Radar Standards & Concepts FAQs

ASTERIX is a family of Eurocontrol standards for exchanging surveillance data such as radar plots, tracks and video. Each type of data has its own category (CAT), for example CAT-48 for radar plots and tracks, and CAT-240 for radar video.

ASTERIX was originally developed for air traffic control in Europe but is now used more widely to exchange radar video and track data. Cambridge Pixel products use it extensively: for example, SPx Server distributes radar video and tracks in ASTERIX formats, and RadarView displays ASTERIX radar video and tracks.

More information is available from Eurocontrol's ASTERIX page. For individual categories, see our Radar Terminology glossary.

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CAT-240 is the ASTERIX category for sending radar video across a network. Plots and tracks use other categories, such as CAT-48.

The HPx-346 converts analogue radar video to ASTERIX CAT-240, SPx Server distributes radar video in ASTERIX, and the free RadarView-240 displays CAT-240 radar video as a PPI. Developers can add CAT-240 support to their own applications with the SPx Development Library.

See ASTERIX CAT-240 for more detail.

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CAT-48 is the ASTERIX category for monoradar target reports: plots and tracks from a single primary or secondary radar, usually sent across a network. CAT-48 is normally used with CAT-34, the category for the radar's service (status) messages.

CAT-34 and CAT-48 replaced the older CAT-1 and CAT-2 pair. Other ASTERIX categories for target data include CAT-10 (surface movement), CAT-21 (ADS-B) and CAT-62 (system tracks, often from several radars). Radar video uses CAT-240.

SPx Server can distribute its tracks in ASTERIX CAT-48, and RadarView can display them. For other categories, see Radar Terminology.

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SPx software supports the NMEA-0183 standard for navigation data such as position, velocity and heading, received over serial or network interfaces, including the Lightweight Ethernet (LWE) protocol.

For example, SPx Server accepts NMEA-0183 RMC, GGA, HDT, HDG, XDR and PASHR sentences by serial port or Ethernet, and can use the navigation data to convert ship-referenced radar video to North-referenced.

To check the NMEA-0183 data arriving at a system, use the free Navigation Data Monitor.

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An A-Scan (or A-Scope) display shows radar signal amplitude against range for each radar return, as an oscilloscope connected to the radar video would. An A-Scan is useful for checking radar video signal levels to verify that the radar is correctly configured.

The trace updates for each azimuth at the radar's pulse rate. RadarView provides A-Scan views alongside PPI and B-Scan, and developers can add A-Scan displays to their own applications with SPx Radar Scan Conversion.

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A B-Scan (or B-Scope) display shows radar video with azimuth along the horizontal axis and range along the vertical axis. B-Scan displays are commonly used with fire control radars, including in naval fire control.

RadarView provides B-Scan views alongside PPI and A-Scan, and developers can add B-Scan displays to their own applications with SPx Radar Scan Conversion.

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An RHI (Range Height Indicator) display shows range along the horizontal (X) axis and altitude on the vertical (Y) axis. In RadarView, unlike its other displays, the RHI shows tracks only, not radar video.

SPx Viewer-3D also has an RHI view showing a 2D cross-section of terrain and track positions, and the browser interface of SPx Tracker-3D includes a range-height indicator.

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RPM stands for Revolutions Per Minute. Typically it is used to describe how fast a radar rotates, i.e. how many times it sweeps through 360 degrees in 60 seconds. It is inversely related to the Period.

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Period is the number of seconds that an event takes to repeat. It is often used to describe how long a radar takes to sweep through 360 degrees. For example, a 2 second period means a radar rotates completely every 2 seconds, which is the equivalent of a 30RPM rotation rate.

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PIM is an acronym for Polar Interface Module. This is one of the classes (SPxPIM) within Cambridge Pixel’s SPx Development Library and is used for the storage of radar video data for processing and display.

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Several do. SPx Alarm Server, RadarWatch and the Maritime Display Framework calculate CPA (Closest Point of Approach) and TCPA (Time to CPA) and can raise alarms on them, and SPx Server's proximity detection supports collision avoidance.

• SPx Server: proximity detection for collision avoidance and autonomous navigation, in the Detection and Tracking versions of SPx Server.
• SPx Alarm Server: alarms on proximity or CPA to a point, shape or area, or to any other track, sent as NMEA-0183 TTM messages; a supplied utility can drive Modbus/TCP relays for alarm sounders.
• RadarWatch: CPA/TCPA between pairs of targets and between targets and reference points, with visual, audible and network alarms.
• Maritime Display Framework: predictive vectors and CPA/TCPA alarms, as typically found in ARPA displays.

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Radar scan conversion transforms radar video from polar format (range and azimuth) into Cartesian form for display on a computer screen, for example as a PPI. Scan conversion must represent targets accurately and introduce no gaps in the video.

Scan conversion is a core Cambridge Pixel capability. The SPx scan converter runs in software on standard processors and graphics hardware, with no special hardware, and supports fade, motion and trail-history modes, multiple windows and radars up to 240 rpm. It is used in RadarView and RadarWatch, and the technology is in service on over 100 military vessels.

Developers can add scan-converted PPI, A-Scan or B-Scan displays to their own Windows or Linux applications with SPx Radar Scan Conversion, part of the SPx Development Library.

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Plot extraction (also called target detection) finds blobs of radar video that meet set size and strength criteria. Each plot may be a real target or noise, and has no speed or course. Tracking correlates plots over time to reject false targets, producing tracks with position, speed and course.

In SPx Server, plot extraction finds spans of processed video above a threshold on each radar return, joins them across returns into connected shapes, and records each plot's centre of gravity, bounding box, weight and timestamp. SPx Server then tracks the plots with a multi-hypothesis tracker and can send plots and tracks to displays and other systems, for example in ASTERIX CAT-48.

Plot extraction is also available in the HPx-700 and the SPx Development Library. See Plot Extraction for more detail.

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A multi-hypothesis tracker considers several possible interpretations of the radar measurements at the same time, rather than committing to one, which significantly improves performance over simpler single-hypothesis trackers. SPx Server uses multi-hypothesis association in its radar tracker.

For each hypothesis, the tracker updates the estimated target position with the new measurement, using a filter whose gain adapts to the expected system and measurement noise. In SPx Server, single or multi-hypothesis association is configurable, and the Model Based Tracking (MBT) extension handles more provisional tracks than standard multi-hypothesis initiation, so that small, low-observable targets such as drones can be tracked alongside conventional targets.

See Radar Terminology for related terms such as Kalman filter and tracking.

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Sector blanking forces radar video to zero over a chosen sequence of azimuths, so that a sector of the radar picture is blanked.

In Cambridge Pixel products, sector blanking is available in the HPx-346 analogue-to-network converter and as a processing module (area, sector and range blanking) in the SPx Development Library. Cambridge Pixel's radar controller software can also offer sector blanking as a radar control, depending on the radar model.

See Radar Terminology for related terms.

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