Sart principle of operation – Kannad Marine Rescuer 2 SART User Manual

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2. SART principle of operation

Activating a SART enables a to be displayed on a search vessel's radar screen as an
easily recognised series of 12 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 centre of the screen, with the echoes dotted around it.
Echoes further from the centre 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 further away. 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
conjunction with a swept frequency transmitter. Each radar pulse 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
frequency of operation (i.e. within its receive bandwidth), so a "pulse" is produced at
the radar input each time the SART sweep passes through the correct frequency. The
text and diagrams on Page 14 show this in more detail.

A slow sweep would give the radar a stronger echo to deal with as the sweep would
be inside the operating bandwidth for a longer period. The delay for the sweep to
reach the operating frequency may however lead to an unacceptable range error, as
delayed echoes appear to be coming from more distant objects.

To minimise this problem, the SART uses a "sawtooth" response, sweeping quickly,
then slowly for each of its twelve forward and return sweeps. At long range, only the
slow sweeps, giving the strongest returns, are picked up

. At close range, where

errors are more important, the fast sweeps are also detected. As the first sweep is a
fast one, then the range error is minimised and should be less than 150 metres.