The Range of Primary Radar

Principles Radar 11

It can be seen from Figure 11.6 that energy is absorbed and scattered by raindrops; the total effect depends upon the size of the water droplets and the transmitted wavelengths. At wavelengths longer than 10 cm the attenuation is negligible. If the wavelength is between 10 cm and 4 cm the attenuation is significant only in tropical rain. However, with wavelengths less than 4 cm, attenuation is significant in rain in the temperate latitudes. One conclusion is that wavelengths less than 3 cm should not be used for long range systems. Airfield Surface Movement Indicator (ASMI) radars operate at 1.75 to 2 cm wavelengths. Airborne Weather Radars (AWR) and Precision Approach Radars (PAR) use 3 cm wavelengths. Surveillance radars (ground) use 10, 23 or 50 cm wavelengths.

•Atmospheric conditions. Certain atmospheric conditions can actually increase the range of radar pulses by refracting the waves which would normally travel in straight lines. This is called super-refraction and it gives radar ranges beyond normal line of sight i.e. it gives over the horizon radar capability by causing the radio waves to refract downwards towards the earth’s surface. Such conditions occur when there is a temperature inversion and a decrease in humidity with height. On the other hand, atmospheric conditions can also

cause sub-refraction in which the theoretical range of the radar is reduced by causing the waves to refract upwards away from the surface.

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Figure 11.7 Pulse Width Decides Minimum Range

•Restoration Time is a design factor that affects the time taken for a receiver to recover to normal after transmission has occurred.

•Pulse width determines the minimum range. With reference to Figure 11.7, it can be shown that a pulse 1 µs wide would extend 300 metres. Thus an object at 150 metres reflecting this pulse would cause it to arrive back at the receiver as its tail was leaving the transmitter.

Any object closer than 150 metres would reflect a pulse that could not be received as the transmitter would still be transmitting. Furthermore, two objects in line 150 metres or less apart would appear as a single return. As a result, if short range operation is required for target resolution and accuracy, short pulses are used, e.g. 0.1 µs.

Radar Principles

Note: 1 or 2 µs are used for medium range radars and about 5 µs for long range radar.

Question A surface movement radar is required to measure down to 500 m. Calculate the maximum pulse width in microseconds.

Radar Measurements

Bearing

Bearing measurement is obtained by using the searchlight principle. Radio pulses are concentrated into very narrow beams which are produced by shortening the wavelength or increasing the aerial size and in advanced systems this is done electronically. The beam is rotated at a constant speed. The PPI display is synchronized with the antenna rotation. The direction of an object is the direction of the beam, measured from a fixed datum, at the time when the echo is received.

Range

Calculated from the time interval between the transmission and reception of the radar pulse.

Harmonization

In order that bearing and range information can be determined from the radar system it is necessary to harmonize the rotary speed of the antenna, the pulse duration or width, the pulse repetition frequency, focusing and transmission power.

Radar Resolution

The image painted on a PPI display from a point target will not be a single point but will appear as a rectangle, known as the radar resolution rectangle i.e. the target appears to be stretched both radially and in azimuth. The dimensions of the rectangle depend upon the pulse length, the beamwidth and the spot size.

The radial resolution is dependent upon half the pulse length. For example, a pulse length of 1 μs would stretch the target by 150 metres (distance that an electromagnetic wave travels in 0.5 µs). If two targets happen to be within half pulse width they will be illuminated simultaneously by the pulse and return only a single echo to the receiver.

The azimuth resolution is dependent upon the full beamwidth. Therefore a 3° beamwidth at a range of 120 km would stretch the target in azimuth by 6 km (using the 1 in 60 rule).

It follows therefore that in order to resolve adjacent targets the radar should have short pulse lengths and narrow beamwidths. However shortening the pulse length reduces the time the target is illuminated by the pulse and reduces the chance of a good return being received. Beamwidths can only be narrowed by increasing the size of the antenna.

The spot size and the target size also increase the size of the echo displayed on the PPI screen.

Moving Target Indication (MTI)

Surveillance radar equipment incorporates circuitry designed to eliminate returns from stationary objects such as hills or buildings which would give returns that would mask the smaller returns from aircraft. By erasing the permanent echoes the radar is able to display only the moving targets such as aircraft.

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Radar Principles 11