Radio Line-of-Sight and Data Link in a Hilly Corridor
You are operating a 7 kg multirotor in a valley for a search-and-rescue support mission. The ground control station is at the valley floor, 1.2 km from the search area. The terrain rises 180 m on both sides of the valley, and the search area is behind a ridge that is 90 m above the ground control station. The control link uses a 2.4 GHz system with a nominal range of 8 km in line of sight, and the telemetry shows RSSI of -68 dBm at 400 m, -79 dBm at 800 m, and -88 dBm at 1.2 km when the aircraft is at 120 m AGL with the ridge between it and the ground station. The video link is separate and shows latency of 350 ms and occasional frame loss. The mission requires the aircraft to descend into a hollow behind the ridge to 30 m AGL.
1. Explain why the link degrades as it does, and predict what will happen at 30 m AGL behind the ridge.
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The RSSI figures show a loss that is steeper than free-space path loss. Free-space loss increases by about 6 dB per doubling of distance, so going from 400 m to 800 m should cost about 6 dB; the observed loss is 11 dB, and from 800 m to 1.2 km it is a further 9 dB for a 50 percent increase in distance. The extra loss is diffraction and shadowing from the ridge, not distance.
The ridge is 90 m above the ground station and the aircraft is at 120 m AGL, which may put it just above the ridge line at 1.2 km, but the Fresnel zone is obstructed. At 2.4 GHz the first Fresnel zone radius at 1.2 km is roughly 6 m, so even a near-grazing path loses several dB. When the aircraft descends to 30 m AGL behind the ridge, the path becomes fully obstructed and the loss will increase by 15 to 25 dB or more, depending on diffraction geometry. An RSSI of -88 dBm could fall to -105 dBm or lower, which is at or beyond the receiver sensitivity for many systems, and the link will fail or become intermittent.
The video latency of 350 ms and frame loss confirm that the link is already stressed and that the margin is thin.
2. How do you restructure the operation to complete the mission safely?
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The first option is to move the ground control station. If I can place it on the ridge or on high ground with line of sight into the hollow, the diffraction loss disappears and the link budget recovers by 15 to 25 dB. That may require a relay or a second operator, but it is the most reliable fix.
If I cannot move the ground station, I can use a tethered or airborne relay: a second aircraft or a relay payload positioned above the ridge with line of sight to both the ground station and the search area. This is common in search-and-rescue support and converts one obstructed path into two clear ones.
The third option is to change the flight profile. I would keep the aircraft above the ridge line for the transit, then descend vertically into the hollow only after establishing a clear link, and set a minimum altitude and a maximum range at which the link is known to be reliable. I would fly the descent in short steps, checking RSSI at each step, and define a lost-link behavior that climbs rather than holds, because climbing restores line of sight.
I would also reduce the data rate: lower the video resolution and frame rate to free link margin for control and telemetry, since control is the safety-critical function.
3. What lost-link and emergency procedures must be briefed before the descent?
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The lost-link procedure must be explicit: if the link fails, the aircraft should climb to a pre-set altitude above the ridge, hold for a defined period, and then return to the launch point or to the last known good position. Climbing is the correct first action because it restores line of sight; holding at 30 m behind the ridge would leave the aircraft unrecoverable.
I would brief the visual observer on the aircraft's expected behavior and on the handover of visual contact. I would define a loss-of-link timeout, a maximum time to regain control, and the point at which we declare the aircraft lost and notify the coordinating agency.
I would also brief the search team on the aircraft's planned track and altitude so that they are not surprised by a low pass, and I would confirm that the aircraft's return path does not conflict with any helicopter operating in the area. Finally, I would verify the battery reserve for the climb and return, because a climb from 30 m to above the ridge costs energy and time.