You may find some explanations on Internet or AI, so I’ll just address two points that you may not see anywhere else. To understand the difference between surface low medium and high altitude ADS-B stations, one must recognize two critical technical factors: the difference in message formats between airborne and surface aircraft, and Challenge of Close Targets.
The Foundation: Surface vs. Airborne ADS-B Message Formats
A fundamental distinction exists between the data transmitted by an aircraft on the ground and one in the sky. Aircraft equipped with ADS-B transponders switch between different transmission states based on phase of flight. When aircraft in air, it transmits Airborne ADS-B Messages. When aircraft on airport ground, it transmits Surface ADS-B Messages.
Therefore, to function as a surface ADS-B ground station, the equipment hardware and software must explicitly support the decoding of surface messages. This capability is mandatory for airport surface surveillance systems like ASDE-X.
The Challenge of Close Targets: Antenna Theory and Near-Field Detection
When selecting equipment for a surface ADS-B ground station, engineers cannot focus solely on "how far" the ADS-B ground station can see. They must also ask, "how close can it see?"
To reliably monitor aircraft at extremely close range — such as when a plane is taxiing directly past an ADS-B ground station mounted on the runway edge — the ADS-B ground station must be capable of handling very strong signal power levels.
For a surface ADS-B ground station:
High Maximum Receive Power is Essential: The ADS-B ground station must be designed to tolerate a very high maximum input power. This ensures that when an aircraft transmits from only 20 meters away, the signal does not overwhelm (saturate) the receiver's front-end electronics, causing data loss or corruption.
The Sensitivity Penalty: However, there is a physical and electronic trade-off. In many ADS-B receiver designs, extending the ability to handle very strong signals can impose constraints on the ability to handle weak signals. Often, the higher the maximum receivable power, the higher the minimum detectable signal (the noise floor) becomes. This means that while the ADS-B ground receiver excels at seeing aircraft right next to it, its ability to see weak signals from distant aircraft is reduced, effectively shortening the receiver's maximum range.
The Role of the Antenna: Minimizing Gain
This is where antenna selection becomes a strategic tool to manage signal strength.
The Problem with High Gain:If a high-gain antenna were used, it would amplify the already powerful signal from a nearby aircraft even further. This would almost certainly saturate the receiver, making close-in detection impossible.
The Solution: Low-Gain Antennas: To keep the signal within the ADS-B ground receiver's manageable range, surface stations are paired with low-gain antennas. By using an antenna that does not amplify the signal (or even slightly attenuates it), the energy reaching the receiver's RF front end is kept at a level that the electronics can handle. This prevents saturation and ensures that the aircraft is successfully tracked during the critical phases of landing, taxi, and take-off roll.
In summary, designing a surface ADS-B ground station involves a careful balancing act. It requires a receiver capable of handling extremely strong close-range signals without sacrificing too much sensitivity, paired with a low-gain antenna to physically attenuate those close-range signals to a manageable level. This combination ensures seamless surveillance from the runway centerline all the way to the far reaches of the airport surface.
ADSB-RE1090 is an ADS-B ground receiver that can be used as surface ADS-B ground station. Not showing its ability with theoretical parameters, it really worked in airports to monitor close range aircrafts. There’s a parameter we called “gain” in ADSB-RE1090 design. The “gain” can be adjusted based on specific customer requirement on the balance of how close and how far customer want the surface ADS-B ground station to monitor.