ADS-B Technology

ADS-B is a nextgen technology that changed tracking in aviation completely with Real-time precision tracking and shared situational awareness.


HOW DOES IT WORK

Automatic Dependent Surveillance–Broadcast (ADS–B) is a surveillance technology in which an aircraft determines its position via satellite navigation or other sensors and periodically broadcasts it, enabling it to be tracked. The information can be received by air traffic control ground stations as a replacement for secondary surveillance radar, as no Interrogation signal or Two-way contract is required.


Signal Structure

An ADS-B frame is 112 bits long and consists of five main parts

±---------±---------±------------±-----------------------±----------+
| DF (5) | CA (3) | ICAO (24) | ME (56) | PI (24) |
±---------±---------±------------±-----------------------±----------+

Bit No. bits Abbreviation Information
1–5 5 DF Downlink Format
6–8 3 CA Transponder capability
9–32 24 ICAO ICAO aircraft address
33–88 56 ME Message, extended squitter
(33–37) (5) (TC) (Type code)
89–112 24 PI Parity/Interrogator ID

It is worth noting that the ADS-B Extended Squitter sent from a Mode S transponder uses Downlink Format 17 ( DF=17 ). Non-Transponder-based ADS-B Transmitting Subsystems and TIS-B Transmitting equipment use Downlink Format 18 ( DF=18 ). By using Downlink Format 18 instead of 17, an ADS-B/TIS-B Receiving Subsystem will know that the message comes from equipment that cannot be interrogated.


Message Types

To identify what information is contained in an ADS-B message, we need to take a look at the Type Code of the message. The Type Code is located at bits 33–37 (or the first 5 bits of the ME segment)

Type Code Data frame content
1–4 Aircraft identification
5–8 Surface position
9–18 Airborne position (w/Baro Altitude)
19 Airborne velocities
20–22 Airborne position (w/GNSS Height)
23–27 Reserved
28 Aircraft status
29 Target state and status information
31 Aircraft operation status

Technical Specfication

Frequency: 1090 Mhz

Broadcast Format: ADS-B Downlink / Mode-S

Bandwidth: 4.3 Mhz ( centered at 1090 )


Benefits

  • Real-time Precision tracking
  • Improved safety
  • Situational awareness
  • Search and Rescue

Environmental impact

ADS-B technology provides a more accurate report of an aircraft’s position. This allows controllers to guide aircraft into and out of crowded airspace with smaller separation standards than it was previously possible to do safely. This reduces the amount of time aircraft must spend waiting for clearances, being vectored for spacing and holding. Estimates show that this is already having a beneficial impact by reducing pollution and fuel consumption.


Security Risk

A security researcher claimed in 2012 that ADS-B has no defense against being interfered with via spoofed ADS-B messages because they were neither encrypted nor authenticated. The FAA responded to this criticism saying that they were aware of the issues and risks but were unable to disclose how they are mitigated as that is classified. A possible mitigation is multilateration to verify that the claimed position is close to the position from which the message was broadcast. Here the timing of received messages is compared to establish distances from the antenna to the plane.

The lack of any authentication within the standard makes it mandatory to validate any received data by use of the primary radar. Because the content of ADS-B messages is not encrypted, it may be read by anybody.


SOURCE: Wikipedia, FAA, Eurocontrol, Skybrary, AINonline, Mode-S.org, ICAO
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