The coverage of an ADS-B ground receiver is a commonly asked question and serves as a key technical specification. However, while it may seem straightforward, it's actually quite complex. Let's try to break it down in simple terms.
When discussing the coverage of an ADS-B ground receiver, it refers to the radius around the receiver's location. ADS-B signals, whether on the 1090MHz or 978MHz UAT frequency, operate on a line-of-sight principle. The coverage area of an ADS-B ground receiver, or any radio equipment operating on line-of-sight frequencies, is determined by three main factors: transmitter power, receiver sensitivity, and the altitude of the transmitter/receiver (due to the curvature of the Earth). Let's break down each factor individually.
Firstly, let's consider transmitter power. Commercial aircraft use Mode-S transponders with a standard transmitter power of 250W for ADS-B transmission. However, general aviation aircraft and UAVs/drones may use an ADS-B transmitter with transmitter power less than 250W. With the same ADS-B ground receiver installed at the same site and aircraft flying at the same altitude, varying transmitter power can result in different coverage areas, ranging from teens of miles to hundreds of miles.
Next, let's discuss receiver sensitivity. With the same transmitter, the signal power received by the ADS-B ground receiver antenna varies based on the distance between the transmitter and the receiver. A more sensitive ADS-B ground receiver can detect signals from transmitters located farther away, resulting in increased coverage area.
Transmitter is like mouth, receiver is like ear. Just like when person A (transmitter) shouts louder (bigger transmitter power), person B (receiver) can hear person A from a greater distance. And when person B has better hearing (better receiver sensitivity), person B can pick up the sound from even farther away. This illustrates how transmitter power and receiver sensitivity both contribute to the coverage area of an ADS-B ground receiver.
Then, altitude. In simple terms, when an aircraft flies at a higher altitude, its signals can reach the ADS-B ground receiver over longer distances due to the curvature of the Earth. So, higher-altitude aircraft contribute to expanding the coverage area of the ADS-B ground receiver.
Consider the example illustrated above. When an aircraft flies to position A at an altitude of 1,000 meters, its ADS-B signal can be received by the ADS-B ground receiver antenna, enabling the ADS-B ground receiver to "see" the aircraft. However, when the aircraft moves to position B at the same altitude of 1,000 meters, its signal is obstructed by the Earth, preventing reception by the ADS-B ground receiver antenna, and thus the ADS-B ground receiver cannot "see" the aircraft. Therefore, the ADS-B ground receiver coverage at an altitude of 1,000 meters extends only to the distance from the receiver to position A. Conversely, if the aircraft ascends to position C at an altitude of 2,000 meters, its signal can once again be received by the ADS-B ground receiver antenna, expanding the coverage. This example illustrates that ADS-B coverage varies depending on the altitude of the aircraft.
Applying the same principle, installing the ADS-B ground receiver antenna at a higher elevation further enhances coverage, allowing the ADS-B ground receiver to detect signals from a wider area.
There’re some other issues that would affect ADS-B ground receiver coverage. Close frequency interference can disrupt the reception of ADS-B signals, especially if there are other radio devices operating nearby on similar frequencies. Additionally, the length of the cable connecting the ADS-B ground receiver antenna with the receiver itself is crucial. Longer cables can introduce signal loss, diminishing the receiver's ability to capture ADS-B signals effectively. Therefore, it's essential to minimize interference and ensure optimal cable length to maximize ADS-B ground receiver coverage.
While ADS-B signals primarily operate on a line-of-sight principle, there can be instances where signals are reflected and still reach the ADS-B ground receiver antenna. These reflections might occur due to various atmospheric conditions or surface features that can bounce the signal towards the receiver. As a result, even in scenarios where direct line-of-sight communication is obstructed, ADS-B receivers may still detect and capture signals through reflection, extending the effective coverage area to some extent.