length is not wave length although 10 minimum pulse length is longer.
If you get this close you are already to darn close.

 

[lw395]

What hole would that be?

A RADAR does have a minimum range, they physical cannot measure a distance less than a pulse length. Pulse length is not wave length although 10 minimum pulse length is longer.
If you get this close you are already to darn close.

The oft-quoted Qinetiq report explains it far better than I can be bothered to.

 

[AntarcticPilot]

What hole would that be?

A RADAR does have a minimum range, they physical cannot measure a distance less than a pulse length. Pulse length is not wave length although 10 minimum pulse length is longer.
If you get this close you are already to darn close.

All radars have to blank the receiver while transmitting; otherwise the receiver's front-end would be overloaded and destroyed. This blanking period is at least the length of the transmitted pulse, which far exceeds the wavelength (basically, it's the radial size of the "blob" of a target on the screen). To avoid large reflections from nearby objects, the gain of the receiver also varies with time after transmit , so that weak reflections near the transmitter won't be seen. So there is necessarily a hole in the centre of a radar's coverage.

Incidentally, why do you capitalize radar? Although it started as an acronym, it has entered English as a common noun and should not therefore be capitalized.

 

[bbg]

I disagree with your central conclusion that one only needs an X-Band RTE. In fact, I would prefer a decent passive reflector to X-RTE only and most certainly a passive plus AIS transceiver.

It is true that the vessels you cite all carry X-Band: partly for reg compliance, partly because X-Band is generally better for collision avoidance and pilotage, and of course partly because the 9-12' scanners required to achieve the necessary gain and their associated heavy power usage are a bit much for small vessels!

So what gives?

The bridge of a big ship will typically be swinging X and S scanners. The operational parameters of the two systems are quite distinct, and while X-band is often thought of as better for pilotage and collision avoidance, its signal is attenuated more by moist air than S so it becomes limited in adverse weather. Interestingly, this feature is harnessed by more sophisticated systems which compare the attenuation and scatter pattern differences between the two radar bands to resolve weather returns into rain, snow, hail, etc.

So S is better in poor weather/vis, also better in heavy seas, and having a wavelength about 3x as long will nominally have a 1.3x greater range (fourth power relationship).

Back to the bridge. Good practice requires the OOW to be watching the two, radars, typically with the S on a longer range. Moreover, for optimum results the OOW should be constantly fiddling with his/her X-Band's gain, tuning (AFC), rain clutter (FTC), and sea clutter (STC). In practice he/she often starts to primarily rely on the S-Band at times the X-Band becomes compromised.

So the yacht equipped with only an X-RTE is invisible on the ship's S-Band and is now 100% reliant on said OOW glancing across to / switching-to / overlaying his X-Band and thinking WTF is that big RTE return on my screen! Worse, as he does this, he may have tightened the range on the X-Band to a level that he's now hard up against the maneuvering parameters of his ship.

This is now the last place a small yacht wants to be!

Fortunately, it can remain entirely hypothetical if one transmits on AIS and/or is visible on S

You seem to believe that a passive reflector will give a better return on S band than the boat / rigging / mast / engine would. Can I ask what you base this on? Have you seen any reports that indicate the performance of passive reflectors on S band?

Genuine question.

 

[Ladybirdgaffcutter]

Yes, the S-band value of passive reflectors seems to need addressing to further this discussion..

 

[dom]

I think this is probably deeply flawed.
A yacht is quite likely to disappear into a hole close to a big ship on S band. (Read the whole Qinetiq report!)
Yachts very, very rarely get hit by big ships.
When they do, it's usually either a total FUBAR on the part of the ship (IOW ferry recently?) or the main problem is the yacht being unpredictable or altering course wrongly.
It's often the smaller 'ships' which are more of a problem to avoid.

Big ships generally display a high level of seamanship as do most yachts. In big ships this encompasses the use of sophisticated equipment and most yachts take steps, some required by law, to ensure their visibility. So accidents are thankfully low.

To this end, there is no silver bullet, but specific arrangements can drastically minimise risks. Others possess weaknesses most mariners will wish to address:

1. Reliance upon an X-RTE controller can leave a yacht invisible on S-Band. Not sure which QientiQ report you are referring to, but the March 2007 report commissioned by the MAIB makes exactly this point as Conclusion 1, “The Sea-Me is a good example of an active reflector (RTE) exceeding the requirements of the current and future ISO 8729..... Drawbacks are that it requires power to operate (which on a yacht is at a premium), it will only operate at X-Band and will offer no performance at S-Band.”
https://www.rya.org.uk/sitecollecti...ations and Safety/Radar_Reflectors_Report.pdf


2. An XS-Band RTE will ensure that the vessel is displayed on both S and X-Band radars. Seems self-evident?

3. Fitting an AIS transceiver will alert ships to the vessel’s presence: also self-evident which is why more and more yachts fit them.

