Radio technology has advanced into astounding new realms since the first broadcasts, but a century later traditional AM and FM stations still provide music, news, and talk shows to listeners on a terrestrial basis. But a satellite radio service offers something no terrestrial station can: coast-to-coast coverage with a clean, digital signal that delivers CD-quality sound and text information like stock quotes, sports scores, traffic updates, weather reports, and road conditions.
A company called Sirius and a rival called XM began to offer satellite radio in the early 2000s. They got placement in GM, Honda, Toyota, and BMW vehicles and sold subscriptions to listeners who installed a satellite radio tuner in their cars or connected it to home stereo systems.
Satellite radio is broadcast from satellites orbiting Earth, which bounce the signals to specialized receivers found in car and home stereo systems and portable boomboxes. The receivers can pick up three signal beams, each of which carries more than 60 channels of programming. Each of the beams covers 5.4 million square miles (14 million kilometers) of the continent, and terrestrial repeaters in urban areas can augment the coverage when line-of-sight to the satellites is lost.
To avoid dropouts in the audio stream, Sirius and XM developed a system of transmitting different waveforms on their satellites. The receivers can detect the difference in frequency and amplitude, then combine the stronger signals with the weaker ones using an algorithm known as maximal ratio combining. This technique, combined with forward error correction, ensures that the satellite radio system can deliver high-quality audio even when driving under bridges or into a tunnel, for example.
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