3D155 Volume 1. Introduction to Radar Systems Flashcards
What is the purpose of AC&W radar?
To control friendly aircraft, detect hostile aircraft, and control interceptors.
What is used with proper receiving equipment to detect the presence of a distant object?
Reflected energy
At what speed do radio waves travel?
The speed of light, or 162,000 nautical miles per second.
How long does it take for a radar pulse to travel one radar mile?
12.36 us.
What radar assembly supplies timing signals to coordinate the operation of the complete system?
Synchronizer.
When a transmitter uses a high-power oscillator to produce the output pulse, what switches the
oscillator on and off?
Modulator high voltage pulse.
What radar component permits the use of a single antenna for both transmitting and receiving?
Duplexer.
What are the functions of the antenna on transmit?
To concentrate the energy in a predetermined beam shape and to point this beam in a predetermined
direction.
What are the functions of the antenna on receive?
The antenna forms a beam in a particular direction to gather selectively transmitted energy that has been
reflected from various targets.
What does it mean if an antenna is said to be reciprocal?
The transmit and receive patterns of the antenna are usually identical.
Which pattern is specified and measured for a reciprocal antenna?
Only one pattern, usually the transmit.
Why are reflector antennas extremely important and practical devices for use in radar systems?
They offer an economical method of distributing energy over a large aperture area and can produce shaped
or pencil beams with high gain.
What generally does the reflector do with its energy?
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.
What is the most common reflector shape?
The paraboloid.
How is the paraboloid formed?
The paraboloid is formed by rotating a two-dimensional parabola about its focal axis.
Why is the paraboloid shape useful?
It’s useful since all rays leaving the focal point and striking the reflector are reflected along a path parallel
to the focal axis.
How is a paraboloid reciprocal?
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.
What happens when you change the physical shape of the antenna?
It gives a fixed change to the radiated beam pattern.
How can you provide an amount of control over the received beam pattern?
Use two feedhorns, one active (low beam) and one passive (high beam).
What is the purpose of the active feedhorn?
It is the horn normally used for transmitting and receiving.
What is the purpose of the passive feedhorn?
It is used only for receiving.
What is an advantage of using MTI?
It reduces the clutter.
What is a disadvantage of using MTI?
It reduces system sensitivity.
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?
The clutter would be reduced or eliminated because it is below 3.5 degrees.
What problem in the dual-feedhorn antenna can be eliminated by using a 12-feedhorn antenna?
Multipath reflections and holes caused by them.
How is range gating adjusted in the 12-feedhorn system?
The range gating will be individually adjustable in each of four azimuth quadrants, relative to the north
reference.
What do you call the use of contiguous beams stacked in elevation?
A stacked-beam radar.
Why is stacked-beam a good technique?
It uses simultaneous pencil-beam radiation patterns from a single aperture to cover the elevation angles of
interest.
What is each beam considered?
A separate radar.
How does the transmitter give the desired elevation coverage?
The transmitter radiates a fan beam from the summation of all overlapping pencil beams.
How many receivers are used for the pencil beam?
A separate receiver is provided each pencil beam.
How is sidelobe cancellation employed?
It must be used separately in each receiving channel.
What is a disadvantage of using automatic detection, sidelobe cancellation, or MTI in the radar?
It adds to the cost and complexity of the radar.
What is an advantage of the individual pencil beams when handling rain clutter or chaff?
It limits the volume of space observed.
How does the gain of the individual pencil beams compare to the fan-beam antenna?
The individual pencil beams have a higher gain than a fan-beam antenna.
What phrase-array ability is an important advantage if the required antenna is large?
Its inherent ability to steer a beam without the necessity of a large mechanical structure.
What are attractive features of the phased-array antenna?
The capability to generate more than one beam with the same array and flexibility in the control of the
aperture illumination.
Which array is particularly useful in radar applications?
The two-dimensional planar array.
How does the two-dimensional planer array work in rectangular form; in circular aperture form?
It can generate fan beams. It can generate pencil beams.
What kinds of radiators have been used in phased arrays more than others?
The dipole, the open-ended waveguide, and the slotted waveguide.
Which waveguide antenna is more suited for one-dimensional scanning than scanning in two
coordinates?
A slotted array antenna.
Give four reasons why it is important that minor lobes of radar antennas be small compared to the
main lobe.
(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
What do we call the fictitious surface located on or near an antenna?
The antenna aperture.
How is the fictitious surface used?
It is often useful in computing the performance of the antenna.
How is the pattern of the antenna determined?
The distribution of electromagnetic energy from the antenna over the aperture determines the pattern of the
antenna.
How can the antenna designer modify the shape of the pattern?
Altering the distribution of energy over the aperture.
What are the three primary performance parameters for an antenna?
Gain, beamwidth, and sidelobe level
Name two basic functions of the radar antenna.
(1) To efficiently launch and receive electromagnetic energy into the atmosphere or space. (2) To direct the
energy into an appropriately shaped beam.
What determines the shape of the beam of radar energy and its antenna pattern?
The purpose of the radar.
In the search radar what do we need to measure?
Range and azimuth but not height.
What is a practical beamwidth and vertical height?
A practical beamwidth is 1 degree to 2 degrees, and the average vertical beam height is about 30 degrees to 35 degrees.
How do we shape this beam?
Using a parabolic reflector.
What refers to the orientation of the electromagnetic wave as it travels through space?
Polarization.
What refers to the motion of the antenna axis (of the beam) as the radar looks for an aircraft?
The scanning method used by the system.
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?
The conical scan.
What is identified at the ECM receiver by its regular intervals between illuminations?
The circular scan.
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?
The raster scan.
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?
The monopulse scan.
In what radar does the antenna rotate on an azimuth sweep, while the elevation angle rises slowly
from 0 to 90?
The helical scan.
What happens to the speed of electromagnetic energy traveling through air as the altitude
increases?
Signal speed increases.
What effects can ducting have on radar coverage?
Extend coverage or create holes.
Name 4 types of propagation anomalies?
Ducting, subrefraction, superrefraction, and multipathing.
What do we call propagation of a wave from one point to another by more than one path?
A multipath.
When multipath occurs in radar, of what does it consist?
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.
When may a multipath also include more than one path through the ionosphere?
At frequencies below about 40 MHz.
What can near-simultaneous reception of “pulse-type” information cause?
Delayed, but separate, pulses.
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?
A direct path from the target and an indirect path involving energy reflected from the surface of the earth.
What attempts are made to reduce multipath effects on radar tracking accuracy?
The use of frequency agility, polarization agility, high-resolution antennas, clutter fences, and complex
indicated angle processing techniques.
