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Radar Terminology

A helpful glossary of frequently used radar and related terms. Can't see something that you're looking for here? Email us your questions; we will be happy to help.

  • 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. 
  • ASTERIX: A family of standards for radar data formats, originally developed for Air Traffic Control in Europe, but now used more widely for radar video and track format exchange in other market sectors. The specific standards are given a CAT (Category) number, for example CAT 240 is the standard for radar video.
  • 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.
  • CAT-240 is the ASTERIX message format for network distribution of rotating radar video distribution in ASTERIX.
  • CAT-48 is the ASTERIX message format for network distribution of radar plot and tracks from primary or secondary radars.
  • CAT-21 is the ASTERIX message format for the distribution of ADS-B messages.
  • 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. 
  • IGMP (Internet Group Management Protocol) is used to control multicast group membership.
  • Kalman Filter – A well-established method of filtering noisy measurements to get a best estimate of a position or state. The filter takes into account uncertainty in the current estimate and uncertainty in the measurement, effectively providing a dynamic adjustment of filter gain to best reflect the balance of confidence in the existing estimate and the new measurement.
  • KML is a file format used to display geographic data, for example in Google Earth and similar products.
  • Latency is a time lag between receipt of a signal or data and its appearance on a display or at the next stage of processing.  In the context of radar video, latency would normally mean the time between receipt of the video form the radar and its rendering on screen.

  • Multicast transmission allows an IP packet to be sent simultaneously to a group of hosts on a network.

  • Network Distribution is the process of sending data (e.g. radar video, tracks, AIS, navigation data) across an Ethernet network. 

  • NMEA-0183: A maritime standard for the transmission of navigation and related information, including GPS, heading, speed, position, AIS and tracks.
  • 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.
  • Packetization is the process of wrapping up a data payload (e.g. a radar video return) by adding network headers before sending it out over Ethernet.  
  • 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 Interface is the signal output of the radar, to which external equipment may be connected.  It may be a set of analogue signals, comprising amplitude video, trigger (synchronization) and azimuth information or it may be an Ethernet output providing network video or detection data in a specific format.

  • 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.
  • RDF (Radar Direction Finding) is a method of measuring the direction of a radio source using a radio receiver. Several RDF receivers at different locations can be used to estimate a position using triangulation.
  • RPM: Revolutions Per Minute.
  • Scan Conversion is a core capability provided by Cambridge Pixel, it is the process of transforming polar format radar video data into Cartesian form, suitable for display on a modern computer screen. Scan conversion must ensure that targets are accurately represented on the display and that no gaps are introduced into the video.  Cambridge Pixel’s scan conversion process is highly-optimised and supports different fade modes, motion modes and trail history modes.
  • 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.
  • SHM: The Ship Heading Marker (SHM) signal is a reference pulse that occurs at a reference point on the ship, normally the bow crossing. The reference pulse is used to define the specific angle of the radar at the time of the pulse. Typically, a separate Azimuth Change Pulse (ACP) or Bearing Pulse (BP) is then used to indicate a change in the azimuth position around the rotating radar sweep.
  • 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.
  • Transceiver: A combined transmitter and receiver.
  • Unicast transmission involves sending an IP packet to a single specific recipient on a network.
  • WAN (Wide Area Network) is a means of connecting local networks, allowing IP traffic to flow between them.
  • X-band or S-band: Although a radar generally operates at a particular frequency, the frequency spectrum is partitioned into bands. For example X-band denotes the range of frequencies 8 to 12 GHz and S-band denotes the frequency range 2 to 4 GHz. There are generally various positives and negatives associated with each band.

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