One big misconception here seems to be that it's believed that the Sirius Satellite is in a fixed orbit over just the USA. This is not true at all and it may come as a surprise to some but there 3 satellites, yes there are 3 of them, are constantly moving using elliptical orbits not geostationary. There 3 satellites spend much of there time in an obit over Canada.
Now to get "Sirius" about satellites... here you go!
There are two basic ways to cover a given area with satellites. The traditional approach and the one used by most space communications systems 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 (yes, 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 30?) 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 60?) and thereby avoid most earth obstacles. Yet at such distances, the attenuation from satellite to earth is enormous. Typical signal strength in the U.S. is −102 dBi, meaning that a hot receiver is needed.
All communications satellites are space-based repeaters that receive an uplinked signal, which is translated to another frequency and retransmitted back to earth. In the Sirius system, the digitized music and talk is uplinked from studios in New York City and retransmitted back to earth.
Mike Ledford, VP of engineering for Sirius, explains the system. Sirius is assigned 12.5 MHz of spectrum from 2320 to 2332.5 MHz centered on 2.32625 GHz (2326.25 MHz). This spectrum is roughly divided into thirds. One third is assigned to transmitting satellite #1 (TDM1) centered at 2322.3 MHz, one third to the terrestrial repeater network (more on that later) centered on 2326.25 MHz, and one third to transmitting satellite #2 (TDM2) centered on 2330.2 MHz. This gives each satellite roughly 4-MHz bandwidth.
The satellites use quadrature phase shift keying (QPSK) modulation. It's the most common for satellite communications, due to its robustness against signal degradation over long distances with minimal interference.
The receiver can receive and decode one, two, or all three signals simultaneously and recombine them internally into one signal. The receiver also accounts for phase delays (the satellites have large and constantly changing distances from the receivers), frequency shift (Doppler effects), and absolute time delays. (TDM1 is broadcast with an approximate 4-second delay from TDM2 and buffered internally, a Sirius patent.) Also, it accommodates a huge dynamic range. The receiver must work with a range of signals that are separated by more than a billion times the power of one another.
part 2 below....
