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Marine Radar

Marine Radar
Some agencies use police radar to measure water craft speeds. Marine radar must meet the same Federal Communication Commission standards as police radar. Additionally marine radars operate in a harsh environment must be hermetically sealed.

Marine radar is measuring much smaller radar size targets then land radar. Trucks and cars are 10 to 10,000 times larger radar targets than boats of comparable size, largely due to shape and materials differences. Land vehicles are typically 10 m2 to 200 m2 radar size, boats vary from about 0.02 m2 to 10 m2. Radars measuring boats will not have the same performance as measuring land vehicles.

Water is highly reflective, about 100 times greater than dry ground, aggravating multi-path problems. Multi-path occurs when reflections return by indirect paths instead of directly from radar to target and back. Multi-path can go from radar to water surface to target and back, or from radar to target back to radar via the water surface. Multi-path is unavoidable on water.

Moving water reflections introduce an error proportional to water speed. The measured speed is high when the water and target are in the same direction, and low when the water and target speeds are in opposite directions. Water that is perfectly still and calm will not cause any speed errors but will cause the target reflection to fade in and out.

Wave Water Speed
Water wave speed is a function of water wavelength, the longer the wavelength the greater the speed. Water wavelength is the distance between wave peaks, or wave valleys.

  • Wind Waves
  • Swell Waves -- generated outside area
  • Gravity Waves -- typically greater than 2 inch wavelength
  • Capillary Waves -- surface tension ripples typically less than 1 inch wavelength
  • other waves riding waves

Classic Ocean-wave Theory
Vw = ( g Lw / 2 pi )0.5
Vw = Water Speed
Lw = Water Wavelength
pi = 3.141592
g = Acceleration Due to Gravity

fig 3.9-1

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Radar in a Tunnel

Microwave police traffic radar should not be operated inside a tunnel. These radars are designed to operate in free space -- no obstructions and as high above (away from) ground as practical. Radar beam pattern (beamwidth) in a tunnel would be greatly affected (distorted) by the tunnel side walls and ceiling.

Operating inside a tunnel virtually guarantees multipath problems. The radar receiver would get the target vehicle direct line-of-sight echo as well as the target vehicle echoes bouncing off the walls and/or ceiling and/or ground. This causes the received signal to fade and increase -- forcing the AGC (Automatic Gain Control) to vary wildly unbeknownst to the operator. During close target fading the receiver gain (amplification) will increase making distant vehicles more detectable.

Other vehicles would also have a direct line-of-sight echo and one or more indirect echo path(s) -- multipaths. The more vehicles the more multipath echoes and the greater the chance of generating radar receiver harmonics and spurs -- false echoes or speed readings.

Strong reflections or interfering signals have the potential to saturate (overpower) a radar receiver (low noise amplifier and/or mixer) producing harmonics and spurious signals (false targets). These false targets literally come out of nowhere, and can produce various (false) speed readings depending on speeds and echo amplitudes of any/all moving vehicles (objects).


Police Radar Information Center

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