UpToWhereRF line of sight calculator, on real terrain
This page checks whether two points have line of sight over real terrain, and whether the first Fresnel zone is clear enough for a radio link. It is free and unlimited, needs no account, and runs entirely in your browser on public 30 m elevation tiles. Paths up to 150 km, at any frequency from optical to 24 GHz.
- DataAWS Terrain Tiles, about 30 m
- Path limit150 km
- CostFree, no sign-up
- RunsIn your browser
Radio horizon, before any terrain
Two antennas can only see each other if the planet is not already in the way. This is the ceiling on a path with no hills in it at all.
The two horizons add: A sees over the bulge to its own horizon, B sees back to the same point. Beyond this distance the Earth blocks the path however flat the ground is. Below it, terrain decides, which is what the map does next.
Click the map to place A, or drag the markers.
Place two points and press Check.
Elevation from AWS Terrain Tiles at about 30 m per pixel, read in your browser. Geometry only: no buildings, no trees, no signal strength. Paths up to 150 km.
How the radio horizon is computed
Before terrain matters at all, the planet sets a ceiling. An antenna at height h can see over the bulge to a horizon at a distance that grows with the square root of that height, and a link between two antennas reaches as far as the two horizons added together.
The atmosphere lengthens both. Air thins with altitude, which bends a radio beam gently downward and lets it follow the curve a little way. ITU-R P.530 models the median atmosphere as an Earth with 4/3 of its real radius, which is where the 4.12 constant below comes from. An optical sightline bends less, and surveyors use about 1.15.
| Antenna height | Radio horizon (k = 4/3) | Optical horizon (k = 1.15) | Geometric (k = 1) |
|---|---|---|---|
| 1 m | 4.1 | 3.8 | 3.6 |
| 2 m | 5.8 | 5.4 | 5.0 |
| 5 m | 9.2 | 8.6 | 8.0 |
| 10 m | 13.0 | 12.1 | 11.3 |
| 20 m | 18.4 | 17.1 | 16.0 |
| 30 m | 22.6 | 21.0 | 19.6 |
| 50 m | 29.1 | 27.1 | 25.2 |
| 100 m | 41.2 | 38.3 | 35.7 |
| 300 m | 71.4 | 66.3 | 61.8 |
One antenna, in kilometres. Doubling the height buys about 41% more reach, which is the reason mast height is the first lever on every marginal link and the second one is patience.
Fresnel clearance on the profile
A path that clears the ground can still fail. Radio energy travels as a beam with width, and the first Fresnel zone is the ellipse around the direct line where an obstruction changes what arrives. The working rule is that 60% of it has to stay clear.
That is why the profile above draws three lines and not one: the sightline, the full first zone, and the 60% envelope. Terrain crossing the third one is the case a plain line-of-sight check calls a pass and the installed link calls a problem.
When the path is blocked or marginal, the useful answer is not the word blocked. It is the height that fixes it, at each end, which the calculator gives you as the extra metres at A or at B that bring the worst point back over the criterion.
Link budget, and where this stops
Geometry decides whether a link is possible. It does not decide whether it works. Once the path is clear, the remaining question is a power one: transmit power, antenna gains at both ends, cable and connector losses, free space path loss over the distance, and the receiver sensitivity you have to stay above.
Free space path loss alone is 92.45 + 20 log10(distance in km) + 20 log10(frequency in GHz), in decibels. A 10 km link at 5.8 GHz starts from about 128 dB before anything else is counted.
This page does not compute any of that, and no terrain tool should pretend to. Rain fade, foliage, interference, multipath and the actual radios in the actual boxes decide the rest, and the honest scope line is that this tells you whether the geometry is on your side.
The data, and what it cannot see
Elevation comes from AWS Terrain Tiles, a global mosaic assembled from SRTM, ASTER and national datasets at about 30 m per pixel, read directly by your browser. It is a good global model and it is not a survey.
Bare earth only
No buildings, no trees, no masts. A path that clears the terrain by 3 m across a forest does not clear the forest. Add the canopy height yourself, or treat it as a site visit.
A mosaic, not one instrument
Accuracy varies by source and by region. Treat a result within a few metres of the criterion as go and look, not as a yes.
150 km, then it stops
Beyond that the atmosphere decides more than the terrain does, and a fixed refraction constant stops being a useful model of a real day.
Geometry, not signal
Clear here means clear of ground and clear of the zone. It says nothing about how strong the received signal will be.
What the app adds
The same check, run on the engine the rest of this site is built on: Copernicus GLO-30 with the half-pixel correction, a Fresnel margin shaped like the elevation model own error rather than added flat, the height trade-off between the two ends, obstacles and exclusion polygons you draw yourself, and a saved project you can come back to.
Above one link it becomes a different problem. A network is a matrix: every pair between every site, tested at once, sorted worst first, with the paths that fail separated from the ones that only just pass. That is what Studio does, and it is the reason to have an account rather than a bookmark.
Frequently asked questions
How do I check line of sight between two points?
Put both points on the map, set the height of each antenna above the ground, choose the frequency, and read the profile. The calculator walks the terrain between them at about 30 m steps, applies Earth curvature and refraction, and reports whether the direct line clears the ground and whether 60% of the first Fresnel zone is clear too.
What is the maximum range of a line of sight radio link?
The horizon of one antenna plus the horizon of the other. At the radio convention that is 4.12 times the square root of each height in metres, in kilometres. Two 30 m masts reach about 45 km over flat ground. Terrain almost always shortens it.
Is line of sight enough for a radio link?
No. A path can clear the ground and still lose signal to the first Fresnel zone, which is why this tool draws the 60% envelope as well as the sightline. Clearing the line but not the zone is the single most common reason a link that looked fine on a map underperforms.
Does this account for the curvature of the Earth?
Yes, and for refraction. The profile is drawn in the curved frame, so the ground genuinely rises between the two ends. Mid-path bulge over 50 km is about 49 m at the radio constant, which is more mast than most people expect.
Are buildings and trees included?
No. This is a bare-earth terrain model. Trees, buildings and masts are not in it and have to be added by hand or checked on site.
Is it really free?
The check on this page is free and unlimited, with no account. Point to point inside the app is a paid feature, because there it runs on the validated engine with margins, obstacles, a link matrix and a report.
Related
Run this path on the validated engine
GLO-30 elevation, a clearance margin sized to the data, the height that fixes each end, obstacles you draw, and every link in a network tested at once.
Open this path in UpToWhere