Satellite radio is the most significant development in mass media since cable television. Many of the technical hurdles that had to be cleared have, in fact, been higher. For one, it is necessary to have a universal signal that can be received anywhere by standard receivers. For another, the antennas that receive the signal have to be more or less omnidirectional so that the signal can be received in a moving vehicle without having to track the satellites. These accomplishments have required a tremendous effort by some pretty sharp engineers working over a period of over ten years. Here is a little discussion to enlighten those interested in looking “under the hood” of satellite radio.

To start with, the system is called satellite digital audio radio services (SDARS). Why digital? Because the analog signals used for traditional AM and FM broadcasts are degraded by fading and multipath. In other words, the signal bounces off mountains and buildings, creating interference that degrades the intended transmission from the station. This is why FM radio sounds so good at home but not in a car. Multipath effects can be removed from a digital signal through the use of error correction and detection, permitting a high-quality sound reproduction regardless of the location of the receiver. Digital signals can also be compressed, permitting better use of the available bandwidth. Why satellite? This is a tougher question. Sirius and XM both use terrestrial repeaters to reach areas in which the satellite signals are blocked. With enough repeaters there’s really no need for satellites. It’s really a matter of economics. The cost of installing and maintaining a large network of ground transmitters is just too expensive. Besides, the idea of receiving radio from outer space is just more appealing to the average technophile.

The satellite constellations used by Sirius and XM are quite different. Sirius uses three satellites in highly elliptical orbits. This means that the satellites are always in motion relative to the earth albeit in orbits that cause them to stall, or dwell, for a while in orbit. This particular configuration provides good satellite coverage over North America and reduces the need for repeaters. It is also, however, a more difficult system to manage because the transmission has to be “handed off” from one satellite to another. Within a year or so, Sirius is planning to launch a fourth satellite. Unlike Sirius, XM uses two geostationary satellites that remain fixed over the equator at one particular position. Because the satellites are always over the same place on the surface the system is easier to manage because XM doesn’t have a satellite hand off problem. The problem is that geostationary satellites also orbit around the equator so their coverage of North America is not as good as that of satellites in elliptical orbits. It is for this reason that XM uses more ground repeaters than Sirius.

As shown in Figure 1, the Sirius satellites move about a specific longitude, 100 degrees, while moving across latitudes. Each satellite rises and sets approximately every 16 hours. Two of the three satellites are visible to receivers in North America at any given time. The satellites have an elevation angle of 60 degrees. The XM satellites are in conventional geostationary orbits. One satellite is at 85 degrees W longitude and the other is at 115 degrees W longitude. The elevation angle of both satellites is 45 degrees. The distribution of the signal between two satellites provides spatial diversity. This improves likelihood that a receiver will be within line-of-sight of at least one satellite.

Figure 1 – The Sirius Satellite Configuration

Sirius and XM both use quadrature phase shift keying (QPSK) to modulate the signal transmitted from the satellites to the receivers. Frequency and time diversity are also used to reduce the effects of multipath. Each satellite transmits in a slightly different frequency band and at a slightly different time. Because the frequency is shifted a fading signal from one satellite may be supplemented by the signal from the other. In addition, the time shift enables a receiver to fill in blocked bits of a digital transmission from a given satellite by waiting a short time for the next signal to arrive. The time shift is no more than 4 or 5 seconds but it introduces a delay in the receiver that is a little annoying to someone wanting to listen to the play-by-play of a sporting event.

As mentioned earlier, both satcasters use terrestrial repeaters to supplement their satellite signals. The repeaters use coded orthogonal frequency division multiplexing (CODFM) as their modulation scheme. Because the XM satellites are geostationary, the repeaters can operate by receiving the signal from space and simply re-transmitting it. To provide physical isolation between receiver and transmitter a high-gain directional antenna is used to receive the signal from space. Physical isolation is necessary to prevent the high-power transmitted signal from overloading the receiver. Because the satellites are in constant motion, the Sirius repeaters cannot easily use directional antennas. Rather than use a complicated antenna tracking system Sirius simply makes use of commercially available bandwidth from geostationary communications satellites to feed the repeaters.

Figure 2 – Summary of Sirius and XM Technical Characteristics

Both Sirius and XM continue to upgrade. Sirius is planning to launch a fourth satellite to make their system more robust. Due to technical difficulties with their satellites, XM has had to build and launch replacement satellites. This has been done successfully, insuring that all the satheads out there will continue to be able to get Sirius – and XM.