ASTERIX Category 021: ADS-B target reports, in and out

A Category 021 data block carries one or more target reports. Each report describes one aircraft as the ground station understood it: an identity, a decoded position, a set of times, and the station's own assessment of how good the data is.

The category is the downstream sibling of the raw 1090ES broadcast. The ground station has already performed compact position reporting decoding, merged several message types into one report and added quality indicators, so three of the hard problems in raw ADS-B are gone before the format is read.

In their place the category introduces its own difficulties: seven separate time items, none carrying a calendar date, and a quality vocabulary whose meaning depends on an optional item that names the standard version in use.

Cooperative air traffic is the background against which uncooperative air traffic is recognised. An air picture that cannot ingest ADS-B derived reports has to treat every civil airliner as an unknown.

Counter-uncrewed-aircraft work depends on the same distinction. Being able to subtract what has identified itself is the first step in isolating what has not.

Ground station quality indicators matter to a commander because they say how much of the picture is measurement and how much is assessment. Carrying them through unaltered is the difference between a decision made on data and a decision made on a summary of data.

Ingest takes one data block and produces one Entity and one Event per record. Every item the canonical model has no field for is parked in the object's attributes rather than dropped, and the coverage document records where each one lands.

Every Category 021 time is elapsed time since the previous midnight, with no date anywhere in the payload. The date therefore comes from the injected clock, and the instant chosen is the one bearing the stated time of day nearest the receipt instant, considering the receipt date, the day before and the day after. That single rule handles midnight rollover in both directions without a special case, and the basis is recorded on the object rather than left implicit.

A time value at or beyond 86,400 seconds is refused rather than reduced modulo a day. The field can express a time of day that does not exist, and quietly wrapping it would move a contact by hours while every other check still passed.

The quality indicator whose quantity depends on the optional version item is carried with its ambiguity intact. The adapter does not guess which quantity was meant.

The station's own judgements, including range checks, position validation and black list results, are carried and never re-decided.

Egress emits one data block. A Track becomes one block of several records, which is the shape this format has for a history and the shape a raw 1090ES stream does not have.