RADAR SCAN TYPES

Introduction

1. To measure a target’s bearing accurately radar needs to have a narrow beamwidth. Unfortunately narrow beamwidths reduces the instantaneous look-area; therefore to cover a wider area, a radar system needs to scan. Different types of radars use different types of scan, depending upon the function of the radar.

What is Scanning?

2. Scanning can be described as the search pattern of radar to detect targets. It may involve physically moving the antenna to change the orientation of the transmitted beam or alternatively alter the direction of the radar beam by electronic means.

3. Scanning is different from tracking. Tracking involves the precise and continuous measurement of a target’s range, angle and/or velocity to determine its flight path and predict its future position. Some radar systems, particularly airborne radars, can perform both the tracking and scanning functions at the same time these are called track-while-scan radars.

Why Scan?

4. Currently no radar system can cover 360º in azimuth and elevation to detect targets. To ensure maximum coverage wide beamwidths are used, this results in poor angular resolution. Narrow beamwidths have good resolution, but less space is searched, therefore the radar beam must be scanned in order to detect targets.

Scan Duration

5. Different radar functions require different types of antenna, which produce different beam shapes that are scanned in different ways. Not only are the methods of scanning different, but also the duration of the scan differs. The following terms may be used to describe the scan duration:

a. Scan Period. The time taken to complete a scan pattern and return to the starting point, expressed in seconds.

b. Antenna Rotation Period (ARP). Another term for scan period, although only used when describing circular scanning radar.

c. Rotation Rate. The number of complete scans in one minute, expressed as revolutions per minute (rpm). Also used to describe circular scanning radars.

d. Scan Rate. The number of complete scans in one minute or per second. Scan rate is often used to describe non-circular scanning radars.

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Scan Types

6. Circular Scan means that the radar antenna is scanned through 360º in azimuth whilst continuously transmitting is said to have a circular scan. Long-range early warning, air defence and surveillance radars usually use it. The ARP of circular scanning radars depends upon the required maximum range and can vary from 10 to 20 seconds for long-range systems.

a. Older circular scanning radars used a fan beam with narrow beamwidths in azimuth (about 2º for good azimuth resolution) and wide beamwidths in elevation (about 10º to cover the maximum volume searched). Unfortunately this gave poor elevation resolution.

b. Shorter-range radar systems may also use circular scanning for target acquisition. AAA and SAM search radars may have ARPs of around one second.

7. Sector scan is usually used to search in a particular direction or area, typically by target Acquisition Radars (TAR). TARs may scan sectors that contain potential targets, allocated by early warning or long-range air defence radars. Scan rates of sector scanning radars vary according to angle scanned and function of the radar. There are 3 types of sector scan:

a. Bi-Directional Sector Scan. Bi-directional sector scan radar transmits as the antenna moves in both directions through a fixed point. Typical of this application are weather or ground mapping radars.

b. Unidirectional Scan. The radar antenna is rotated through 360 with the beam only being transmitted in the sector of interest. Unidirectional sector scan may be used without the target knowing it is being sector scanned. The E-3 Airborne Warning and Command System (AWACS) and ground-based long-range air defence radars may use unidirectional scanning.

c. Vertical Sector Scan. This is used by height-finding radar, which has narrow elevation beamwidths. Scan rates vary from 5 to 30 scans per minute.

8. Conical Scan means that the beam is rotated rapidly around a central boresight. The rapid rotation can be achieved by mechanically rotating the radar feed or by rotating the antenna reflector itself. A narrow beamwidth is used for good angle resolution and the angle between the centre of the beam and the boresight is called the squint angle. The squint angle is always less than half the beamwidth. Scan rates for conical scanning radars generally vary from 50 to 100 Hz.

9. Raster Scan is used by air intercept (AI) radars to search a large volume of airspace in azimuth and elevation with good angle resolution. The beam is scanned in azimuth through the sector of interest. At the end of each scan the elevation of the beam is altered by less than the overall beamwidth and the beam swept in the opposite direction. Raster scanning can accurately fix a target position in 3 dimensions: range, azimuth and elevation. Scan rates vary according to the angle of sector, but may generally be in the region of 4 to 5 scans per second.

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10. Electronic scanning is quicker and more agile than mechanical methods. By electronically controlling the phase of the signal sent to individual antenna elements the radar beam may be steered. The shape and polarisation of the beam can also be altered. Electronic scanning can also be combined with mechanical scanning to produce a combination of scan patterns, such as circular scan with raster scan.

Summary

11. Scanning can be described as the search pattern used by radar to detect targets. The duration of a scan is also known as: scan period, antenna rotation period, and rotation rate or scan rate.

12. Circular scanning is usually used by long-range early warning, air defence and surveillance radars, with ARPs of 10 to 20 seconds. Short-range TARs for AAA and SAM systems may also use circular scanning, with ARPs of around one second. There are 3 types of sector scan: bi-directional sector scan, unidirectional sector scan and vertical sector scan. Conical scan involves rapidly rotating a narrow beamwidth around a central boresight. Raster scan is used by AI radars and involves sector scanning in azimuth, with successive alterations in elevation by less than the overall beam width. 13. Electronic scanning is quicker and more agile than mechanical methods. It allows radar to steer, change beam shape and polarisation. Modern multi-mode or multifunction radars use it.

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