satellite radio

System design Satellite radio uses the 2.3 GHz S band in North America, and generally shares the 1.4 GHz L band with local Digital Audio Broadcast (DAB) stations elsewhere. It is a type of direct broadcast satellite, and is strong enough that it requires no satellite dish to receive. Curvature of the Earth limits the reach of the signal, but due to the high orbit of the satellites, two or three are usually sufficient to provide coverage for an entire continent. Local repeaters similar to broadcast translator boosters enable signals to be available even if the view of the satellite is blocked, for example, by skyscrapers in a large town. Major tunnels can also have repeaters. This method also allows local programming to be transmitted such as traffic and weather in most major metropolitan areas, as of March 2004. Each receiver has an electronic serial number (ESN)-Radio ID to identify it. When a unit is activated with a subscription, an authorization code is sent in the digital stream telling the receiver to allow access to the blocked channels. Most services have at least one "free to air" or "in the clear" (ITC) channel as a test, and some outside the U.S. have a few free programming channels, (Sirius uses channel 184 "Sirius Weather & Emergency" though this may end up being a loss leader tactic to lure more listeners until satellite radio gains more widespread use. Most (if not all) of the systems in use now are proprietary, using different codecs for audio data compression, different modulation techniques, and/or different methods for encryption and conditional access. Like other radio services, satellite radio also transmits program-associated data (PAD or metadata), with the artist and title of each song or program, and possibly the name of the channel.

A satellite radio or subscription radio (SR) is a digital radio that receives signals broadcast by communications satellite, which covers a much wider geographical range than terrestrial radio signals. SR functions any place there is line of sight between the antenna and the satellite, given there are no major obstructions, such as tunnels or buildings. SR audiences can follow a single channel regardless of location within a given range. Because the technology requires access to a commercial satellite for signal propagation, SR services are commercial business entities (not private parties), which offer a package of channels as part of their service—requiring a subscription from end users to access its channels. Currently, the main SR providers are WorldSpace in Europe, Asia and Africa, and XM Radio and Sirius in North America. All are proprietary and non-compatible signals, requiring proprietary hardware for decoding and playback. These and other services have news, weather, sports, and several music channels. Satellite radio is currently at the forefront of the evolution of radio services in some countries, notably the United States, but is less prominent in countries with relatively high population densities, where it is easier to reach the bulk of the population with terrestrial broadcasts, such as the United Kingdom. In the UK and some other countries the contemporary evolution of radio services is focused on Digital audio broadcasting (DAB) services rather than on satellite radio.