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Flashcards in Basic Radar Theory Deck (14):

What is PRI?

PRI - pulse recurrence interval

It is the time from the start of transmission of one pulse to the start of the transmission of the next.

PRI aka PRP (Pulse recurrence period)


What is PRF?

The pulse repetition frequency (PRF) is defined as the no. Of pulses occurring in 1 sec, measured in PPS (pulse per sec).

PRF aka PRR (pulse recurrence period)



What is max unambiguous Theoretical Range?

Have a prob of too high a PRI will miss reflection, too low, will miss a target...

If a pulse propagation delay is greater than the PRI, then range ambiguity can be generated as it receives return echoes from the wrong pulse.

Max unambiguous theoretical range of a radar is therefore related to PRF. The greater the range required, the lower the PRF used.


What is the formula for Max theoretical unambiguous range?


R (unamb) = C ÷ 2x PRF
(KM). (KM)


R (unamb) = 81 000 ÷ PRF
(NM). (NM)


What is the formula used for radar range?

Range = Time of travel of pulse x speed of light ÷ 2


What is the difference between Max and Min theoretical range when talking about radar transmissions?

Max Theoretical range is the max distance a transmission wave will travel. Keeping in mind that these are space waves and transmitter and receiver must be in line of sight with each other.

Min Theoretical range is the (min) distance that becomes a 'dead area' where no transmissions are made as the antenna is switched from transmitting to receiving.

The greater the pulse width, the greater the min range. Pulse width is proportional to min range.

PW: Min Range
1µS 150m
2µS. 300m
3µS. 450m......


Radio waves are degraded/attenuated by...?

• WV
• temp
• px changes


How does atmosphere affect radio wave range?

Energy is absorbed by WD turning into WV (absorbing latent heat).

Thus radio wave's energy is absorbed by the atmosphere (WV) and becomes attenuated. The further a signal travels, the more atmospheric attenuation.

More attenuation also occurs at high freq as these signals are higher thus hotter.


Radar range can be extended by a phenomenon known as super refraction, what is this?

Where normally a radar beam's trajectory should spread out in a fan like shape the higher (in height) as it travels due to the curvature of the earth. How ever with super refraction the beam bends closer to the earth's surface and sometimes even bouncing off the earth's sfc.


What does ducting refer to in radar theory?

Ducting is the extending of a radar range cause by strong low level inversions which allows the radar waves to travel much further in distance than normal.

Ducting cause ground clutter as it stays closer to the ground, and is most common in the morning when low level inversions are more common (NB: think radiation fog/inversion!!)


What is sub refraction?

Cause reduction in radar max range, it is the opposite effect of super refraction where radar signals refracting away from the earth's sfc more than normal.

Formed by warm moist air moving over cold, dry sfc (think advection fog)


What are the types of ATC radars?

• Area/Terminal Area Radar:
Usually long range with low PRF and low RPM

• Surveillance radar:
shorter range with higher PRF and faster rotating antenna

• Ground radar has very high PRF


What is MTI?

The problem with radars is that it can pick up obstructions e.g. Hangars, terrain, stationary objects

So primary radar is prone to A/C at lower levels being masked by strong returns from stationary objects.

So special circuitry is incorporated which only illuminates moving targets such as A/C. This function is called moving target indicator (MTI).


In a primary radar using pulse technique, the ability to discriminate between targets in azimuth is a factor of?

Beam width.

The narrower the beam, the more accurate the detection.