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post #21 of Old 09-09-2006
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Radar Transponder (aprox$850) , [s-band only]

The disadvantage of the passive reflector lies in the many directions in which its radar cross section is low, resulting in a minimal return.

The polar plot shows that Sea-me has a much more even response pattern and suffers from none of the gaps so typical of a passive device.

1. Greater detection range.
The strength of a radar signal falls off rapidly with distance. In the case of a passive reflector this degradation takes place over a distance equal to double the range (radar to target and back) whereas in the case of an active device, which is itself a transmitter, it only falls off over the return distance - and from a much higher amplified signal level. This means that Sea-me, an active device, has a greater detection range than the typical passive reflector.

2. A more consistent return.
Ships today use a system known as ARPA (Automatic Radar Plotting Aid). This enables the radar to emit an alarm when it acquires a target within its guard zones and thus to alert the watchkeeper. Target acquisition will only take place if the radar receives returns on at least 50% of scans. Two factors contribute to a consistent return - the strength of the returned signal and, because small boats yaw and heel, particularly in rough seas, the evenness of the return.

3. Better performance in clutter.
Performance in clutter (rain and wave) is all about signal strength. The single fact that Sea-me emits a stronger signal means that it performs better in clutter.

4. Better performance at heel.
The effectiveness of a passive reflector falls off rapidly as the boat heels, as you would expect of a device which is effectively a mirror reflecting in a single direction. The Sea-me device shapes the beam to provide an effective return even when the boat is heeled.

5. Ship detection alert.
Passive reflectors cannot give you any warning that you have been struck by a radar signal indicating that there is a ship in your area.
Sea-me, being active, knows when it has received a signal and can alert you with a flashing light, and if required, audibly. Sea-me thus enables you to sharpen up your lookout when necessary.
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