Where every number on the workbench comes from

A technical reference for investigators, pilots and engineers using the RealTraffic Flight Analysis Workbench. Every parameter is traced to the radio message that carried it and the aircraft system that produced it.

The workbench draws on two data sources, selectable per target:

RT raw ADS-B (RAW) — a per-message decode of the Beast data recorded by our own receiver network. One row per decoded 1090 MHz message, at its exact millisecond arrival time, carrying only the fields that message actually contained. This is the preferred basis for timing-critical work (CPA, rates, event sequencing). Coverage is limited to where our feeders hear the aircraft.
Aggregated ADS-B (AGG) — our merged multi-provider feed (our feeder network plus several commercial/community ADS-B sources), snapshotted every 2 seconds. Broadest coverage, but each record is a merged state vector: the winning source can switch between snapshots and individual field timestamps can differ from the position timestamp by a second or two. Use it where raw coverage is absent; the workbench flags the provenance of every target.

Everything displayed is ADS-B / Mode S derived unless explicitly noted. The only exceptions are marked throughout: the GFS-model QNH used for the corrected-altitude trace, the navdb-derived geometry (runways, ILS), and quantities the workbench computes from the transmitted samples (acceleration, flight-path angle, separations — all flagged workbench-computed).

Badges used below: transmitted value measured/produced on the aircraft and broadcast in a message · decode-derived value computed by the readsb decoder from transmitted fields · workbench-computed computed client-side from the series · model from the GFS weather model, not from the aircraft.

The 1090 MHz decode chain

AIRCRAFT SYSTEMS                         TRANSPONDER                    RF            GROUND
────────────────                         ───────────                    ──            ──────
GNSS receiver ──── position, GS vector,
                   geometric alt, HAE ─┐
Air data (ADC/ADIRS) ─ pressure alt,   │  loads GICB registers          1090 MHz      our feeders (RTL-SDR /
                   IAS/TAS/Mach, SAT,  ├─ 0,5 0,6 0,8 0,9 6,1 6,2 6,5 ─ extended ───▶ Beast receivers, NTP-
                   baro rate ──────────┤  → DF17 extended squitter      squitter      synced ingest)
Inertial ref (IRS/AHRS) ─ roll, track  │  (self-broadcast, ~6/s)            +              │
                   rate, mag heading,  │                                replies to         ▼
                   inertial VS ────────┤  answers SSR / TCAS            SSR + TCAS    readsb decode
FMS / FCU-MCP ──── selected alt & hdg, │  interrogations:               interrog.     (CPR position pairing,
                   baro setting, modes ├─ DF0/4/5/11/16 short replies,                Gilham/Q-bit altitude,
TCAS computer ──── RA (register 3,0) ──┤  DF20/21 Comm-B w/ BDS                       Comm-B register
Transponder panel ─ squawk, IDENT ─────┘  4,0/5,0/6,0/4,4 content                     inference, dedupe)
                                                                                           │
                                                                                           ▼
                                                                          RAW per-message series · AGG merge

DF17 = extended squitter from a Mode S transponder; DF18 carries the same formats from non-transponder emitters (and ADS-R/TIS-B rebroadcasts). ADS-B messages are self-broadcast at fixed rates; Mode S replies (DF0/4/5/11/16/20/21) only exist where a ground radar or another aircraft's TCAS interrogates, so their cadence varies by region.

Timing, deduplication, signal fields

Timestamps are receiver arrival times stamped by our NTP-synchronised ingest — not an onboard clock. For 1090ES this is within a few ms of transmission time. Raw rows carry the exact per-message time; aggregated rows carry the merged snapshot's position timestamp, which can be jumbled by ~1–2 s when the winning source switches — the reason RAW is preferred for CPA work.
Deduplication: the same transmission is heard by many feeders; identical payloads arriving within 1.5 s are collapsed into one sample. The earliest copy is kept, nrx counts the copies (receivers that heard it), and the strongest RSSI of the set is retained. Receiver identities are stripped. The copies are not independent measurements.
RSSI is the received signal strength at the feeder's front end, in dB relative to full scale (dBFS). It is uncalibrated and receiver-dependent — useful for relative trends and range/ antenna diagnostics, not as an absolute power measurement.
Gaps longer than 10 s are marked in every series; the workbench never interpolates across them. Values between samples are shown as held (stepped), and sample dots can be displayed so the true report cadence is always visible.

Airborne position — DF17/18, TC 9–18 & 20–22

The airborne position message (register 0,5) carries the TYPE code, surveillance status, the 12-bit altitude subfield, and CPR-encoded latitude/longitude (17 bits each, alternating even/odd frames that the decoder pairs to obtain an unambiguous position). Broadcast twice per second while airborne (DO-260B Table 2-79).

TYPE codes 9–18 carry barometric pressure altitude; TC 20–22 carry GNSS height above ellipsoid (HAE) instead, used only when valid pressure altitude is unavailable (DO-260B Table 2-14, §2.2.3.2.3.4.1/.4.2). Within each group the TYPE code encodes the containment radius R_C, i.e. the NIC.
Altitude encoding: Q-bit set → 25 ft LSB, range −1000…50175 ft; Q-bit clear → 100 ft Gilham code (DO-260B §2.2.3.2.3.4.3, Fig 2-4). The value is pressure altitude referenced to 1013.25 hPa — the standard is explicit that this “specifically does not refer to barometric corrected altitude” (DO-260B §2.2.3.2.3.4.1). Source: air data computer (ADC/ADIRS) static pressure, via the transponder's altitude source.
Surveillance status (2 bits): permanent alert (emergency), temporary alert (squawk change), or SPI (DO-260B Table 2-15) — set by the transponder from the Mode A code state.
Position source: the GNSS receiver (typically via ARINC 743A); integrity radius from its HPL/HIL output (DO-260B §2.2.3.2.3.1.1).