4. OOWs simultaneously use X and S-Band radars and this carries risk for small vessels not be safely identified on S-Band at longer ranges: They do, and it does.

On this last point, the operational parameters of X and S-Band radars are different owing to differences in technology and the sets themselves. No surprise that shipping co.’s like to streamline equipment across their fleets to ensure adequate training in the specific operational parameters of their sets.

The dark hole (dead zone) you refer to is, as Antarctic pilot explains, simply a function of the need to electronically blank the radar receiver to the transmitted signal for the time (pulse width) it is transmitting. Early pioneers reputedly discovered this by blowing-up their sets! For this reason, short pulses are generally preferable at short-range with the added benefit that this helps with target resolution. But short pulses have a drawback; the quantity of energy that can be delivered to a potential target is a function of the equipment’s maximum power and the duration of the pulse. One can increase range by increasing the pulse width, at the price of an increased dead zone and reduced target resolution. Sometimes pulse width is adjustable and most sets increase pulse width with increasing range-scale; details will be in the radar’s documents.

Then there are problems with radar pulses being propagated close to the sea’s surface, which we all know as sea clutter. This is complex and frequency dependent, and arises from the main beam being split into multiple lobes with radar waves reflected off the sea interfering with the main wave. The sea clutter control helps here by lowering the gain for a few microseconds after each pulse and then gradually ramping it up to its former level. Naturally this increases the dead zone. Newer solid state sets are of course entirely different.

The key point here for risk-averse small vessel is to absolutely avoid getting into the radar dead zone close to a ship, all the more so in inclement weather!

 

[dom]

You seem to believe that a passive reflector will give a better return on S band than the boat / rigging / mast / engine would. Can I ask what you base this on? Have you seen any reports that indicate the performance of passive reflectors on S band?

Genuine question.

Believe? Well to the extent that I've been told so by a manufacturer, in this case Echomax. Their marketing documentation refers to MCA sea trials in 2002, but I haven't personally seen the report:

"In MCA sea trials in January 02 at 5 miles plus the EM230 responded 100% to X band radar and 80% to S band radar. QinetiQ who carried out the tests stated that the S band results defied all the laws of physics as the 'expected norm' was 10-15% S - X band. At their request two further EM230 were sent to the Anechoic Chamber which confirmed the sea trial results. The high S band response was attributed to John Firths 'Glint Effect' which is a reaction of the phases as the Echomax array excites in real sea conditions, rather like the mirror on the hill scenario."
https://www.echomax.co.uk/faq.html

I know the RYA has also looked into this so might be woth following up if interested.

 

[Topcat47]

Good well thought out replies, Dom.

 

[Uricanejack]

The oft-quoted Qinetiq report explains it far better than I can be bothered to.

It took me a while to find it on google. I’ve now read the report from 2007. It hardly mentions S Band and I saw no reference to a hole.
The test was using a Bridge Master X Band. In simulate 16 knot wind sea state. So sea clutter taken into some account.

It was interesting. Gives comparative data on types of reflector. The only surprise might be the performance of the fender type which have been criticized by other report. Not sure if it was RYA RNLI or a PBO test.

Don has already posted the conclusions.

 

[Uricanejack]

You seem to believe that a passive reflector will give a better return on S band than the boat / rigging / mast / engine would. Can I ask what you base this on? Have you seen any reports that indicate the performance of passive reflectors on S band?

Genuine question.

I know of no report giving scientific data. Just personal observation.
It is the wrong question.
The question should be.
Does a passive radar reflector improve the return from a small boat?
I believe from my observation the answers is yes. Quite significantly. I can’t quantify this.
Even an empty kayak will give a return in calm ideal conditions. A seagull siting on a log will give a better return than a seagull siting on the water.

So a small boat with no reflector will give a return to both X and S band radars. This return in the case of wooden or plastic boats will be significantly weaker than a steel boat or a bigger boat. So a reflector is advised and even mandatory to improve the return.

