ADS-B Data Link

What is ADS-B data link?
As introduced in the Simple Explanation of ADS-B, ADS-B needs to send out aircraft's own ADS-B information via communication equipment. The communication is ADS-B data link.

How many data links does ADS-B have?
Three. 1090,UAT and VDL M4.

What are the advantages and disadvantages of the three ADS-B data links?
1090, frequency is 1090MHz
Advantages: The 1090 ADS-B communication equipment on-board air carrier aircraft is the Mode S transponder, originally used for Secondary Surveillance Radar, which most of air carrier aircrafts have already installed. So, the cost of air carriers modification to support ADS-B is low. Just need to upgrade the software of the transponder, if it’s not a too old version.
Disadvantages: Secondary Surveillance Radar, TCAS and Multilateration all use the 1090MHz frequency. This makes 1090MHz frequency very crowded. It’s like too many different vehicles running on the same road.
UAT, frequency is 978MHz
Advantages: This data link is designed specifically for ADS-B in the United States and so has sufficient bandwidth.
Disadvantages: Aircrafts need to add UAT communication equipment, which greatly increases the cost of aircraft modification. Also, it adds much complexity for aircrafts to see each other. In U.S.A. ADS-R (Automatic Dependent Surveillance- Rebroadcast) has to be implemented for aircrafts with 1090 ADS-B and UAT ADS-B to see each other.
Almost no one is using this anymore.

Which datalink should ADS-B use?
The short answer is 1090. UAT is only used in the U.S.A. for general aviation. And as mentioned above, VDL M4 is not used anymore. A simple reason is, air carrier aircrafts have already almost been installed with Mode-S transponder, so 1090 is the cheap and easy answer. Also, if all use 1090, the whole nation ADS-B system would be easy. U.S.A. uses 1090 for air carrier aircrafts and UAT for general aviation, so the U.S.A. ADS-B system is complicated, because it has to deal with two data links. U.S.A. uses UAT because U.S.A. has too many aircrafts, both air carrier and GA, and too many radars. So the 1090MHz is already very crowded.

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