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Radar Terminology

A glossary of frequently used radar and related terms.

  • A-Scan is a view of radar video traditionally associated with an oscilloscope display. It shows signal intensity on the vertical axis and range (time) along the horizontal axis. The trace is updated for each azimuth at the pulse rate of the radar.
  • ACP/ARP are sets of pulses used to encode the bearing/azimuth angle of the radar antenna. The ACP (Azimuth Change Pulse) is a pulse train where the interval between pulses is a fixed angle. Commonly there are 2048, 4096 or 8192 pulses per 360 degrees. The ARP (Azimuth Reset Pulse) is a reset pulse that occurs at a reference point. The angle of the radar is therefore derived by counting the number of ACP pulses since the last ARP pulse.
  • ADS-B: Automatic Dependent Surveillance Broadcast (ADS-B) is a method used by aircraft to report their GPS-derived position to a remote receiver, allowing the aircraft to be identified and tracked. 
  • Azimuth Correlation may be employed if there are more input returns than required for display or processing, or if some degree of processing is required for filtering. The process combines a number of input returns for each output using either a peak-picking, averaging or other method of combination.
  • B-Scan is a view of radar video that shows azimuth along the horizontal axis and range along the vertical axis. It is commonly seen in fire control radars.
  • Fast Time Constant (FTC) is a processing operation that can be applied to a radar video signal to remove low frequency components, for example due to weather effects. The FTC processing will filter these low frequency components, so that only pulses that rise and fall quickly will be displayed.
  • Frequency Modulated Continuous Wave (FMCW) radars transmit a continuous beam of RF energy with a linearly changing frequency. The transmitted frequency at any given time is always known. By comparing the received frequency to the current transmit frequency a reliable measurement of distance may be obtained. The advantage of FMCW over pulsed radars is that they can use much less powerful transmitters. Since they are continuously transmitting, an equivalent amount of energy can be delivered to a target from a lower powered transmitter, compared to one that uses short pulses.
  • Heading-referenced radar video is radar video where the azimuth is measured relative to the current heading of the platform on which the radar is mounted. Each time the radar antenna sweeps past the bow of the ship (typically) the azimuth is reset to zero. The radar video is therefore always aligned to the platform's direction.
  • IFF: Identification Friend or Foe. A system for interrogating aircraft to get information. Different modes  (1, 2, 3/A, 4 and 5) in transmission are used to request different information from the aircraft. 
  • North-referenced radar video is radar video where the azimuth is measured relative to North. Each time the radar antenna sweeps past North the azimuth is reset to zero. The radar video is therefore always aligned to North.
  • Plot Extraction (also known as "target detection") is the process of extracting contiguous blobs (plots) of radar video that meet defined size and strength criteria. A plot is potentially a target of interest but may also be noise that happens to meet the size/strength criteria. Plots need to be correlated over time in order to filter out false targets (see "Tracking"). Plots do not contain any information about target dynamics.
  • PPI (Plan Position Indicator) is the view of radar video that shows the radar as it would appear on a plan view, that is a polar coordinate display of the area surrounding the radar platform. The radar position is represented as the origin of the sweep, which is normally located in the centre of the scope, but may be offset from the centre.
  • Pulse Repetition Frequency (PRF) is the rate at which the radar generates new returns. In a pulse radar it is the frequency of transmission of new pulses.
  • Radar Video is derived as a sequence of returns, sometimes called spokes, each of which contain a set of amplitude samples as a function of range for a specific azimuth. The radar may rotate or scan, and in so-doing creates a sequence of returns. In the situation of a search radar, for example, the rotating radar creates returns at a rate defined by the pulse repetition frequency (PRF) and the rotation of the radar increases the azimuth from 0 to 360 degrees.
  • Random Scan is a feature of electronically steered radars that can very quickly "look" at different azimuth numbers that are not necessarily increasing steadily over time. In general, the radar can produce any arbitrary azimuth sequence and the display is expected to show this effect. In this situation the radar is said to be working in random scan mode.
  • Range Correlation may be used if there are more samples per return than required for display or processing. A number of adjacent range samples are combined using either a peak-picking, averaging or other method of combination.
  • Range Resolution is the ability of the radar to discriminate two targets that are closely spaced in range. For example, a range resolution of 10 metres means that two targets that are on the same azimuth and 10 metres apart in range can be resolved.
  • Range Sampling is when an analogue radar video signal is measured at discrete time intervals. The frequency of sampling is limited by the capture hardware and is ideally chosen to ensure that the full bandwidth of the radar video is captured. For example, a radar video signal that is bandwidth limited to B Hz can be fully reproduced by sampling at 2B Hz.
  • Scan Rate/Period is the rotation rate/period of a search radar, for example as the time between two North crossings.
  • Sector Blanking is the process by which a sequence of azimuth values are blanked, meaning that the video is forced to value 0.
  • Sector Scan is the situation where the radar is scanning between a start and stop azimuth, rather than a continuous 360 degrees.
  • Sparse Azimuths describes the situation when the number of returns being generated by the radar is less than a pre-defined store dimension. For example, if a store dimension is set to 2000 but the radar only generates 500 returns per scan, then only 1 in 4 of the azimuths in the store is populated. Special processing can be invoked to fill-in the sparse azimuths.
  • Sensitivity Time Control (STC) is a processing technique commonly used to reduce sea clutter at shorter ranges. In general STC applies an attenuation to the video amplitude that varies with range. At short ranges, where sea clutter is strongest, the STC process applies greater attenuation. The attenuation will decrease with range, falling to zero at some defined limit.
  • Tracking is the process of correlating plots over time to filter out false targets (noise) from targets of interest. Physical targets of interest will move in a consistent, predictable manner and may therefore be observed and correlated over time. After a number of observations a track may be created, containing information about the target's speed and course, as well as its position. The input to the tracking process is plots, typically from the plot extraction process.
  • Transciever: A combined transmitter and receiver.

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