To a passive reflector it makes no difference which band is in use. The difference between X and S band. Are choices for the observer. S Band is significantly less effected by clutter than X band so a small target is more likley to be detected by SBand in conditions where there is more clutter. I can’t quantify this it’s just a personal observation of a generally accepted differences.

 

[Uricanejack]

All radars have to blank the receiver while transmitting; otherwise the receiver's front-end would be overloaded and destroyed. This blanking period is at least the length of the transmitted pulse, which far exceeds the wavelength (basically, it's the radial size of the "blob" of a target on the screen). To avoid large reflections from nearby objects, the gain of the receiver also varies with time after transmit , so that weak reflections near the transmitter won't be seen. So there is necessarily a hole in the centre of a radar's coverage.

Incidentally, why do you capitalize radar? Although it started as an acronym, it has entered English as a common noun and should not therefore be capitalized.

Just old enough and out of date enough to be an anachronism

 

[dom]

Good well thought out replies, Dom.

Cheers, never realised this stuff was so contentious!

 

[bbg]

I know of no report giving scientific data. Just personal observation.
It is the wrong question.
The question should be.
Does a passive radar reflector improve the return from a small boat?
I believe from my observation the answers is yes. Quite significantly. I can’t quantify this.
Even an empty kayak will give a return in calm ideal conditions. A seagull siting on a log will give a better return than a seagull siting on the water.

So a small boat with no reflector will give a return to both X and S band radars. This return in the case of wooden or plastic boats will be significantly weaker than a steel boat or a bigger boat. So a reflector is advised and even mandatory to improve the return.

To a passive reflector it makes no difference which band is in use. The difference between X and S band. Are choices for the observer. S Band is significantly less effected by clutter than X band so a small target is more likley to be detected by SBand in conditions where there is more clutter. I can’t quantify this it’s just a personal observation of a generally accepted differences.

This was the point of my question. Since your answer is not scientific, I'm afraid I have to discard it.

X and S band operate on different wavelengths. X band 2.5 to 4 cm; S band 8 to 15 cm.

I expect that some reflectors, due to their size, will reflect certain wavelengths better than others.

In fact I wonder whether a reflector that doesn't have a surface that is at least 8-15 cm will reflect S band at all.

And when you look at the size of a boat and rigging - and bear in mind that there is no data on whether reflectors help with S band - I wonder whether they do anything at all for S band.

Anyway, passive reflectors seem like toys next to RTEs. If you are really worried about showing up on radar, dual band RTE is the only way to go.

 

[dom]

I know of no report giving scientific data. Just personal observation.
It is the wrong question.
The question should be.
Does a passive radar reflector improve the return from a small boat?
I believe from my observation the answers is yes. Quite significantly. I can’t quantify this.
Even an empty kayak will give a return in calm ideal conditions. A seagull siting on a log will give a better return than a seagull siting on the water.

So a small boat with no reflector will give a return to both X and S band radars. This return in the case of wooden or plastic boats will be significantly weaker than a steel boat or a bigger boat. So a reflector is advised and even mandatory to improve the return.

I think what you say is broadly consistent with test data, anecdotal evidence, and theory. Other things being equal one would expect the radar return to fall off with the square of the wavelength. On this basis X-Band on c.3.2cm would be expected to have a 10x greater return than S-Band on c.10cm. This I presume is why Echomax say that the "laws of physics" would suggest an "expected norm" on S-Band of "10-15%" of X-band performance.

But as you say S-Band does better with clutter and its transmissions attenuate less in grotty conditions. The real performance of S-Band is therefore probably more like 20% and possibly much higher in adverse weather conditions.

Worth mentioning that the little Plastimo tubes mentioned in this thread have a diameter of 5cm or 10cm, which would suggest some pretty serious problems returning S-Band's 10cm waves!

Re your friendly seagull, love it . But. What one is looking for with any reflector is a decent performance as measured by free-space radar cross section (RCS) across all azimuths from 0-360 degrees and all angles of heel up to 20 degrees.

Big nulls spanning large angles of azimuth and heel - MAIB report clearly depicts these - can mean that a bobbing yacht returns blip-zero-blip, or perhaps a zero-zero-blip on a ship's radar. Even if those blips are above say 5m^2 radar cross section, the ARPA may not include the vessel as a potential collision risk until it receives the required three in a row blip-blip-blip confirmation.

Your seagull will of of course show up on a radar with software designed to spot him, but to trigger the ship's radar he'd need to maintain an appropriate flight vector and wing combo to trigger the ARPA