Datalink Surveillance
✈️ DATA LINK & SURVEILLANCE — ONE-PAGE MEMORY SHEET
ACARS (Aircraft Communications Addressing and Reporting System)
Role: Data link carrier
- Routes messages between aircraft and ground
- Uses VHF / HF / SATCOM
- Managed by CMU / ATSU
- Supports CPDLC, ADS-C, AOC, maintenance
CPDLC – Communication
Purpose: Replace/reduce voice ATC
- Text messages pilot ↔ ATC
- Pre-formatted ICAO messages
- Requires logon
- Crew must read, accept, respond
- Some clearances loadable into FMS
Messages:
- Requests (level, route, speed)
- Clearances
- Reports (position)
- Responses (WILCO / UNABLE)
Memory hook: 👉 CPDLC = TALK by text
ADS-C – Surveillance (Contract)
Purpose: Track aircraft without radar
- Automatic position reports
- Requires logon
- Uses contracts
- No crew action (except emergency)
Contract types:
- Periodic (time-based)
- Demand (ATC request)
- Event (altitude, waypoint, deviation)
- Emergency (crew-initiated)
Memory hook: 👉 ADS-C = TRACK by contract
ADS-B – Surveillance (Broadcast)
Purpose: Radar replacement / enhancement
- Aircraft broadcasts position every second
- Uses GNSS
- No logon, no contract
Types:
- ADS-B OUT – transmit position
- ADS-B IN – receive traffic display
Limits: Line-of-sight (VHF)
Memory hook: 👉 ADS-B = SHOUT position
CPDLC vs ADS-C (Key Exam Table)
| Feature | CPDLC | ADS-C |
|---|---|---|
| Function | Communication | Surveillance |
| Crew action | Yes | No |
| Automatic | ❌ | ✅ |
| Uses contracts | ❌ | ✅ |
| Replaces | Voice ATC | Radar |
Transponder Roles
- Mode A: Squawk only
- Mode C: Squawk + altitude
- Mode S: Unique ID + data
- ADS-B: Uses Mode S
- TCAS: Uses transponder replies
- ADS-C: ❌ No transponder
Handling CPDLC Clearances (Golden Rules)
- Both pilots read & understand
- Both pilots confirm achievable
- Set correctly in FMS / MCP
- Check FMA for correct mode
- Acknowledge compliance to ATC
Ultra-Short Exam Mnemonics
- ACARS = carrier
- CPDLC = communicate
- ADS-C = contract
- ADS-B = broadcast
- Transponder = reply / broadcast