ADSb-Vue

Terrain & the predicted horizon

▲ Predicted horizon draws the lowest altitude an aircraft can fly and still be in your antenna's line of sight, given the terrain around you and the earth's curvature. It's the physical limit your low-altitude coverage is measured against — a smooth bowl that's ~ground level at the receiver and rises with distance.

◑ Compare to horizon

Recolours your measured detection cone by how it stacks up against that horizon:

green — you hear low traffic right down to the horizon: confirmed good coverage.

amber — the lowest aircraft you heard sits higher than terrain says it should.

blue — you heard something below the predicted horizon: over-performing.

grey — only high traffic flew here, so low-altitude coverage can't be judged.

How to read it

Grey isn't bad. You have coverage there — you're hearing aircraft — but no low traffic came through to grade it against the horizon. Low-altitude performance there is simply unknown, not poor.

Amber is a clue, not a verdict. It can mean a real gap (terrain or an obstruction blocking you from hearing low), or just that the lowest plane that happened to fly that way was still fairly high — which is why it's understated, not an alarm. From the top-down view you can't tell which; the per-bearing profile below can.

Tip: switch to the side view to see coverage as a bowl — the coloured part near the centre is the low-altitude floor, the only region with enough data to grade. Green usually traces your busy arrival/departure corridors.

⋰ Bearing profile

Opens a side-on slice along one direction: the terrain going outward, the horizon line, and the actual aircraft hits. Pick a direction with the slider or by clicking the map. It's the way to settle an amber patch — if hits sit right down on the horizon line you have good low coverage there; if the low region is simply empty, no low traffic came through (not a gap).

◌ HWT range rings

The same horizon rings tar1090 draws, from your site's HeyWhatsThat panorama: each floating ring is the boundary where a plane at that altitude drops below your horizon. An independent model of the same physics as the predicted horizon — a good cross-check against it and against the measured cone. Needs ADSB_HEYWHATSTHAT_ID set to your panorama id.

Antenna height