# What do the radiation patterns of ADS-B transmissions on large airliners tend to look like?

In the recent article Flightradar24 gears up for a new wave of flight tracking I've read about FR24's experimental Wave Glider ADS-B receiver - essentially a self-propelled, solar power buoy as a remote ground station.

What do the radiation patterns of ADS-B transmissions on large airliners tend to look like? In particular, the transmissions that would be received by satellite or surface ADS-B receivers collecting traffic information.

Are there any examples of these radiation patterns published? Are there guidelines, best practices or regulations that apply to the directionality of the radiation?

An example of a general requirement might be EASA's CS-ACNS:

CS-ACNS.D.ELS.060 Antenna installation

(See AMC1 ACNS.ELS.)

(a) The installed antenna(s) has (have) a resulting radiation pattern which is (are) vertically polarised, omnidirectional in the horizontal plane, and has (have) sufficient vertical beam width to ensure proper system operation during normal aircraft manoeuvres.

(b) Antenna(s) is/are located such that the effect on the far field radiation pattern(s) by the aircraft structure are minimised.

which only addresses directions above/below horizontal to the extent that they apply to "normal aircraft maneuvers." There may be more.

Below I've added a similar but different gain pattern as an example of the kind of data I am looking for. However it is the gain pattern for a downwards-looking ADS-B receiving aboard a satellite, showing good reception from below (nadir: $$\theta,\phi \approx 0$$). What I am looking for is a similar type plot, but for the radiation from an aircraft.

above: Example from this paper of an receiver's gain pattern. In this case it is an ADS-B receiving antenna system, aboard a satellite, showing gain optimization looking straight down (nadir).

• Are you under the assumption that multiple antennas can't be installed? One on the top of the aircraft for satellite and one on the bottom for ground stations? It may be a purely academic topic though, I know the FAA has been planning a roll-out of ADS-B satellite receivers in the Iridium or GlobalStar constellation, but I don't believe they've launched a single capable satellite as of today. – Ron Beyer Sep 25 '16 at 4:44
• @RonBeyer Good point! It's a big topic - more than can be covered by a single question. I'm really interested in the overall shape of the pattern and expressed my 'guess' as a reference point only. Do large aircraft often have a "cone of silence" device, and film. Pardon the humor - any helpful information to get me started would be appreciated! – uhoh Sep 25 '16 at 5:16
• Transport-category aircraft (i.e. airliners) are required by ICAO to have "diverse" 1090ES systems with one antenna pointing up and one pointing down. – pericynthion Sep 25 '16 at 7:08
• To start with you may have a look at this draft document from EASA. It specifies draft requirements for "ADS-B out", including antenna requirements (there are vertically polarized and omnidirectional). See page 28: "ADS-B TRANSMIT UNIT". Be prepared to look into other documents referenced here for each aspect. EASA is targeting a mode called 1090ES, that is frequency is 1090 MHz and data rate is "extended" (extended squitter) to support more data to be transmitted (compared to classic mode-S transponders). – mins Sep 25 '16 at 10:10
• Also this 1090ES presentation. Someone will come with more information, but this is a good start. – mins Sep 25 '16 at 10:17

In airliners, like in most aircraft, the ADS-B transmission come from the ATC radar transponder. The transponders in aircraft with an Maximum Takeoff Mass exceeding 5700kg /12500 lbs are required to have antenna diversity. This means that the aircraft will have a top and a bottom antenna connected to the transponder.

ADS-B transmissions are sent out of either the top or bottom antenna, the selection of which should be random with a 50-50 distribution.

Radar and TCAS replies are sent out the antenna on which the incoming interrogation was strongest.

The antennas are vertically polarised and have an approximately omnidirectional radiation pattern in the horizontal plane. In the vertical plane, most radiation energy will be focused 45 degrees around the horizon. They have a cone of silence.

The aircraft structure blocks and reflects part of the radiated energy. This is causing the far field pattern to be not uniform and varies from aircraft to aircraft. On the lower antenna the main effect is of the fuselage and the engines. On the top antenna the fuselage, the tail and the wings/winglets are the main obstacles.

On small aircraft this is more of a problem, especially when the transponder antenna is between large fixed gear struts.

This study contains an antenna pattern of the top antenna of an aircraft

• Thanks! This is a very helpful description of the general pattern, and about the handling of the top and bottom antennas. I see you reviewed my comments carefully - I have just now found out that "cone of silence" is an actual aviation term for this. I'd thought it was a remarkable coincidence that it suited the situation so well – uhoh Sep 25 '16 at 14:25
• You're welcome. This is very closely related to work I did. I worked with the people who wrote the documents @mins linked to. Establishing the exact radiation pattern of aircraft is not easy. I have tried to do this from received signal power in terrestrial ADS-B receivers but it is not straightforward to distinguish effects of signal multipath, receiver antenna pattern and aircraft antenna diversity. – DeltaLima Sep 25 '16 at 14:32
• If received signal strength were recorded and logged as standard, and could be accumulated, over time one could build a sort-of "reception cloud" of data around an aircraft. It it had a dead spot, over an extended period of time and variety of routes the dead spot would start to show up if one data-mined extensively. It's not a proper measurement of course, but it could be interesting. I don't know if received signal strength is really accessible for such a large volume of data required. – uhoh Sep 25 '16 at 14:40