Datalink

Data Link Applications (ADS-B, ADS-C, FANS):

Data Link Applications – Simple Explanation

FANS / CNS-ATM (background)

• ICAO introduced FANS to modernize aviation.

• It led to CNS/ATM:

  • Communications (data link)
  • Navigation (GNSS)
  • Surveillance (ADS)

• Result: less voice, more automatic data reporting.

ADS-B (Automatic Dependent Surveillance – Broadcast)

• Aircraft broadcasts its position automatically
• Uses GNSS
• Sent every second
• No contract, no request

Key points:

• One-way broadcast (aircraft → everyone)
• Replaces or supplements radar
• Ground stations and other aircraft receive it
• ADS-B OUT = transmit
• ADS-B IN = receive traffic display
• Limited to line-of-sight (VHF)
• Mandatory in most airspace

ADS-C (Automatic Dependent Surveillance – Contract)

• Aircraft sends reports only when agreed with ATC
• Requires logon to ATC
• Uses contracts
• Mainly for oceanic / remote airspace

Types of contracts:

• Periodic – sent at time intervals

• Demand – ATC requests immediate report

• Event – sent when something changes:

  • vertical rate
  • lateral deviation
  • altitude range
  • waypoint change

• Emergency – crew-initiated, frequent updates

Key points:

• Two-way system
• Automatic after logon
• Handover between ATC units is automatic

ADS-B vs ADS-C (one line)

• ADS-B: continuous broadcast, no ATC control
• ADS-C: controlled reports, based on contracts

Transponder – Main Role

A transponder automatically replies to ground or airborne interrogations and/or broadcasts aircraft data, allowing ATC and other aircraft to see you.

Roles by System

Mode A

• Sends identity code (squawk) only
• No altitude
• Basic identification

Mode C

• Sends squawk + pressure altitude
• Enables altitude display on radar

Mode S

• Sends unique aircraft address
• Sends altitude, identity, and additional data
• Enables selective interrogation (less congestion)
• Required for advanced surveillance

ADS-B OUT

• Uses Mode S transponder
• Broadcasts GNSS position, altitude, velocity
• Automatic, continuous
• Used by ATC and other aircraft
• Does not require interrogation

ADS-B IN

• Receives ADS-B data from other aircraft
• Provides traffic awareness to pilots
• No ATC function

TCAS / ACAS

• Uses transponder replies from other aircraft
• Calculates collision risk
• Issues Traffic Advisories (TA) and Resolution Advisories (RA)

ADS-C

• Transponder not used
• Uses data link (ACARS / SATCOM / VHF)
• Included here for comparison only

One-Line Summary (Perfect for Exams)

• Transponder: replies or broadcasts surveillance data
• Mode A/C: identity and altitude
• Mode S: selective, data-capable surveillance
• ADS-B: position broadcast via transponder
• TCAS: collision avoidance using transponder data