niques, each representing a different kind of redundancy known as diversity, are used by the SDARS services to enhance performance. With its two satellites transmitting essentially identical signals from two, widely spaced locations (though in different frequency bands, so they won't interfere with one another), XM has implemented spatial diversity. This arrangement reduces the probability that the satellite signal will be completely blocked from the receiver. (Sirius uses spatial diversity as well, but in a different manner.) Under ideal circumstances, an XM receiver in a moving vehicle will "see" both satellites and be continuously receiving and processing the signals from both. Because the receiver is in motion, from time to time at least one of the satellites will be blocked by an obstacle; in this circumstance, the receiver will have to rely upon the signal from just the one (unblocked) satellite. Because the probability of signal blockage to a receiver in motion served by just a single satellite is so great, spatial diversity is a key attribute of this system. Without it, during these times of blockage the audio output of the receiver would simply disappear, situation listeners would hardly tolerate. The two other forms of diversity used by both systems are frequency diversity and time diversity. As mentioned for XM, each of its two satellites is transmitting the same signal but in different frequency bands. This is a form of frequency diversity and can help to combat problems associated with multipath fading, since sometimes multipath fades are frequency selective and limited to one band or the other. Finally, time diversity is implemented by introducing a time delay between the otherwise identical signals broadcast from each satellite and between the satellite and terrestrial repeater signals. (Bonsor, K 112-300) With time diversity, when a short signal blockage occurs, it will affect the two signals at different t...
