RADAR reflectors

[marco422]

[quote="Anna Leigh"]
If you are traveling in or through Canadian waters they are required. Although I don't know what they do to enforce it. I guess if you have an incident that gets investigated you better have one.
[/quote]

It's not quite that draconian:

Rule 40 Collision Regulations, Canada Shipping Act 2001 requires a vessel that is less than 20 metres in length or is constructed primarily of non-metallic materials to be equipped with a radar reflector if practicable, unless (i) it operates in limited traffic conditions, daylight, and favourable environmental conditions and where compliance is not essential for the safety of the vessel, or (ii) the small size of the vessel or its operation away from radar navigation makes compliance impracticable.

[journey on]

As far as the amount of signature increase a radar reflector shows, let's look at the Davis reflector, which is 11.5" in dia. This is slightly less than 1 sq. ft., MAX. and if it's not pointed with one of the panels perpendicular to the radar beam, can go down to ~1/2 sq. ft.

Now a reflector just reflects energy, it can't increase it beyond what it intercepts, so the MAX reflected radar signal is equal to a 1 sq. ft. target, at the appropriate range. Now compare that with a metal ship that's 50' high by 40' wide by 300' long with lots of metal things sticking out (bridges, etc.)

I do not believe that a small reflector can give the same return signal as a big (or even small) metal ship at the same range. No matter how you look at a ship, one can't get the cross-sectional area down to 1 sq. ft.

That's why I feel the best radar signal is from the radar on your boat. YOU can see the big guys quickly, and can take the appropriate action quickly. I also feel the cheapest radar you can get is the best safety investment for your boat.

Boris

[marco422]

[quote="journey on"]As far as the amount of signature increase a radar reflector shows, let's look at the Davis reflector, which is 11.5" in dia. This is slightly less than 1 sq. ft., MAX. and if it's not pointed with one of the panels perpendicular to the radar beam, can go down to ~1/2 sq. ft.

Now a reflector just reflects energy, it can't increase it beyond what it intercepts, so the MAX reflected radar signal is equal to a 1 sq. ft. target, at the appropriate range. Now compare that with a metal ship that's 50' high by 40' wide by 300' long with lots of metal things sticking out (bridges, etc.)

I do not believe that a small reflector can give the same return signal as a big (or even small) metal ship at the same range. No matter how you look at a ship, one can't get the cross-sectional area down to 1 sq. ft.

Boris[/quote]

I think you are looking at it too simplistically. Radar reflections are a very complicated matter and have to compete with sea clutter at short ranges. A typical octohedral reflector has an RCS (radar cross section) anywhere from 2 to 10 square metres depending on heel and dips in its profile. It doesn't have to be facing the radar transmitter, the signal is reflected within the corners back to its source. A ship's side, on the other hand has to be directly facing the radar to reflect properly. The ship's signal is more likely to come from all the angled surfaces above the deck line than the hull itself.

The point of a radar reflector may be to provide a minimal consistent reflection that distinguishes a fibreglass boat from sea clutter at short ranges (2-10 miles). Of course, the attentiveness and superior maneuverability of a small boat operator is going to be far more effective in collision avoidance than dependence on the radar operator on a seagoing vessel.

[rogerbum]

[starcrafttom]

Last year my father picked up a radar signal booster. It take a signal that is pointed at it, reads and amplifies it and then sends it back at a higher power there by making you look bigger or more to the point sends a stronger signal that may not other wise be seen at long distance or in big seas. We have not had time of the place to put it to a test yet but its a neat idea.

[journey on]

Actually the way a radar works is pretty simple. It sends a beam of energy out, some of that energy is reflected back, and the time of flight is measured and there you have it, distance. Implementing that is complicated, though.

Well, a radar transmits RF energy from its antenna, which is designed to aim that energy in as tight a beam as possible. That is the only energy the radar has to work with. The only energy the receiver will see is that which is transmitted from that radar and reflected back via some conducting target, (typical boat radar, I don't speak for the whole DoD world.) Think of a mirror reflecting a flashlight beam.

This electric field energy is measured like the surface of an expanding bubble, in watts/sq. cm. And this bubble surface keeps expanding as the distance increases, which means the watts/sq. cm decreases as the square of the distance increases, because we're dealing with the bubble surface, not the radius.

Now as the electric field travels away from the radar antenna, objects get in its way and if it's a conducting surface reflects the energy equal to its surface perpendicular to the travel of the radar pulse electric field. This is the MAXIMUM it can reflect. That's the total signal the radar receiver gets back at its antenna. Of course, the return energy has some expanding surface, but to provide an upper bound, let's say all the reflected energy gets back to the radar.


For a perfect 1 sq. ft. reflector, that reflected energy is the total energy that hits the target and is reflected. It's in the microwatt range, and I'm impressed how well radar works.

So, whatever the type of reflector, it can't catch and reflect the radar's transmitted power outside of it's cross-sectional area. So if its maximum dimension is a 1' diameter, it can't reflect the same power as an object with 2 or 3 sq. ft. of area. It's physically impossible.

And, yes corner reflectors can turn a signal around. That's because the angle of the two reflecting surfaces always equal 180 deg. And the energy ALWAYS has to be reflected 180 deg, cause that's where the radar is. But it can't reflect energy that's outside it's projected cross-section. And If you turn a iron sheet edge on to the radar, it will not reflect any signal back. I feel that a large iron ship will reflect more that 1 sq. ft. worth of energy, is all I'm saying.

