Documentation sections

Line of Sight Calculator — Radio Horizon & Earth Bulge

POSThttps://rftools.io/api/py/v1/calculate

Slug line-of-sight. Radio horizon of each antenna, maximum line-of-sight distance between two masts, Earth bulge and first Fresnel-zone radius at any point along the path, and the clearance a link needs there, over a smooth Earth with the k-factor of your choice.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of line-of-sight
Name	Unit	Range	Default	Description
antennaHeight1number	m	0 … 10000	40	Height of antenna 1 above the datum (ground or sea level)
antennaHeight2number	m	0 … 10000	30	Height of antenna 2 above the same datum
kFactornumber	none	at least 0.1	1.3333333333333333	k = 4/3 for the standard atmosphere; lower k bends rays less and raises the bulge
pathLengthnumber	km	at least 0.001	30	Distance between the two antennas
pointDistancenumber	km	at least 0	15	Where along the path to evaluate the bulge and Fresnel zone; must lie between 0 and the path length
frequencynumber	GHz	at least 0.001	6	Link frequency, for the Fresnel-zone radius
fresnelFractionnumber	none	at least 0	0.6	Fraction of the first Fresnel-zone radius to keep clear

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of line-of-sight
Field	Unit	Description
maxLosDistanceheadline	km	Maximum Line-of-Sight Distance
horizon1	km	Radio Horizon of Antenna 1
horizon2	km	Radio Horizon of Antenna 2
bulge	m	Earth Bulge at the Point
fresnelRadius	m	First Fresnel-Zone Radius
requiredClearance	m	Required Clearance (bulge + Fresnel)
rayHeight	m	Ray Height Above Smooth Earth
maxObstacleHeight	m	Highest Obstacle Allowed at the Point

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "line-of-sight",
    "inputs": {
      "antennaHeight1": 40,
      "antennaHeight2": 30,
      "kFactor": 1.3333333333333333,
      "pathLength": 30,
      "pointDistance": 15,
      "frequency": 6,
      "fresnelFraction": 0.6
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("line-of-sight", {
    "antennaHeight1": 40,
    "antennaHeight2": 30,
    "kFactor": 1.3333333333333333,
    "pathLength": 30,
    "pointDistance": 15,
    "frequency": 6,
    "fresnelFraction": 0.6,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "line-of-sight",
    "inputs": {
      "antennaHeight1": 40,
      "antennaHeight2": 30,
      "kFactor": 1.3333333333333333,
      "pathLength": 30,
      "pointDistance": 15,
      "frequency": 6,
      "fresnelFraction": 0.6
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "line-of-sight",
	  "inputs": {
	    "antennaHeight1": 40,
	    "antennaHeight2": 30,
	    "kFactor": 1.3333333333333333,
	    "pathLength": 30,
	    "pointDistance": 15,
	    "frequency": 6,
	    "fresnelFraction": 0.6
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "line-of-sight",
  "values": {
    "maxLosDistance": 48.64472092771843,
    "horizon1": 26.068627377238972,
    "horizon2": 22.576093550479456,
    "bulge": 13.243603829854028,
    "fresnelRadius": 19.35821718289161,
    "requiredClearance": 24.858534139588993,
    "rayHeight": 21.756396170145972,
    "maxObstacleHeight": 10.141465860411007
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:line-of-sight",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "ITU-R P.526-15, Eqs. (2), (21) and (22); ITU-R P.530-17, §2.2.2; ITU-R P.834-9, §2",
    "assumptions": [
      {
        "code": "smooth-earth",
        "text": "The Earth is a smooth sphere of effective radius k × 6371 km; terrain, buildings and vegetation are not modelled."
      },
      {
        "code": "uniform-refraction",
        "text": "Refraction is a single k-factor along the whole path; ducting (negative or very large k) is not modelled."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "antennaHeight1": {
          "min": 0,
          "max": 10000,
          "unit": "m"
        },
        "antennaHeight2": {
          "min": 0,
          "max": 10000,
          "unit": "m"
        },
        "kFactor": {
          "min": 0.1,
          "max": null,
          "unit": ""
        },
        "pathLength": {
          "min": 0.001,
          "max": null,
          "unit": "km"
        },
        "pointDistance": {
          "min": 0,
          "max": null,
          "unit": "km"
        },
        "frequency": {
          "min": 0.001,
          "max": null,
          "unit": "GHz"
        },
        "fresnelFraction": {
          "min": 0,
          "max": null,
          "unit": ""
        }
      },
      "outside": [],
      "model": null
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "antennaHeight1": 40,
      "antennaHeight2": 30,
      "kFactor": 1.3333333333333333,
      "pathLength": 30,
      "pointDistance": 15,
      "frequency": 6,
      "fresnelFraction": 0.6
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "antennaHeight1"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pageLine of Sight Calculator — Radio Horizon & Earth Bulge, at these values