Radar Reflectors and Why you need one.

First, How Does Radar Work?

Radar is used to detect obstacles and other ships when navigating. It is particularly useful in foggy conditions or at night when visibility is reduced. Many radar frequencies are not affected much by rain so radar is very useful in rainy weather.

Radar works by sending out a radio pulse and listening for the echo of the pulse which gets bounced off objects in the area. The reflected pulse is much smaller than the original. There can be noise and interference to complicate the interpretation of the echo.

Large metal structures such as ships and buoys are good radar reflector but small crafts made from wood or fiberglass are notoriously poor radar reflectors and are often mostly invisible to radar.

The shape of the object also has an effect on its ability to reflect a signal back to the radar. Round objects are poor reflectors. That's why stealth bombers have funny rounded shapes. This means that a mast even though it is quite large does not act as a good reflector. Flat metal plates can be effective radar reflectors if they are in the right orientation, if not then the reflected pulse can miss the radar antenna and not be detected at all.

In small watercraft any reflective structures such as motors tend to be low or even below the waterline making it harder to "see" with the radar.

How Does a Radar reflector Work?

There are several types of Radar Reflectors such as passive and active reflectors

Passive Radar Reflectors

By creating a number of flat reflecting planes a radar reflector hopes to offer a few well oriented plates to the pulse in order to send the maximum possible echo back to the radar transmitter. By placing several reflective planes at different angles the likelyhood of reflection is improved significantly.

Anyone who has played with a mirror in sunlight knows that the angle of the mirror has to be just right to reflect the sunlight.

By orienting the reflecting plates at an angle to each other the chances of reflections are also increased.

What are Radar Enhancers?

A more effective way of appearing on radar is using a RTE, Radar Target Enhancer. Radar reflectors passively reflect received signals without any enhancement. By the time the receiving ship gets the signal it can be very weak. Radar enhancers improve the signal.

When a radar signal is received, it is electronically enhanced and amplified and sent back. This greatly increases the chances of being seen.

Here is a link to Echomax who produces a radar target enhancer. Sea Me is another brand.

In Radar Reflectors, Bigger is Better.

The reflected radar echo is always much smaller than the transmitted signal. In order to improve the odds of sending a useable echo back, making the reflector as large as possible is always an advantage.

Orientation also matters.

Since reflection depends on a plate being at the right angle to reflect a radar signal, it makes sense that the orientation of the reflector matters. A great deal of research has been done on this and some positions are definitely better than others. Orientation of a radar reflector is particularly tricky in a sailboat because it tends to lean while underway. For this reason reflectors are sometimes installed in gambelled assemblies. (We have seen gambelled stands which hold a stove flat even when the boat leans over.)

It is also possible to install the radar reflector such that it can be adjusted as the angle of the boat changes on different tacks.

A successful mounting position is referred to as the rain catching position.

Masts and other metallic structures will interfere with the pulse/echo and make a radar shadow where no signal is seen or reflected. For this reason it is often recommended that the radar reflector be mounted above the mast.

This blind spot can be in the order of 150 Meters. This varies according to the frequency of the wave, and the type of switch used. This is significant for small boat navigation particularly in crowded waters.

Calculating the Speed of Objects

The speed of the object is calculated using the Doppler effect: the frequency of an object approaching the receiver will be greater than the original signal that was sent out. From this shift the actual velocity can be calculated. if the object is moving away from the radar unit, the received frequency is less. If the object is not moving relative to the radar, the received signal will have the same frequency as the transmitted signal.

Some Technical Details on Radar used by Pleasure Boats

Frequency of Radar pulse used for pleasure boats is in the range of 8-12 GHz. This is the I/J Band. For more detail on radar frequency Radar Tutorial has a good page.

As I was looking at the various models currently on sale, the chosen frequency was 9.4 KHz.

