What’s the coverage of one ADS-B ground receiver is an usually asked question and is always used as a technical specification of ADS-B ground receivers. While this is a seems simple but actually complicated issue. We’ll try to explain it in simple words.
When we talk about ADS-B ground receiver coverage, it actually means radius centered on the ADS-B ground receiver. ADS-B, no matter its frequency is 1090MHz or 978MHz UAT, both are line of sight. The coverage of one ADS-B ground receiver, or any radio equipment on line-of-sight frequency, is decided by transmitter power, receiver sensitivity and transmitter/receiver height (because of earth curvature). More transmitter power would bring more coverage, better receiver sensitivity would bring more coverage and transmitter/receiver installed on higher altitude position would bring more coverage. We’ll explain these three factors one by one.
First, transmitter power. For commercial planes, the ADS-B transmitter is actually the Mode-S transponder with standard transmitter power 250W. But for general aviation planes and UAV/drones, the ADS-B transmitter could be a 250W Mode-S transponder or a smaller transmit power ADS-B transmitter. For the same ADS-B ground receiver installed on the same site, for aircraft fly at the same height, different transmit power would bring different coverage, from teens of miles to hundreds of miles.
Second, receiver sensitivity. For the same transmitter, the signal power left when the signal reached the ADS-B ground receiver antenna end is different when the transmitter distance from the ADS-B ground receiver is different. When the ADS-B ground receiver is more sensitive, it can receive signals from further away transmitters, thus means more coverage.
Transmitter is like mouth, receiver is like ear. When person A (transmitter) shout louder (bigger transmitter power), person B (receiver) can hear person A from longer distance. When person B is younger with good hearing (better receiver sensitivity), person B can also hear person A from longer distance.
Third, altitude. This is probably the most complex part to explain. A simple explanation is, the higher the aircraft flies, the more coverage ADS-B ground receiver would get.
Above picture is a simple example. When aircraft flies to position A with altitude 1,000 meters, its ADS-B signal can be received by the ADS-B ground receiver antenna, hence the ADS-B ground receiver can “see” the aircraft. While when the aircraft flies to position B with the same altitude 1,000 meters, its signal is blocked by the earth and can’t be received by the ADS-B ground receiver antenna, hence the ADS-B ground receiver can’t “see” the aircraft. So, the ADS-B ground receiver coverage on altitude 1,000 meters is the distance from the ADS-B ground receiver to position A. But when the aircraft flies to position C with altitude 2,000 meters, its signal can be received by the ADS-B ground receiver antenna again, hence the ADS-B ground receiver on altitude 2,000 meters is the distance from the ADS-B ground receiver to position C. This is a simple example of the higher aircraft flies, the more ADS-B coverage ADS-B ground receiver can get. So, ADS-B coverage is actually different in different aircraft altitude.
With the same theory, the higher ADS-B ground receiver antenna is installed, the more coverage ADS-B ground receiver would get.
There’re some other issues that would affect ADS-B ground receiver coverage. Is there any close frequency interference? How long is the cable that connect ADS-B ground receiver antenna with ADS-B ground receiver? The cable would bring signal strength loss.
One more thing, ADS-B signal is line of sight, but it’s actually not that strict. Because the signal transmitted by ADS-B transmitter might be reflected to reach the ADS-B ground receiver antenna and the ADS-B receiver can still “see” the aircraft with reflected ADS-B signal.