Editing Radar (section)

====Transit time====
{{further|Time of flight}}
[[File:Radaroperation.gif|thumb|right|Pulse radar: The round-trip time for the radar pulse to get to the target and return is measured. The distance is proportional to this time.]]

One way to obtain a [[distance measurement]] (ranging) is based on the [[time-of-flight]]: transmit a short pulse of radio signal (electromagnetic radiation) and measure the time it takes for the reflection to return. The distance is one-half the round trip time multiplied by the speed of the signal. The factor of one-half comes from the fact that the signal has to travel to the object and back again. Since radio waves travel at the [[speed of light]], accurate distance measurement requires high-speed electronics.
In most cases, the receiver does not detect the return while the signal is being transmitted. Through the use of a duplexer, the radar switches between transmitting and receiving at a predetermined rate.
A similar effect imposes a maximum range as well. In order to maximize range, longer times between pulses should be used, referred to as a pulse repetition time, or its reciprocal, pulse repetition frequency.

These two effects tend to be at odds with each other, and it is not easy to combine both good short range and good long range in a single radar. This is because the short pulses needed for a good minimum range broadcast have less total energy, making the returns much smaller and the target harder to detect. This could be offset by using more pulses, but this would shorten the maximum range. So each radar uses a particular type of signal. Long-range radars tend to use long pulses with long delays between them, and short range radars use smaller pulses with less time between them. As electronics have improved many radars now can change their pulse repetition frequency, thereby changing their range. The newest radars fire two pulses during one cell, one for short range (about {{convert|10|km|miles|abbr=on}}) and a separate signal for longer ranges (about {{convert|100|km|miles|abbr=on}}).

Distance may also be measured as a function of time. The '''radar mile''' is the time it takes for a radar pulse to travel one [[nautical mile]], reflect off a target, and return to the radar antenna. Since a nautical mile is defined as 1,852&nbsp;m, then dividing this distance by the speed of light (299,792,458&nbsp;m/s), and then multiplying the result by 2 yields a result of 12.36&nbsp;μs in duration.