Surface position — TC 5–8

On the ground the transponder switches to the surface format (register 0,6): no altitude; a 7-bit Movement field (ground speed, non-linear quantization: 0.125 kt steps near zero, 0.25–0.5 kt steps through the taxi range, 1–5 kt steps above 15 kt DO-260B Table 2-16) and a 7-bit Heading/Ground Track field (LSB 360/128 = 2.8125°, with a status bit; true-vs-magnetic reference given by the HRD flag in the operational status message) (DO-260B Tables 2-17/2-18, Table 2-75/2-76). Rate: 2/s while moving, one per 5 s stationary (Table 2-79). Surface CPR has a 90° ambiguity, so the raw replay is seeded with a reference position — without it the taxi/takeoff roll would have no raw positions.

Identification — TC 1–4

Callsign (8 characters, 6-bit subset of IA-5) and emitter category (wake/size class). The flight ID is entered in the FMS/transponder panel; if absent the registration is broadcast (DO-260B §2.2.3.2.5.1). Rate: one per 5 s airborne (Table 2-79).

Airborne velocity — TC 19, subtypes 1–4

Register 0,9; broadcast twice per second (Table 2-79). The subtype determines what the message carries (DO-260B Table 2-20, Figs 2-7/2-8):

Subtype 1 (2 = supersonic): velocity over ground as East/West and North/South components, 1 kt LSB (4 kt supersonic), referenced to WGS-84 (Tables 2-23…2-26, 2-32/2-33). Source: the GNSS receiver's velocity solution. Ground speed and track shown by the workbench are composed from these two components by the decoder — the message itself carries the vector, not the angle.
Subtype 3 (4 = supersonic): used when velocity-over-ground is unavailable — carries heading (10 bits, LSB 360/1024 ≈ 0.35°, true-vs-magnetic per the HRD flag) and airspeed with a type bit selecting IAS or TAS, 1 kt LSB (Tables 2-34…2-39). Airliners with a valid GNSS solution send subtype 1, so IAS/TAS on the workbench usually come from Comm-B EHS instead (below).
Vertical rate (all subtypes): 9 bits, 64 ft/min LSB, max 32,608 ft/min, with a sign bit and a source bit: 0 = “Vertical Rate information from Geometric Source (GNSS or INS)”, 1 = “from Barometric Source”; the standard notes a blended baro/inertial rate should be marked barometric (DO-260B Tables 2-27/2-28/2-29). The raw series keeps these apart as vs_baro / vs_gnss.
Geometric − baro difference: 7 bits, 25 ft LSB + sign — the offset between the GNSS altitude (HAE preferred) and the pressure altitude (Tables 2-30/2-31, §2.2.3.2.6.1.15). This is how a decoder tracks geometric altitude while the position message carries baro (gbd in the raw series).
NAC_V: 3 bits, velocity accuracy bound (<10 / <3 / <1 / <0.3 m/s) (Table 2-22).

Target state & status — TC 29 subtype 1

Register 6,2; broadcast every 1.2–1.3 s when airborne and either selected altitude or selected heading is valid (DO-260B §2.2.3.3.1.4.1, Table 2-79). This is the “what is dialled into the autoflight system” message (Fig 2-10):

Selected altitude: 11 bits, 32 ft LSB, 0–65,472 ft, with a one-bit Selected Altitude Type: 0 = value comes from the MCP/FCU (the altitude window on the glareshield), 1 = from the FMS (Tables 2-42/2-43). The standard cautions that this does not necessarily reflect the true target level in certain VNAV/approach modes, and that many installations never receive FMS data and only ever transmit the MCP/FCU value (§2.2.3.2.7.1.3.2 Note). The workbench amcp / afms traces follow this type bit.
Barometric pressure setting: 9 bits, 0.8 hPa LSB, offset +800 hPa (range 800–1208 hPa) — the value set on the baro knob; the standard notes it can represent QFE or QNH/QNE per local procedure (Table 2-44). This is the crew's setting (nqnh), independent of the GFS model QNH the workbench uses for altitude correction.
Selected heading: 8 bits + sign, LSB 0.703125°. There is no reference bit — the standard encourages magnetic, which is the de facto convention, but the message cannot indicate its reference (Tables 2-45…2-47 and Note 2). Source: MCP/FCU heading bug.
Mode bits (with a status bit indicating they are deliberately populated): autopilot engaged, VNAV, altitude hold, approach mode, LNAV, and TCAS operational (set only when TCAS can issue an RA, RI = 3/4) (Tables 2-48…2-54). These come from the AFCS/FMGC via the transponder interface and drive the workbench's nmod readout.
Also repeats NAC_P, NIC_baro, SIL between operational-status messages.

