3D155 Volume 1. Introduction to Radar Systems

Question 1

What is the purpose of AC&W radar?

Answer 1

To control friendly aircraft, detect hostile aircraft, and control interceptors.

Question 2

What is used with proper receiving equipment to detect the presence of a distant object?

Answer 2

Reflected energy

Question 3

At what speed do radio waves travel?

Answer 3

The speed of light, or 162,000 nautical miles per second.

Question 4

How long does it take for a radar pulse to travel one radar mile?

Answer 4

12.36 us.

Question 5

What radar assembly supplies timing signals to coordinate the operation of the complete system?

Answer 5

Synchronizer.

Question 6

When a transmitter uses a high-power oscillator to produce the output pulse, what switches the
oscillator on and off?

Answer 6

Modulator high voltage pulse.

Question 7

What radar component permits the use of a single antenna for both transmitting and receiving?

Answer 7

Duplexer.

Question 8

What are the functions of the antenna on transmit?

Answer 8

To concentrate the energy in a predetermined beam shape and to point this beam in a predetermined
direction.

Question 9

What are the functions of the antenna on receive?

Answer 9

The antenna forms a beam in a particular direction to gather selectively transmitted energy that has been
reflected from various targets.

Question 10

What does it mean if an antenna is said to be reciprocal?

Answer 10

The transmit and receive patterns of the antenna are usually identical.

Question 11

Which pattern is specified and measured for a reciprocal antenna?

Answer 11

Only one pattern, usually the transmit.

Question 12

Why are reflector antennas extremely important and practical devices for use in radar systems?

Answer 12

They offer an economical method of distributing energy over a large aperture area and can produce shaped
or pencil beams with high gain.

Question 13

What generally does the reflector do with its energy?

Answer 13

The reflector is used to redirect and reshape energy from one or more point sources located near the focal
point into a desired far-field pattern.

Question 14

What is the most common reflector shape?

Answer 14

The paraboloid.

Question 15

How is the paraboloid formed?

Answer 15

The paraboloid is formed by rotating a two-dimensional parabola about its focal axis.

Question 16

Why is the paraboloid shape useful?

Answer 16

It’s useful since all rays leaving the focal point and striking the reflector are reflected along a path parallel
to the focal axis.

Question 17

How is a paraboloid reciprocal?

Answer 17

It intercepts an electromagnetic plane wave traveling parallel to its axis and redirects it so that all of the
energy passes to the focal point, where it may be collected.

Question 18

What happens when you change the physical shape of the antenna?

Answer 18

It gives a fixed change to the radiated beam pattern.

Question 19

How can you provide an amount of control over the received beam pattern?

Answer 19

Use two feedhorns, one active (low beam) and one passive (high beam).

Question 20

What is the purpose of the active feedhorn?

Answer 20

It is the horn normally used for transmitting and receiving.

Question 21

What is the purpose of the passive feedhorn?

Answer 21

It is used only for receiving.

Question 22

What is an advantage of using MTI?

Answer 22

It reduces the clutter.

Question 23

What is a disadvantage of using MTI?

Answer 23

It reduces system sensitivity.

Question 24

What could you expect if you’re using the clutter-reducing aspect of the dual-feedhorn system
discussed on a mountain range reaching 3 above the horizon?

Answer 24

The clutter would be reduced or eliminated because it is below 3.5 degrees.

Question 25

What problem in the dual-feedhorn antenna can be eliminated by using a 12-feedhorn antenna?

Answer 25

Multipath reflections and holes caused by them.

Question 26

How is range gating adjusted in the 12-feedhorn system?

Answer 26

The range gating will be individually adjustable in each of four azimuth quadrants, relative to the north reference.

Question 27

What do you call the use of contiguous beams stacked in elevation?

Answer 27

A stacked-beam radar.

Question 28

Why is stacked-beam a good technique?

Answer 28

It uses simultaneous pencil-beam radiation patterns from a single aperture to cover the elevation angles of interest.

Question 29

What is each beam considered?

Answer 29

A separate radar.

Question 30

How does the transmitter give the desired elevation coverage?

Answer 30

The transmitter radiates a fan beam from the summation of all overlapping pencil beams.

Question 31

How many receivers are used for the pencil beam?

Answer 31

A separate receiver is provided each pencil beam.

Question 32

How is sidelobe cancellation employed?

Answer 32

It must be used separately in each receiving channel.

Question 33

What is a disadvantage of using automatic detection, sidelobe cancellation, or MTI in the radar?

Answer 33

It adds to the cost and complexity of the radar.

Question 34

What is an advantage of the individual pencil beams when handling rain clutter or chaff?

Answer 34

It limits the volume of space observed.

Question 35

How does the gain of the individual pencil beams compare to the fan-beam antenna?

Answer 35

The individual pencil beams have a higher gain than a fan-beam antenna.

Question 36

What phrase-array ability is an important advantage if the required antenna is large?

