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Twice as fast updates of radar and lightning with SiriusXM

Updated: May 1, 2019

A number of pilots couldn't attend my presentation at AirVenture this year and were wondering if my presentation had been recorded in some way...unfortunately, it was not.  But I wanted to pass along some information about changes to the SiriusXM (SXM) lightning and radar products you get via the satellite datalink broadcast.


You may have gotten an email or snail mail from SXM about these changes that affect both the lightning and composite reflectivity radar product.  It is being marketed with the "twice as fast" catch phrase to denote that their refresh rate is twice as fast as FIS-B regional composite reflectivity (and the new FIS-B lightning product).  Just as a refresher, composite reflectivity examines the base reflectivity (dBZ, where Z is the reflectivity parameter) from every elevation scan in the volume coverage pattern (VCP).  It then extracts the highest dBZ in each column over the radar coverage region.  That could have been from the lowest base reflectivity elevation angle (lowest tilt) or the base reflectivity from one of the higher elevation angles. As I discussed in my presentation at AirVenture, the term "base" does not mean lowest as most pilots assume...every elevation angle has a base reflectivity product.  To create a mosaic, the composite reflectivity from each radar site must be stitched together (keep in mind, there can be some nasty assumptions with this process as well).


As you might imagine, the WDR-88D NEXRAD Doppler radars are asynchronous.  That is, one radar site may be scanning the atmosphere on the lowest elevation angle while a neighboring radar site may be scanning the atmosphere on the 4th elevation angle.  There are many different scanning strategies depending on the type of weather expected in the area (severe, drizzle, snow, etc).  Nevertheless, TWC (SXM's provider) doesn't care about the asynchronous aspect of the radars.  They simply grab the last known complete base reflectivity scan from whatever elevation angle and count backwards in time from there until they've grabbed an entire "volume scan's" worth of data.  For example, if the radar just completed the 4th elevation angle, they grab the base reflectivity from that scan and go back to elevation 3, 2, 1, 14, 13, ..., 7, 6, 5 to grab the base reflectivity from those scans.  Perhaps a neighboring radar was finished the 7th elevation angle, they would grab 7, 6, 5, 4, 3, 2, 1, 14, 13, ..., 9, 8 and so on.  In this case I'm assuming that every radar has 14 elevation angles per volume coverage pattern which is not always the case.


So to get the twice as fast updates, they simply schedule this process described above every 2.5 minutes.  They could do it once a minute, however, both FIS-B and SXM are highly bandwidth challenged...you can only put so much data in the small pipe. As a result, each broadcast has to be scheduled accordingly.  Of course, any particular pixel you see on your display could have been from the oldest elevation scan or the newest.  And yes there are delays in processing and uplink/downlink that occur.


Let me first talk about the delay experienced for the lightning broadcast.  First, SXM includes both cloud-to-ground and intracloud lighting (FIS-B only includes cloud-to-ground lightning at the moment).  At a scheduled time, TWC pulls all data collected in past 2 minutes and puts it into a file, which is then queued for SXM broadcast. There's about a 10 seconds delay for processing and queuing.  All said, the lightning data a pilot would see could be anywhere from 25 seconds to 2 minutes 55 seconds old at reception in the cockpit. The age (I call this the virtual age) you see on some displays is based on the time of reception and not the natural age of