Aircraft operational status — TC 31

Register 6,5; every 2.4–2.6 s (0.7–0.9 s bursts for 24 s after a change) (Table 2-79, §2.2.3.3.1.4.2). Carries the quality and capability metadata the workbench shows in the integrity panel and opstat column (Fig 2-11):

NAC_P — 95% horizontal accuracy bound (EPU/HFOM from the GNSS receiver): e.g. 8 → <92.6 m, 9 → <30 m, 10 → <10 m, 11 → <3 m (Table 2-70).
NIC — integrity containment radius R_C, encoded jointly by the position-message TYPE code and the NIC supplements A (here), B (position message) and C (surface) (Table 2-69). The workbench shows both NIC and the decoded R_C (rc, km).
SIL — probability that the true position lies outside R_C without alert: 1 → ≤10⁻³, 2 → ≤10⁻⁵, 3 → ≤10⁻⁷ per hour or per sample (SIL supplement selects which); GNSS HPL supports SIL 3 (Tables 2-72, 2-41).
SDA — system design assurance of the whole position transmission chain (0 unknown … 3 hazardous-level assurance) (Table 2-65).
GVA — geometric vertical accuracy from the GNSS VFOM: 1 → ≤150 m, 2 → ≤45 m (Table 2-71).
NIC_baro — 1 when the pressure altitude is either cross-checked against a second source (Gilham installations) or comes from a non-Gilham (Synchro/DADS) source; 0 for un-cross-checked Gilham encoders (Table 2-73).
Capability/mode bits — TCAS operational, 1090ES/UAT IN, single antenna, IDENT switch active (18±1 s after the crew presses IDENT), TCAS RA active, ADS-B version (2 = DO-260B), aircraft length/width class (surface), and the HRD flag declaring whether headings are referenced to true or magnetic north (applies to the surface heading and velocity subtypes 3/4) (Tables 2-56, 2-63/2-64, 2-66…2-68, 2-74…2-76).

Aircraft status — TC 28: emergency & ACAS RA

Subtype 1 — Emergency / priority status + Mode A

Event-driven; broadcast every 0.7–0.9 s while an emergency is active (§2.2.3.3.1.4.3.1). Carries a 3-bit status — no emergency · general emergency · lifeguard/medical · minimum fuel · no communications · unlawful interference · downed aircraft — plus the 13-bit Mode A code (Fig 2-14, Table 2-78). Entering 7500/7600/7700 on the transponder panel forces the status to unlawful-interference / no-communications / general-emergency respectively; the same squawk also arrives via DF5/21 replies. This feeds the workbench's emerg column and the orange EMERG markers.

Subtype 2 — 1090ES TCAS RA broadcast (and BDS 3,0 / DF16 MV)

When TCAS issues a resolution advisory, the transponder broadcasts the RA within 0.5 s, repeats it every 0.7–0.9 s, and keeps transmitting until 24±1 s after the RA terminates (§2.2.3.2.7.8.2, §2.2.3.3.1.4.3.2). The message content mirrors transponder register 30₁₆ (the ACAS active-RA register, ICAO Annex 10 Vol IV): the same 56-bit RA report also appears as BDS 3,0 in DF20/21 Comm-B replies and in the MV field of DF16 air-air replies. Fields (Fig 2-15):

ARA (14 bits) — the active resolution advisory itself (climb/descend, corrective/ preventive, sense reversal, crossing, increase-rate…). The workbench decodes this into the advisory text (ra) and shows the raw bits (ra_ara).
RAC (4 bits) — resolution advisory complements received from other TCAS units (“do not pass below/above, do not turn left/right”) (ra_rac).
RAT — RA terminated flag (“clear of conflict”) (ra_rat).
MTE — multiple-threat encounter (ra_mte). When 0 the RA is against a single intruder and ARA encodes one advisory (climb, descend, level off…). When 1 TCAS is resolving two or more threats at once and the ARA bits change meaning: they become a set of flags — correct upwards / climb required / correct downwards / descent required / crossing / increase-or-maintain vs reduce-or-limit vertical rate — which is why such RAs decode to compound text like “Correct upwards; Descent required; Crossing; Reduce / Limit vertical rate” and the livemap chip just says MTE. Some implementations also set MTE on the clear-of-conflict message of an ordinary encounter (RAT = 1), so MTE with RAT = 0 is the real multiple-threat case.
TTI (2 bits) — threat type; when 1 the following 24 bits carry the threat's ICAO address (ra_tti, ra_threat) — this lets an investigator identify the conflicting aircraft directly from the RA message.

Source system: the TCAS/ACAS computer loads register 30₁₆; the transponder broadcasts it. The workbench decodes the 56-bit payload itself (identical layout in all three carriers) and draws red RA markers on the time panels.

What the RA messages mean

The advisory text is decoded from the ARA/RAC/RAT/MTE bits (same rules as readsb). The chip column is the abbreviation the livemap shows on the aircraft during the RA; the workbench shows the full text. Corrective RAs demand a manoeuvre; preventive RAs restrict one.

