Automatic Dependent Surveillance - Broadcast

In the Next Generation environment, starting in 2020, the FAA will require all aircraft (FAR 91.225) to continuously announce their position, velocity, and altitude in order to operate in certain airspaces.  Those airspaces include Class A, in and above B, and C to 10,000’, airspace above 10,000’ (but not below 2,500’), and from the surface to 10,000’ within 30 nm of certain airspaces listed in FAR 91.225.  This ADS-B Out system will eventually replace Radar ground stations for traffic separation. Equipment requirements are specified in FAR 91.227.

There are two independent ADS-B technology paths to choose from; Mode-S transponders with Extended Squitter reports, and a  978 Mhz Universal Access Transceiver (UAT) developed by UPSAT (later purchased by Garmin) for the FAA in the Capstone project in Alaska.  Let’s look at each in turn.

Transponder Based

Mode-S transponders are interrogated on 1030 Mhz and reply on 1090 Mhz with a message that includes your transponder code and barometric altitude -- just like Mode C transponders.  But they also send out a  56 bit squitter message that uniquely identifies you (it’s tied to your tail number), and the 1030 MHz channel has data link capability to receive Traffic Information System (TIS) reports from terminal radars.  The squitter message is sent automatically (without being interrogated) at least every second, depending on the data being sent.

To determine where you are, the ground station uses your transmitted altitude plus its determination of your position by the timing delay from their pulse and your response. There is some inherent uncertainty in angle and range information, with TCAS as well, and there is latency in the system in delivering traffic information to you. So errors in the Mode S information you receive are not negligible.

The ADS-B Out solution for improved position reports is to extend the Mode S squitter message to 112 bits and include your WAAS GPS horizontal position and velocity in the extra bits. Not all of this information will fit in a single Mode S squitter so it breaks the data up into multiple messages, each with its own schedule for repeating.  Along with your barometric altitude this sends more accurate position information twice per second. Your 1090 MHz squitters go to an ADS-B ground station and directly to any aircraft that can receive it. The extended squitter can send  47 individual parameters, compared to 3 for Mode C and 7 for Mode S.  These extra parameters can also transmit your flight plan route, a capability that is being planned.

To receive the extended squitter reports, either directly from another aircraft or from ground stations, requires a 1090 MHz “ADS-B In” receiver that meets TSO-C166B requirements. The ground stations tell you about Mode C traffic, and any traffic reporting on the 978 Mhz system described below. Once you purchase a Mode ES transponder and a 1090 MHz ADS-B In receiver (and antenna), you need to connect WAAS GPS and altitude encoder signals to each of them.

So, ADS-B traffic is a major improvement over receiving Mode S traffic only in the vicinity of Terminal Area Radars, which are slowly being phased out.

UAT Based

The FAA-funded Capstone project in Alaska (1999 – 2006) was used for early trials of a full ADS-B system. A 978 MHz UAT and an MFD with a built-in GPS was developed, which the FAA provided (free) to pilots there for a system demonstration of the concept.  The transceiver sends and receives the required signals.  Currently, UAT’s for ADS-B are a variant of the one used in Alaska, requiring 978 MHz for both In and Out channels (separated by timing of the code messages).  These units must meet the requirements of TSO-154C. Along with the transceiver and an antenna, you need a moving map display and WAAS GPS and altitude encoder signals to the unit.

The 978 MHz system has a much larger bandwidth (1.2 MHz) than the 1090 MHz channel, so weather products and NOTAM information, in addition to traffic, can be sent to you from a ground station for display on a moving map.  That’s not possible on the 1090 MHz channel, which has a much narrower bandwidth.

There are both advantages and disadvantages to the 978 Mhz system. Since 1090 MHz is the ICAO standard for traffic reporting, you can’t use it for a position report above 18,000’. But you could report position on a 1090 MHz Out unit, and purchase a 978 MHz In receiver only for traffic and weather. (FAA software for this option is not ready, so you can't do it now). The weather information is only for the area around the ground station, and at most airports you won’t even see a ground station until you’re in the air.  So you won’t have traffic and local area weather before takeoff and in the initial climb – but for now it is free. Ground stations covering the entire US should be completed in a year or so, according to FAA plans.  There will still be coverage issues however, either because you are too low or in an area of sparse stations.

Garmin currently offers its GDL 90 system, and there are certified 978 MHz systems coming from NavWorx, Trig, and FreeFlight within the year. But another issue for the UAT based system is the display unit. The Garmin MX-20 and GMX-200 displays traffic and weather from its own GDL-90 UAT. But according to NavWorx, they can display legacy weather products (Nexrad, Metars, Tafs) plus textual reports on a Garmin MX-20, GMX-200, and Chelton EFIS screens. All other displays are currently either hand-held devices, or units for the experimental market only. So be sure to get good answers on the display options from each manufacturer you’re considering.

What to Do?

Your options will surely expand well before the 2020 mandate on ADS-B Out, as more manufacturers bring out products in response to these basic requirements. Many manufacturers have units awaiting their TSO’s, so it appears there will be lots of choices by 2012.  Experimental aircraft can jump in right now, whenever the units are available. They can also use non-certified displays and hand held devices like the iPad. But the FAA has "portable installation guidelines" under which you could velcro mount a NavWorx ADS600-B transceiver in certified aircraft.

One advantage of participating in the ADS-B program now is to display traffic information from all transponder equipped aircraft around you without the expense of a TCAS unit. To do that you need both ADS-B Out and In. Trig has their TA60 series of 1090 MHz receivers with certification expected at the end of the year.  This is one choice if you plan to continue using Satellite weather. But if you can accept the limitations of ADS-B weather information you can install a UAT transeceiver and get both traffic and weather now.

Home