Definition
A datalink service provided by the FAA's ADS-B ground network that retransmits ADS-B position reports between the two ADS-B frequencies (978 MHz UAT and 1090 MHz Extended Squitter) so that aircraft equipped on one frequency can see traffic equipped on the other. The service is provided automatically when an aircraft is within coverage of an ADS-B ground station and is broadcasting its own ADS-B Out signal.
Plain English
ADS-B aircraft use two different radio frequencies, and an aircraft tuned to one frequency can't directly hear aircraft on the other. ADS-R is a ground service that listens to both frequencies and rebroadcasts the traffic across to the other frequency so everyone sees each other.
Context Anchor
Pilots encounter ADS-R when learning about ADS-B In traffic displays and why some traffic information may depend on nearby FAA ground stations.
Derivation
Rebroadcast means 'to broadcast again.' The ground station receives an ADS-B signal on one frequency and broadcasts it again on the other frequency so traffic on both frequencies can see each other.
Why Pilots Care
Without ADS-R, a pilot using 978 MHz UAT would not see traffic broadcasting only on 1090 MHz ES, reducing situational awareness and increasing the chance of an undetected conflict in mixed-equipage airspace.
Grounding Statement
A ground station hears ADS-B traffic information on one radio link and repeats it on the other so more aircraft can receive it.
Intuition Check
Do not read dependent as meaning weak or unreliable. Here, dependent means the surveillance report depends on information supplied by the aircraft's own navigation equipment.
Example Sentence 1
Because his aircraft was broadcasting ADS-B Out within ground station coverage, ADS-R let him see nearby 1090 ES traffic on his 978 UAT receiver.
Example Sentence 2
In areas without ADS-R coverage, the pilot knew some ADS-B equipped aircraft might remain invisible on the cockpit display.