So, I don't know how a small reflector can reflect as much energy as something much bigger, unless the bigger thing is turned sideways. And that can always happen, but I don't think it's relevant to this discussion. All we're trying to do is set an UPPER bound on the reflected energy.

As far as transponders go, they create their own energy and thus amplify the radar's signal. One can find that on sea bouys, which actually provide their location and name. I'm not aware of any in my price range, but they do have them for aircraft.

Sea clutter is reflected energy from the ocean surface, and is treated as noise. Modern radars try to filter it out, based on it's random properties. But I agree it's noise, and the target signal need to be picked out from that. However, it has nothing to do with the amount of energy a reflector reflects.

Again, I wouldn't count on a small radar reflector letting that tanker or ferry being able to see you in the sea clutter, and then avoid you. A Furuno 1623 LCD Radar is 1300 bucks, let's you see the danger and take corrective action. If you operate in fog or at night, it's the best saftey device you can have. Oh, that and your depth sounder.

That's all, Folks

Boris

[marco422]

As I said before, while the basic theory is fairly simple, the details are complex. Radar reflections are measured by RCS which is an equivalent reflection from a conductive sphere of a certain cross-sectional area. So to say something has an RCS of 5 square metres doesn't mean it reflects as much energy as a flat plate of that area facing the radar transmitter. But a sphere would be a very approximate equivalent of a real world object with all its angled surfaces. So when we say that a reflector has an RCS of 5 sqm it is approximately the same as the average reflection from a metal boat about 5 metres long by 1 metre high.

Of course the reflection from the boat will vary widely depending on its attitude, just like that of the radar reflector. So the number gives us a false sense of exactitude because the signal will vary by very large amounts in both cases (as shown even in several of the test reports found on the Internet which were done in test chambers rather than at sea).

The reason I mention sea clutter is not because it affects the returning signal but it provides a background from which our reflector's signal must be differentiated. Sea clutter will vary depending on sea state, angle of wave faces at various distances from the radar unit, etc and is going to be staccato. Hopefully our boat or reflector will provide a more consistent signal so that the radar processor and the operator can distinguish it.

The reason I say that radar reflections are complex (despite the simple theory you outlined) is that there are many diffraction effects caused by corners, hidden parts of the target and target materials that complicate things. I think the article in Wikipedia about radar cross section mentions some of them.

Nothing in the real world seems to comply with simple mathematical models, so your intuitive assessment is probably just as useful.

[thataway]

I certainly agree with Boris, that a radar on your vessel beats any passive reflector. Of course we now have AIS, and that is probably one of the best safety features which has come along in quite some time.

I am curious as to what Radar Transponder Tom has. Active Echo is one commercially available, ACR and Weems Plath both have emergency transponders designed for SAR and lifeboats.

The Lense type of radar reflectors, do better than the 90 degree spherical or square aluminum reflectors. Either of these, is susposed to assure that the radar energy comes back at maximum intensity from the reflector. Even a steel ship may not bounce radar microwaves back at max intensity, because the ship is not perpendicular to the beam.

We often asked ships how far away they could see us. The vessel was 62 feet LOA, and afiberglass hull, with two wooden masts, and round rigging--not a lot of metal which would be reflective above the water. The engine and water tanks were below the water line. There was some metal--such as stoves, refigeration plates and batteries which were above the water line. However, the radar signal was definately enhanced by one of the lense reflectors--which was up in the rigging (in that case about 40 feet above the water. Ships with good radar would regularly see us at a range of 13 miles--far more than just what would have been seen for the hull alone.

Last summer when we were crossing San Francisco bay, I kicked the Radar on to see if I could track the other C Dories. We were farily close, but I could see all of the boats (no reflectors) fairly easily. There is thread about seeing lobster pots on radar (calm water) on recreational boats on The hull truth. Most radar operators claim that they can see 5" lobster pots, and birds.

[rogerbum]

[Sea Wolf]

HOW TO MINIMIZE YOUR RADAR SIGNATURE TO OTHER BOATERS...

We encountered this Navy Stealth Research Vessel on the Alameda Naval Air Station nook of S.F. Bay in 1993 while practicing racing sailing techniques.

This was one of the first days they had taken the stealth boat out on the bay. Before that, it had been tested in a test chamber / closed in building over in Redwood City.

One look and we knew it was a stealth boat, looking much like the F-117A stealth fighter, or a Star Wars movie prop.

It illustrates many of the points made in the discussion in the posts above.

Joe.

[matt_unique]

[Alyssa Jean]

The whole point of a reflector is to make you appear larger on another vessels radar than you would without one. They work, so I use one.

[Sea Angel]

I question of application comes to mind...

Could we take AL duct tape and cut it to the 'S' and 'X' band radar wave
lengths and set a design within the cabin's inside walls for it to act as
[mulitple] dipoles?

In theory it seems that it should work; especially, if you had a shower
within the cabin to apply the tape to [for cosmetic purposes] .

Art

[thataway]

There would be no correlation with reflection of radar microwaves and cut lengths of foil--they wold make a good receiver antenna, but not a reflector.

If you lined the boat with foil--it still might not be as good a reflector as a multi faceted true 90 degree ball--where all microwaves which hit the reflector, are sent back to the scanner antenna.

Often when you get echos off fiberglass boats, it is flat pieces of metal in the boat--engine block, tanks, stoves etc.

[Anita Marie]

Radar Flag