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Will A see B?

This tool answers one question: can two points on the ground see each other, over the real shape of the land in between. It walks the great circle between them in steps of about one terrain cell, reads the ground height at every step from our elevation store, adds the earth's curvature, and compares the result with the straight line between the two antennas. If something is in the way it tells you where, how high it is, and how high each end would have to be for the line to get through.

For rocketry it answers the practical versions of that question: will telemetry reach from the flight line to the landing area on the far side of a rise, will an LTE flight computer get a signal from the bottom of a valley, will a ground tracker hear a rocket that has come down behind a hill.

The earth gets in the way before the hills do

Over long paths the limit is not terrain at all, it is the planet. The surface falls away from a straight line by roughly d squared over 8R, which is about 8 m at 20 km and 78 m at 63 km. Radio reaches a little further than optics because the atmosphere is denser near the ground and refracts the beam gently downwards; the standard way to model that is to pretend the earth has 4/3 of its true radius, which is what the radio setting here does. Two antennas at 1.5 m see each other to about 10 km and no further, whatever the terrain does. Get one end into the air and the horizon opens up fast: the geometric horizon goes as the square root of height, so 300 m of altitude is worth about 71 km on its own.

Fresnel clearance, not just a clear line

A radio link wants room around the line, not only the line. The first Fresnel zone is the ellipse around the path inside which a reflected wave arrives within half a wavelength of the direct one; its radius at any point is the square root of (wavelength times d1 times d2 divided by D), which at 868 MHz over a 10 km path is about 29 m at the midpoint. The working rule is that 60 percent of that zone should be clear of obstructions. A path that grazes a ridge is optically fine and several dB down in practice, so the tool reports the geometric answer and the Fresnel answer separately.

What the terrain data is, and is not

Heights come from our own elevation store, built from 1 arcsec global samples: raw 1 arcsec (about 30 m) cells across the UK, Ireland, the Alps and around known launch sites, 3 arcsec across the rest of Europe, the US and the steepest ground anywhere else, 6 arcsec on other hills and 18 arcsec elsewhere on land. Sea and anywhere without data read zero. The same data is available through our free terrain elevation API.

Two limits are worth knowing. First, every cell holds the mean height of the ground inside it, so a sharp ridge crest reads lower than it really is, by a couple of metres at 1 arcsec and by more at the coarser tiers; the chart draws that margin as a band above the ground and calls a path that only just clears it marginal rather than clear. Second, this is bare earth: no trees, no buildings, no masts. A line that clears a ridge by 5 m does not clear the forestry plantation on top of it.

Frequently asked questions

How far will 868 MHz telemetry go? Ground to ground, until a hill or the horizon stops it, usually under 10 km with both ends at head height. Ground to rocket, tens of kilometres, because altitude buys horizon.

Why does it say blocked when I can see the hill? Seeing the hill is not seeing over it. The chart shows where the ground rises above the line between the two antennas.

What height does B need? The tool gives the minimum height above ground at B that puts the line clear of everything in between, and the same figure for A. If that number is large, the obstruction is close to the other end and no mast will fix it.

Can I use it for LTE or mobile coverage? For the terrain part, yes, with the frequency set to the band you care about. It knows nothing about where the masts are or how loaded they are.

Is there an API? Yes, the same check is public at /api/los/, and the ground heights alone are at /api/terrain/.