The focus of our discussion so far has been on telephone communications, but space communication is not just about this. For example, images taken by interplanetary spacecraft or Earth-observation satellites are commonly conveyed by space communications links, and this is again a digital process. The cameras on spacecraft use digital photography, so the pictures already come in a handy digital form, allowing the same type of method that we described above to transmit them from the spacecraft to the desktop computers on the ground.
The design of the spacecraft's communications subsystem is strongly influenced by the ground communications systems that it will talk to during the mission. Figure 9.13 shows typical spacecraft and ground communications antennas, used by GEO communication satellites. The main job of the communications subsystem engineer is to determine the size of the spacecraft antenna and the amount of power it needs to radiate to achieve a good-quality link. The physical characteristics of the overall (spacecraft and ground) system play a major part in this design process, and these include the ground-to-spacecraft range, the frequencies used, the size and power of ground antennas, and the losses caused by atmospheric absorption.
The two most important attributes of the spacecraft communications subsystem are radiated power and gain. If you use a mobile phone, you have some idea of what the radiated power of a communications system is all about; they have a little scale indicator on the screen to tell the user if there is a signal. If there is one, then somewhere nearby there is a mobile phone mast that has been designed to have sufficient radiated power to cover a particular area so that calls can be made and received. Further away from the mast, the signal strength will drop, so another mast has to provide ground coverage area there so that the service can be maintained. In the same way, an important measure of the effectiveness of a spacecraft's communications subsystem is the amount of radiated power it generates. However, a number of watts of radiated power is not the end of the story; the effectiveness of the system can be further enhanced if the spacecraft's communications antenna (usually dish-shaped) has high gain. The simple rule is that a large dish has high gain, whereas a small dish has low gain. We can understand the idea of gain if we consider the small light bulb inside a flashlight. The amount of radiated power it generates, in terms of the amount of light it gives off, is small, and certainly much smaller than that of a standard domestic light bulb. If we take the small bulb out of the flashlight and connect it to a battery, it provides insufficient light in a darkened room. However, if we put the bulb back into the flashlight and turn it on, the flashlight's dish-shaped reflector focuses the light from the small bulb into a beam, which can be dazzling if pointed directly into your eyes. The flashlight's reflector has a measure of gain, effectively increasing the radiated power of the bulb along the axis of the flashlight where the beam is concentrated.
This is very much like a dish antenna on a spacecraft. The radiated microwave power generated by the spacecraft is focused into a beam by the dish, and this beam can then be pointed at a receiving dish on the ground. This has the effect of increasing the received power on the ground. To achieve the level of received power on the ground required to ensure a good quality of link, spacecraft designers have a choice: they can achieve the required level either by having a small amount of radiated power and a large gain (a big dish), or by having lots of radiated power and a small gain (a small dish). This power-gain tradeoff is one of the main design issues for the spacecraft communications subsystem engineer. This issue affects spacecraft design, in particular that of interplanetary spacecraft that travel to distant parts of the solar system. For example, probes such as Cassini/ Huygens (see Fig. 7.6 in Chapter 7), and the New Horizons spacecraft (Fig. 9.14) recently launched to investigate Pluto, look almost like flying communications dishes. At great distances from the Sun, generating large amounts of electrical power onboard the spacecraft is difficult, so the
Figure 9.14: An artist's impression of the New Horizons spacecraft at Pluto. (Image courtesy of NASA.)
communications subsystem is likely to have low radiated power. To achieve the link quality required to return science data across such great distances, these spacecraft require large, high gain antennas.
Let's move on now and briefly discuss the subsystem that is tasked with the job of moving all the binary digits around the spacecraft.
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