imes (relative to the underlying modulation), allowing the receiver to eliminate the effect of the blockage by making use of the unblocked portions of each signal. One consequence of a time diversity system is the introduced delay. Typically on the order of 4-5 seconds, the delay makes it difficult, but not impossible, for broadcasters to use the broadcast signal as an in-studio monitor (a common practice in which the on-the-air and the control room staffs listen to the received broadcast signal to monitor its quality), or for sports fans at a sporting event to listen to a broadcast (since the audio will be lagging the action). A three-satellite approach Sirius has taken a more novel approach to spatial diversity by using three satellites in a highly elliptical orbit [see figure]. In contrast, other existing broadcasting and most communications satellites are in geostationary orbits--that is, they always appear to be above the same spot on the earth. As the Sirius satellites orbit the earth, they move about a specific longitude (100) while moving across latitudes and, with respect to the northern hemisphere, rise and set approximately every 16 hours; thus two of the three satellites are visible to receivers in the United States at any given time (hence providing spatial diversity). Consequently, these satellites have an elevation angle of about 60 on average, higher than the typical 45 or so angle of the geostationary satellites used by XM, but are constantly in motion with respect to earth stations. (http://www.techtv.com/products/consumerelectronics/story/0,23008,3360750,00.html by Hahn Choi) With this higher elevation angle, the Sirius satellites are less likely to be blocked. The constant motion of the satellites is not a problem because the receiver antennas are nearly omni directional and, in fact, the motion of a vehicle speeding at 100 km/hour will have a much greater impact on reception than will the apparent motion ...