Decoded text	Chip	Meaning for the pilot
Climb / Descend	CLIMB / DESCEND	Corrective, positive: establish a vertical rate of at least 1,500 fpm in the given sense (initially ≈ 0.25 g, within 5 s).
Increase Climb / Increase Descend	INCR CLIMB / INCR DESCENT	Strengthening of an existing Climb/Descend RA: increase the vertical rate to 2,500 fpm because the first RA is not achieving enough separation.
Climb; Climb NOW / Descend; Descend NOW	CLIMB NOW / DESCEND NOW	Sense reversal: the previous RA was in the opposite direction; reverse immediately (the intruder manoeuvred, or a coordinated sense changed).
Climb; Crossing Climb / Descend; Crossing Descend	X-CLIMB / X-DESCEND	The commanded manoeuvre will pass through the intruder's altitude — the aircraft are currently on the “wrong” sides of each other. Uncomfortable but intentional; do not reverse it.
Level Off	LEVEL OFF	Corrective, non-positive: reduce the vertical rate to zero (TCAS 7.1 replaced the old “Adjust vertical speed, adjust” with this).
Maintain vertical speed (; Crossing Maintain)	MAINTAIN VS	Preventive, positive: keep the current climb/descent rate, do not level off; “Crossing” = the maintained manoeuvre crosses the intruder's altitude.
Monitor vertical speed	MONITOR VS	Preventive, non-positive (vertical speed limit): do not change the vertical rate into the restricted band; nothing to do if already complying.
Clear of Conflict	CoC	RAT = 1: the threat is resolved; return promptly to the ATC clearance. Transmitted for ≈ 24 s after the RA ends, which is why every episode finishes on it.
Correct upwards; Climb required; Correct downwards; Descent required; Crossing; Increase/Maintain or Reduce/Limit vertical rate	MTE	MTE = 1, multiple-threat encounter: the ARA bits are individual flags describing the composite manoeuvre against several intruders (see MTE above). Read them together; there is no single-word advisory.
Do not pass below / above, Do not turn left / right	—	RAC complements: restrictions received from the other TCAS unit so that the two coordinated manoeuvres do not conflict. Shown after the advisory (`ra_rac`), never as a chip.

Decoding follows ICAO Annex 10 Vol IV / DO-185B ARA bit semantics as implemented in readsb's sprintACASJson; the workbench reproduces the same logic client-side from the raw 56-bit field. A “threat” address (TTI = 1) identifies the intruder; the livemap links two RA episodes into a pair when each names the other.

Do RAs need an ADS-B intruder?

No. TCAS does not use ADS-B to decide an RA: it actively interrogates nearby transponders (Mode C and Mode S) and tracks intruders by range (reply time), bearing (directional antenna) and altitude (the Mode C/S altitude reply). ADS-B enters only as “hybrid surveillance” (a TCAS 7.1 option) to reduce the interrogation rate against non-threatening targets — a threat is always confirmed by active interrogation before any TA/RA. So the intruder can be:

Mode S without ADS-B — full RA capability; if both aircraft carry TCAS II they coordinate the sense (one climbs, one descends) over the Mode S air-air link (DF0/DF16). That coordination and the intruder's Mode S address are what fill TTI = 1 / ra_threat — a linked pair can therefore have a Mode S-only aircraft on the other side.
Mode C only — still triggers an RA on the TCAS-equipped aircraft, but no coordination and no threat address (TTI = 2 with the intruder's range/bearing/altitude, or TTI = 0).
Mode A only / no transponder — no TA or RA at all; TCAS is blind to it.

Consequences for what the livemap and this workbench can show:

An RA episode with a threat address but no matching aircraft on the map is expected: TCAS heard the intruder via Mode S, but it may not transmit ADS-B positions (or not to our feeders). The pair line and the two-target airprox analysis need positions for both aircraft; the threat's identity is still recorded. A Mode S-only intruder can sometimes be placed by MLAT when four or more feeders hear it.
The RA broadcast itself (TC28 subtype 2) is sent by the ADS-B-equipped aircraft. A Mode S-only aircraft's RA is only visible when a ground interrogator asks for BDS 3,0 (DF20/21) or via DF16 coordination replies, so RA counts are biased towards the ADS-B side of each encounter.

Mode S surveillance replies — DF 0/4/5/11/16/20/21

Unlike ADS-B squitters, these are replies — they exist only where a secondary radar or another aircraft's TCAS interrogates. Their cadence therefore varies from several per second (dense radar coverage) to none (oceanic). They give the raw series its high-rate altitude and squawk between position messages.

DF	Reply	Elicited by	Content used by the workbench
DF0	Short air-air surveillance	TCAS of other aircraft	13-bit altitude (25 ft or 100 ft Gilham)
DF4	Surveillance, altitude	SSR ground radar	Altitude + flight status (alert / SPI / airborne–ground)
DF5	Surveillance, identity	SSR ground radar	Mode A squawk (incl. 7500/7600/7700) + flight status
DF11	All-call reply	Acquisition interrogations	ICAO address (presence/RSSI only)
DF16	Long air-air	TCAS	Altitude + MV field (can carry the ACAS RA report)
DF20	Comm-B, altitude	Ground GICB interrogation	Altitude + 56-bit MB field (a BDS register, below)
DF21	Comm-B, identity	Ground GICB interrogation	Squawk + 56-bit MB field

Comm-B register inference. A DF20/21 reply does not say which BDS register its 56-bit MB field contains — the interrogator asked for a specific register and only it knows which. The decoder infers the register from content plausibility. This is usually right but occasionally wrong; we have observed a bogus 438 kt “BDS 5,0 ground speed” on a departing A320. All Comm-B speed/track decodes are therefore kept in separate *_cb columns (gs_cb, trk_cb) and never mixed into the primary ADS-B velocity traces. The BDS 6,0 vertical rates are also register-inferred and are kept source-explicit (vs_baro60, vs_inert60).

Enhanced surveillance Comm-B registers (ICAO Doc 9871 / Annex 10 Vol IV)

Where the local ANSP interrogates for EHS (much of Europe, Australia and elsewhere), DF20/21 replies carry these registers, typically once per radar scan (~4–12 s) per interrogator. Field layouts and LSBs below are from ICAO Doc 9871; they are not part of DO-260B.

