Instruction manual

PROFICIENCY IN SURVIVAL CRAFT AND RESCUE BOATS OTHER THAN
FAST RESCUE BOATS
14 19
during launching. Alternatively the transponder should form an integral part of
each lifeboat.
14.3.1 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 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 Pages 10 & 11 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