Note: I'm not an Air Traffic Controller, nor have I trained to be one, nor do I play one on TV. Don't use this information to plan a flight, please. If you'd like a professional's view of this gear, ask a professional like our own archiewood or other trained ATC types. If you have a problem with this writeup, the fault is all mine and I should get the slap.
ADS-B stands for "Automatic Dependent Surveillance-Broadcast." It is a system that is part of the U.S. FAA's 'NextGen' program, intended to increase the safety, efficiency and reliability of air traffic systems inside the U.S. It consists of a protocol and equipment specification.
The basic intent of ADS-B is to shift some of the burden of collision avoidance (and possibly routing) from the central air traffic control to the pilots of each individual aircraft. In order for this to happen, each aircraft needs to have a complete picture of the traffic in its immediate area. Contrary to what some folks think, general aviation and civil air transports do not carry radar systems intended or designed to show other airplanes. They may carry weather radar, but that's about it. As a result, they're dependent on their own eyes, and on the air traffic controller monitoring them, to determine if any other aircraft are nearby and a danger to them. Some aircraft carry TCAS, which while not a radar uses other aircraft transponders to collect position and course information in order to generate warnings of imminent collisions.
Since all aircraft cleared to operate in U.S. IFR flight areas must carry a transponder, ADS-B builds on that. For an aircraft to be capable of employing ADS-B, it must have a transponder which broadcasts not just Mode C but ADS-B information. Essentially, a GPS receiver (or the output from the aircraft's primary GPS) is piped into the transponder, along with other information from flight instruments. Then, once per second, the transponder transmits a code which contains the aircraft ID, its altitude, and its position as derived from GPS - along with information such as its rate of climb or descent and whether it is presently turning (and in what direction).
This transmission is intended to be received by two different types of receivers. First, it is intended to be received by other aircraft in the vicinity (within approximately 200 nautical miles). Those aircraft, if they are also equipped for ADS-B, will have a special computerized display in their cockpits which is able to display the position of every ADS-B transponder-equipped aircraft and its relative position and heading from the display aircraft. This differs from TCAS in both the complexity and range of the display as well as in operating mode. TCAS works by having every aircraft with TCAS interrogate other aircraft and receive an answer on a separate designated frequency. This process may happen more often than the ADS-B update of once per second, but it also risks overloading the frequencies used to up/download as more aircraft come into close proximity. TCAS uses internal extrapolation of aircraft position based on its own data of each aircraft's positional track - it doesn't always receive course, turn and climb information. TCAS is intended for terminal collision avoidance - that is, it is intended to warn pilots when they are in imminent danger of colliding with another aircraft. ADS-B is a longer-range solution intended to allow for a 'single unified picture' of the airspace, especially through its interface with TIS-B (below).
In addition, ADS-B transmissions are meant to be received by ground stations. These ground stations, typically air traffic control locations, take the ADS-B targets they receive and add in information on non-ADS-B targets - aircraft without the requisite transponders, who have been located by ground radar. They then broadcast this more-complete picture of air traffic back out to local aircraft, who can choose to display this synthesized version rather than their own local receiver's information. The broadcast from the ground back out to the aircraft in the region is called TIS-B, for Traffic Information Service - Broadcast. In addition to basic TIS-B, some ground stations may be equipped to include information on current weather and local flight restrictions. This service is called FIS-B, or Flight Information Service - Broadcast.
ADS-B is scheduled for testing aboard aircraft of several different U.S. Airlines starting in 2009. The FAA is providing funding to equip these aircraft with prototype or early-model ADS-B gear to determine how well the system works in practice. Initially, the service will be mostly used at or near airports, where air traffic is most crowded. ADS-B should offer sufficient resolution to be used on the airport surface while taxiing in order to increase the reliability of ground control.
Sources:
- Popular Mechanics - http://www.popularmechanics.com/science/research/4300722.html
- FAA - http://www.faa.gov/news/fact_sheets/news_story.cfm?newsId=7131
- Aviation Week - http://www.aviationweek.com/aw/generic/story_generic.jsp?channel=awst&id=news/aw012609p3.xml
- ADS-B Technologies, LLC - http://www.ads-b.com/home.htm