Terrain Shadowing on a VHF Sector Frequency
You are operating a scheduled turboprop at FL190 over mountainous terrain, 85 NM from the sector VHF ground station. The station antenna is on a ridge at 4,200 ft AMSL; your aircraft is at 19,000 ft. For the last 4 minutes ATC has acknowledged only about one call in three, and on two occasions the controller reported your transmission as "readable but broken, background hiss." ATC's own transmissions reach you clearly and continuously. The flight is in a non-radar environment, so position reporting by voice is the only separation tool. Fuel is normal; there is no emergency. The next reporting point is 12 minutes ahead. You have a second radio on the company frequency and an HF set available but not selected.
1. Diagnose why ATC hears you poorly while you hear ATC clearly, and state the physical mechanism.
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The asymmetry is the key clue. Ground-to-air reception is good while air-to-ground is degraded, which points to a problem on the uplink path from aircraft to ground rather than a receiver failure at either end.
- VHF is essentially line-of-sight. At FL190 the radio horizon is roughly 1.23 x sqrt(19000) ≈ 170 NM, so geometric range to an 85 NM station is not the limiting factor.
- However, the station antenna at 4,200 ft sits on a ridge surrounded by higher terrain. The first Fresnel zone on the aircraft-to-ground path can be obstructed by intervening ridges even when the geometric line clears them. Obstruction produces diffraction loss and multipath, which shows up exactly as the controller described: readable but broken with hiss.
- The reverse path is not necessarily reciprocal in practice because the ground transmitter is far more powerful and the aircraft receiver has a much better noise floor margin, so a path with, say, 10 dB of extra loss still gives a good downlink but a marginal uplink.
- A second contributor is that your transmitter power is fixed and modest compared with the ground station, so the same path loss hurts the weaker link first.
Conclusion: terrain diffraction and multipath on the aircraft-to-ground path, aggravated by the power asymmetry between the two ends.
2. List, in priority order, the actions you would take to restore reliable voice communication, and justify the order.
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Priority is driven by the fact that loss of position reporting in a non-radar environment is a separation risk, so restore ATC contact first.
- Re-check the obvious on the current radio: correct frequency (including any 8.33 kHz channel spacing), correct mic gain, squelch not blocking, antenna selector on the right antenna, no stuck PTT. Cheap to verify and occasionally the real cause.
- Climb if operationally possible. Increasing altitude improves the radio horizon and, more importantly, lifts the path above the obstructing ridge line, restoring Fresnel-zone clearance. Even 2,000-3,000 ft can convert a diffracted path into a clear one.
- Change lateral position slightly if cleared to do so, to move off the azimuth where the ridge shadow is deepest. A few miles can change the diffraction geometry materially.
- Try the second VHF radio / second antenna to rule out a set or antenna fault.
- Relay through another aircraft on the sector frequency if one is in contact, giving your position report via the relay.
- Use HF as a fallback if the oceanic/remote HF network covers the area and ATC can be reached through it; note HF has its own propagation variability and is slower to establish.
- If contact cannot be restored, follow the published lost-communication procedure for the route and continue on the last acknowledged clearance, while squawking the appropriate code and maintaining the assigned level.
Justification: steps 1-4 are fast and non-disruptive; relay is the standard mitigation when direct contact fails; lost-comms procedure is the last resort because it changes the separation picture for everyone.
3. Explain why "readable but broken" should not be accepted as adequate for a position report.
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Readability scales exist precisely because "readable" is not binary. A report graded as readable but broken with background interference means individual digits or letters can be misheard, and in a position report a single misheard digit changes the aircraft's reported location by tens of miles.
- In a non-radar environment the controller builds the separation picture entirely from your reported positions and times. An error of one digit in a lat/long or a misheard reporting point name can place you in conflict with another aircraft without anyone noticing.
- Standard practice is to read back and have critical items confirmed, and if readability is marginal, to repeat the report or ask the controller to confirm the elements received.
- If readability cannot be improved, the correct response is to say so plainly and use the mitigation chain (relay, altitude change, alternate frequency) rather than let a doubtful report stand as fact.
A report that is accepted but wrong is more dangerous than a report that is acknowledged as unreadable, because the latter triggers a known procedure while the former silently corrupts the traffic picture.