AOPA Pilot Magazine - July 2016 - 81

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Line echo/squall line

altitude, if weather forces you to consider
landing short of your destination, the nearest airports may be metar- and TAF-free
zones at lower altitude tiers.
XM VERSUS FIS-B RADAR IMAGERY

Bow echo

Hook echo

BOTH FIS-B AND XM WX can alert pilots to these phenomena, shown here as Nexrad images.

The tiers can cause other issues. They're
set up to ensure that aircraft at all altitudes
can receive weather products, from at or
near the surface to 24,000 feet, and beyond
(usable signals can even reach as high as
40,000 feet). But line-of-sight and bandwidth considerations complicate matters.
Two types of Nexrad radar mosaics are
issued by FIS-B. The continental United
States (CONUS) mosaic takes in the entire
nation, and has a 10-kilometer resolution,
each of its pixels measuring 10 km (about
11 nm) square. It takes a lot of bandwidth
to assemble and transmit the imagery covering the whole United States, so CONUS
Nexrad mosaics are only made available to
aircraft flying in the medium and high coverage tiers. Regional mosaics cover smaller
areas, use data from nearby Nexrad radars,
have 2-km (about 1 nm) resolution, and
are intended for use closer to the receiving aircraft's position. Because of its finer
resolution, regional Nexrad has more definition and detail. Depending on altitude,
regional Nexrad coverage can extend as far
as 250 nm around an aircraft's position.
When storms threaten, pilots naturally
want to zoom in on any datalinked images
to have a closer look. That's where the resolution factor kicks in. Zoom in on a CONUS

view and contours turn out blocky. Regional
Nexrad gives a better rendition, but zoom
in too much and you'll still see some pretty
fair-sized pixelation.
THE 'LOOK-AHEAD' FACTOR

Your distance and altitude affect what
Nexrad views you'll be able to receive.
For example, if you're at 18,000 feet agl
you should be able to see regional Nexrad
imagery 250 nm ahead. At 3,000 feet agl and
below, your look-ahead range should be 150
nm. But if your destination is 300 nm away,
then you'll be stuck with CONUS Nexrad
and its coarse shapes.
As for METARs and TAFs, the same
dynamics hold, but with an extra caveat.
METARs and TAFs for airports within
Class B and C airspace get preferential treatment at the 500-nm range. If you're 400 nm
from a Class B or C airport you'll receive
its METARs, TAFs, airmets, sigmets-and
a CONUS Nexrad feed-but you'll have to
wait until you're within 375 nm of a Class D
airport to get its metars, TAFs, and regional
Nexrad if you're flying in the medium altitude tier.
This means that FIS-B information may
be available for a distant airport, but absent
from closer airports. Depending on your

There are some big differences between XM
WX and FIS-B's Nexrad presentations. Both
providers use composite reflectivity scans;
however, FIS-B begins assembling its imagery only after each radar site completes a full
scan of all radar antenna angles. This could
be as many as 12 different scan angles, and
the antenna rotations to complete them take
time. As a result, the images from the scans
can be five to 10 minutes old before they
are made into a mosaic and beamed up to
the cockpit. XM WX checks each radar site
every five minutes, and begins making its
images using the latest sweep of the antenna.
Then it adds the scans from the previous
five minutes to make what it calls a "rolling" composite mosaic every five minutes.
This, SiriusXM says, means that its Nexrad
images are never more than five minutes old.
XM WX also has none of FIS-B's lookahead issues, because its data is sent down
from satellites instead of being beamed
up from ground stations. This also means
that XM WX Nexrad and text data can be
received by aircraft on the ground.
Finally, XM WX offers better image
quality than FIS-B, with 2-km resolution
over the entire United States and Canada.
Then again, you get what you pay for.
Subscriptions for XM WX include $34.99
per month for basic Aviator LT service,
$54.99 per month for the Aviator service
(which adds lightning, airmets, sigmets,
freezing level, echo tops, and satellite imagery), and $99.99 per month for Aviator Pro
Service (which adds convective outlooks,
turbulence, current icing potential, and
mesoscale discussions). FIS-B promises
lightning, turbulence and icing nowcasts,
and echo top products in the future.
Whether you opt for FIS-B or XM WX,
let there be no doubt: The more information
you have, the better. It sure beats the bad old
days, when all you had was your eyeballs, a
Flight Watch frequency, and some convective sigmets over certain VOR frequencies.
And with FIS-B, the price is right. AOPA
EMAIL tom.horne@aopa.org

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Table of Contents for the Digital Edition of AOPA Pilot Magazine - July 2016
