Because of atmospheric refraction, electromagnetic waves get bent downwards and can propagate beyond the geometric horizon. As a result, the refractive index of the atmosphere falls with height. With the standard atmosphere (we will talk about the standard atmosphere later), as we move to higher altitudes, the temperature and pressure are lowering and the air becomes thinner. Now we will try to consider the refraction of the electromagnetic waves propagating in the atmosphere. The refractive index of the atmosphere n falls with height and because of this fact, the radio waves get refracted or bend downwards That is, we can calculate the radar horizon for satellites and space stations on low Earth orbit. The percentage error is less than 1% when h r<250 km and h t<250 km. When h r and h t are small compared to the Earth radius, these can be approximated by Note the right angles between the line of sight and the Earth radius R 0 at the tangent point, which means we have two right triangles.
The line of sight to the horizon is the blue line labelled d t- d t where d t is the distance from the point of tangency to the target, D = d h + d t is the target visibility distance. The radar antenna is elevated at the height h r and the target is flying at the height h t. In the picture above, the brown circuit is the Earth surface. We will also assume (at this point) that there is no refraction in the atmosphere. To make our calculations easier, we will assume that the Earth is a perfect sphere with the radius of R 0=6,371.009 km. The nautical mile is abbreviated as M, NM, nm, or nmi.Įxample: Calculate the radar horizon and the target visibility if the height of the radar antenna above the surface is 10 m and the height of the target is 15 m above the surface.įor a radar system, the radar horizon is defined by the distance at which the radar beam “touches” the Earth’s surface and the detection of targets moving below the beam is impossible. Note that in air and marine navigation, a nautical mile is used as a unit of distance because the nautical mile is one minute of latitude.
If the target altitude is zero (a ground target), we are talking about geometrical and radar horizon. This radar horizon and target visibility calculator determines the geometric target visibility and radar target visibility (taking into account the refraction of radio waves in the atmosphere) taking into account the radar and the target elevation. D t is the distance from the point of tangency to the target,ĭ= d h + d t is the target visibility distance and R 0 is the mean radius of the Earth
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