The FAA’s ADS-B requirement moves the bulk of air traffic control hardware and technology from the ground and puts it into the airplane. Instead of using a radar network on the ground to locate the position of aircraft, ADS-B relies on equipment in each airplane to broadcast its location, altitude, and movements.
ADS-B will provide a more accurate picture of the location and velocity of aircraft than radar can because new position reports will be broadcast from every aircraft at least once each second. Radar antennas take many seconds to complete a sweep so aircraft position can only be updated as often as the radar beam scans each aircraft within range.
The ADS-B acronym stands for automatic dependent surveillance-broadcast. The reporting of aircraft position, altitude, and so on is automatic. The system is dependent on every aircraft reporting its location accurately based on a common grid, which will be supplied by GPS. The dependent part of ADS-B also means that each airplane must have equipment functioning properly or it will be invisible to controllers.
But the FAA has taken this concept of every airplane broadcasting its location to every other airplane and to the controllers and divided the sky in half. The ADS-B equipment in some airplanes can communicate with other cockpits, but other pilots will not be able to receive this information even though they have fully functioning certified ADS-B equipment installed.
The reason some ADS-B equipped airplanes will be invisible to other equipped airplanes is that the FAA has created a two-tier system. Actually, it’s a dual language system. Depending on what type of airplane you fly, your ADS-B won’t talk to other airplanes without an FAA-supplied ground station in between to translate the two languages.
The ADS-B languages are actually radio frequencies. The ADS-B in all higher performing airplanes – both airlines and GA – will broadcast on 1090 MHz, the frequency used by all transponders. Lower performing airplanes that do not fly above 18,000 feet can opt to install a universal access transceiver (UAT) that broadcasts on 978 MHz. If the UAT-equipped and 1090-equipped airplane are flying high enough so that the line-of-sight signals can reach an FAA ground station, be translated, and rebroadcast, the pilots in each airplane can see each other. If the airplanes are too low, or too far from a station, each is invisible to the other.
How did this dual ADS-B system develop? Obviously, the UAT is not really “universal” because it can’t receive the 1090ES (extended squitter) position broadcast that all higher-performing airplanes will make. The move to ADS-B will eliminate the need for ground-based radar, but replaces radar with a ground-based translating system so all airplanes and the controllers can see each other.
The long road to the current ADS-B rules and dual system go back decades to when the technology for TCAS, the airborne traffic warning system, was being developed. ADS-B was a contending technology to provide TCAS. But, as is still true, for ADS-B to provide useful collision warning every airplane must be equipped with compatible participating avionics.
But an alternate technology we used to call BCAS, for beacon collision avoidance system, was adopted as the traffic warning standard. BCAS operates on the transponder frequency – 1090 MHz – and it can interrogate any transponder-equipped airplane and calculate the relative position of that airplane from the BCAS-equipped airplane. The first airplane to install BCAS – since called TCAS – gained collision protection from every transponder-equipped airplane. That was an obvious and winning advantage for BCAS. All traffic awareness systems use this same basic technology.
Meanwhile, ADS-B technology remained in the background, hanging around waiting for an application. The FAA really didn’t want ADS-B messing around with the transponder frequency so it was assigned 978 MHz. That dedicated frequency also allowed bandwidth for ADS-B to receive useful information such as weather reports and other alerts. It was the 978 UAT version of ADS-B that the FAA deployed in Alaska for its reasonably successful Capstone Program to demonstrate that ADS-B could work in a non-radar environment. It is also UAT that the helicopters are using for traffic separation over the Gulf of Mexico, and that UPS is using in its fleet of freighters and that several major aviation flight academies have installed to keep track of their fleets.
But when the FAA and other national aviation authorities decided to convert to an ADS-B Nextgen world, it became clear that UAT was the wrong choice for many aircraft. The reason is that the traffic warning systems already installed in all jets and many propeller airplanes – TCAS – must remain as a final collision warning system in case the ADS-B system in an airplane fails. TCAS operates using a Mode S transponder on – you guessed it – 1090 MHZ. So it makes perfect sense to send out the ADS-B position and other information on 1090 MHz since the traffic warning systems are already using that frequency.
That left the FAA in a pickle because it has been promising GA that a change to ADS-B would provide free weather and traffic warning display, but that is only possible on UAT because of bandwidth crowding on 1090 MHz. At the time of the original “free weather”
promotion there was no XM Weather or Sirius satellite weather, so it sounded like magic to us in GA. The “free” part of weather in the cockpit still sounds good, but any pilot who flies much at all for transportation already has at least a handheld device that can receive satellite weather. Heck, even an iPhone gets a good Nexrad radar picture in the cockpit.
We’re all required to install an approved ADS-B “out” system to broadcast our location and velocity in order to fly in regulated airspace after January 1, 2020. GA pilots have the choice of selecting a UAT system, or a transponder type of system operating on 1090 MHz. If you go with a 1090 system you play with the big boys and all higher-performance airplanes can see you without need to be within range of a ground station. If you go with UAT you will be able to receive weather and other information from those same FAA ground stations.
I can’t predict how the cost of UAT versus a 1090 system will play out over the next eight years. The 1090 system has some advantages because all airplanes will need to keep the same transponder capability required now and have it tested for accuracy and performance every two years as we do now. But there will almost certainly be some clever UAT designs that meet the basic requirements while minimizing costs.
But the bottom line is that I find it to be frustrating that a system – ADS-B – that is supposed to show every pilot the location of all aircraft around him ends up being divided into an A system and a B system that needs ground-based intervention to link us together. This system is a little better than the current radar system, but not nearly as good as it should be.