of the satellites. In the case of XM, each of the satellites will be broadcasting the same signals, but in different frequency bands. There are some operational consequences of the moving Sirius satellites. Because only two frequency bands are set aside for the satellite signals, only two of the three satellites can transmit at any given time; otherwise, two of the satellites would have to transmit in the same frequency band and would interfere with each other. To avoid this problem, as the satellites rise and set, a "hand-off" must occur between their rising and setting--one satellite ceases transmission and the other initiates transmission. This is likely to cause a brief interruption (on the order of milliseconds) in signal reception at the receiver for the signal being handed off. As long as the signal from the third satellite is not blocked during this hand-off time, the listener will be unaware of the switch; this is likely the case since the third satellite, at the hand-off time, will be at its highest elevation angle and will therefore have its lowest probability of blockage. Repeaters for those really hard-to-reach spots Also affected by the motion of the Sirius satellites is the system's use of terrestrial repeaters. Both XM and Sirius will use a network of ground-based transmitters to rebroadcast the satellite signal into hard-to-reach areas such as tunnels or urban canyons, adding yet another leg to the spatial diversity of the system. These terrestrial repeaters use a modulation technique different from the one used by the satellites, being optimized for use by terrestrial transmitters--namely, coded orthogonal frequency division multiplexing, or COFDM, versus the satellites' QPSK, or quadrature phase shift keying. For XM, since the satellites are geostationary, a high-gain (that is, a directional) antenna can be used at the repeater site to receive the incoming satellite signal. This is necessary sinc...
