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 illustrated below shows that Sea-me has a much more even response pattern and suffers from none of the gaps so typical of a passive device.
 
Radar Cross Section in Square Metres

Sea-me
  Radar Cross Section is the key parameter used to measure the relative effectiveness of different reflectors.
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.