Let’s pick this up right where the physics starts to bite — Page 198, Lesson 191

Let’s pick this up right where the physics starts to bite. We’re talking about ASMI — that’s the Airfield Surface Movement Indicator, the radar that watches aircraft and vehicles moving on the airfield surface, the taxiways and runways, in poor visibility. The key idea here is attenuation. Attenuation means the signal gets weaker as it travels. And for ASMI, the transmissions — the radar pulses it sends out — are increasingly attenuated and absorbed as the intensity of precipitation increases. So the heavier the rain or snow, the more of that transmitted energy gets soaked up before it can bounce back off a target. Now, what does that do to the radar? It reduces the radar’s range. The effective distance the radar can see shrinks because the signal is being eaten away. But here’s the important qualifier: this is not a significant problem for ASMI. Why? Because these radars are only required to cover the environs of the airfield — the immediate surroundings of the airfield, not hundreds of miles out. So even though heavy precipitation cuts the range, the range that’s left is still enough for the job. Then we get a contrast, and this is the crucial limitation. The EHF band — that’s Extremely High Frequency, the highest band of microwave frequencies — 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 in the EHF band reduces its effective operational range and capabilities so much that it can’t do the ASMI job reliably. So the logic chain is: precipitation causes attenuation and absorption of the radar transmission → that reduces range → for ASMI that’s acceptable because the coverage needed is only the airfield environs → but for EHF, the attenuation is so severe in most precipitation types that it knocks the effective range and capabilities down below what’s usable, so EHF is ruled out for ASMI. That’s the whole relationship: the choice of frequency band for an ASMI radar is governed by how much attenuation the precipitation will cause, and EHF fails that test.