Avionics News January 2023 - 57

With HF, there is the initial cost of
system purchase and installation
with no reoccurring costs. Satellite
systems require a subscription and
a monthly fee for continued use. In
the long run, an HF system is less
expensive.
Those using their aircraft often
What is
squelch?
for overseas or remote terrain
operation may prefer a satellite
system. ASTM concentrates on four
providers. Inmarsat (International
Maritime Satellite) maintains 11
satellites in geosynchronous orbit.
Globalstar uses 44 satellites in
low-Earth orbit, and Iridium uses 66
LEO satellites. All use PSK digital
modulation, and all are duplex. Until
2020, the Iridium system was used
by Gogo, which combined satellite
communication with ground-based
communication. In 2020, Intelsat
purchased Gogo.
Sirius/XM also uses PSK, but it
is a one-way communication system
like a broadcast system.
All of the satellite systems are
multiplexed, which means multiple
users can use the same frequency
without interfering with each other.
Some satellite systems will use
directional antennas, and all are on
UHF or higher frequencies.
Like the other stories in this
series, this is simply a summary
of the material a technician should
study before testing for the RCS
endorsement. Those studying for the
RCS should read about the aircraft
communication addressing and
reporting system and the selective
calling system. ACARS is a digital
non-voice type of communication
system. SELCAL is a type of
squelch used primarily on HF, where
normal squelch doesn't work well.
Good luck with the test. q
Those of us knowledgeable with radios can immediately
identify one who is not radio savvy by listening to how one
uses the word squelch. According to the Merriam-Webster
Dictionary, the definition of squelch is to completely
suppress, quell or silence. Squelch on radio receivers does
the same thing. Often, we hear people refer to squelch
noise or reporting they can hear squelch. Either phrase is a
misuse of the term.
Squelch refers to the circuitry used to silence a radio
when a signal is not being received. We do not hear
squelch, but we do hear noise when squelch is not working.
In addition, we miss valid incoming signals if the squelch is
adjusted improperly.
Improper squelch adjustment was one factor leading
engineers to develop modern automatic squelch. The
primary automatic squelch circuit works using the following
logic: If noise is present, no signal is present; therefore, the
audio must be shut off. This circuit is referred to as noise
squelch or carrier/noise squelch, and it works because
an incoming signal tends to quiet down the rushing noise
present on an unused frequency.
Noise squelch can be fooled by a noisy incoming signal;
therefore, automatic squelch systems use a second squelch
circuit using the following logic: If a valid signal is present,
the automatic gain control circuitry will be active. This
circuit is referred to a carrier squelch.
When adjusted properly, automatic squelch does a great
job of allowing the pilot to hear valid incoming signals and
avoid listening to constant annoying and deafening noise.
Sometimes, you can hear a burst of noise when there is
a lag between the time a signal is being received and the
squelch closes. This noise is called a squelch tail and can
be reduced or eliminated with proper adjustment.
To become believable as a technician who knows what
he or she is talking about, please be aware that if you
hear noise, the squelch is off, not working, open or not
squelched. If you do not hear noise, the squelch is on,
working, closed or squelched. There is no such thing as
squelch noise. q
avionics news * january 2023 57
Avionics News January 2023

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