How it works
The Physics Behind Your Result
Last updated: July 1, 2026
The short version: CanIReach combines real elevation data with a seven-factor RF propagation model — radio horizon, Fresnel zone clearance, link budget, and more — to give you a confidence estimate, not a guarantee. It's a planning tool built on real physics, not a certified engineering model, and it should never replace an actual field test.
Where the terrain data comes from
Ground Path results are built on real elevation data — Shuttle Radar Topography Mission (SRTM) data at 30-meter resolution, served through OpenTopoData. This is the same underlying global elevation dataset used by many professional propagation and mapping tools.
30 meters reliably captures hills, ridges, and most large-to-medium terrain features, but it will not catch details smaller than that resolution — a single building, a narrow ravine, or a small rise in an otherwise flat field may not show up. For open rural paths, mountain-to-mountain shots, and most POTA/SOTA-style activations, this level of detail is generally very solid. For tight urban paths or anything with fine terrain nuance, treat the result as a strong first estimate, not the final word.
The seven-factor model
Every Ground Path result is a weighted combination of seven scored factors, each rated 0–100 and combined according to how much it actually matters for whether a signal gets through:
| Factor | Weight | What it measures |
|---|---|---|
| Geometry | 30% | Radio horizon — how far you can see given antenna heights and Earth's curvature |
| Fresnel zone | 25% | Whether the terrain clears the invisible zone around the signal path that needs to stay open |
| Link budget | 20% | Whether transmit power and antenna gain are enough to overcome the calculated path loss |
| Obstruction | 10% | How much a partial terrain block degrades the signal, adjusted for band |
| Frequency | 7% | How well-suited the chosen band is for the path type and distance |
| Environment | 5% | A general penalty for the surroundings you select (open, suburban, woods, etc.) |
| Power & antenna | 3% | A smaller adjustment for your specific station setup |
Geometry — radio horizon
Beyond the radio horizon, no amount of transmit power helps — the curvature of the Earth is simply in the way. CanIReach calculates this using the standard Bullington 4/3-Earth formula, which accounts for the way the atmosphere bends radio waves slightly beyond true line of sight:
Fresnel zone clearance
Even when you technically have line of sight, a signal needs clearance around the direct path, not just along it — that's the Fresnel zone, an ellipse-shaped region around the line connecting your two radios. A hill or ridge that clips this zone without fully blocking the line of sight still causes real signal loss through diffraction. CanIReach checks this against the actual terrain samples loaded for your path:
Link budget
This is the classic radio engineering question: is there enough margin between what you're transmitting and what the receiver needs to hear you? CanIReach calculates free-space path loss and compares it against your power and antenna gain, using a standard reference receiver sensitivity of −116 dBm (12 dB SINAD, per ITU-R SM.329) — a reasonable assumption for a typical VHF/UHF FM receiver, not a spec pulled from your exact radio model:
Obstruction, frequency, environment, and station setup
The remaining factors apply smaller, more situational adjustments: how much a partial terrain block matters depends on your band (VHF diffracts more forgivingly than UHF or SHF), how well-suited your chosen frequency is for the path, a general penalty based on the surroundings you select, and a small adjustment for your specific antenna and power setup.
What the model does not account for
Being upfront about the limits of this model matters as much as explaining how it works. CanIReach does not currently model:
- Foliage and tree canopy — there's no per-path vegetation data. Instead, you select a general environment category (open, suburban, woods, etc.) that applies a broad confidence adjustment, not a measurement of actual trees along your specific path
- Buildings and structures — individual buildings aren't in the terrain data at 30m resolution and aren't separately modeled
- Multipath and reflections — signals bouncing off buildings, water, or terrain can sometimes help a path that looks marginal on paper, or hurt one that looks clear
- Your exact radio's receiver sensitivity — the model uses a standard reference value, not your specific radio's datasheet
- Local noise floor and interference — RF noise at your specific location and time isn't measured
- Weather-driven propagation anomalies — tropospheric ducting and similar atmospheric effects that can occasionally extend VHF/UHF range well beyond normal
HF Skywave is a different kind of estimate
Ground Path is a geometry and link-budget problem — the terrain and the math don't change from day to day. HF Skywave is fundamentally different: it depends on the ionosphere, which changes hour to hour based on solar activity, time of day, and season. CanIReach's Skywave planner uses live space weather data and simplified MUF (Maximum Usable Frequency) guidance to suggest favorable bands and time windows, but this is inherently an approximation. Treat Skywave results as a starting point for choosing a band and window to try, not a guarantee that a contact will happen.
Why results are a confidence band, not a promise
CanIReach reports results as Excellent, Very Likely, Likely, Possible, Weak, Unlikely, or Very Unlikely rather than a single number or a flat yes/no. That's intentional — real-world radio propagation has enough variables outside any model's control (exact antenna placement, momentary interference, atmospheric conditions, the operator on the other end) that a confident-sounding precise percentage would be misleading. The confidence band tells you how strongly the physics favor your path, so you know whether it's worth trying, worth adjusting first, or worth planning around a repeater instead.
For critical planning — emergency communications, safety-of-life operations, or anything where a wrong prediction has real consequences — cross-check CanIReach's result against a dedicated RF propagation tool such as Radio Mobile, HeyWhatsThat, or SPLAT!, which use more detailed terrain diffraction modeling. CanIReach is built to be fast and useful in the field, not to replace professional RF engineering software.
Questions or feedback
If something in your results doesn't match what you're seeing in the field, or if you have suggestions for improving the model, we want to hear about it: canireach.app@gmail.com