Frequently Asked Questions

Why are recreational vessels so difficult to detect with radar?
Recreational vessels have poor radar images; they are hard to detect with radar. Part of the problem is that recreational (i.e. small) powerboats and sailboats have small radar cross section compared to large ocean-going vessels. The main problem, however,  is that the radar-reflective material is concentrated close to the waterline. The multipath effect caused by radar waves reflecting from the surface of the water makes the effective RCS of even large targets very small when the target is concentrated close to the surface of the water, as is the case with recreational vessels.
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What is multipath propagation and why is it important?
In the marine environment a radar pulse travels from the radar to the target (and also in the reverse direction) by two paths. One path is direct from the radar antenna to the target; the second path involves reflection from the surface of the water. The two pulses combine at the target before being reflected or processed by an active RTE. The combined pulse may be much stronger than would be the case in the absence of the water. It may also be zero, i.e. absolutely no radar power hits the target, in which case the target is invisible to radar regardless of its size. The power in the combined pulse, which may be anything from zero to four times the power in each individual pulse, depends on the height of the radar antenna and the height of the radar-reflecting target..
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What does a radar reflector accomplish?
A radar reflector, or active RTE, provides a radar target that is higher above the surface of the water than the hull.  Even though the RCS of the radar reflector may be no larger than the RCS of the vessel itself, being mounted above the hull produces a stronger radar image.
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What is the difference between an RTE and a radar reflector?
I use the term "radar target enhancer" or RTE to mean any device that improves the radar image of the object it is mounted on. "Passive RTE" or "radar reflectors" are devices that works by reflecting incoming radar pulses. "Active RTE" are devices that consist of a receiver antenna, radar receiver, amplifier, transmitter, and transmitter antenna. They work by receiving the incoming radar pulse, amplifying it in the electronic amplifier, and retransmitting the amplified pulse.
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What are typical values for radar cross section (RCS)?
Recreational vessels, i.e. under 15 m (50 feet) have RCS values on the order of 2 to 10 m².  For comparison, a 10 kT naval cruiser is about 160,000 m² and a person is about 1 m².  Clearly recreational vessels are tiny compared to large ships.
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Where does one measure a device's RCS?
The RCS of a target is obtained by measuring the strength of the reflected radar signal in an indoor "compact radar range" laboratory. The system is carefully calibrated so that the strength of the reflected and received signal equates to RCS. The indoor lab isolates the measurements from environmental conditions. The lab is designed as an anechoic chamber, i.e. no radar energy is reflected from the walls that would contaminate the measurements. A final point is that a radar system behaves differently when the target is close to the antenna than when the target is far from the antenna, i.e. "in the far field". The indoor radar labs are designed so that the measurements correspond to far-field measurements, that is as if the target were at several miles from the antenna rather than several meters possible in an indoor lab.
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How does one measure a device's RCS?
The device is mounted on a turntable in an indoor radar range. Usually the radar beam is horizontal, the test device is aligned vertically, and the center of the test device is level with the radar antenna. The radar is turned on and measurements of the received signal strength are recorded as the turntable is rotated slowly through 360 degrees. The data are processed through calibration algorithms to convert the raw signal strength into calibrated RCS. This single set of data may be presented as a numerical table, long strip chart (convenient as it may be produced by the recording instrumentation), or as a polar diagram. With the standard test configuration the single polar diagram corresponds to 0° elevation of the radar beam relative to the target. This polar diagram is the most common way manufacturers present test data. If data at other elevation angles are desired the test device is repositioned on the turntable and the measurement process repeated.
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What is a Target Pattern Map (TPM)?
Presenting a lot of RCS data using numerous polar diagrams, each at a different elevation angle does not work too well as it is difficult for someone to interpret multiple overlying graphs. A better approach is to present the data three-dimensionally using color. That is the display shows elevation and azimuth on two axes and uses color to code RCS.
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