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Radar Terminology

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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.

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Browse our educational 'Engineering Insights' articles.

Engineering Insights
  • A-Scan: 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: sets of pulses used to encode the bearing/azimuth angle of the radar antenna. The Azimuth Change Pulse (ACP) 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 Azimuth Reset Pulse (ARP) 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 is a method used by aircraft to report their GPS-derived position to a remote receiver, allowing the aircraft to be identified and tracked. 
  • AIS: the Automatic Identification System is a tracking system that uses transceivers on ships and is mainly used by Vessel Traffic Services (VTS). Information provided by AIS includes unique identification, position, course, and speed. Having AIS is a legal requirement for many vessels.
  • 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.
  • Bistatic radar: consists of a transmitter and receiver located at a distance comparable to that between the transmitter and the target.
  • B-Scan: 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: the ASTERIX message format for network distribution of rotating radar video distribution in ASTERIX. Also written as CAT240.
  • CAT-48: the ASTERIX message format for network distribution of radar plot and tracks from primary or secondary radars. Also written as CAT048.
  • CAT-21: the ASTERIX message format for the distribution of ADS-B messages. Also written as CAT021.
  • CCD: Coherent Change Detection is most often used with Synthetic Aperture Radar, to detect and measure changes of one geographic region on the surface of the Earth.
  • CFAR: Constant False Alarm Rate is a way for adaptively adjusting the detection sensitivity for a radar signal by adjusting the threshold to maintain a constant false alarm rate in an area.
  • CMS: Combat Management Systems mostly refer to naval central command and decision-making systems that typically support sensor interfacing and signal processing for detection and tracking, data distribution, situational displays, action support and weapon control for naval missions. 
  • Course Up Display: in course-up display, the 'up' direction represents the direction which has been input as the vessel's desired course.
  • Digital Radar Signal Processing: involves the conversion of radar signals to digital form after IF amplification and phase sensitive detection.
  • Dynamic Thresholding: a process for concerting an analogue signal into a two-level signal by comparing the signal level with a locally-derived threshold value. The input signal becomes 1 where the input exceeds the threshold and 0 where it does not, with the threshold itself taking a different value for each sample according to the local signal around the test point. 
  • FTC: Fast Time Constant 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.
  • FMCW: Frequency Modulated Continuous Wave 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: 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.
  • Head-up Display: a form of display in which the vertical axis of the screen corresponds to the heading of own-ship. Own ship always appears to move upwards and other targets and background map are adjusted accordingly. 
  • IFF: Identification Friend or Foe is 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: Keyhole Markup Language is a file format used to display geographic data, for example in Google Earth and similar products.
  • Latency: 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.

  • Monostatic Radar: a conventional radar in which the transmitter and receiver are co-located.

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

  • Multistatic radar: Also known as multsite or netted radar, a multistatic radar system fuses data from multiple monostatic radar or bistatic radar components to provide a shared area of coverage.

  • MSSR: Monopulse Secondary Surveillance Radar (or Monopulse SSR) uses signal processing and a reduced rate of radar transmissions to improve accuracy over other types of secondary surveillance (transponder-based) radars.

  • NCTR: Non Cooperative Target Recognition is the process of identifying unknown targets without the requirement of establishing communication with them.  It includes a wide range of technology which will typically compare observed target data with a database of potential targets to try and identify the closest match.

  • 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.
  • North-up Display: a form of display in with the vertical axis of the screen corresponding to North. The heading of own ship is then shown as a vector pointing in the appropriate direction. 
  • Packetisation: the process of wrapping up a data payload (e.g. a radar video return) by adding network headers before sending it out over Ethernet.  
  • Passive Radar: a radar system without a dedicated transmitter. The receiver relies on third-party transmitters, measuring the time difference between the signal arriving directly from the transmitter and the signal reflected from the targets.  As there is no dedicated transmitter, passive radar is used for covert surveillance.
  • 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.
  • PRF: Pulse Repetition Frequency is the rate at which the radar generates new returns. In a pulse radar it is the frequency of transmission of new pulses.
  • Radar: Originally RADAR was an acronym for 'Radio Detection and Ranging', now used a noun, for a system that transmits radio waves and receives the reflections, to determine the distance (ranging), direction (azimuth and elevation angles), and radial/line-of-sight velocity of objects relative to the radar.
  • Radar Data Processing: pertains exclusively to handling of information about a target. 
  • Radar Interface: 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 (synchronisation) and azimuth information or it may be an Ethernet output providing network video or detection data in a specific format.

  • Radar Signal Processing: the process of calculating an evaluatable image on a radar display is called radar signal processing, typically dealing with the analogue and digital magnitude of the echo signal.

  • Radar Video: 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: 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: 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: 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.
  • SAR: Synthetic Aperture Radar is a form of radar that is used to create high-resolution 2D images or 3D reconstructions of objects, such as the Earth's surface. Used in various applications like mapping, surveillance, and environmental monitoring, SAR works by moving the radar antenna along a path and combining the received signals to simulate a large antenna aperture.
  • Scan Conversion: 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: the rotation rate/period of a search radar, for example as the time between two North crossings.
  • Sector Blanking: the process by which a sequence of azimuth values are blanked, meaning that the video is forced to value 0.
  • Sector Scan: the situation where the radar is scanning between a start and stop azimuth, rather than a continuous 360 degrees.
  • SHM: the Ship Heading Marker 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.
  • Slew-to-cue: this term refers to the integration of a radar or other target-detection device that directs an Electro-Optical/Infra-Red (EO/IR) Pan Tilt Zoom (PTZ) camera where to point, hence the term. An electronic detection sensor, like a radar, is programmed to identify and determine the location of the target and transmit that information to the PTZ camera, which in turn tracks, identifies and recognizes the target. 
  • 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.
  • STC: Sensitivity Time Control 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: 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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