Difference between TCAS and ADS-B

When ADS-B (Automatic Dependent Surveillance – Broadcast) becomes widely used in aviation, what’s the difference between TCAS (Traffic Collision Avoidance System) and ADS-B is an often-asked question. As we mentioned in the Simple Explanation of ADS-B, ADS-B can be used for ground-air surveillance and air-to-air surveillance. While TCAS can only be used for pilot. So when we talk about the different between TCAS and ADS-B, we focus on the difference for pilot.

1. TCAS and ADS-B have different working ways. You can easily find their working ways on Internet. Simple words, TCAS is like an airborne secondary radar and ADS-B gets aircraft information from airborne GPS.

2. TCAS and ADS-B have different applications. Both TCAS and ADS-B can make pilots “see” other aircrafts around, or what we called situation awareness. But pilots can only rely on TCAS for situation awareness and collision avoidance, while can rely on ADS-B for a lot of applications, like ITP (In-Trail Procedure).

3. The development phase is different. TCAS is already a mature technology that requires mandatory installation and use on civil aviation aircraft. However, the relatively mature application of ADS-B is still limited to ground-to-air surveillance. Air-to-air surveillance is still in the experimental phase. It is still a thing of the future that pilots rely on ADS-B to execute some flight procedures more flexibly and autonomously, and even use ADS-B to avoid collisions and achieve the concept of free flight.

4. The onboard equipment is similar. For TCAS, the onboard equipment includes an interrogator (transmitting interrogation 1030MHz signals and receiving 1090MHz signals) and a transponder (receiving 1030MHz signals and transmitting 1090MHz signals). For ADS-B OUT, it includes a GPS receiver and an ADS-B transmitter (transmitting 1090MHz ADS-B signals). While for ADS-B IN, it includes a receiver (receiving 1090MHz ADS-B signals). The TCAS transponder, ADS-B OUT transmitter, and secondary radar transponder all refer to the same device, Mode-S transponder. ADS-B IN is still in the testing phase, and it is generally preferred to use TCAS interrogators as ADS-B IN receivers on air carrier aircrafts. For general aviation aircrafts and UAV/drones, they don’t have TCAS onboard and the ADS-B IN receiver onboard is a specific ADS-B IN receiver, so it’s not included in this difference topic.

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