BDS 4,0 — Selected vertical intention

Field	Resolution / range	Aircraft source
MCP/FCU selected altitude	16 ft LSB, 0–65,520 ft	MCP/FCU altitude window
FMS selected altitude	16 ft LSB	FMS (where interfaced)
Barometric pressure setting	0.1 hPa LSB, offset +800 hPa	Baro-set knob (finer than the 0.8 hPa TC29 field)
Mode bits	VNAV · altitude hold · approach (+ status bit)	AFCS/FMGC
Target altitude source	unknown / aircraft altitude / FCU-MCP / FMS	Autoflight logic (`tasrc`)

BDS 5,0 — Track and turn report

Field	Resolution / range	Aircraft source
Roll angle (`roll`; left = negative on the workbench)	45/256 ≈ 0.176° LSB, ±90°	Inertial reference (IRS/AHRS)
True track angle (`trk_cb`)	90/512 ≈ 0.176° LSB	IRS/GNSS blend
Ground speed (`gs_cb`)	2 kt LSB, max 2046 kt	IRS/GNSS blend
Track angle rate (`trr`)	8/256 = 0.03125°/s LSB	Inertial reference
True airspeed (`tas`)	2 kt LSB, max 2046 kt	Air data computer

BDS 6,0 — Heading and speed report

Field	Resolution / range	Aircraft source
Magnetic heading (`mhdg`)	90/512 ≈ 0.176° LSB	IRS true heading + magnetic variation model (or AHRS/flux valve)
Indicated airspeed (`ias`)	1 kt LSB, max 1023 kt	Air data computer (pitot-static)
Mach (`mach`)	0.004 LSB	Air data computer
Barometric altitude rate (`vs_baro60`)	32 ft/min LSB, ±16,384	ADC — differentiated pressure altitude; inherently noisy
Inertial vertical velocity (`vs_inert60`)	32 ft/min LSB	IRS / baro-inertial blend — smooth, small lag

Four vertical-speed sources, kept apart. An aircraft has physically different vertical rates on the wire: the ADS-B velocity message carries one rate with a source bit (vs_baro or vs_gnss — and the “geometric” source is defined as GNSS or INS), while BDS 6,0 carries a barometric rate and an inertial vertical velocity (vs_baro60, vs_inert60). Which physical source feeds the velocity message varies by installation — some aircraft send the GNSS rate there and the baro rate only via BDS 6,0, others the reverse. In the aggregated feed the upstream decoder blends these: the velocity message's GNSS rate and the BDS 6,0 inertial VS both land in geom_rate, and baro sources land in baro_rate — so AGG vs/gr are source-blended, while the RAW columns are source-explicit. Both blended rates are still ADS-B/Mode S data. For derived quantities the workbench uses velocity-message baro first, else BDS 6,0 baro (and GNSS first, else inertial).

BDS 4,4 — Meteorological routine air report measured

The only measured/transmitted meteorological data in the system. Registers: wind speed (1 kt LSB) and true wind direction (180/256 ≈ 0.70° LSB) — the aircraft's own computed wind (FMS/IRS: ground vector minus air vector); static air temperature (0.25 °C LSB) from the ADC-corrected TAT probe; average static pressure (1 hPa LSB); turbulence (2-bit category); and humidity (≈1.6% LSB, rarely fitted). A source/FOM nibble (metsrc) qualifies the data. (ICAO Doc 9871.)

Availability depends entirely on regional ground interrogation: BDS 4,4 exists in the raw data only where the local ANSP's radars request it. Sydney's interrogators, for example, do not — Australian samples contain none — while parts of Europe do. Everything else labelled wind or temperature on the workbench is derived, not transmitted (see below).

Parameter glossary

Series keys shown as raw / agg where they differ. LSB = resolution of the transmitted field; displayed precision never exceeds it.

