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Let’s explain ADS-A, ADS-B, and ADS-C

🧭 1️⃣ What “ADS” Means

ADS = Automatic Dependent Surveillance

It’s a method for ATC to track aircraft position automatically without radar. Each type (A, B, C) refers to a different generation or method of how the position data is sent and used.

Let’s break it down 👇

🛰️ 2️⃣ ADS-A — (Automatic Dependent Surveillance – Addressed)

💡 Also known as ADS-C (Contract) in modern use.

• Uses a data link between the aircraft and a specific ATC center.
• The aircraft automatically sends position reports at agreed times or events.

Example: Every 5 minutes or when crossing waypoints. Used in: Oceanic or remote areas (no radar coverage).

✅ Key idea:

Aircraft automatically reports its position to one ATC station it’s in contact with.

🌍 3️⃣ ADS-B — (Automatic Dependent Surveillance – Broadcast)

• The aircraft broadcasts its position, speed, and altitude continuously to anyone listening.

• The signal is received by:

  • ATC ground stations
  • Other aircraft (for traffic awareness via TCAS or cockpit display)

Data source: GPS position Frequency: 1090 MHz (Mode S Extended Squitter)

✅ Key idea:

The aircraft broadcasts its position to everyone — not just one controller. Used for surveillance and traffic situational awareness.

📡 4️⃣ ADS-C — (Automatic Dependent Surveillance – Contract)

This is the modern name for ADS-A. It’s used in FANS systems via satellite or HF/VHF data link.

• A “contract” defines when reports are sent:

  • Periodic (every X minutes)
  • Event (altitude change, waypoint)
  • Demand (when ATC requests)

✅ Used for: Oceanic routes, long-range operations (e.g., North Atlantic). ✅ Data link used: CPDLC / SATCOM / HF Data Link.

🧠 5️⃣ Summary Table

✅ In short:

• ADS-A → old, point-to-point system (replaced by ADS-C)
• ADS-B → broadcasts position to everyone (ATC + aircraft)
• ADS-C → automatic position reports via data link to one ATC (used in FANS)