The August 2011 Wx Watch (" Datalink Lowdown") in AOPA Pilot provoked comments from the two major suppliers of datalink weather. Here, Bob Baron of Baron Services—the provider behind WxWorx’s XM WX service—and Paul Devlin of WSI each explains why his service is best. Read on, and you’ll see why XM WX favors composite reflectivity Nexrad radar depictions, and WSI feels base reflectivity is just as good. Just remember, steering clear of dangerous radar signatures is the hands-down winner when it comes to using datalink radar in the cockpit. —Ed.
Baron Services is committed to delivering the most accurate, timeliest products possible, which is why we developed the first true metadata stream designed for the pilot.
Nexrad radar returns are divided into individual packets (range bins), of individual arcs (radials) of individual elevational sweeps (volume scans). In order to find the severe parameters in storms, we committed to analyzing everything on the fly, all the time, beginning nearly two decades ago. This unique (we have many patents in this area) capability has made it possible for our broadcast clients to spot and track dangerous storms accurately and in real-time.
It is this experience and commitment that we brought to aviation. Because of bandwidth limitations and our design requirement of five-minute updates, the product suite was carefully defined. We felt then (and still do) that the radar information was a critical weather tool for the pilot, and that we should make maximum use of every bit of data available to us. To generate nationwide Nexrad data, we effectively stitch together radar returns from across the country into mosaics.
This brings us to the difference between a national composite mosaic, and a national base reflectivity mosaic. A national mosaic of current radar returns from each radar can be made with only the lowest sweep of the atmosphere, called a base reflectivity mosaic.
A composite mosaic, on the other hand, is a merging of returns from all sweeps produced by each radar, thus filling in any gaps where a single sweep might undershoot (close to the radar) or overshoot (at greater distances from the radar) a weather feature. We create composite mosaics every five minutes, at a substantially increased cost, because we believe it is important to get as much information as possible to the pilot.
The National Weather Service (NWS) and FAA feel composited data is important. That’s why they produce a single-site composite of their own, as well as three products that attempt to show radar returns from set blocks of altitudes that are built at the end of each radar volume scan. However, to our knowledge, only Baron provides a nationwide composite mosaic. Because of latency and resolution issues, we do not use any of the NWS composite products.
Here's a little "what and why" regarding the published radar image. When generating a nationwide composite mosaic, the first task is to remove false returns, commonly called anomalous propagation (AP). You can see a noncorrected example from the NWS.
There are a number of techniques we use to remove AP, including the subtraction of nonmoving pixels, comparisons of multi-level returns, and man-in-the-loop interpretation. But it’s still pretty challenging at times. Our system is mostly automated, so there’s no lost time between receipt of data and its inclusion in the compositing process.
If we used NWS composites (composed at the end of each volume scan), some radar information would be 10 minutes old by the time it reached the cockpit, and 15 to 20 minutes old by the time it was updated. To maximize timeliness, a few seconds before our five-minute composite mosaic is generated, our servers access the current sweep of each radar across the country, and then backtrack through all previous sweeps to complete our own volume scan, in which all the data is no older than five minutes.
To minimize blurring and maximize accuracy, each sweep is then converted from a radial product to a conformal grid called "Cartesian coordinates." Basically, this process accurately squares up pixels of data. Each sweep is then laid over the other, and the highest value in each grid is selected.
Finally, each composited single site is combined into a mosaic for the whole country. Given the speed of modern computers, the whole process takes only a few seconds to produce, and 30 seconds or less to broadcast over the XM satellites. It is also appropriate to mention here that the pilot has the benefit of redundant distribution over both XM satellites.
There are similar innovative processes that determine our "rain/ice/snow line," satellite imagery that defines its altitude, lightning, next generation storm cell identification and tracking, and as our Nexrad upgrade installations become operational, some exciting new data streams for pilots that might be fuel for some future articles.
We can't speak to other weather companies and how they process data, nor can we speak to how any avionics manufacturer chooses to depict the data we supply, but we can speak to the care we take in preparing and distributing it. We've had a saying around here for a long time that "all data is good, but some data is ‘gooder’ than others." We spend a lot of time ensuring ours is of the ‘gooder’ kind.
There are the "visible" thunderstorm threats of lightning and hail; but a significant, unseen threat is that posed by wind shear and turbulence. A sudden and significant change in relative wind is something that pilots want to avoid. Weather radar shows the reflectivity of water in the atmosphere. The simplified rule is the greater the reflectivity, the more water is present. The distance in the change of reflectivity is as telling as the reflectivity itself. This change rate over distance is called gradient. A storm, in which there is a change from light to intense precipitation over a very short distance, is said to have a steep gradient. Areas near, or in, steep gradients have intense turbulence. The reason is that in very close proximity, air is moving a great deal of water (up or down) next to air that is moving little or no water.
It takes a strong column of rising air to hold aloft large water droplets. Conversely, when a lot of water falls through the atmosphere, it moves along with it a lot of that atmosphere. The rapid change in air circulation, speed, or direction is the definition of shear, which translates into turbulence and becomes a hazard to aircraft. This creates an amazing correlation. It can be a smoother, safer flight in a large area of moderate returns than in clear air next to an echo with a steep gradient.
It takes less than 20 seconds for a weather radar site to complete the base level scan. This allows WSI to provide a snapshot of the precipitation in the lower atmosphere. In comparison, as " Datalink Lowdown" notes, composite reflectivity uses many scans at various tilt angles, which are then combined. The Nexrad site’s scans are sequential, not concurrent; so a composite image has a much longer "exposure" during the time in which the storm moves. Storms always appear larger on a composite reflectivity image compared to a base reflectivity image because of this.
By using base reflectivity as the basis of our WSI NOWrad product, which is delivered via the WSI InFlight service, WSI is able to accurately depict the structure of a storm in the lower atmosphere. Pilots are able to accurately interpret the gradient of all regions; and with the help of the associated products, echo tops and lightning, are able to deduce the relative risk from shear that each section of the storm presents. The gradient in composite imagery is masked because of the time exposure effect in the collection process. Composite reflectivity will show the highest level of intensity echo in a storm, but there is no way of knowing its actual extent or altitude.
Finally, there is another benefit to the use of base reflectivity. It is a very common depiction with which even nonpilots are familiar. Almost everyone in the United States has used and understands base reflectivity imagery. This is because it is a time lapse movie of the base mosaic that every television meteorologist uses in every broadcast to explain the position, intensity, and movement of storms. The depiction is prevalent in all forms of media. In aviation it is the most referenced image by pilots, dispatchers, and controllers as they plan their operations. When, in conversation, someone talks about the ‘radar’ showing a storm, they are talking about base reflectivity imagery. Common situational awareness leads to better decision making.