Scans/Scan Types

Question 1

What is a scan?

Answer 1

It’s the process of how a RADAR searches it’s environment by shifting its radiation pattern.

Question 2

How are scans measured?

Answer 2

Scan Period and Scan Rate.

Question 3

What is a Scan Period?

Answer 3

It represents the time it takes for one complete scan cycle.
It’s normally used to describe slow RADARs ( RADARs that take one second or longer to complete a full cycle).

Question 4

What is a Scan Rate?

Answer 4

This indicates how many complete scan cycles occur within a given time interval, most often expressed in cycles per second, or Hz.
Also known as illumination rate
Normally applies to faster RADARs that take less than a second to complete.

Question 5

What are the two distinct groups of scanning?

Answer 5

Mechanical and Electronic.

Question 6

What consists of Mechanical Scanning?

Answer 6

Physically moving the antenna or part of the antenna to “steer the beam”.

Question 7

What are the three ways to “steer the beam”?

Answer 7

Moving the entire antenna (feed and reflector).
Moving the feed relative to a fixed reflector.
Moving the reflector, relative to the feed.

Question 8

What consists of Electronic Scanning?

Answer 8

“Steering the beam”, by shifting the radiation pattern through phase interactions or other non-mechanical means.

Question 9

What are the three primary ways to form an Electronic Scan?

Answer 9

Frequency Scanning
Phase Shift Scanning
Time Delay Scanning

Question 10

What is a non-scanning emitter?

Answer 10

An emitter that does not have a scanning radiation pattern and are said to have stead scans.

Question 11

What is a Circular Scan?

Answer 11

They’re the most simplest and common form of RADAR antennas in use today.
It sweeps the beam in a full 360° rotation around a vertical access.
It uses a narrow, fan shaped, vertical beam that yields acceptable azimuth resolution and sufficient altitude coverage.

Question 12

What is lobe duration?

Answer 12

The time it takes for the beam to cross, or “light up” a target.

Question 13

What is a Helical Scan?

Answer 13

It combines vertical motion with a circular scan, allowing it to search a large volume of space with a small beam.
The RADAR rotates the same as a circular scan, however, also slowly changes its elevation angle of the antenna.
The vertical scan component is either bi-directional or unidirectional.

Question 14

With a helical scan, how is bi-directional and unidirectional different?

Answer 14

Bi-directional: RADAR will scan each elevation going up and then down, creating a sinusoidal helical scan.

Unidirectional: RADAR will scan each elevation going up, but will fly back down and begin the cycle again, creating a “sawtooth” helical scan.

Question 15

What is a Uni-directional scan?

Answer 15

The beam moves in one direction only, normally used by a RADAR to obtain a high data rate.

Question 16

What are the three ways to achieve a uni-directional scan?

Answer 16

Blanking a circular scan
Using a rotating feed
Electronically steering the beam

Question 17

Describe the three ways to achieve a uni-directional scan.

Answer 17

Circular scan blanking: Blocking the transmitted beam for a significant portion of the scan cycle. It is an inefficient use of power, so it’s not normally used.

Rotating feed: Uses a stationary reflector with a feed element rotating in front of it. The power to feed the element is synchronized so it only radiates when the beam is properly aligned with the reflector. This is a much more common way of generating the scan.

Electronic Steering:

Question 18

What is a Bi-Directional Scan?

Answer 18

It is a two-way sweeping scan used to search a specific region of space within an arc that’s less than 180 degrees.
It can sweep in either a horizontal or vertical plane.

Question 19

What is a Raster Scan?

Answer 19

A radiated beam, that’s used in both the horizontal and vertical plane.
Each horizontal sector scan is called a bar, a complete raster sequence is called a frame.
It uses a small “pencil beam” to cover large areas and is normally used for Target Acquisition.
A “Bar Period” is the time required to sweep one “Bar”.
The “Scan Period” is the time required to scan a complete “Frame”.
The “Illumination Rate” is the actual number of bars swept per second.
It begins the pattern again once the scan completes all bars.

Question 20

What is a Lobe-Switching Scan?

Answer 20

When the radar beam is electronically switched between two or more overlapping points in space.
These techniques were among the earliest scans used for anti-aircraft fire control radars.

Question 21

What are two methods used in lobe switching scans?

Answer 21

Two-way and four-way lobe switching patterns.

Question 22

How does a two-way lobe switching pattern work?

Answer 22

Radiates two beams alternatively, positioned so that their coverage area overlaps.
Switching the lobes horizontally gives accurate azimuth data, switching vertically, gives accurate elevation data.
It can provide adequate azimuth or elevation resolution to aim the antenna directly at a target.
It can NOT give accurate data in both planes, only one plane.

Question 23

How does a four way lobe switching pattern work?

Answer 23

It combines horizontal and vertical two way lobe switching to provide accurate tracking data in both planes.
It compares the target echos amplitude in all four beams. If they are all of equal amplitude, the antenna assembly is pointed directly at the target.

Question 24

What is a Conical Scan?

Answer 24

Similar to Lobe-Switching Scans, Conical Scans take this lobe concept and rotate the beam around a central axis.
There are two variations to this scan; Rotating and Nutating.