An SDR on a balcony really does pick up ADS-B at 200 km. But the radio horizon and urban noise set hard bounds. Fusion mostly serves to cut false positives.
From a fixed point you capture more than you think, and less than you hope. Measuring what an SDR actually sees keeps you from overrating a setup.
What gets through and what blocks
An SDR on a balcony receives the ADS-B of an aircraft 200 km away, because the craft is high and the clear line of sight reaches far. For an aircraft, altitude pushes the radio horizon well beyond the ground.
At ground level, the radio horizon is short. A low transmitter behind a building is invisible. Urban noise adds a floor: electronics, power supplies and networks pollute the low bands and drown weak signals.
What fusion is for
Stacking several sensors does not magically increase range. Each one stays bounded by its physics and its horizon. Fusion does not create range that does not exist.
Its real value is to reduce false positives. A detection confirmed by two independent modalities is credible. A single modality stays a hypothesis. Multi-sensor buys confidence, not distance.