AOPA Turbine Pilot Magazine - December 2023 - T-8

/ A IR WAYS /
Get smart
Why you want this weather radar
BY J. MAC MCCLELLAN
THE ONBOAR D WE ATHER radar is the
only piece of avionics that requires a
reasonably high level of pilot skill and
understanding to be used effectively. But
thanks to the ongoing miracles of digital
electronics, " smart " radars eliminate
nearly all the guesswork when using the
radar to avoid dangerous weather.
I compare using conventional weather
radar to searching a totally dark room
with only a flashlight. We can learn about
the contents of the room only where the
flashlight beam points. And even then,
something may be hiding behind an object
the beam illuminates. A person skilled in
using the flashlight will discover more
in the dark room than the untrained and
inexperienced. Still, there is much that
may escape the detection of the narrow
beam of light.
But smart radars automatically scan
the large parcel of air ahead searching for
precipitation, and even for turbulence, in
the atmosphere the airplane is about to fly
through. The pilot need not fiddle with the
tilt angle of the radar, or even adjust the
gain, to understand the weather picture
ahead. And wondering if you're looking at
a radar return from a line of hills, or even
tall buildings, or a significant area of precipitation
is a thing of the past because the
smart radar knows the difference and suppresses
the ground returns.
The most important technology in
smart radars is the use of solid-state
transmitters in place of the vacuum tubetype
magnetron. For decades weather
radars needed to blast out a massive
pulse of energy in the hope that enough
signal would bounce back to the antenna
to " paint " a target. A magnetron can
produce megawatts of energy. In fact,
T-8
AOPA PILOT / December 2023
it's a magnetron that generates the heat in
a microwave oven.
But the frequency of the pulse produced
by a magnetron is not stable. That
means the receiver of a radar using a magnetron
must be listening for a wide range
of frequencies to detect the return " echo "
of the transmitted energy. The wide spectrum
receiver also detects a bunch of noise
from other sources. Weather radars can
separate the noise from the desired " echo, "
but detail is lost.
Smart radars use a solid-state digital
electronic device to create the transmitted
signal. This signal is very stable, so the
radar receiver is listening for a returned
" echo " over a narrow frequency range.
That narrow range eliminates most of the
ever-present electronic noise and makes
it possible for the radar to paint a very
detailed picture of the precipitation that
is reflecting the transmitted signal.
A very important side benefit of the stable
solid-state transmitter is the ability to
measure the Doppler shift of the returned
energy. As you probably know, sound and
radio frequencies appear to change when
the source is in motion. The usual explanation
is how we hear a changing tone from
a moving train horn even though the horn
is emitting a constant frequency.
Smart radars look for a Doppler shift
in the frequency of the returned energy.
If there is a frequency shift the radar can
know-and show us-that the precipitation
that reflected that shifting signal is
moving at a different velocity than the
overall precipitation. Rapidly moving rain
drops mean something very important to
pilots: turbulence. The only phenomenon
that can cause some precip to move at different
speeds or in different directions is
powerful moving air currents, and that is
the very definition of turbulence.
The more recent technology that
makes smart radars really smart is the
ability to rapidly scan at different angles,
and then assemble the results of those
scans into a radar picture of the weather
ahead. This is not new to ground based
radars. The Next Generation Weather
Radar (Nexrad) ground stations also scan
the sky at a huge range of angles, but it can
take those ground stations more than five
minutes to complete and assemble a single
scan. That won't do any good when you're
flying toward a possible thunderstorm
at more than 400 knots. The new smart
radars complete their scans of the area of
interest so quickly that detail is not lost.
All three leading makers of avionics
for turbine aircraft, Collins, Garmin, and
Honeywell, now make smart radars. Collins
was early to the technology and trademarked
MultiScan for its radar. Garmin
and Honeywell refer to their systems as
" volumetric scanning, " meaning they scan
ahead at multiple angles to evaluate a volume
of the atmosphere. But all three use
essentially the same multi-scan techniques.
Detection of turbulence is still limited in
range, and by the necessity of some precipitation
to reflect a radar pulse. Depending
on the radar, and usually the size of antenna
that will fit in your airplane, turbulence
detection is limited to 40, or at most 60
nautical miles. The reason is the amount of
returned energy for the radar to measure a
Doppler shift is attenuated over distance.
But when making weather avoidance decisions,
even 20 nm warning of turbulence in
an area of precipitation is very useful.
Smart radars also employ electronic
analysis of a return to predict if a
AOPA Turbine Pilot Magazine - December 2023

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