Parameter	Carried in	Aircraft source system	Units / LSB	Notes & caveats
Position & altitude
lat, lon transmitted	Airborne position TC9–18/20–22; surface TC5–8 (CPR-encoded)	GNSS receiver	deg; CPR grid ≈5 m airborne, ≈1.3 m surface	Even/odd frame pairing by the decoder; surface decode needs a reference position. Accuracy bound given by NAC_P, integrity by NIC/R_C.
alt transmitted	Position msg altitude subfield; also DF0/4/16/20 replies	Air data computer (static pressure)	ft; 25 ft (Q=1) or 100 ft Gilham	Pressure altitude, standard 1013.25 hPa reference — never baro-corrected on the wire (DO-260B §2.2.3.2.3.4.1). NIC_baro flags cross-checking.
balt model	— (computed)	—	ft	QNH-corrected altitude: `alt + (QNH − 1013.25) × 27 ft` with QNH from GFS model mean-sea-level pressure sampled ~every 30 km along track. The correction is model-derived, not aircraft data; the uncorrected 1013 trace is always available.
qnh model	— (GFS)	—	hPa	GFS MSLP used for `balt`. Distinct from `nqnh`, the crew's transmitted baro setting.
galt_m transmitted	Position msg TC20–22	GNSS receiver	ft (HAE); 25 ft	Measured GNSS height above ellipsoid — only broadcast when pressure altitude is invalid (rare on transport aircraft).
gbd transmitted	Velocity msg “difference from baro alt”	GNSS vs ADC	ft; 25 ft, ±3138 max	Geometric minus barometric altitude. HAE-referenced preferred; MSL permitted for TC11–18 (DO-260B §2.2.3.2.6.1.15).
galt / agg galt decode-derived	— (readsb)	—	ft	Decoder's geometric-altitude estimate: baro altitude + learned geometric−baro offset (from `gbd` / TC20–22). Still entirely ADS-B-derived; marked derived because the combination is done in the decoder.
agl, aglG workbench	— (computed)	—	ft	Height above runway threshold: `balt` (or geometric) minus navdb threshold elevation. Depends on the GFS QNH correction and AIRAC threshold data.
Speeds
gs transmitted	Velocity TC19 st.1/2 (E/W + N/S components); surface Movement field	GNSS receiver (velocity over ground, WGS-84)	kt; components 1 kt (4 kt supersonic); surface non-linear 0.125–5 kt steps	GS magnitude composed from the two transmitted components by the decoder. Surface (taxi) GS is coarsely quantised by design — expect visible steps.
ias transmitted	BDS 6,0; or velocity TC19 st.3/4 (type bit = IAS)	Air data computer (pitot-static)	kt; 1 kt	On jets, almost always from BDS 6,0 (EHS interrogation required) — subtype 3/4 is only sent when GNSS ground velocity is unavailable.
tas transmitted	BDS 5,0; or velocity TC19 st.3/4 (type bit = TAS)	Air data computer	kt; 2 kt (BDS 5,0) / 1 kt (TC19)	BDS 5,0 origin carries the register-inference caveat.
mach transmitted	BDS 6,0	Air data computer	—; 0.004	EHS coverage required.
gs_cb, trk_cb transmitted	BDS 5,0 (DF20/21 Comm-B)	IRS/GNSS blend	kt 2 kt; deg ≈0.176°	Register inferred by the decoder — occasionally wrong (observed bogus 438 kt GS on an A320). Kept separate from the primary ADS-B velocity traces for this reason.
Vertical rates — all five variants
vs_baro transmitted	Velocity TC19, source bit = barometric	ADC (may be baro/inertial blend per DO-260B Table 2-27 note)	ft/min; 64	Raw series only; which physical source feeds the velocity message varies by installation.
vs_gnss transmitted	Velocity TC19, source bit = geometric	GNSS or INS (DO-260B Table 2-27)	ft/min; 64	“Geometric source” is defined as GNSS or INS — not guaranteed pure GNSS.
vs_baro60 transmitted	BDS 6,0 barometric altitude rate	ADC (differentiated pressure alt)	ft/min; 32	Noisy by nature; EHS coverage required.
vs_inert60 transmitted	BDS 6,0 inertial vertical velocity	IRS / baro-inertial blend	ft/min; 32	Smooth, slight lag. Register-inference caveat applies.
agg vs (baro_rate), gr (geom_rate) transmitted blended	All of the above, blended upstream	mixed	ft/min	The aggregated feed's decoder stores velocity-message GNSS rate and BDS 6,0 inertial VS both as `geom_rate`, baro sources as `baro_rate` — source-blended but still entirely ADS-B/Mode S data. Use RAW for source-explicit rates.
Track, heading, attitude
trk transmitted	Velocity TC19 st.1/2 (from components); surface TC5–8 heading/track field	GNSS receiver	deg true; components 1 kt → sub-degree; surface 2.8125°	True track over ground, composed from the transmitted E/W–N/S vector.
mhdg transmitted	BDS 6,0; or velocity TC19 st.3/4 heading	IRS true heading + magvar model (or AHRS/flux valve)	deg; ≈0.176° (BDS 6,0) / ≈0.35° (TC19)	Magnetic by convention; TC19 st.3/4 reference is declared by the HRD flag in TC31.
agg thdg	Surface heading (HRD = true) or upstream-derived	IRS	deg true	The workbench uses agg `thdg − mhdg` as the local magnetic declination when deriving wind from raw data.
roll transmitted	BDS 5,0	Inertial reference (IRS/AHRS)	deg; ≈0.176°, ±90°	Left = negative on the workbench. Register-inference caveat.
trr transmitted	BDS 5,0 track angle rate	Inertial reference	deg/s; 0.03125	Turn-rate cross-check against roll and track derivative.
Autoflight intent (selected state)
amcp transmitted	TC29 selected altitude (type bit = MCP/FCU); BDS 4,0	MCP/FCU altitude window	ft; 32 (TC29) / 16 (BDS 4,0)	What is dialled on the glareshield — not necessarily the true target level in VNAV/approach modes (DO-260B §2.2.3.2.7.1.3.2 note).
afms transmitted	TC29 selected altitude (type bit = FMS); BDS 4,0	FMS	ft; 32 / 16	Many installations never feed FMS data to the transponder and only ever transmit MCP/FCU (verified DO-260B note) — absence of `afms` is normal.
tasrc transmitted	BDS 4,0 target altitude source	Autoflight logic	enum	unknown / aircraft altitude / FCU-MCP / FMS.
nqnh transmitted	TC29 baro pressure setting; BDS 4,0	Baro-set knob	hPa; 0.8 (TC29) / 0.1 (BDS 4,0), range 800–1208	The crew's actual setting; may be QFE or QNH/QNE per local procedure (DO-260B Table 2-44 note). Compare against the GFS `qnh` to spot mis-sets.
nhdg transmitted	TC29 selected heading	MCP/FCU heading bug	deg; 0.703125°	No true/magnetic reference bit exists in the message — magnetic by de facto convention (DO-260B §2.2.3.2.7.1.3.7 Note 2).
nmod transmitted	TC29 mode bits; BDS 4,0 mode bits	AFCS/FMGC	flags	autopilot · VNAV · alt hold · approach · LNAV · TCAS operational. A clear bit can mean “off” or “unknown/not wired” (DO-260B Tables 2-49…2-54).
