Demystifying the strange, unusual or confusing data you may see from your ADS-B receiver.
The "Ident" field may be the source of the most obvious strange things that you see when viewing data from your ADS-B receiver (whether you are using FlightAware's Skyview or another application). We gave an overview of the "Ident" field in our July blog post. The data in this field is typically set by the pilot(s) before a flight so generally you expect to use an ICAO flight number or a registration/tail number. Frequently this field will be blank and this is more common for Mode S aircraft without ADS-B. Aircraft without ADS-B will transmit the Ident less frequently so it may be harder for your receiver to catch a message with this information. The field may be populated with strange data though. You may see anything from "TEST1234" to short phrases. Sometimes the pilots will only enter a flight number without the airline code prefix. You may also see military aircraft transmitting their callsign (frequently these are also abbreviated due to length so you may have to think cleverly to expand the full callsign). In some areas, particularly in Europe, the ATC callsign may be different than the published schedule's flight number (to learn more about this, see Eurocontrol's Call Sign Similarity Service).
Altitude is another confusing piece of data. While at first glance it may seem straight-forward, there are actually several different ways altitude can be reported. Most people instinctively think of altitude as "height above ground level" however this is not what is transmitted in Mode S/ADS-B data. The most common altitude type is "uncorrected pressure altitude" from the aircraft's altimeter. You may see strange things such as negative values or lower values than you would expect because this data is not corrected for local atmospheric pressure. An airplane landing in a mountainous region may be on the ground at an altitude of several thousand feet! ADS-B is standardizing around "altitude above ellipsoid" based on GPS satellite calculations (and again, this is not altitude above ground level). Unfortunately, when viewing Mode S/ADS-B data it is not necessarily obvious what type of altitude you are seeing. You may also see altitudes based on height above mean sea level from older ADS-B equipment. Always take the altitude value with a grain of salt!
Even the "unique" ICAO code can be wrong. It is not common but definitely a problem! Even major airlines have aircraft flying with the wrong ICAO "hex code" programmed. In some cases it is a mistake when the transponder was installed. In some cases, an aircraft will be re-registered with a new tail number but the transponder's ICAO code is not updated to match the new registration. This can result in multiple aircraft flying with the same ICAO code. In applications like Skyview, the ICAO code tells us the registration/tail number (for many but not all aircraft) which in turn tells us what type of aircraft icon to display. Unfortunately, if the ICAO code is wrong your software may lookup data about a different aircraft and this is definitely confusing!
Hopefully this helps clear up some common oddities you may see when viewing data from your receiver.
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