ASDI what is it?

When we go to FlightAware.com, FlightExplorer or any of the other flight tracking web sites or apps, they have a lot of good information. The information all comes from the FAA, for free! Ever think about the FAA and how they collect that information? How does is all come together? 

All over the country, there are RADAR sensors. These RADAR sensors are scanning the skies 24 hours a day, 7 days a week. The output of the RADAR sensor is sent to a computer, where the range and azimuth data is correlated to the transponder and altitude data. In the drawing below, the black line coming out of the RADAR dish is the interrogation signal, the black line coming back it the "skin paint" reflection signal, and the blue line is the transponder broadcast from the aircraft.

The computers correlate the transponder code to the flight plan, and take the RADAR returns and calculate a speed, altitude and track that the aircraft would be on. The flight plan helps determine where the airplane will be, based on the speed and time since last sample. 

All of the track information for all of the RADAR systems are sent to the FAA command center where they are made available for display in the various FAA systems that need the information (IE URET, TRACONs, ERAM, TFMA, etc).

One of those system that get the FAA RADAR data is the Aircraft Situation Display to Industry (ASDI). The ASDI data is availble to the airlines and other organizations in the aviation industry. The information includes flight plans, position reports, departure, and cancel flight plans. Using this information sites like FlightAware can present aircraft on maps for the general public. 

The ASDI data only contains non-blocked RADAR and flight plan data for aircraft with IFR flight plans in the US and some of Europe. There is an option available to private aircraft allowing them to block the ASDI data for competitive reasons (IE the president of ATT doesn't want the Verizon corporation to know about some special meetings with a partner or something).  

There are about 4 different kind of feeds of ASDI data. There is the internal FAA feed, the need to know real-time feed, the need to know real-time with European data, and a delayed feed. The real-time feeds are for the airlines and such to use for business reasons. The delayed feed is for the web sites visited by the general public. The delay is like 5 minutes, so it is good enough for people to know when to show up at the airport to get their loved one. 

There is a bit of information that can be derived from the ASDI data. Looking at the ASDI data, someone can determine which airports are taking delays with many aircraft holding. Other things can include looking at the projected track, and weather data to see when it might be best to re-route an aircraft because it is heading toward some convective activity. Airport operators can use the data to count operations relative to other airports, to help improve service. 

The ASDI data feed contains a lot of data. The position reports will be updates of aircraft positions every time the computers are aware of a position update (IE every 12 seconds for enroute RADAR, 5 seconds for TRACON, or 1 second for ADS-B). Typically Monday through Friday in the US there will be 3000-5000 aircraft in the air from 6am to 6pm. 

The FAA has many other services similar to ASDI. Most of it isn't as useful. ASDI is a great resource. 

What do you think.