Samyung ENC SAR-9, Instruction Manual

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SAR-9 7
3. SART principle of operation
Actuating a SART enables a survival craft to show up on a search vessel
’s radar
display as an easily recognised series of dots.
Radar (radio detection and ranging) is a device carried by most ships which is used to
determine the presence and location of an object by measuring the time for the echo
of a radio wave to return from it, and the direction from which it returns.
A typical ship
’s radar will transmit a stream of high power pulses on a fixed frequency
anywhere between 9.2GHz and 9.5GHz. It will collect the echoes received on the
same frequency using a display known as a Plan Position Indicator (PPI), which
shows the ship itself at the center of the screen, with the echoes dotted around it.
Echoes further from the center of the screen are thus further from the ship and the
relative or true bearing of each echo can be easily seen.
The SART operates by receiving a pulse from the search radar and sending back a
series of pulses in response, which the radar will then display as if they were normal
echoes. The first return pulse, if it sent back immediately, will appear in the same
place on the PPI as a normal echo would have done. Subsequent pulses, being
slightly delayed, appear to the radar like echoes from objects furthernaway.
A series of dots is therefore shown, leading away from the position of the SART.
This distinctive pattern is much easier to spot than a single echo such as from a radar
reflector. Moreover, the fact that the SART is actually a transmitter means that the
return pulses can be as strong as echoes received from much larger objects.
A complication arises from the need for the SART to respond to radars which may be
operating at any frequency within the 9GHz band. The method chosen for the SART is
to use a wideband receiver (which will pick up any radar pulses in the band), in
conjuction with a swept frequency transmitter. Each radar pulses received by the
SART results in a transmission consisting of 12 forward and return sweeps through
the range 9.2GHz to 9.5GHz. The radar will only respond to returns close to its own