Answer 36

Its inherent ability to steer a beam without the necessity of a large mechanical structure.

Question 37

What are attractive features of the phased-array antenna?

Answer 37

The capability to generate more than one beam with the same array and flexibility in the control of the aperture illumination.

Question 38

Which array is particularly useful in radar applications?

Answer 38

The two-dimensional planar array.

Question 39

How does the two-dimensional planer array work in rectangular form; in circular aperture form?

Answer 39

It can generate fan beams. It can generate pencil beams.

Question 40

What kinds of radiators have been used in phased arrays more than others?

Answer 40

The dipole, the open-ended waveguide, and the slotted waveguide.

Question 41

Which waveguide antenna is more suited for one-dimensional scanning than scanning in two coordinates?

Answer 41

A slotted array antenna.

Question 42

Give four reasons why it is important that minor lobes of radar antennas be small compared to the main lobe.

Answer 42

(1) Have an antenna with high directivity. (2) Reduce the susceptibility of the antenna to interfering signals. (3) Reduce the possibility of detecting a target in a minor lobe. (4) Reduce the probability of interference with other nearby systems

Question 43

What do we call the fictitious surface located on or near an antenna?

Answer 43

The antenna aperture.

Question 44

How is the fictitious surface used?

Answer 44

It is often useful in computing the performance of the antenna.

Question 45

How is the pattern of the antenna determined?

Answer 45

The distribution of electromagnetic energy from the antenna over the aperture determines the pattern of the antenna.

Question 46

How can the antenna designer modify the shape of the pattern?

Answer 46

Altering the distribution of energy over the aperture.

Question 47

What are the three primary performance parameters for an antenna?

Answer 47

Gain, beamwidth, and sidelobe level

Question 48

Name two basic functions of the radar antenna.

Answer 48

(1) To efficiently launch and receive electromagnetic energy into the atmosphere or space. (2) To direct the energy into an appropriately shaped beam.

Question 49

What determines the shape of the beam of radar energy and its antenna pattern?

Answer 49

The purpose of the radar.

Question 50

In the search radar what do we need to measure?

Answer 50

Range and azimuth but not height.

Question 51

What is a practical beamwidth and vertical height?

Answer 51

A practical beamwidth is 1 degree to 2 degrees, and the average vertical beam height is about 30 degrees to 35 degrees.

Question 52

How do we shape this beam?

Answer 52

Using a parabolic reflector.

Question 53

What refers to the orientation of the electromagnetic wave as it travels through space?

Answer 53

Polarization.

Question 54

What refers to the motion of the antenna axis (of the beam) as the radar looks for an aircraft?

Answer 54

The scanning method used by the system.

Question 55

What precision radar system uses continuous rotation of a pencil beam (narrow angle in both dimensions) around the aircraft and uses the phase of the return signal modulation to maintain track in both azimuth and elevation?

Answer 55

The conical scan.

Question 56

What is identified at the ECM receiver by its regular intervals between illuminations?

Answer 56

The circular scan.

Question 57

In what system does a thin beam cover a rectangular area by sweeping it horizontally with the angle of elevation being incrementally stepped up or down with each horizontal sweep of the sector?

Answer 57

The raster scan.

Question 58

Which radar gets its name from the fact that each echo pulse from the aircraft being tracked yields a new azimuth and elevation correction signal?

Answer 58

The monopulse scan.

Question 59

In what radar does the antenna rotate on an azimuth sweep, while the elevation angle rises slowly from 0 to 90?

Answer 59

The helical scan.

Question 60

What happens to the speed of electromagnetic energy traveling through air as the altitude increases?

Answer 60

Signal speed increases.

Question 61

What effects can ducting have on radar coverage?

Answer 61

Extend coverage or create holes.

Question 62

Name 4 types of propagation anomalies?

Answer 62

Ducting, subrefraction, superrefraction, and multipathing.

Question 63

What do we call propagation of a wave from one point to another by more than one path?

Answer 63

A multipath.

Question 64

When multipath occurs in radar, of what does it consist?

Answer 64

It usually consists of a direct path and one or more indirect paths by reflection from the surface of the earth or sea or from large man-made structures.

Question 65

When may a multipath also include more than one path through the ionosphere?

Answer 65

At frequencies below about 40 MHz.

Question 66

What can near-simultaneous reception of “pulse-type” information cause?

Answer 66

Delayed, but separate, pulses.

Question 67

When a low-altitude target is illuminated by a radar system, or for higher-angle situations involving appreciable antenna sidelobes, by what two paths can energy enter the tracking antenna?

Answer 67

A direct path from the target and an indirect path involving energy reflected from the surface of the earth.

Question 68

What attempts are made to reduce multipath effects on radar tracking accuracy?

Answer 68

The use of frequency agility, polarization agility, high-resolution antennas, clutter fences, and complex indicated angle processing techniques.