Editing Radar (section)

==Signal processing==

===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.

===Speed measurement===
[[Speed]] is the change in distance to an object with respect to time. Thus the existing system for measuring distance, combined with a memory capacity to see where the target last was, is enough to measure speed. At one time the memory consisted of a user making [[grease pencil]] marks on the radar screen and then calculating the speed using a [[slide rule]]. Modern radar systems perform the equivalent operation faster and more accurately using computers.

If the transmitter's output is coherent (phase synchronized), there is another effect that can be used to make almost instant speed measurements (no memory is required), known as the [[Doppler effect]]. Most modern radar systems use this principle into [[Doppler radar]] and [[pulse-Doppler radar]] systems ([[weather radar]], military radar). The Doppler effect is only able to determine the relative speed of the target along the line of sight from the radar to the target. Any component of target velocity perpendicular to the line of sight cannot be determined by using the Doppler effect alone, but it can be determined by tracking the target's [[azimuth]] over time.

It is possible to make a Doppler radar without any pulsing, known as a [[continuous-wave radar]] (CW radar), by sending out a very pure signal of a known frequency. CW radar is ideal for determining the radial component of a target's velocity. CW radar is typically used by traffic enforcement to measure vehicle speed quickly and accurately where the range is not important.

When using a pulsed radar, the variation between the phase of successive returns gives the distance the target has moved between pulses, and thus its speed can be calculated.
Other mathematical developments in radar signal processing include [[time-frequency analysis]] (Weyl Heisenberg or [[wavelet]]), as well as the [[chirplet transform]] which makes use of the change of frequency of returns from moving targets ("chirp").

===Pulse-Doppler signal processing===
{{main|Pulse-Doppler signal processing}}
[[File:Pulse doppler signal processing.png|thumb|Pulse-Doppler signal processing. The ''Range Sample'' axis represents individual samples taken in between each transmit pulse. The ''Range Interval'' axis represents each successive transmit pulse interval during which samples are taken. The Fast Fourier Transform process converts time-domain samples into frequency domain spectra. This is sometimes called the ''bed of nails''.]]

Pulse-Doppler signal processing includes frequency filtering in the detection process. The space between each transmit pulse is divided into range cells or range gates. Each cell is filtered independently much like the process used by a [[spectrum analyzer]] to produce the display showing different frequencies. Each different distance produces a different spectrum. These spectra are used to perform the detection process. This is required to achieve acceptable performance in hostile environments involving weather, terrain, and electronic countermeasures.

The primary purpose is to measure both the amplitude and frequency of the aggregate reflected signal from multiple distances. This is used with [[weather radar]] to measure radial wind velocity and precipitation rate in each different volume of air. This is linked with computing systems to produce a real-time electronic weather map. Aircraft safety depends upon continuous access to accurate weather radar information that is used to prevent injuries and accidents. Weather radar uses a [[Pulse repetition frequency#Low PRF|low PRF]]. Coherency requirements are not as strict as those for military systems because individual signals ordinarily do not need to be separated. Less sophisticated filtering is required, and range ambiguity processing is not normally needed with weather radar in comparison with military radar intended to track air vehicles.

The alternate purpose is "[[look-down/shoot-down]]" capability required to improve military air combat survivability. Pulse-Doppler is also used for ground based surveillance radar required to defend personnel and vehicles.<ref name="Syracuse Research Corporation; Massachusetts Institute of Technology">{{cite web|url=https://www.mit.edu/~lrv/cornell/publications/Ground%20Surveillance%20Radars%20and%20Military%20Intelligence.pdf|title=Ground Surveillance Radars and Military Intelligence|publisher=Syracuse Research Corporation; Massachusetts Institute of Technology|url-status=dead|archive-url=https://web.archive.org/web/20100922174712/http://www.mit.edu/~lrv/cornell/publications/Ground%20Surveillance%20Radars%20and%20Military%20Intelligence.pdf|archive-date=22 September 2010}}</ref><ref>{{cite web|url=https://www.youtube.com/watch?v=B0q1Pgz6Cm8| archive-url=https://ghostarchive.org/varchive/youtube/20211030/B0q1Pgz6Cm8| archive-date=30 October 2021|title=AN/PPS-5 Ground Surveillance Radar| date=29 December 2009|via=YouTube; jaglavaksoldier's Channel}}{{cbignore}}</ref> Pulse-doppler signal processing increases the maximum detection distance using less radiation close to aircraft pilots, shipboard personnel, infantry, and artillery. Reflections from terrain, water, and weather produce signals much larger than aircraft and missiles, which allows fast moving vehicles to hide using [[nap-of-the-earth]] flying techniques and [[stealth technology]] to avoid detection until an attack vehicle is too close to destroy. Pulse-Doppler signal processing incorporates more sophisticated electronic filtering that safely eliminates this kind of weakness. This requires the use of medium pulse-repetition frequency with phase coherent hardware that has a large dynamic range. Military applications require [[Pulse repetition frequency#Medium PRF|medium PRF]] which prevents range from being determined directly, and [[range ambiguity resolution]] processing is required to identify the true range of all reflected signals. Radial movement is usually linked with Doppler frequency to produce a lock signal that cannot be produced by radar jamming signals. Pulse-Doppler signal processing also produces audible signals that can be used for threat identification.<ref name="Syracuse Research Corporation; Massachusetts Institute of Technology"/>

===Reduction of interference effects===
[[Signal processing]] is employed in radar systems to reduce the [[#Interference|radar interference effects]]. Signal processing techniques include [[moving target indication]], [[Pulse-Doppler signal processing]], moving target detection processors, correlation with [[secondary surveillance radar]] targets, [[space-time adaptive processing]], and [[track-before-detect]]. [[Constant false alarm rate]] and [[digital terrain model]] processing are also used in clutter environments.

===Plot and track extraction===
{{Main|Radar tracker|Track algorithm}}

A track algorithm is a radar performance enhancement strategy. Tracking algorithms provide the ability to predict the future position of multiple moving objects based on the history of the individual positions being reported by sensor systems.

Historical information is accumulated and used to predict future position for use with air traffic control, threat estimation, combat system doctrine, gun aiming, and missile guidance. Position data is accumulated by radar sensors over the span of a few minutes.

There are four common track algorithms:<ref>{{cite web|url=http://www.aticourses.com/fundamentals_radar_tracking.htm |title=Fundamentals of Radar Tracking |publisher=Applied Technology Institute |url-status=dead |archive-url=https://web.archive.org/web/20110824221707/http://www.aticourses.com/fundamentals_radar_tracking.htm |archive-date=24 August 2011 }}</ref>
* [[Nearest neighbour algorithm]]
* [[Probabilistic data association filter|Probabilistic Data Association]]
* Multiple Hypothesis Tracking
* Interactive Multiple Model (IMM)

Radar video returns from aircraft can be subjected to a plot extraction process whereby spurious and interfering signals are discarded. A sequence of target returns can be monitored through a device known as a plot extractor.

The non-relevant real time returns can be removed from the displayed information and a single plot displayed. In some radar systems, or alternatively in the command and control system to which the radar is connected, a [[radar tracker]] is used to associate the sequence of plots belonging to individual targets and estimate the targets' headings and speeds.