Pilot's Guide to Avionics 2017-2018 - 62
Continued from page 61
within storms by measuring the speed of moisture,
water droplets and debris within weather systems.
Modern Doppler airborne weather radar performs this
task in real time. With airborne weather radar, pilots
can see weather ahead, learn cloud height, terrain
height and the intensity of the weather.
Those color variations detailed earlier correspond to
the severity of precipitation detected by the radar.
Green translates to mild precipitation; yellow to
moderate; red to heavy and magenta to severe - in
line with the way storms rated from Level 1 to Level
4. Airborne weather radar returns gauge cloud height
so pilots can decide whether to maintain an altitude or
request a change.
For that 30-to-120-degree arc straight ahead of
an aircraft, the image from Doppler digital airborne
weather radar is considered highly accurate. The realtime nature of the image allows the flight crew to avoid
the worst weather or, if possible, climb above the storm.
What's 90 degrees abeam the aircraft and what's
directly behind is relevant - but invisible to a radar,
which sees nothing outside its sweep.
Datalink Doppler weather radar:
the world as it was
Contrast the real-time view of airborne digital
weather radar with the datalink weather radar.
The first thing to understand is that those images
originate with the National Weather Service system.
That image your local television meteorologist, and The
Weather Channel's local feeds broadcast, originates as
the same product you see on the FIS-B feed.
These images originate as six individual one-minute
sweeps of the NWS Doppler weather radar antenna,
starting at an angle just a couple of degrees above the
horizon. Through each additional scan, the antenna
angle increases, providing height data on the weather.
After the sixth scan, the six individual images
undergo computer processing that turns them into a
single aggregate, or composite, image. That's why we
see the television weather radar animations built out of
What pilots see from datalink weather can never
be younger than sixJIM
it is minutes older by the extra time needed for the
six radar images to undergo processing into a single
composite graphic and then to be broadcast by the
Federal Aviation Administration's FIS-B network through
hundreds of ADS-B ground stations.
FIS-B images have aged anywhere from 10 to 12
minutes by the time it's broadcast ... every six minutes.
Thanks to some display systems' ability to replay a
sequence of those six-minute-spaced updates, airborne
datalink weather consumers gain the ability to replay a
series of those composite images and see the weather
system's speed, direction and, most importantly,
FIS-B also provides other valuable tools to the
aviator, including METAR and TAF reports, AirMets,
SigMets, NOTAMs and TFR information.
While it's never real time, it brings another benefit
unavailable from airborne weather radar: a nationwide
view. FIS-B lets the pilot choose what part of the
country to examine, rather than simply what's in
front. That big-picture view lets pilots make strategic
decisions about avoiding weather on the fly.
The shortcomings of airborne weather radar
Airborne weather radar, even Doppler airborne
systems, for all their advantages over datalink Doppler
weather radar, still suffer some shortcomings. Some
examples of airborne weather radar handicaps include:
Attenuation. The definition we need is the first, "the
reduction of the force, effect or value of something."
Both apply to what happens to airborne weather
radar signals under certain circumstances. Variation
in the type and nature of precipitation also effects the
quality of the airborne radar return.
When trying to peer into a large storm with heavy
rain, that precipitation can attenuate the radar
transmission, preventing it from penetrating the storm -
and eliminating radar's ability to see beyond the band
Another contributor to attenuation troubles is the
radome protecting the radar antenna, whether dish
or electronically scanning. Frozen precipitation can
attenuate the signal, as can frost and a heavy moisture
Doppler datalink weather suffers less from these
problems because of the higher transmission power
of NWS stations and those stations' antenna location,
inside a radar-transparent geodesic dome.
Range. Airborne weather radar is useful out to about
320 nautical miles - in optimal conditions. Some airborne
weather radar systems are limited by smaller antenna.
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