Identity & status
sq transmitted	DF5/21 identity replies; TC28-1 Mode A subfield	Transponder control panel	4-digit octal	7500/7600/7700 trigger the emergency machinery (see `emerg`).
cs transmitted	Identification TC1–4	FMS / transponder panel entry	8 chars	Flight-plan callsign, or registration when no flight ID is entered.
cat transmitted	Identification TC1–4 (category sets A–D)	Installation config	class	Wake/size/emitter class (A3 = large, A5 = heavy, B1 = glider, …).
emerg transmitted	TC28-1 emergency/priority status	Pilot Mode A entry / panel selection	enum	Table 2-78 states; broadcast every 0.7–0.9 s while active. Orange EMERG markers on the panels.
gnd transmitted	Airborne-vs-surface format choice; DF4/5/20/21 flight status; DF17 CA	Transponder air/ground logic (weight-on-wheels, GS/radio-alt)	0/1	Misdeclared air/ground states occur; cross-check against GS and AGL.
opstat transmitted	TC31 summary	Transponder/installation	text	Version, capability and mode summary as printed by the decoder.
Quality & integrity metrics
nic, rc transmitted	Position msg TYPE code + NIC supplements (TC31/position/surface)	GNSS HPL/HIL	NIC 0–11; R_C km	Integrity containment radius (DO-260B Table 2-69). NIC 7 → R_C < 0.2 NM, NIC 8 → < 0.1 NM.
nacp transmitted	TC31 (repeated in TC29)	GNSS EPU/HFOM	0–11	95% horizontal accuracy bound: 8 → <92.6 m, 9 → <30 m, 10 → <10 m, 11 → <3 m (Table 2-70). Shown on the integrity panel.
nacv transmitted	Velocity TC19	GNSS velocity figure of merit	0–4	1 → <10 m/s … 4 → <0.3 m/s (Table 2-22).
sil transmitted	TC31 (repeated in TC29)	Position-source integrity certification	0–3	P(outside R_C unalerted): ≤10⁻³/10⁻⁵/10⁻⁷ per hour or sample (Tables 2-72/2-41).
sda transmitted	TC31 operational-mode subfield	Installation design assurance	0–3	Design-assurance level of the whole position chain (Table 2-65).
gva transmitted	TC31	GNSS VFOM	0–2	Geometric vertical accuracy: 1 → ≤150 m, 2 → ≤45 m (Table 2-71).
nicb transmitted	TC31 / TC29 NIC_baro	Altimetry installation	0/1	1 = pressure altitude cross-checked or from a non-Gilham source (Table 2-73).
nica transmitted	TC31 NIC supplement A	—	0/1	Combines with the TYPE code to resolve the exact R_C band.
Reception metrics (our network — not aircraft data)
rssi receiver	— (feeder front end)	—	dBFS	Uncalibrated, receiver-dependent; strongest copy kept when duplicates are collapsed. Trend and dropout diagnosis only.
nrx receiver	— (dedupe counter)	—	count	Feeders that received this message within the 1.5 s dedupe window. Coverage-quality indicator; receiver identities are stripped.
ts, sample Δt receiver	— (ingest clock)	—	UTC, s	NTP-synchronised arrival times. Δt between position reports is plotted on the integrity panel; nominal ADS-B cadence is 0.4–0.6 s per position/velocity message.
src receiver	— (decode label)	—	DFn/TCn/BDSxy	Message provenance of each raw row, e.g. `DF17/TC11`, `DF20/BDS50`.
Meteorological
ws44, wd44 measured	BDS 4,4	FMS/IRS onboard wind (ground vector − air vector)	kt 1; deg ≈0.70°	Only where the regional ANSP interrogates BDS 4,4 (Sydney does not; parts of Europe do).
sat44 measured	BDS 4,4	TAT probe → ADC static air temperature	°C; 0.25	The only transmitted temperature.
p44, turb44, hum44, metsrc measured	BDS 4,4	ADC static pressure; turbulence category; humidity sensor (rare)	hPa 1; 2-bit; ≈1.6%	`metsrc` = source/FOM nibble.
agg wd/ws · raw wd_d/ws_d derived	— (computed from TAS + heading vs GS + track)	—	deg / kt	Wind vector = ground-velocity vector − true-airspeed vector. Needs TAS and heading, so it exists only with EHS coverage; true heading for raw is mag heading + declination taken from the aggregated `thdg − mhdg`. Sensitive to heading error: 1° ≈ 8 kt crosswind error at 450 kt TAS. The AGG values are derived the same way upstream — labelled derived, never measured.
agg oat/tat · raw oat_d/tat_d derived	— (computed from Mach + TAS)	—	°C	OAT from the TAS/Mach relation (TAS = 38.97·M·√T); TAT = OAT·(1 + 0.2M²). Valid only for M ≥ 0.395 — blank below that by design.
ACAS resolution advisory
ra transmitted decoded	TC28-2 / BDS 3,0 / DF16 MV (identical 56-bit layout)	TCAS computer (register 30₁₆)	text	Advisory decoded from the ARA/RAT/MTE bits: Climb / Descend / Level Off / Monitor VS / crossing / increase / reversal (“NOW”) forms. Red RA markers on the panels.
ra_ara	same	TCAS	7 bits shown	Raw active-RA bits for expert interpretation.
ra_rac	same	TCAS coordination	flags	Complements received from other aircraft: do not pass above/below, do not turn left/right.
ra_rat	same	TCAS	0/1	RA terminated (“clear of conflict”); broadcast continues 24±1 s after it sets.
ra_mte	same	TCAS	0/1	Multiple-threat encounter.
ra_tti, ra_threat	same	TCAS threat file	enum; ICAO hex	When TTI = 1 the threat's ICAO 24-bit address is carried — direct identification of the conflicting aircraft.
Workbench-computed series (never transmitted)
acc, accPosg workbench	— (d(GS)/dt)	—	g	Central difference of GS on the actual sample times, never across a gap; optional smoothing stops at gaps. `accPosg` repeats the computation from positions as a cross-check.
fpa workbench	—	—	deg	atan2(vertical speed, GS) from the transmitted rates.
altdev workbench	—	—	ft	1013-referenced altitude minus MCP selected altitude — level-bust indicator. Both inputs transmitted.
s, y, drem, dthr workbench	— (navdb geometry)	—	m / nm	Runway-frame coordinates: along-track from threshold, lateral offset, distance remaining, distance to threshold. Depend on AIRAC runway data.
loc, gsd workbench	— (navdb ILS geometry)	—	deg / dots	Localizer / glideslope deviation computed from antenna positions and course/angle in the navdb — a geometric reconstruction, not the aircraft's own ILS receiver deviation.
ke, stopreq, stopmargin, iso-decel workbench	—	—	kJ/kg, m	Energy and stop-distance quantities from GS and entered deceleration assumptions.
sep H/V/slant, closure, tau workbench	—	—	nm / ft / kt / s	Pairwise separations from the two aircraft's transmitted positions/altitudes; closure = −d(slant)/dt; tau = slant/closure. Timing quality follows the underlying series — RAW preferred.

