Mode-S

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1m 33s

What is a Mode-S transponder?
Mode-S is the most common form of transponder used amongst GA aircraft in the UK and Europe.

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

  • Squawk. A code assigned by air traffic controllers to identify an aircraft's uniquely in a flight information region (FIR).
  • Altitude (height relative to 1013.2mB): The transponders altitude encoder is used when calculating the vertical separation between the host and the target aircraft.
  • Aircraft identification though an ICAO 24 bit aircraft address: The ICAO code is used to provide a unique identity which is linked to the aircraft's registration. G-INFO It is this ICAO element that is one of the factors that separate Mode-S from Mode-C transmissions which do not transmit the ICAO code.

The limitations of Mode-S.

Mode-S transponders do not contain a GPS position, because of this, when detected 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 which is referred to referred to as 3D traffic. Also, Mode-S only transmits (at 1090MHz) when interrogated by a ground radar signal or an aircraft's TCAS System. This is key to the understanding of the operation of Mode-S because if the transponder is not being interrogated, it will not transmit. This lack of response can happen at low levels and/or in remote areas.

How PilotAware detects and processes Mode-S.
PilotAware receives the Mode-S transponder messages directly using the integrated 1090MHz receiver. The PilotAware uses the altitude encoder data to determine the relative vertical separation to the host aircraft this data is then passed directly onto navigation applications.

Because Mode-S doesn't contain a GPS derived position, PilotAware uses signal processing algorithms that take the ICAO, signal strength and rate of change of power received to work out a distance relative to configurable parameters. This relative distance is then passed onto navigation applications along with the absolute vertical separation to display. The vertical separation will be within the accuracy of the target aircrafts encoder which is usually +\- 100ft.

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

How PilotAware makes Mode-S a 3D target!


Mode-S/3D is a tehnique unique to PilotAware that reboradcasts the location of Mode-S traffic using multilateration of its position. Using the multilateration technique, the locations of Mode-S equipped aircraft can be calculated through precise timing methods. Once the Mode-S locations are calculated, they are sent to the ATOM stations where they are then broadcast up to PilotAware equipped aircraft. The multilateration data used is provided by 360 Radar Ltd as a best effort service. This does not meet any aviation standard.


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