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Downlink Broadcast

The downlink broadcast frame is resent at regular intervals by the aircraft for a specific period time, at a nominal rate of 1 Hz. It includes the transmission of Extended Squitter - the unsolicited downlink broadcast of positional reports. The system „depends” on other aircraft systems, like a barometric encoder and GPS equipment for the position data.

The preamble of the Mode S downlink allows a synchronisation to a clock for decoding the Data Block which is coded with the Pulse-Posistion-Modulation (PPM). The Data format of an ADS-B message is the format number DF17. The content of the datafield DF is here the decimal number 17, in binary code 10001b.[1]

Preamble
Data Block
5
3
24 bits
56 bits
24 bits
DF
CA
AA
ADS-B data
PI
clock

Figure 1: Data Block of an ADS-B message

Datenblock eines ADS-B Reports
Preamble
Data Block
5
3
24 bits
56 bits
24 bits
DF
CA
AA
ADS-B data
PI
clock

Figure 1: Data Block of an ADS-B message

The three following bits (called “Capability”, or “CA”) ist the number of sub-type of the ADSB- message. The length 3 bit gives eight different kinds of reports. The next 24 bits are the individual Aircraft (ICAO) Address. The next Data field of 56 bits is the carrier of the ADS-B report, depending on the content of the “CA” field.

This report can include:

The last 24 bits are the error detection code (Parity Information, PI).

Traffic Information Broadcast (TIS-B)

Traffic Information Broadcast (TIS-B) is used by airfield ground equipment, e.g. the Follow-me-Car or aircraft tow tractors.[2] The message uses the same structure as ADS-B, but the format is named DF18 instead of DF17. This format designator is intended to support the broadcast of extended squitter ADS-B messages by non-transponder devices, i.e. they are not incorporated into a Mode S transponder of an aircraft. This separate format is used to clearly identify this non-transponder case to prevent Mode S interrogators from attempting to interrogate these devices.

preview of a flyer for a real example of a virtual radar
(click to enlarge: PDF = 235 kByte
Figure 2: preview of a flyer for a real example of a virtual radar

SBS-1

The squitter information can be received and shown as a Real Time Radar display on your PC screen of Mode-S/ADS-B equipped aircraft the airspace immediately around your home. Such a receiver is described at www.javiation.co.uk. A lot of users of this ADS-B receiver are associated with www.virtual-radar.de.

The traffic around Zurich is represented in internet at radar.zhaw.ch, based on a diploma thesis from the suiss school of engineering in Zurich. The shown “radar informations” are originated on ADS-B “squitter” reports.

Source:

  1. ICAO Annex 10, Volume IV, 3.1.2.8.6 EXTENDED SQUITTER, DOWNLINK FORMAT 17, (p. 72)
  2. ICAO Annex 10, Volume IV, 3.1.2.8.7 EXTENDED SQUITTER/SUPPLEMENTARY, DOWNLINK FORMAT 18, (p. 77)