XM satellite weather lightning
is a degraded product
Closely behind the Global Positioning System (GPS),
satellite-based weather is probably one of the most significant
contributions to aviation safety in the last 20 years. For
many pilots, a summertime flight without
satellite-based weather is like turning off all of their fancy
electronics and flying soley by reference to a sectional.
Pilots have come to rely on this technology especially when a
challenging convective environment presents itself.
Nevertheless, satellite-based weather
has some important limitations that can't be overlooked especially for
which is degraded in many ways. First, all of the data
received over the XM satellite weather datalink is broadcast.
Lightning data is broadcast at a five-minute interval
normally right before the NEXRAD data is sent. For lightning,
this implies that the data is a smear over time. That is, it is
made up of data collected since the last broadcast or five-minute
interval. Therefore, there is no way to determine the exact
age of the product you see on your display. The time stamp on
the product simply tells you the last time it was broadcast.
As a result, the data has an intrinsic latency.
One of the next observations is that
the lightning (yellow plus sign shown above) is presented on a grid (click here for a larger image).
This is a picture of the satellite-based weather as shown on
a Garmin G1000 multi-function display. The ground-based
lightning data from the National Lightning Detection Network (NLDN) is
collected by WxWorx in Huntsville, Alabama. It has
a native resolution of 500 meters and consists of cloud-to-ground
lightning strikes. Lightning data with this resolution is very
expensive. As a result, WxWorx through XM Satellite Radio
broadcasts the product with a resolution of four kilometers to keep the
In addition to the resolution, not every strike is
displayed. Instead, a single strike symbol will be rendered
at each grid point as long as at least one strike is captured within
that 4-km grid. If there are a dozen or more strikes in that
grid, the display will still only show a single lightning strike
symbol. Unlike a spherics device such a Stormscope which
attempts to display every lightning strike (including cloud-to-cloud
strikes), XM satellite weather only shows a single symbol every 4
Keep in mind that a strong convective
cell may be in the developing stage showing little or no precipitation
and potentially no lightning strikes. Due to the delay in the data, a
harmless looking cell (lower left) can grow into a mature thunderstorm
(below right) within 20 minutes or less. These two images are
10 minutes apart and there were no lightning strikes until the next
update five minutes after the image on the right. Also keep
in mind that XM-based lightning shows only cloud-to-ground strikes.
Storms in the Midwest often contain 10 times more
cloud-to-cloud or intracloud lightning than cloud-to-ground