Both Sirius & XM Satellite* radio are intended (and licenced) to provide service
to only North
Siruis utilizes inclined elliptical geosynchronous orbiting satellites, which provide a larger coverage area than XM’s geostationary satellites.
Neither system actually “turns off” their up-down link at the equator ~ the satellites just lose their line-of-site.
Notwithstanding, as currently configured, Sirius is generally available through Mexico
& Pueto Rico, while XM has an even smaller footprint.
Satellite Radio Coverage Maps:
satellites are space-based repeaters that receive an uplinked signal, which is translated to another frequency and retransmitted back to earth.
There are two basic ways to cover a given area with satellites. The traditional approach is to put up a geostationary satellite over the desired area. Geostationary satellites are put into a circular orbit around the equator about 22,300 miles from earth. In such an orbit, the satellite speed matches the rotation of the earth, so the satellite is always overhead to any observer or station on earth. Sirius' competitor XM Radio uses this system with two satellites providing full U.S. coverage. The equatorial geostationary orbit is unique and currently jammed with satellites side by side only a few degrees apart. As with the frequency spectrum, we're simply running out of space.
Sirius takes the other approach of using elliptical orbits. The company has three elliptical orbits over the U.S. They are geosynchronous, meaning that their rotational period is 24 hours just like a geostationary satellite. The satellite apogee (high point) is 29,200 miles over Canada
and the perigee (low point) is 14,900 miles. The orbits function in a way where two satellites are over the U.S. at all times. The satellites are spaced eight hours from one another, and each satellite is over the U.S. for about 16 hours. All three transmit the same data.
The elliptical orbits offer the advantage of a very high angle of coverage. With a conventional geostationary satellite, the line-of-sight path runs at a very low angle of elevation above the equator (about 30̊) to the south from the U.S. Because microwave transmissions are direct-line-of-sight, signals from geostationary satellites encounter many more obstacles like trees and buildings. With elliptical orbits, the satellites are more directly overhead (always above 60̊) and thereby avoid most earth obstacles. Yet at such distances, the attenuation from satellite to earth is enormous.
international satellite-licensing regulations
, a geostationary orbiting satellite has priority over a non-geostationary orbiting satellite.
Interoperable Technologies is a company that is actually working with both SIRIUS and XM to create a dual-service receiver that supposedly is nearing production. Interoperable Technologies has gone and filed for a patent to operate a dual-service receiver via a cellphone/PDA.
* Digital Audio Radio Service (DARS)