Radar Principles 11

Radar Frequencies

Radar systems are in the VHF and above frequency bands because:

•these frequencies are free from external noise/static and ionospheric scatter.

•the shorter wavelengths produce narrow, efficient beams for target discrimination and bearing measurement.

•the shorter wavelengths can produce shorter pulses.

•efficient reflection from an object depends upon its size in relation to the wavelength; shorter wavelengths are reflected more efficiently.

Pulse Technique

Primary and secondary radar systems use the pulse technique which is the transmission of radio energy in very short bursts. Each burst of energy is in a pulse form of a predetermined shape. The duration of the pulse is equal to the pulse length or width. Although a pulse is of short width (time) it can contain many cycles.

+
0	TIME

-	PULSE
	WIDTH
+

0		TIME
-	PULSE	PULSE RECURRENCE INTERVAL(PRI)
	RECURRENCE	or
		PULSE RECURRENCE PERIOD(PRP)

Figure 11.2 Pulse technique

Pulse Recurrence Interval (PRI) is the time interval between two pulses.

Pulse Recurrence Frequency (PRF) is the number of pulses transmitted in one second (pps). Example. If the PRF is 250 pps what is the PRI of the transmission?

PRI = 1 / 250 s

PRI = 1 000 000 / 250 µs = 4000 µs

Radar Principles 11

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11 Radar Principles

Distance Measurement - Echo Principle

Principles Radar 11

Figure 11.3

The distance to an object is found by timing the interval between the instant of the pulse’s transmission and its return as an echo; this is shown in Figure 11.3.

For example, if the echo (the time between transmission and reception) is 500 µs then:

Distance	=	300 000 000		×		500			m
							1 000 000	× 2
	=	75 000 m		=		75 km
or
Distance	=		162 000 × 500
			1 000 000 × 2
	=		40.5 NM

(c = 300 000 000 m/s or 162 000 NM/s)

Other methods of calculating the range are:

	Range =	500 × 300	=	75 km	Range =	500 × 300		=	40.5 NM
		2				2	× 1852

A radar mile (one NM out and back) = 12.36 µs.

500

Range = 12.36 = 40.5 NM

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Radar Principles

Theoretical Maximum Range

Relationship to PRF

Maximum theoretical range is determined by the PRF i.e. the number of pulses transmitted in one second (pps) Each pulse must be allowed to travel to the most distant object planned before the next pulse is transmitted; to do otherwise makes it impossible to relate a particular echo to a particular pulse. The maximum range is therefore related to the PRF such that the greater the range required, the lower the PRF used.

Examples

1.We wish a radar to measure a range of up to 187 km. What should the PRF (PRR) be?

2.What is the maximum PRR for a radar required to measure up to 200 NM?

3.Maximum range for a radar is to be 170 km. What is the maximum PRR?

4.An AWR has a 400 pps PRR. Calculate the maximum range in nautical miles for this equipment.

Answers

1.The pulse must travel 374 km (2 × 187) before the next pulse transmission.

The time for the journey, T = D/S	=	374 000/300 000 000 seconds
	=	0.0012466 s = 1246 µs
i.e. PRI	=	1246 µs.

Thus the second pulse can only leave 1246 µs after the first.

PRF (pps) = 1/ PRI = 1/ 1246 µs = 1 000 000 / 1246 = 802 pps

Alternately we can say that PRF = 300 000 000 / 374 000 = 802 pps

2.405 pps

3.882 pps

4.203 NM

Practical Range

The practical range for the radar is less than the maximum theoretical range because the trace on the CRT (cathode ray tube) needs a period of time to return to the point of origin. This period is called the fly-back or dead time. During this period returning echoes cannot be displayed thereby reducing the range achievable for a given PRF.

11

Radar Principles 11

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11 Radar Principles

Primary Radars

The pulses are concentrated into the beam dimensions designed for the particular radar. The beam uses the ‘echo’ principle to determine range and the ‘searchlight’ principle to indicate bearing or height. Figure 11.4 shows the Plan Position Indicator (PPI) display and Figure 11.5 shows the ATC radar antennae. The long structures at the top of the primary radar antennae are the secondary radar antennae.

The transmitter and receiver share the same antenna. The receiver is energized to accept ‘echoes’ from objects in the pulses’ path as soon as the transmitter pulse exits the antenna. The reflected pulses are very weak due to the double journey.

The shape and size of the radar antennae determines the size of the main and side lobes as well as the width of the radar beam generated by the system. The larger the aerial, the narrower will be the beam.

Principles Radar 11

Figure 11.4 A PPI display of primary raw radar

Figure 11.5 Typical radar antennae

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