ADS-B 1090ES: one frame in, one frame out

A 1090ES message is a 112-bit frame. A small number of type codes carry identification, surface position, airborne position, velocity and operational status, and each frame stands alone on the air.

The format states no time at all. The one time-like bit says only whether a position is synchronised to a fixed epoch, which is a flag about time rather than a time.

It also states no unambiguous position in a single frame. Latitude and longitude arrive as compact position reporting values, which describe a position within a zone rather than a position.

Raw extended squitter is what a locally deployed receiver produces, which matters in environments where a national surveillance feed is not available or not trusted.

The distinction between what has identified itself and what has not is the foundation of counter-uncrewed-aircraft work, and it starts with reading the cooperative broadcast directly.

Working from raw frames also makes it possible to compare a local receiver against a processed feed, which is a way of noticing that one of them is wrong.

Ingest takes one frame in the standard hexadecimal text form and produces one Entity and one Event.

Because the format carries no time, the observation instant is the receipt instant on every frame, and the object records that this is what it is rather than implying the aircraft stated a time.

The receiver timestamp in the extended text form is a free-running counter since the receiver started rather than a wall clock. It is parked and never read as a time, because an adapter that treated it as one would date every frame to 1970.

Global position decoding, which pairs an even frame with an odd frame received within a few seconds, is out of scope, and the reason is structural rather than effort. Two frames are two transmissions that must be joined across time, and an adapter is a pure function of one payload, so a global decoder would either emit a half-populated object or hold a cache. A cache inside a translator is fusion performed where nothing audits it. A payload holding two frames is refused outright, so the pair cannot be smuggled in through the framing.

Local decoding, which needs one frame plus a reference position already known to be nearby, is the supported path.

Egress emits extended squitter frames. An Entity emits the type code it arrived as, and a Track emits one position frame per sample in the track's own order, which is the only shape this format has for a history.