Page 334 - Physics Coursebook 2015 (A level)

Cambridge International A Level Physics
The bandwidth available increases as the frequency of the wave increases. As microwaves have a high bandwidth, they can carry many telephone conversations at once. They are also very secure and difficult to tap into, as the beam of microwaves that travels between the two dishes is narrow and does not spread out. Until fibre optic cable was available, microwave links carried the majority of long- distance telephone conversations.
To summarise, high-frequency radio waves and microwaves:
■■ have high bandwidth and can carry a large amount of information
■■ can be transmitted as narrow beams which are more secure
■■ are line-of-sight and often use a satellite or microwave link.
Satellites and optic fibres
Transmissions in the LW and MW bands use a surface wave and do not travel further than about 1000 km. For long-distance communication, it is possible to transmit using a sky wave or using a space wave and a satellite.
Figures 20.1 and 20.18 both show a communications satellite in space. The satellite receives a space wave from a transmitter on Earth, the uplink, with a carrier frequency
322 in the microwave region. Because the satellite can only reflect a tiny fraction of the signal sent from Earth, the reflected signal received back on Earth would be far
too small. Instead, the satellite re-transmits the signal
it receives as the downlink back to Earth, on another frequency and with more power than it received. If the downlink and uplink frequencies were the same, then the much larger signal sent from Earth would swamp the signal sent from the satellite, so different frequencies are used. The satellite transmits the signal back to an individual satellite dish back on Earth or to many dishes, over a wide area, particularly when broadcasting television programmes.
The first communications satellites used a frequency of 6 GHz for the uplink and 4 GHz for the downlink, but now even higher frequencies are used.
Here are some advantages of communication by satellite rather than by sky wave:
■■ The concentration of ions in the ionosphere is constantly changing and reflection of the sky wave is not always possible; sometimes layers in the ionosphere even absorb radio frequencies.
■■ The satellite boosts the signal for its return to Earth and provides a stronger signal than is obtained by reflection from the ionosphere.
■■ Satellite communication uses higher frequencies, which have higher bandwidth and can carry more information per second.
■■ Only a few frequencies in the MW and SW bands are available. More frequencies are available for communicating if a satellite uses higher frequencies.
In order to obtain a constant link between the satellite and the satellite dish on Earth, it is essential that the satellite dish always points towards the satellite. If the satellite moves across the sky, the dish must move to track the movement of the satellite. To avoid this problem many communications satellites rotate around the Earth in
a geostationary orbit. They orbit the Earth in the same direction as the Earth rotates, at a height of 36 000 km above the Earth’s surface. At this height, each satellite
has a period of rotation of 24 hours. This means that each satellite naturally takes 1 day to orbit the Earth, exactly the time that it takes a point on the surface of the Earth to make one complete rotation. A geostationary satellite is in orbit above the equator and it never appears to move when viewed from any point on the Earth. The satellite does not rotate with the same speed as a point on the Earth because its orbit is far larger than the circumference of the Earth.
There are many other satellites around the Earth. Some of these are in polar orbit. These satellites commonly travel above the North and South Poles in a time much shorter than a day. They are usually closer to the Earth than geostationary satellites and are used for surface observation and as weather satellites. At a commonly used height of 1000 km above the Earth’s surface, the period of rotation around the Earth is only 100 minutes. Being closer to the Earth, polar-orbit satellites can see smaller detail when used for observation and espionage. As they orbit from the North to the South Pole, the Earth rotates underneath them and so they pass over the whole Earth in a 24-hour period.
There is always a delay in sending a message to a satellite because the satellite is high above the Earth. This delay can be annoying when talking by telephone. For example, if the satellite is directly overhead and the signal travels a distance of 72 000 km up to the satellite and then down to the other person, the time delay is 0.24 s. The reply
uplink
satellite dish
satellite
downlink
satellite dish
Figure 20.18 A satellite system.