About ADS-B (Automatic Dependent Surveillance-Broadcast)

Before reading about ADS-B, please read a bit about transponders and TCAS collision avoidance here. Below is a (highly condensed) summary of ADS-B as relevant for gliders operating in USA. Apologies in advance: this is not simple topic nor a short summary; nor can it be. Beware that ADS-B is a generic term. ADS-B does not describe a specific piece of equipment. You cannot "buy an ADS-B box".  The ADS-B umbrella includes many different pieces of equipment, not all interoperable. And, in different countries, the term ADS-B will be taken to mean very different sets of equipment.

Why ADS-B ?

ADS-B brings many benefits, but ADS-B is being implemented primarily for these aircraft surveillance advantages:

1. Provide Air Traffic Control (ATC) with more precise information about aircraft than possible with radar systems, and
2. Provide pilots with information about other aircraft in their vicinity.

ADS-B is for traffic awareness, not collision avoidance. For at least the next decade, GA collision avoidance will be provided by TCAS (on larger aircraft) and transponders. 

ADS-B Background

In ADS-B , aircraft autonomously broadcast their identification, position, and other information.
A complete airborne ADS-B setup would include:

1. Position and track source (usually a GPS, interfaced to the ADS-B-Out device below),
2. A pressure-altitude encoder, 
3. A transmitter that periodically sends the aircraft's identification, position, track, pressure-altitude, and other information (referred to as ADS-B-Out),
4. A receiver that listens for nearby aircraft and other traffic information (referred to as ADS-B-In), and
   
5. A traffic display for the pilot.
   
6. Note: Collision hazard warning for the pilot is not included under the ADS-B umbrella term, though some ADS-B traffic display units also provide traffic alerts.

Because the focus is on surveillance, initial requirements mandate ADS-B-Out equipment which broadcasts the aircraft's identification, position and other information. Thus, many aircraft will be required to add ADS-B-Out equipment, but ADS-B-In and traffic display equipment are optional.

In USA, the FAA has announced that effective on January 1, 2020, any aircraft operating in airspace where a Mode C transponder is required today (in controlled airspace, above 10,000', in a mode C veil), will also be required to carry an ADS-B-Out transmitter (this effectively requires replacement of older Mode C transponders with Mode S transponders with 1090ES, ie ADS-B-Out over 1090). Above 18,000 Mode S with 1090ES is required, but below 18,000 UAT may be used - more details below.

How does ADS-B Provide Traffic Information ?

You may receive ADS-B traffic information directly from another aircraft, or from a ground station who thinks you are listening for and need the info. There is no broadcast of all traffic in an area - that would be too easy. Ground stations transmit traffic data about only some of the aircraft in their area, and in any case much glider flying in USA is not in airspace ground stations can reach.

Traffic information is transmitted by individual aircraft with specific equipment, and by ground-stations under a few very specific circumstances:

1. Aircraft equipped with ADS-B-Out autonomously broadcast identification and position information every couple of seconds. Anybody within earshot can receive this information. In most of the world and USA always over FL180 (and sometimes below), ADS-B-Out is broadcast on 1090MHZ (e.g. by a Mode S transponder with 1090ES, aka ADS-B-Out via 1090). When you receive a message directly from another aircraft its called ADS-B-Direct.
2. Traffic Information Service -Broadcast (TIS-B) ground stations broadcast traffic information about a target aircraft "X" where:
   1. X is tracked by ground radar, and
   2. X is in the vicinity of some known-ADS-B-In-equipped aircraft Y who should know about X because they're close, and
   3. X is *not* sending ADS-B-Out information on a data channel Y can receive directly because:
      1. X doesn't have ADS-B-Out (perhaps X has a Mode A/C transponder, or a Mode S transponder without ADS-B-Out), or
      2. X is sending ADS-B-Out on a data channel that Y cannot receive (e.g. X is transmitting ADS-B-Out over UAT, but Y is listening on 1090 - this retransmission is called ADS-B-R).

Whew. What this boils down to is:
You can receive relevant ADS-B traffic information about aircraft "X" only if:

1. You have ADS-B-In and receive traffic information from X directly (X has ADS-B-Out on the same channel as your ADS-B-In), or
2. You have both ADS-B-In and ADS-B-Out but on different channels than X, and you're in a service area where a ground-station can see you, and the ground station sees that information about X is relevant for you, so broadcasts that information (that retransmission is called ADS-B-R), or,
3. You're nearby someone else who has ADS-B-Out and has indicated they'd like to receive this info on a channel you can receive.

