Editing Radar (section)

===Distance measurement===

====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.

====Frequency modulation====
[[File:Sonar Principle EN.svg|thumb|right|Continuous wave (CW) radar. Using frequency modulation allows range to be extracted.]]

{{main|Frequency modulation}}
Another form of distance measuring radar is based on frequency modulation. In these systems, the frequency of the transmitted signal is changed over time. Since the signal takes a finite time to travel to and from the target, the received signal is a different frequency than what the transmitter is broadcasting at the time the reflected signal arrives back at the radar. By comparing the frequency of the two signals the difference can be easily measured. This is easily accomplished with very high accuracy even in 1940s electronics. A further advantage is that the radar can operate effectively at relatively low frequencies. This was important in the early development of this type when high-frequency signal generation was difficult or expensive.

This technique can be used in [[continuous wave radar]] and is often found in aircraft [[radar altimeter]]s. In these systems a "carrier" radar signal is frequency modulated in a predictable way, typically varying up and down with a [[sine wave]] or sawtooth pattern at audio frequencies. The signal is then sent out from one antenna and received on another, typically located on the bottom of the aircraft, and the signal can be continuously compared using a simple ''beat frequency'' modulator that produces an audio frequency tone from the returned signal and a portion of the transmitted signal.

The [[Frequency modulation#Modulation index|modulation index]] riding on the receive signal is proportional to the time delay between the radar and the reflector. The frequency shift becomes greater with greater time delay. The frequency shift is directly proportional to the distance travelled. That distance can be displayed on an instrument, and it may also be available via the [[Transponder (aeronautics)|transponder]]. This signal processing is similar to that used in speed detecting Doppler radar. Example systems using this approach are [[AZUSA]], [[MISTRAM]], and [[UDOP]].

Terrestrial radar uses low-power FM signals that cover a larger frequency range. The multiple reflections are analyzed mathematically for pattern changes with multiple passes creating a computerized synthetic image. Doppler effects are used which allows slow moving objects to be detected as well as largely eliminating "noise" from the surfaces of bodies of water.

====Pulse compression====
{{main|Pulse compression}}

The two techniques outlined above both have their disadvantages. The pulse timing technique has an inherent tradeoff in that the accuracy of the distance measurement is inversely related to the length of the pulse, while the energy, and thus direction range, is directly related. Increasing power for longer range while maintaining accuracy demands extremely high peak power, with 1960s [[early warning radar]]s often operating in the tens of megawatts. The continuous wave methods spread this energy out in time and thus require much lower peak power compared to pulse techniques, but requires some method of allowing the sent and received signals to operate at the same time, often demanding two separate antennas.

The introduction of new electronics in the 1960s allowed the two techniques to be combined. It starts with a longer pulse that is also frequency modulated. Spreading the broadcast energy out in time means lower peak energies can be used, with modern examples typically on the order of tens of kilowatts. On reception, the signal is sent into a system that delays different frequencies by different times. The resulting output is a much shorter pulse that is suitable for accurate distance measurement, while also compressing the received energy into a much higher energy peak and thus improving the signal-to-noise ratio. The technique is largely universal on modern large radars.