User manual: Installation and Operation of Direction Finder System RHOTHETA RT-600 / SAR-DF 517
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2.1.4
Squelch Operation
The main challenge in direction finder operations is to only use the signal transmitted by the destination for
calculation of the bearing. Noise and other disturbances shall not cause misleading bearing indications.
In order to prevent the direction finder to calculate bearing results which are mislead due to noise and
disturbances, several means are given:
If a signal can be distinguished from noise and other signals by its message content (e.g. LoJack ID), only
transmissions containing the required message content are used for bearing.
In other cases, the squelch can be used to suppress unwanted weak signals and noise. A squelch level, called
threshold, has to be set by the user or automatically by the system. All signals which have a level below this
threshold value will be ignored. Signals above this level will cause a bearing indication.
As a result, a signal to be used for bearing has to be so much stronger than the noise or disturbing signals that
the receiver can clearly distinguish between wanted and unwanted signals.
The result of this requirement is that the sensitivity is reduced to the level set by the squelch threshold. This
results in a reduced maximum distance to the transmitter.
However, it might be desirable to achieve bearing information even for weakest-possible signals which are very
close to the noise level. In this case, it is possible to reduce the squelch level below the noise level. As soon as
the modulation of a signal can be heard in the loudspeaker, it can be expected that bearing is possible.
If the noise itself is a randomly distributed noise (so-called white noise), and there is no other signal receiver, the
bearing indication itself will be randomly distributed. If the noise is not white noise but a disturbance, bearing will
show to the source of the noise.
If there is a very weak signal “in the noise”, this signal will produce a correct bearing indication thanks to
sophisticated bearing technologies used in the RT-600 system. Fluctuations due to noise will be small even at
low signal levels, however the reaction time of the bearing indication can increase.
To verify if a bearing indication is due to white noise or due to an internal disturbance source, respectively, or if it
is due to a real receive signal, the aircraft should make a slow turn. If the bearing indication is due to an external
signal, e.g. a SAR beacon, the bearing indication has to compensate the changed relative flight angle.
Example: Bearing indication is 30°. The signal is w eakly audible in the loudspeaker due to being deep in the
noise. The airplane makes a turn left by 60°. If th e bearing indication is not changing to approximately 30°+60° =
90° after a few seconds, the bearing is likely to b e due to noise or internal disturbances off the aircraft.
Note: The weaker a signal is, the longer it will take to change the bearing. Durations around 5 seconds are not
unfamiliar in such cases. Therefore, it is not useful to use this procedure for signals with short transmission
times and low transmit duty cycles, such as COSPAS-SARSAT transmission. The procedure is best suited for
ELT transmission with a continuous signal and still well-suited for ELTs with intermittent transmission (down to
33%).
In cases where pulsed signals are to be received, the receiver is able to decide himself which squelch level is
best-suited for normal operation. In such cases, an automatic squelch is provided. The threshold is automatically
set to a value higher than the calculated noise level. The difference in dB between noise and threshold level can
be defined in the Setup Menu. Refer to the chapter “Setup Page” for details. During automatic squelch operation,
the user has the possibility to force the squelch into manual mode, but has to care about the operational
limitations described in this chapter.
In cases where the Antenna Unit checks signal validity autonomously and without useful possibilities of user
interaction, user interaction is not possible and the squelch purely follows internal rules appropriate to the kind of
received signal.
