RAWS CDCs Vol. 2 Self Test Questions

What is the purpose of AC&W radar?

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

What is the purpose of the PAR?

To assist aircraft in making safe landings during poor weather conditions.

What does the 'T' in TPN–19 designate?

Transportable.

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 a radar pulse to travel one radar mile?

12.36 μs.

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 two basic types of transmitters?

Keyed oscillator and power amplifier chain.

In addition to a flat top, what characteristics must a modulator pulse have?

Steep leading and trailing edges.

What is the frequency range of magnetron oscillators?

600-30,000 MHz.

What three types of storage elements are most often used in modulators?

Capacitor, artificial transmission line, or pulse formed network.

What type of tube best meets the requirements of a modulator switching element?

Thyratron.

What modulator element controls the rate at which the storage element charges?

The charging impedance.

What two forms of instability are common in magnetrons?

Mode skipping and mode shifting.

What is the typical frequency range about the center frequency of a tunable magnetron?

± 5%.

In the power amplifier, what two signals are mixed to produce the output?

Local oscillator and coherent oscillator.

What type of klystron is used as the final stage of a power amplifier transmitter?

Multicavity klystron.

What is the result of pulsing a pulsed RF amplifier when no RF is present?

Oscillations at an undesired frequency.

What are the two general classes of antennas?

Omnidirectional and directional.

What determines the width of the antennas main lobe?

The systems purpose and degree of accuracy required.

What is the directivity of a directional antenna?

The degree of sharpness of its beam.

What are the two functions of a horn radiator?

They serve as an impedance matching device and as a directional radiator.

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 does it mean if an antenna is reciprocal?

The transmit and receive patterns of the antenna are identical.

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.

Why is the paraboloid shape useful?

All rays leaving the focal point and striking the reflector are reflected along a path parallel to the focal axis.

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

It gives you a fixed change to the radiated beam pattern but does not give you any way to continuously control or vary the 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 passive feedhorn?

Used only for receiving.

What is an advantage of using MTI?

It can reduce the amount of fixed clutter.

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.

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.

How many receivers are used for the pencil beam?

A separate receiver is provided for each pencil-beam.

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 shaped beam.

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

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

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 is the greatest limiting factor in a receiver’s detectable range?

Noise.

What type of receiver is 'almost, always' in radar systems?

Superheterodyne.

Which component of the receiver produces the signal that is mixed with the received signal to produce the IF signal?

Local oscillator.

What is the one major disadvantage of the unbalanced crystal mixer?

Its inability to cancel local oscillator noise.

Which receiver component converts the IF pulses to video pulses?

Detector.

Why is AGC not used as frequently as other types of gain control?

Because of the widely varying amplitudes of radar return signals.

How does FTC affect receiver gain, if at all?

FTC has no effect on receiver gain.

What type of target has a fixed phase relationship from one receiving period to the next?

Stationary target.

What signal is used to synchronize the coherent oscillator to a fixed phase relationship with the transmitted pulse?

COHO locked pulse.

What is the phase relationship between the delayed and undelayed video?

Opposite.

When a large signal and a small signal are applied to a LIN-LOG amplifier at the same time, what is the effect on the small signal?

Amplification of the small signal is reduced.

What happens to the overall gain of a LIN-LOG amplifier as each stage saturates?

Gain decreases.

If a target is on the azimuth axis of the radiated beam, what is the input to the azimuth IF channel?

Zero.

What functions does the ASDP provide for the radar?

REX control, digital pulse compression, target processing, and weather processing for the receiver channel in which it resides.

What does target processing in the ASDP consist of?

Doppler filtering, CFAR detection thresholding, sliding dwell binary integration, plot extraction, and scan-to-scan tracking.

What is used to suppress large in-band interference returns produced by other radar sources?

Radar interface suppressor.

What is the purpose of pulse compression in the ASDP?

To recover the radar range resolution when long pulse transmissions are used.

What is the purpose of the CFAR function?

Remove clutter and weather returns correlated in range.

When does the binary integrator output a detection?

When two or more detections at the same range have occurred in four successive CPI’s.

What are permanent echoes used for?

Monitoring the performance of the PSR.

How does the weather processor remove second time around returns?

At the end of six scans the PRI diverse weather maps are merged into a single weather map.

What is used to coordinate timing events within the ASDP and REX?

Programmable synchronizer FPGA.

What information does a plot consist of?

Range and azimuth of the target as estimated by the plot processor, and a measure of the quality of the plot.

What processor function collects the weather data for all 256 radials and assembles a weather map?

Plot processor.

How many aircraft can the primary radar detect and process in a single 360° scan?

700.

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.

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.

What are the three primary performance parameters for an antenna?

Gain, beamwidth, and side-lobe 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° to 2°, and the average vertical beam height is about 30° to 35°.

What is used to shape the radar beam?

A parabolic reflector.

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.

When using a conical scan, what component of the return signal modulation is used by the radar system to maintain track in both azimuth and elevation?

Phase.

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.