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What is a Mode-C transponder?
Mode-C is the predecessor to Mode-S and is still used amongst the UK GA community.

What does Mode-C transmit?
The main Mode-C transmission information is:

  • Squawk. A code assigned by air traffic controllers to identify an aircraft uniquely in a flight information region (FIR)
  • Altitude (height relative to 1013.2mB): The transponders altitude encoder is used to calculate the vertical separation between the host and the target aircraft.

The limitations of Mode-C.

Mode-C does not contain an ICAO 24 bit aircraft address, this means that there is no way to identify the registration of an aircraft from a Mode-C transponder.

Mode-C transponders do not contain a GPS position, because of this, it is referred to as a bearing-less target. This is unlike modern EC device like PilotAware, FLARM and ADS-B which contain a GPS position and we refer to a 3D traffic. Also, Mode-C only transmits (at 1090MHz) when interrogated by a ground radar signal or an aircraft's TCAS System. This is key to understand because if the transponder is not being interrogated, it will not transmit. This can happen at low levels and/or in remote areas.

How PilotAware detects and processes Mode-C.

PilotAware receives the Mode-C transponder messages directly using the integrated 1090 receiver. The PilotAware uses the altitude encoder data to work out the relative altitude to the host aircraft this data is then passed directly onto navigation applications.

Because Mode-C doesn't contain a position, PilotAware uses signal processing algorithms that take the signal strength and rate of change to work out an approximate relative distance. This relative distance is then passed onto navigation applications to display.

PilotAware gives you the option to configure the sensitivity and filter out unnecessary aircraft.

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