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 the product. The natural age of the product varies as stated above. Of course, there could be other delays. For example, the software vendor could choose to hold that data for a period of time before displaying it (why that happens is a whole different discussion). Once received, you stare at that data for another 2.5 minutes until a new set of data is received. Therefore, moments before the next update is received, the lightning could be 2 minutes 55 seconds to 5 minutes 25 seconds old (although the "age" documented on the display may only say 2 or 3 minutes).
Same is true of the radar mosaic. The faster refresh rate cuts down that "stare time" from 5 minutes to 2.5 minutes. Similar to lightning, the virtual age is based on the time of reception...once again, this could have been from the oldest elevation scan or the newest (you don't know). In a perfect world, the average natural age of the radar mosaic is 3.5 to 5 minutes old when first received and then you stare at that image for 2.5 minutes now instead of 5 minutes. The "twice as fast" aspect cuts the stare time in half and that creates a slightly fresher product than you'd get with FIS-B radar. Unlike onboard radar, this ground-based radar depiction is still not real time enough for tactical use, meaning, making decisions that you will execute in the next 30 to 60 seconds.
Also important to understand is that this SiriusXM radar mosaic is highly filtered (as is the FIS-B composite radar). It's designed to show only those returns that come from actual hydrometeors. They attempt to filter out ground clutter and anomalous propagation, but sometimes it does sneak through even the best filters. And in the worst case scenario, they can add a manual gross filter in a region where precipitation is highly unlikely. This effectively filters out ALL returns...so if they fail to remove it in a timely manner when precipitation starts to become likely, I've have seen them filter out real precipitation returns...even severe storms in some cases. It happens more often than they want to admit. That's why I always have lightning (and storm tracks) turned on...it's not part of that gross filter process and may be your first hint you've been a victim of the dreaded gross filter.
By the way, the NWS is experimenting with lowering the lowest elevation angles for some radar sites. The lowest elevation angle is 0.5 degrees. This test will include dropping the lowest elevation to -0.2, 0 and +0.2 degrees. Since December 2017, the radar in San Francisco (MUX) has already been operating at +0.2 degrees. They will be expanding this to the Medford, Oregon (MAX) radar as well.
Most pilots are weatherwise, but some are otherwise™
Scott Dennstaedt
Weather Systems Engineer
CFI & former NWS meteorologist
