ADS-B’s two-for-one deal

This is the 11th in a series of articles looking at the impact of the Next Generation Air Transportation System (NextGen) on GA pilots.

Sounds like a sale doesn’t it? Well, not really. Instead, it’s a reference to the FAA’s decision as part of the Next Generation Air Transportation System (NextGen) to use two different “systems” within the ADS-B environment, so everyone on both sides of the aisle would be happy.

This is how it came down: The big boys on top, the transport carriers, have been using the newer Mode S 1090ES (Extended Squitter) transponder system that we discussed last month for some time know. Perfectly understandable since they have all the necessary attributes to work in the proposed ADS-B system environment.

But — don’t you just hate those buts? — the Mode S transponders have some limitations.

Mode A, C and S transponders all have the ability to be assigned airframe identities, while C and S systems also have altitude reporting as a feature. However, over time the number of aircraft, both commercial and GA, grew so fast there were tons of airplanes all over the place. Besides radar screens getting loaded up with N numbers, there were many “False Reply Uncorrelated in Time” (FRUIT) issues that popped up. Here, one plane’s transponder would interfere with another plane’s transponder, inducing “garbling.” This occurs when multiple interrogations being requested are all keyed at once. It is similar to what pilots dislike so much when they talk on their radios and someone steps on them with another transmission. Take that to the next level of 10 or 20 aircraft stepping on your interrogation response.

Mode S transponders offer the additional benefit of providing permanently programmed identifiers. Now ATC does not have to assign your airframe a unique squawk number, it already has one fixed inside the unit. To complement that, Mode S transponders have the ability to transport additional cockpit data information over the same system.

As we all know, ATC interrogates your transponder and reads the 4096 code that was assigned to your airplane. These interrogations are done with short 8 to 20 microsecond transmission pulses. This is transmitted to the transponder’s 1030 MHz frequency.

In Mode S transponders, the interrogations are transmitted by using a modulation scheme called Differential Phase Shift Keying (DPSK), designed to improve the efficiency of the interrogation frequency and reduce the FRUIT interference with Mode A and C systems. In addition Mode S offers the ability for up to 4 MB/s of data to be transferred back and forth which supports its data link.

In response to those interrogations, Mode S transponders send out a response verifying the requested information and signal integrity of the system using a Pulse Positioning Modulation (PPM) transmission system by way of the 1090 MHz frequency. The idea with Mode S is to reduce as much FRUIT garbage out there as possible, especially in high traffic areas. Mode S transponders are interrogated 50 times per second (50Hz) and Mode A and C are interrogated 230 times per second (230Hz). You can see how much more efficient it is just by minimizing the interrogated refresh rate.

Mode S transponders also include an “Extended Squitter” function not available on our typical GA Mode A and C transponders.

So what is all this squitter about? It is a form of “Unsolicited Replies” that the airplane’s transponder uses to help acquire and track other Mode S airframes. Now you have TCAS out there listening to all the squittering going on, which contains your airplane’s discrete address. This limits all the unnecessary transmission we had when the airwaves were loaded with interrogation requests, which reduces much of the FRUIT floating around. Remember, one of ADS-B’s big goals is to make things more efficient and this is just one tool that helps with this.

Well kind of, because this is when things went south.

That’s because while Mode S transponders have the ability to use DPSK data transmissions, the overall bandwidth of 1090ES is limited and does not provide all the goodies the FAA intended for GA.

Yeah, that’s right, here they are offering all these goodies, but didn’t have the 1090ES frequency band real estate to support it! Oh, that is just wonderful, isn’t it?

So here comes the band aid: The FAA added a dual link strategy by incorporating the system they used during the original ADS-B testing in Alaska called Universal Access Transceiver (UAT) in addition to the main 1090ES system. Now this puppy has BANDWIDTH, which allows the FAA to offer all kinds of goodies for the GA guys, including Traffic Information Services (TIS-B) and Flight Information Services (FIS-B).

The 1090ES system is not be able to incorporate FIS-B but, then again, it doesn’t really have to. Typically the big guys have all the top-end avionics in their cockpits already. I’m surprised they can’t make burgers while they fly too.

So at the end of the day, both 1090ES and UAT systems can utilize TIS-B for traffic avoidance.

But wait, doesn’t that create another problem? How does UAT tell 1090ES and how does 1090ES tell UAT where they are if they are on two different systems? If we have GA on UATs and the big guys on 1090ES, how the hell are the big guys going to know where the little guys are?

A new band-aid appears with the introduction of ADS-R, with the R standing for REPEATER. Yep, you got that right. All the traffic information from both the big guys and the small guys gets routed through ATC’s master networks so everyone knows where everyone is — and all this occurs in about 1 second.

But things aren’t as rosy as many would think. We will dig a little deeper next month.


Jeffrey Boccaccio is a private pilot and chief engineer at MatchBox Aeronautical Systems ( You can reach him at or



People who read this article also read articles on airparks, airshow, airshows, avgas, aviation fuel, aviation news, aircraft owner, avionics, buy a plane, FAA, fly-in, flying, general aviation, learn to fly, pilots, Light-Sport Aircraft, LSA, and Sport Pilot.

Leave a Reply

Your email address will not be published. Required fields are marked *