A radar wave travels through the air at the speed of light…until it finds a medium that slows it down. As the energy travels through the medium, parts of the wave refract (bend), changing directions until the wave returns to the radar receiver. When it does this, though, the amplitude (a.k.a. how far above or below its central point the wave moves) decays on the way back to the radar, making the range of a radar wave limited.

Once the radar waves return to the receiver, the time difference between the sent wave and the return wave is calculated. After we've got the time difference, we can use it to find the slant range—the linear distance between the radar and the object the wave reflected off of. The speed of light helps translate all that for us in this formula:

 

R_s stands in for slant range, c is the speed of light and T is the round-trip time difference.

Actually, this equation only really works for figuring out the distance of metallic or near-metallic objects because…they have great reflection qualities. Even then the accuracy of the slant range depends on tons of factors, meaning that we'll need to measure how inaccurate the radar's readings can be, which is called the range resolution.

Still, general distance isn't bad.