Mode-S/3D

Most powered GA aircraft in the UK and Europe carry a Mode-S transponder, but a standard Mode-S reply contains no position. On its own, PilotAware can only show these aircraft as bearingless targets: height and approximate distance, but no direction. Mode-S/3D fills that gap by using multilateration (MLAT) to work out where the aircraft is.

For an overview, watch the Mode-S/3D explainer video.

When does a Mode-S transponder transmit?

A Mode-S transponder doesn't transmit all the time. It replies only when interrogated by secondary surveillance radar (SSR) on 1030 MHz, sending its answer on 1090 MHz. The reply includes the aircraft's unique ICAO (hex) code and its pressure altitude referenced to 1013.25 hPa.

If an aircraft isn't being interrogated, because it's low, out of radar range or shielded by terrain, it transmits nothing at all. Aircraft at a remote farm strip or in the lee of a hill may not appear until they climb into SSR coverage.

Why Mode-S is a bearingless target

When PilotAware receives a Mode-S reply, it captures the aircraft's ICAO code, its altitude and the received signal strength. That's enough to estimate vertical separation and, from changes in signal strength, rough distance, but not bearing.

How Mode-S/3D adds a position

Multilateration works out an aircraft's position from the tiny differences in the time its transmission arrives at several ground receivers. The MLAT data for Mode-S/3D is supplied by our partner 360Radar from its network of ground receivers.

1. Every ATOM ground station receives a continuous feed of MLAT positions for Mode-S aircraft within about 50 km, each time-stamped by the server.
2. Your PilotAware detects a Mode-S reply directly, in the normal way.
3. It cross-references the aircraft's ICAO code against the MLAT data uplinked by the ATOM station.
4. If a position is available, the aircraft is plotted on your RADAR screen or EFB. If not, it's reported as a bearingless target as before. On Rosetta, voice alerts are given for Mode-S/3D targets as for any other traffic.

Latency and ambiguity circles

Unlike FLARM rebroadcast, which arrives almost instantly, MLAT positions travel over the internet from the receivers to the MLAT server and on to the ATOM stations. By the time a position reaches your cockpit it may be a few seconds old.

To show this, Mode-S/3D targets are drawn with an ambiguity circle. The circle starts small when fresh data arrives and grows as the data ages, based on the target's speed. The aircraft could be anywhere within the circle, though it's most likely further along its last track. When new data arrives, the icon jumps forward and the circle shrinks again.

Circle sizes follow the NACp accuracy steps in the GDL90 specification (for example 0.1 nm, then 0.3 nm), always rounding up to the next step to err on the side of caution.

Things to bear in mind

• You'll only get a Mode-S/3D position for aircraft your PilotAware is receiving directly, so position your unit or antennas well.
• You need to be in range of at least one ATOM ground station. Out of range, Mode-S targets revert to bearingless.
• Transponders that aren't being interrogated, often below around 500 ft, transmit nothing to detect.
• PilotAware FX doesn't display bearingless targets, so on FX, Mode-S aircraft appear only when a Mode-S/3D position is available.
• Your EFB needs to be set up to receive PilotAware traffic. See the guide for your app under Connecting Apps & Avionics.