Derived vs transmitted — the honest list

Transmitted by the aircraft (everything else in the glossary): positions, pressure and geometric altitudes, velocity vector, IAS/TAS/Mach, all four source-explicit vertical rates, roll, track rate, magnetic heading, selected altitude/heading/baro setting, nav modes, squawk, emergency status, ACAS RA, NIC/NAC/SIL/SDA/GVA, BDS 4,4 met where interrogated.

Derived in the decode chain (readsb) — from transmitted fields, still pure ADS-B:

GS and track composed from the transmitted E/W–N/S velocity components.
galt geometric-altitude estimate = baro altitude + learned geometric−baro offset.
AGG baro_rate/geom_rate — source-blended vertical rates (velocity-message GNSS rate and BDS 6,0 inertial VS both feed geom_rate).
AGG wind (wd/ws) and temperature (oat/tat) — computed from TAS+heading vs GS+track and from Mach+TAS (M ≥ 0.395), not transmitted. The workbench derives the same for raw (wd_d/ws_d, oat_d/tat_d).
Comm-B register identification (which BDS a DF20/21 carries) — inferred, occasionally wrong; quarantined in *_cb columns.

Computed by the workbench — always labelled: acceleration (d(GS)/dt), flight-path angle, alt−selected deviation, AGL, runway-frame distances, LOC/GS deviation (navdb geometry), kinetic energy and stop margins, pairwise separation/closure/tau, event detection (liftoff, touchdown, V-speed crossings).

Model / database derived: GFS QNH and the QNH-corrected altitude balt; runway thresholds, elevations and ILS geometry from the AIRAC navdb.

References

RTCA DO-260B with Corrigendum 1 (1090ES ADS-B MOPS) — message formats, resolutions and source semantics cited above by section/table were verified directly against the standard: Table 2-14 (TYPE codes / altitude type), §2.2.3.2.3.4 & Fig 2-4 (altitude encoding), Tables 2-15…2-18 (surveillance status, surface movement/heading), Figs 2-7/2-8 and Tables 2-20…2-39 (velocity subtypes, vertical-rate source bit, airspeed type, geometric−baro difference), Fig 2-10 and Tables 2-40…2-54 (target state & status), Fig 2-11 and Tables 2-55…2-76 (operational status, NIC/NAC_P/SIL/SDA/GVA/NIC_baro/HRD), Figs 2-14/2-15 and Table 2-78 (emergency status, TCAS RA broadcast), Table 2-79 (broadcast rates).
ICAO Annex 10 Vol IV & ICAO Doc 9871 — Mode S surveillance reply formats (DF0/4/5/11/16/20/21), Comm-B register contents and resolutions (BDS 4,0 / 5,0 / 6,0 / 4,4 / 3,0), Gilham/25-ft altitude coding. Cited at document level.
ACAS RA semantics — ICAO Annex 10 Vol IV (register 30₁₆); decode follows the readsb reference implementation.
Weather model — NOAA GFS mean-sea-level pressure, sampled along track for the QNH correction. Navigation data — ARINC 424 AIRAC cycle database (runways, ILS, navaids).

RealTraffic — flyrealtraffic.com. This page documents data provenance only; it performs no data access.