Guide
ADS-B emitter categories explained
The emitter category is a short code in every ADS-B identification message that says what kind of thing is transmitting. "A" codes are powered aircraft grouped by size and performance, "B" codes are unusual aircraft such as gliders, balloons and drones, and "C" codes are ground vehicles and obstacles.
Why categories exist
Wake turbulence, the invisible swirl left behind by a wing, depends on an aircraft's weight. Controllers and collision-avoidance systems need to know whether an aircraft is a two-seat trainer or a 400-ton widebody. The emitter category, set when an ADS-B system is installed, carries that information in the broadcast. The categories are defined in the ADS-B technical standards (RTCA DO-260B for 1090ES and DO-282B for UAT).
Set A: powered aircraft
| Code | Meaning | Examples |
|---|---|---|
| A0 | No category information | Often older or misconfigured installs |
| A1 | Light (under 15,500 lb) | Cessna 172, Cirrus SR22, Pilatus PC-12 |
| A2 | Small (15,500 to 75,000 lb) | Many business jets, CRJ200 |
| A3 | Large (75,000 to 300,000 lb) | 737-800, A320, E175 |
| A4 | High-vortex large | The Boeing 757, whose wake is unusually strong for its weight |
| A5 | Heavy (over 300,000 lb) | 777-300ER, 787-9, C-17 |
| A6 | High performance (over 5 g and 400 kt) | Fighters and jet trainers such as the T-38 |
| A7 | Rotorcraft | Robinson R44, H135, UH-60 |
Set B: unusual aircraft
| Code | Meaning |
|---|---|
| B0 | No category information |
| B1 | Glider or sailplane |
| B2 | Lighter-than-air (balloons and airships) |
| B3 | Parachutist or skydiver |
| B4 | Ultralight, hang glider or paraglider |
| B5 | Reserved |
| B6 | Unmanned aerial vehicle |
| B7 | Space or trans-atmospheric vehicle |
Set C: surface vehicles and obstacles
| Code | Meaning |
|---|---|
| C0 | No category information |
| C1 | Surface vehicle, emergency (for example airport fire trucks) |
| C2 | Surface vehicle, service (tugs, follow-me cars) |
| C3 | Point obstacle, including tethered balloons |
| C4 | Cluster obstacle |
| C5 | Line obstacle |
| C6–C7 | Reserved |
A fourth set, D, is reserved for future use.
How heywhatsflying uses categories
- Helicopters now starts with every A7 target and adds aircraft whose type is a known helicopter, because some helicopters are installed with the wrong category.
- Balloons and gliders (B2 and B1) feed the "interesting right now" panel on the dashboard.
- A6 targets are a hint for military and government now.
- C-category ground vehicles are excluded from airborne counts.
Categories and wake turbulence
Controllers do not use the broadcast category directly to space aircraft. They use the wake turbulence category from the flight plan, which in the US has been refined into recategorization ("RECAT") groups based on weight and wingspan. The emitter category is a coarser version of the same idea that any receiver can see. A rough rule: if a target shows A5, expect a widebody or a large military transport; if it shows A3, a narrowbody or regional jet; if A1 or A2, a light aircraft, turboprop or business jet. Pilots of heavy aircraft also add the word "heavy" to their radio callsign, so you will hear "United 901 heavy" for a 777 or 787.
Caveats
Categories are configured by the installer and are not always right. Some light aircraft broadcast A0, some business jets broadcast A1, and a few helicopters show up as A1. We treat the category as one signal among several, together with type, altitude and speed. For the other fields in each message, read how to read flight data.
Questions
What does emitter category A7 mean?
A7 means rotorcraft, which in practice is almost always a helicopter.
Why is the Boeing 757 its own category?
The 757 produces an unusually strong wake for its weight, so it has its own "high-vortex large" category, A4, to help controllers apply extra spacing behind it.