By JEFFREY BOCCACCIO.
This is the tenth in a series of articles looking at the impact of the Next Generation Air Transportation System (NextGen) on GA pilots.
ADS-B is the system that literally allows NextGen to become “The Next NextGen.” It stands for Automatic Dependent Surveillance–Broadcast. But what does that all mean?
Automatic: This basically means when this puppy goes live it stays live, operating on its own with no need for external interrogation or pilot involvement.
Dependent: Tracking and positioning is totally dependent on the ship’s own GPS and Nav system. No longer does radar have to track you.
Surveillance: Emulating “Big Brother,” this feature is like a big megaphone telling the world your airplane’s tail number, type, position (you can’t hide), emergency status, altitude, heading, velocity, and whether you brushed your teeth that morning. If you’re in the air, everyone knows!
Broadcast: All this data is transmitted all over the place. It gets transmitted to more than 700 ground stations, all airborne receivers, not to mention the uplink systems to the “birds in the sky.”
There is one important point I want to slam home on this: This puppy is accurate — I mean real accurate. We will discuss that later on, but my point here is that the quality of service ADS-B provides will give us far more and improved surveillance and situational awareness for pilots and ATC.
Although this sounds pretty straightforward, it has a whole lot more depth to it than just the name. ADS-B is a system. It is not something you just purchase over the counter, slap your credit card down, then sign and drive. It gets a little more involved than that when you consider the hardware, software, procedures, and operation that make it up.
It will be a transition by using current technologies, such as WAAS and transponders, along with our current radar system currently in use. So let’s start from there and gradually build into it, rather than everyone just jumping in and getting totally lost.
Primary Surveillance Radar (PSR)
Historically the surveillance system in the U.S. has been implemented by ground-based radar. Originally, after World War II, Primary Surveillance Radar (PSR) was implemented with radar transceivers that transmit an output pulse of RF energy. We technically refer to this as the “Goes Outa.” It goes out, gets reflected off any metallic objects like aircraft, and comes back into the transceiver as a “Goes Inta.” No, you won’t find those terms in Funk and Wagnalls dictionary, for sure!
The system knows what time the energy left the antenna and measures how long it takes to return. The further the object is, the more power and time it takes to “ping” the target. By using this method, radar can determine your airplane’s bearing and distance.
Nice idea, but this baby is a power-hungry tool proportional to its distance of reach. That means that PSR systems must put out an enormous amount of energy when reaching out for long-distance reflections up to 150 nm. Its reflected yield is relatively small and can ride pretty close to the noise floor of the radar system itself. Throw some interference in there and the system’s integrity goes right into the toilet.
You can see from the chart that the propagated radar waves diminish to the square of its distance. The return trip is no better and eventually goes into the dirt if the object is too far away. Later this was changed to 75 nm distances using less power.
One also has to consider its refresh time for each reflection. Radar surveillance systems can take as long as 12 seconds for a sweep or refresh to take place, unlike ADS-B’s refresh rate of twice per second.
In order to get aircraft positioning, ATC would have to ask planes to make turns to identify who was who. DME was eventually implemented, providing a distance position report along a known track, which made things a whole lot better.
Secondary Surveillance Radar (SSR)
Then came Secondary Surveillance Radar (SSR), introducing a new device known as the “transponder.” First used in World War II as an Identification Friend or Foe (IFF), the transponder brought a whole new meaning to surveillance.
It was a Mode A transponder that was nothing more than a radio transmitter operating on the same frequency radar uses. Each aircraft is assigned a four-digit transponder code or “Squawk” number that the pilot sets upon ATC’s request. From there on, that Squawk number identifies the aircraft until radar is terminated.
Incidentally, the term Squawk originated back when using IFF. At the time they developed a code name for the transponder system called “Parrot.” When ATC wanted pilots to turn off their transponders, they would say “Strangle Your Parrot.” However, when instructed to Ident the transponder, it was “Squawk” your transponder. And, as I am sure everyone knows, Squawk is still used today.
Mode C introduced altitude reporting derived from on-board barometric pressure values typically taken right off the altimeter or an external sensor. This information then gets reported, as does the plane’s Squawk number.
The most recent transponder revision Mode S (Select) permanently assigns your transponder with an aircraft ID. Now ATC’s radar can spot these Mode S airplanes very easily, reducing garbling on SSR return reflections responses. In addition, the system was designed to upload specific data from ground-base stations to aircraft.
One of ADS-B’s key features is the ability to have a data link to and from ATC and other aircraft. Mode S transponders support this for ADS-B. In fact, as we go deeper into ADS-B, you will see how Mode S plays its role.
Next month we explain why there are two types of ADS-B — UAT (Universal Access Transceiver) and 1,090 MHz Mode S Extended Squitter or1090ES. What’s the deal here? You will see next month.