CDS (I) 2020 General Knowledge · Question 26 of 120

Correct — C, radio waves. A satellite link is a radio link. Communications satellites work across a wide range of radio and microwave frequencies — the C, Ku and Ka bands for civil traffic, UHF, SHF and EHF for military systems — and the microwave bands used for most of that traffic are themselves the short-wavelength end of the radio spectrum. The physical reason radio is the right choice is that these wavelengths pass through the atmosphere and through cloud with little absorption, and can be focused by a dish of practical size. The whole point of putting a relay in orbit is that terrestrial radio links travel by line of sight and are blocked by the curve of the Earth; the satellite carries the signal over that curve.

• (a)infrared radiations — Infrared is absorbed strongly by water vapour and carbon dioxide in the atmosphere and is scattered by cloud, so it cannot be relied on for an all-weather link to orbit. Its everyday use is short-range, as in a remote control.
• (b)ultraviolet radiations — Solar ultraviolet below about 300 nanometres is absorbed by ozone and molecular oxygen before it reaches the ground, so an ultraviolet link would be stopped by the atmosphere itself.
• (d)visible lights — Visible light is blocked by cloud, fog and rain, which is why free-space optical links stay experimental for ground-to-satellite work. It also spreads and needs precise pointing over such distances.

Radio waves are the longest-wavelength part of the electromagnetic spectrum, running from millimetres upwards, and microwaves are the short end of that range. Radio links between two points on the ground travel by line of sight, so the curvature of the Earth limits them. A communications satellite in orbit is a relay above that curve. To keep different users from interfering, the world is divided into three regions and frequency bands are allotted service by service.

The item is really a question about which part of the spectrum the atmosphere lets through. Two windows are wide open — the optical window that lets visible light reach the ground, and the much broader radio window. Infrared and ultraviolet are heavily absorbed, which removes options (a) and (b) on physical grounds. That leaves visible light against radio waves, and visible light fails on weather and on beam spreading. It is worth being comfortable with the fact that microwave and radio are not rival answers here: microwaves are a subset of radio waves, so an option naming radio waves covers the C-band and Ku-band traffic a satellite actually carries.

• Communications satellites operate across a wide range of radio and microwave frequencies.
• Terrestrial radio links travel by line of sight and are obstructed by the curve of the Earth, which is what a satellite relay overcomes.
• Civil fixed-service satellites use the C band and the lower part of the Ku band; direct-broadcast satellites use the upper Ku band.
• Military communications satellites typically work in the UHF, SHF and EHF bands.
• For frequency planning the world is divided into three regions, with bands allotted to each satellite service.

• Rejecting radio waves because you remember 'microwave' being mentioned; microwaves are part of the radio spectrum.
• Choosing infrared from the association with remote controls, which are short-range and line-of-sight.
• Assuming the satellite generates the signal; it relays what a ground station sends.

As the band used for a stated application, or in reverse — which application uses a named band such as infrared or ultraviolet.

• practice — not a real PYQ

  Long-distance radio communication on Earth is possible without satellites because certain radio waves are reflected by the

  • (a)troposphere
  • (b)stratosphere
  • (c)mesosphere
  • (d)ionosphere

  Answer(d) ionosphere — its ionised layers refract medium and high-frequency radio waves back towards the ground.

• practice — not a real PYQ

  Communications satellites are usually placed in geostationary orbit because such a satellite

  • (a)is closest to the Earth's surface
  • (b)appears fixed over one point on the equator
  • (c)needs no power supply
  • (d)can be seen from every point on Earth

  Answer(b) appears fixed over one point on the equator — so a ground antenna can be pointed once and left there.