| Sensor | Good at | Weakness |
|---|---|---|
| Radar (active RF) | range, velocity, all weather, long range | gives itself away (EW: Electronic Support Measures); struggles with small/low/stealthy targets |
| Passive RF / ESM | emitters, without transmitting itself; identification | sees nothing that is silent (e.g. fibre-optic drone, Lessons from Ukraine (open source)) |
| EO/IR | identification, passive, accurate in angle | short range; dependent on weather and visibility (Infrared) |
| Acoustic | cheap; works against RF-silent targets | very short range; sensitive to ambient noise |
One sensor's weakness is often another's strength: where radar misses a low, silent drone, acoustics or EO/IR can still pick it up. That is exactly why you combine them.