Билет №30

1). Sky wave - Sky waves are radio signals which have been bent or refracted sufficiently to return to the earth. The medium that causes such bending is the ionosphere, a region in the upper atmosphere where free ions and electrons exist in sufficient quantity to cause a marked change in the refractive index. Ultra-violet radiation from the sun is considered to be responsible for the ionization. For a given intensity of ionization, the amount of refraction becomes less as the frequency of the radio wave is increased. If the frequency is raised too much, the bending becomes too slight to bring the wave back to earth.

2). Attenuation - is simply a loss of energy and velocity. As part of the wave comes in contact with the surface it induces a current in it, thereby losing energy and slowing down. This slowing down of the bottom of the wave gives the waveform a forward and downward tilt encouraging it to follow the curvature of the earth. Waves continue until they are finally attenuated and become undetectable. Attenuation depends on two factors: a) The type of surface b) Frequency in use

3). At what Frequency operate MLS - the MLS system operates at a frequency band of 5031,0 – 5090,7 MHz on two separate channels at a mutual interval of 300 kHz.

Билет № 31

1). Direct wave – in the frequency bands of VHF and above, the only propagation path that can be used is the direct wave which travels in a straight line from the transmitter to the transmitter to the receiver. Radio waves at these frequencies are not refracted in the ionosphere and their surface waves are very short due to attenuation. The curvature of the earth limits the range available by direct waves. It is said that range is limited to the "line of sight", that is the straight line joining the transmitter and receiver. In fact, there is a small amount of refraction in the lower layers of the atmosphere due to density changes which extend the radio horizon.

The formula used to calculate direct wave range is:

Range (nm) = 1.25(√h1 + √h2).

h1 is the height of the transmitter is feet AMSL.

h2 is the height of the receiver in feet AMSL.

Direct wave range depends on:

• The height of the transmitter.

• The height of the receiver.

• The height of any intervening high ground.

• The power of the transmitter.

The line of sight limitation has the advantage of preventing interference from oter transmitters which are over the horizon.

2). Doppler effect - Doppler effect is the apparent change in the frequency of radio waves due to relative motion between the transmitter (TX) and receiver (RX). If the distance between TX and RX is reducing, more radio waves arrive at the RX in unit time so the received frequency is greater than that transmitted. Similarly, if the distance is increasing the received frequency is lower.

3). Aerodrome Surveillance Radar (ASR) - airport Surveillance Radar (ASR) is an approach control radar used to detect and display an aircraft's position in the terminal area. These radar sets operate usually in E-Band(108-118 and 118—136,975 МГц), and are capable of reliably detecting and tracking aircraft at altitudes below 25,000 feet (7,620 meters) and within 40 to 60 nautical miles (75 to 110 km) of their airport.