VOR/DME Fix Degraded by Terrain Shielding
You are cruising at FL150, 62 NM from a VOR/DME station, tracking toward it. The DME readout suddenly jumps from 62 NM to 88 NM and then to dashes, while the HSI bearing pointer swings slowly and settles about 4 degrees off your filed radial. The VOR is a conventional CVOR located in a valley with rising ground to your side; your altitude is 15 000 ft and the station elevation is 400 ft. No other navigation aids are within range. Fuel is sufficient for 40 minutes of holding. The aircraft has a GPS receiver that is powered but not yet selected on the navigation display.
1. Explain what is most likely happening to the DME and VOR indications, and why the two behave differently.
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The symptoms point to terrain/obstruction effects rather than receiver failure. DME is line-of-sight UHF (roughly 960-1215 MHz); when the direct path is blocked the interrogator may receive only reflected pulses or none, so the measured slant range can jump to a false long value (multi-path/reflection) or drop to dashes when reply pulses fall below the tracking threshold. The sudden change from 62 to 88 NM is not physically credible at constant groundspeed, so treat it as a false or reflected reply, not a real range change.
The VOR (VHF, 108-118 MHz) is also line-of-sight but the bearing information degrades more gracefully: the receiver can still lock onto a weak or partially obstructed signal, producing a slow, biased bearing. A steady 4-degree error with slow pointer movement is consistent with a distorted or reflected radial rather than loss of signal. Because the two aids use different frequencies and modulation, one can fail or mislead while the other still appears usable. Neither indication should now be trusted for a position fix.
2. State the actions you would take, in order, and justify the priority.
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Stop using the VOR/DME as a primary fix. Cross-check the bearing against heading and known wind; a 4-degree unexplained bias plus an impossible DME jump means the fix is unreliable.
Select and verify GPS on the navigation display, then compare its position with the last good VOR/DME fix and with dead reckoning. Agreement between GPS and DR within a few NM restores situational awareness; disagreement requires further investigation before trusting either.
Advise ATC of the unreliable aid and request a revised clearance or vectors if the route depends on that station.
Climb if terrain clearance and airspace permit, since raising the antenna above the obstruction may restore line-of-sight to both aids.
Priority is given to re-establishing a trustworthy position before any route change, because the aircraft is still 62 NM from the station and a 4-degree bearing error at that range is roughly 4 NM of cross-track uncertainty, growing as you close in.
3. If no GPS were available, how would you proceed and what error would you expect?
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Without GPS, revert to dead reckoning from the last confirmed fix, using heading, TAS, wind and time. Maintain a terrain-safe altitude and request radar vectors or a heading to a known feature from ATC. Treat the VOR as a rough orientation aid only, using the sense of the pointer (fly toward or away) rather than its exact value.
Quantify the risk: a 4-degree bearing error at 62 NM is about 4.3 NM cross-track; at 30 NM it is about 2.1 NM. DME should be disregarded entirely while it is jumping or showing dashes. If the station cannot be re-acquired, the correct decision is to declare the aid unusable and navigate by the remaining means, not to average the bad indications into a false sense of accuracy.