Attenuation just means the signal loses strength as it travels — Page 198, Lesson 191

I want to walk you through a specific point about ASMI radars and why they behave the way they do in bad weather. ASMI stands for Airfield Surface Movement Indication — it's the radar that shows controllers the position of aircraft and vehicles on the airfield, so they can manage ground movements safely. Now, here's the key physical effect we're dealing with. When precipitation — rain, snow, hail, that kind of thing — gets more intense, the radar transmissions get increasingly attenuated and absorbed. Attenuation just means the signal loses strength as it travels. So the heavier the precipitation, the more the radar energy is soaked up before it can bounce back off a target. What does that do to the radar's performance? It reduces the radar's range. The signal simply can't reach as far because so much of it has been absorbed along the way. But here's the important part — for an ASMI radar, that's not a significant problem. Why? Because these radars are only required to cover the environs of the airfield. They don't need to see hundreds of miles out. They just need to see the runways, taxiways, and the immediate area around the field. So even if heavy rain cuts the range down, the radar still covers everything it's supposed to cover. Now, let's contrast that with the EHF band. EHF stands for Extremely High Frequency — that's the frequency range from 30 to 300 gigahertz. And here's the catch: the EHF band is not suitable for an ASMI radar. The reason is exactly the attenuation we just talked about. In most types of precipitation, the degree of attenuation is so severe that it reduces the radar's effective operational range and capabilities to the point where it can't do the job reliably. So even though EHF might offer other advantages, the weather penalty is too heavy for airfield surface movement work. So the takeaway is this: precipitation attenuates and absorbs radar transmissions, and the heavier the precipitation, the worse it gets. That cuts range. For ASMI, that's acceptable because the coverage area is small. But for EHF, the attenuation is so bad in most precipitation that it knocks the radar's effective range and capability down too far — so EHF just isn't used for ASMI. Let me show you what these radar antennae actually look like, so you can picture the hardware we're talking about. And here's the airborne weather radar antenna, which is a different application but shows you the same kind of hardware in a different role. So to tie it all together: ASMI radars work fine despite precipitation attenuation because their range requirement is modest. EHF doesn't work because the attenuation in most precipitation destroys its effective range and capability. That's the whole story of why ASMI stays away from the EHF band.