Various frequency have been tried and are used in other applications. The lower frequency can be more powerful but less accurate. The higher frequency can be extremely accurate at a smaller distance but are sensitive to atmospheric absorption. Very high frequency radar is used where long distance viewing is not required and can actually be a distraction, such as in automatic parking systems in cars.

Distance which can be scanned is limited by the curvature of the earth. Radar is line of sight.

Manufacturers claim ranges of 48-64 nm. approx: 60-100 km. Placement of the unit will have a significant effect of how far it can see. The higher the radar placement the further it can see.

Another factor in the ability of radar to detect objects is the strength of the signal. Other factors being constant, a stronger signal will produce a stronger reflection.

There are some disadvantages to very strong Radar signals. More noise is produced by strong signals. Birds, wake and waves all combine to complicate the reading. Since the strong output create some radiation there is also a health issue. For this reason Radar is installed above where people stay.

In commonly offered models the antenna output will range between 2-6 KW. Some very low output new models are offered but there were no details as to comparison of efficiency and accuracy.

Most radar unit offered rotate at about 20-36 rpm and are often housed in an enclosed dome, a radome. This rotation allows the radar to see a larger slice than the small 2-5 degrees it scans on each pulse.

Radar sends a pulse that is 2-5 degrees on the horizontal and 16-25 degrees in the vertical. Various models have different sized wedges. The smaller the wedge the stronger the signal. This is similar to a powerful narrow spotlight vs a flood light. Both use the same amount of power but have a more or less concentrated beam.

What is HD radar

It is a poorly defined term that refers to how the signal is analyzed and treated after it is received.

In the past Radar was analog rather than digital and options to remove noise and amplify the signal were more limited

With the development of better computers, and improved techniques for treating the received signal, significant improvements have been seen. Noise reduction, multi-target displays, longer range, split screen display, always improving colour monitors are becoming available. More money will buy better software and features such as automatic identification of spotted items and superimposition of charts. Integration with other systems also come with a higher price.

What is the Cost of Radar?

Costs vary enormously. Low end systems can be as inexpensive as $1000, High end can set you back 10,000 or more. Boat stuff rarely has an upper limit!! Often the Radar is sold separately from the display which might serve other instruments such as plotter, gps, depth sounder etc.

How is radar reflected?

Figuring out reflections of radar signals is not simple. A whole branch of science is devoted to this and from this research Stealth technology has been developed.

Here is an article on how Stealth Bombers avoid Radar Detection Its an interesting read.

After leaving the transmitter the radio wave is propagated (at light-speed) in a widening beam. When it hits an object several things happen. Some of the beam is absorbed and some is transmitted. Some of the pulse gets reflected. Depending on the shape of the object the reflection can be be scattered through a wide angle. This is the case for roundish objects. Flat objects will reflect better but not necessarily back towards the antenna.

By rotating the antenna a larger and better area can be scanned, improving the received signals.

The received signal is much less powerful than the original transmitted radar radio pulse. The more powerful the original signal the more powerful the echo reflected back. The more sensitive the receiver, the better the received signal.

Not all substances reflect Radar equally well. Electrical conducting materials such as metals are better than insulators such as wood or fiberglass. Carbon fiber is better than Kevlar or fiberglass. Flat surfaces reflect better than curved surfaces.

What are Radar Reflectors?

Photo by Bin im Garten

Wood and fiberglass boats are not very good radar reflectors and tend to disappear. Round masts even if metal tend to scatter radar signal and not provide good echos. In order to be more visible to radar small boats often install radar reflectors. These are often simple metallic foil with blades placed at right angles to maximize the reflection of radar signals and increase visibility.

In choosing a radar reflector larger is better. It should also be installed as high as possible.

San Francisco Bay buoy with radar reflector (and sea lion).

Davis Echomaster Radar Reflector (link to website at the left side bar) provides sharp corners to help reflect the Radar Echo.

My left side links have interesting articles on radar reflector experiments and reports.

The manner and amount that an object reflects or scatters radio waves is called its radar cross section.