It's a lot like the early days of the Volkswagen. In the late '50s when you drove a VW Beetle and you saw another one on the road, you tooted, flashed your lights and waved. There just weren't that many on the highways so a sighting was an occurrence. Today, when I'm flying and I see an ADS-B target on my GMX 200 display, I want to wave and toot. There just aren't that many of us on the airways, so the sighting is an occurrence.
What's ratcheted up the current interest in ADS-B is the FAA's announcement that ADS-B will be the cornerstone of the NexGen (next generation) air traffic control system (described as "aircraft-centric") and its Notice of Proposed Rule Making (NPRM) mandating that by 2020 all airplanes operating in Class A, B and C airspace over the 48 contiguous states, and in Class E airspace above 10,000-feet msl, will be required to be equipped with ADS-B-Out. The rule would also require airplanes flying within 30 nm of FAA specified "busy" airports to be ADS-B-Out equipped. (More about the "Out" in ADS-B-Out later.)
It looks as if ADS-B, like the Volkswagen Beetle, will become ubiquitous. In the interest of full disclosure, I have to admit I'm in favor of the implementation of ADS-B, and in fact, my Cessna Cardinal has been equipped with ADS-B for more than three years.
Unfortunately, there seems to be a great deal of confusion about exactly what ADS-B is and what it might do for pilots and controllers. The existence of the confusion was driven home to me by a reader poll on flyingmag.com in April (the results of which appeared in the July issue of the magazine); the question asked was: "How would you rate your knowledge of WAAS approach procedures?"
According to the poll, 70 percent of the pilots who responded to the question indicated a "high degree of uncertainty" about WAAS approaches, but even more concerning was that 45 percent, more than half of the 75 percent, voiced "a complete lack of familiarity with the technology and 25 percent admitted having only a passing knowledge of it."
Since WAAS, (the wide area augmentation system) which transmits its data once every second, is an apparent prerequisite for ADS-B, a lack of understanding of WAAS would obviously fuzzy up the pilot population's perception of ADS-B.
Put simply, WAAS is a system of satellites and approximately 25 ground stations positioned across the country that monitor the data being downlinked from GPS satellites. Two master stations on the coasts collect the data from the reference stations and generate a differential correction message that is then broadcast through one of two geostationary satellites. The corrections account for GPS satellite orbit and clock drift plus signal delays caused by the atmosphere and ionosphere.
With the WAAS corrections GPS navigators are able to provide position accuracy that's as much as five times better than without it and at an update rate of five times per second. A WAAS-capable receiver can report a position accuracy of better than three meters 95 percent of the time.
So, if an airplane equipped with a WAAS GPS knows where it is in space to such a high degree of accuracy, and if its position could be depicted on controllers' displays and on multifunction displays in airplane panels, it would work better than radar for traffic information. In fact, with ADS-B, both pilots and controllers can see radar-like displays with highly accurate traffic data that update in real time.
And that's only some of what ADS-B can provide. But I'm getting ahead of myself. Back to basics: ADS-B stands for Automatic Dependent Surveillance-Broadcast. "Automatic" means it's always on; it's "Dependent" because it depends on accurate position data; it provides radar-like "Surveillance"; and it continuously "Broadcasts" its data to any airplane or ground station equipped to receive ADS-B.
In my airplane, the ADS-B data is processed by Garmin's GDL 90 UAT (universal access transceiver) at 978 MHz for presentation on the GMX 200 multifunction display. In addition to providing traffic information the UAT bandwidth is also capable of uplinking real-time Flight Information Services (FIS-B) that provide weather and other data.
The FAA is promoting ADS-B because it offers some real advantages compared to the current ATC radar system. Unlike radar, ADS-B accuracy does not seriously degrade with range, terrain or atmospheric conditions, and the altitude and update intervals of targets do not depend on the rotation time of a radar sweep, which varies from three to 12 seconds. The ground infrastructure to provide and support ADS-B is as little as one-tenth the cost of equivalent radar equipment. And unlike radar, an ADS-B ground station requires little power and can more easily be installed in remote areas.
