Avionics News May 2017 - 20
Continued from page 19
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
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 six-minute segments.
What pilots see from datalink weather can never be younger
than six minutes; in practice, however, 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, intensity changes.
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 of precipitation.
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 layer.
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
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.
Some newer Doppler airborne weather radar units mitigate
some of these shortcomings - or offer features to let the pilot
know the radar isn't seeing everything it should.
To minimize the individual shortcomings of both airborne and
datalink options, safety and avionics experts recommend using
both together wherever possible.
Airborne system options: plentiful -
but not an option for all aircraft
The antenna of airborne weather radar arguably presents
the main challenge to fitting a system to an aircraft. Whether
an oscillating dish, flat plate or a scanning phased-array
antenna, it must be mounted where no other part of the
airframe obstructs its transmission and reception. Solutions
vary according to the airframe.
The most-common location for radar-antenna mounting
is behind the nosecone - at least in multiengine aircraft like
airliners, business-turbine jets and propjets, and piston twins.
Solutions become a bit more complicated for single-engine
prop-driven aircraft. But in their quest to satisfy customers,
airframers and avionics makers have developed solutions
ranging from wing pods to inside the leading edge of a wing cell.
That leads us to some of today's inventory of airborne weather
BendixKing RDR 2100: Power and smarts
BendixKing's RDR 2100 Digital Weather Radar provides
a vertical picture of a pilot-selected cross-section of the storm,
offering one of the best views available to general aviation.