Samyung ENC SAR-9, Инструкция по эксплуатации

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8 SAR-9
frequency of operation (ie. within its receive bandwidth), so a
“pulse” is produced at
the radar input each time the SART sweep passes though the correct frequency.
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.
The timescale over which all this occurs is very short. Each
“fast” sweep takes about
0.4µs, each
“slow” sweep about 7.5µs. The complete series of twelve forward and
return sweeps is therefore complete within 100µs. Displayed on the PPI, the spacing
between each pair of dots will be 0.6 nautical miles.
On a long range setting, a typical radar will be triggering the SART every millisecond
–
but only during the period that the rotating radar scanner is pointing in the correct
direction. Most modern radars use sophisticated noise rejection techniques, which
prevent the display of echoes which are not synchronized with the radar
’s own
transmissions, so one radar will not normally be confused by a SART
’s response to a
neigbouring radar.