The Universal Access Transceiver (UAT) ADS-B using datalink 978 MHz

Unlike Europe, in the US a separate system called UAT is available for use by general aviation traffic below 18,000, in addition to ADS-B over the 1090MHz frequency. UAT operates on its own frequency and protocol, requiring different radio equipment than 1030/1090 components. Some information relating to UAT:

1. A UAT transceiver is invisible to the TCAS collision avoidance on large aircraft. TCAS only hears transponders (Mode C or Mode S).
2. You must have a transponder to be visible to TCAS.
3. GA aircraft will be required to have transponders for the forseeable future (there are exceptions: gliders, some operating areas...).
4. A GA aircraft can replace a Mode C transponder with a Mode S transponder with 1090ES to satisfy all upcoming requirements for ADS-B-Out. The Mode S transponder can use the existing transponder antenna installation, and connect to an existing ARINC GPS. This is currently the least expensive way for many GA aircraft to satisfy the ADS-B-Out requirement. Some Mode S transponders also provide Traffic Information Service alerts (e.g. Garmin GTX330, though this doesn't provide TIS-B), so with appropriate cockpit displays this upgrade provides immediate benefits today - without any UAT gear. If the aircraft flies above FL180, Mode S with 1090ES will be required, so high-fliers have little incentive to install a UAT transceiver.
5. UAT receivers can't hear ADS-B messages on 1090MHz (that is, Mode S transponders with 1090ES ADS-B-Out). UAT receivers can only receive on the UAT frequency, either from other UAT-equipped aircraft or from a ground station in a service area.
6. Outside of a service area UAT receivers receive no information about aircraft using 1090 (ie aircraft that fly over 18,000, ie larger aircraft). Nor will large aircraft receive any UAT information unless they are equipped with UAT receivers. 
7. UAT ground stations can broadcast weather information. This information is of lower resolution but more timely than the commercial XM weather service. Because this weather information is broadcast from a ground station (unlike the satellite-broadcast of XM weather), you probably cannot receive it before takeoff.
8. UAT ground stations can broadcast information about airspace (NOTAMS etc). Because its broadcast from a ground station, you probably cannot receive it before takeoff.
9. In a radar service area, a ground station will rebroadcast UAT traffic information about aircraft "X" on 1090 only under very specific circumstances: an ADS-B-In-and-Out-1090 equipped aircraft must be in the vicinity of X.
10. In a radar service area, a ground station will rebroadcast 1090 traffic information about aircraft "X" on UAT only under very specific circumstances: an ADS-B-In-and-Out-UAT equipped aircraft must be in the vicinity of X.

Important Differences Between ADS-B and FLARM

Unlike ADS-B, FLARM is designed for collision avoidance. ADS-B transmits an aircraft's position and track, which tells where the aircraft will be over time if it continues flying straight. On the other hand, FLARM transmits the expected position of an aircraft over the next 20 seconds. Because FLARM is aware of aircraft established in a turn (especially thermalling), it broadcasts a much more accurate picture of where the aircraft is likely to be. With this more accurate information, FLARM can more effectively warn of collision hazards, and avoid false alarms.

Editorial: The discussion above emphasizes differences between USA and Europe. So why do we care about Europe and the rest of the world ? Because larger and international aircraft will be equipped as required for the rest of the world, and because the world market determines what products will be commercially available in USA. The US General Aviation market is just a fraction of the world market, and US-specific products will by necessity cost more and likely be available later. And the glider market in turn is but a small fraction of the GA market.

Links for More Information

AOPA's excellent ADS-B overview 
Plane&Pilot  - Beyond Today’s Transponder- Demystifying ADS-B
ITT ADS-B Overview
Great summary of current ADS-B status, worldwide
Eurocontrol's ADS-B For Dummies, excellent technical introduction
FAA ADS-B web site
FAA Discussion about TIS (alert service over 1030) and TIS-B (general traffic awareness broadcast)

Please send comments, questions, and corrections regarding this page to Dave.Nadler@Nadler.com