April 30, 2009  


XM satellite weather lightning is a degraded product  XM Lightning on a G1000 MFD Display

    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 XM-based lightning 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 product affordable.  
     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 kilometers.

     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 lightning.

XM Image Cell Before GrowthXM Image Cell After Growth
                          1430 EDT                                               1440 EDT

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