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In Depth

Technology

Satellite radio

From space to your ear

November 24, 2006

An aerial shot of a city, showing the shadows thrown as sunlight hits tall buildings. Satellite transmissions are blocked by buildings and other obstacles in much the same way, creating areas where satellite radio receivers aren't able to get a signal.

In the popular science fiction of yesteryear, radio transmissions from outer space were typically a prelude to an alien invasion. Now, they're just a means of distributing pop songs and talk shows.

Satellite radio, though only half a decade old, provides hundreds of channels of music and spoken-word radio to entire continents, and has already attracted tens of millions of subscribers. Its primary advantage over traditional radio is its ability to deliver a clear, near-CD quality audio signal that can be received almost anywhere, no matter how remote the location.

The technology that drives the satellite radio industry is sophisticated (it is, after all, a space-faring enterprise). Complicating matters, many key components — including audio codecs (the way sound files are digitized and compressed) and signal strength — are proprietary secrets kept under tight wraps by Sirius and XM, the two companies delivering satellite radio service to North America. A third company, WorldSpace, has a virtual monopoly over the European, African, and Asian markets.

Still, the basic elements of satellite radio are open to public scrutiny, and not all that difficult to wrap one's brain around.

Built from the ground up

It all starts Earth-side. Before radio content (in the form of channels, the majority of which are produced by XM and Sirius) is sent to satellites for distribution, audio processing facilities encode it to ensure that the signal can't be received by unlicensed devices. These facilities also apply filters that optimize the sound quality for digital broadcast, while ensuring available bandwidth is used as efficiently as possible — in other words, making sure that things like impurities in an audio track aren't broadcast.

Once content is ready for broadcast, it is sent to an uplink station that relays it to orbiting satellites. The satellites blanket a particular region of the planet with a digital radio broadcast.

Reception

However, reception in blanketed areas is not guaranteed. Satellite radio depends heavily on something called "line-of-sight." In other words, the satellite needs a clear view of the receiving mechanism on Earth in order to reach it — or at least mostly clear. Clouds, light foliage, and tent roofs won't hamper the signal, but severe weather, dense forest canopies and concrete buildings or overpasses probably will.

Both Sirius and XM use multiple satellites to increase the size of the blanketed area; the former employs three satellites, while the latter uses four (two on active duty, two on backup). More important than the number of satellites, however, is their orbit.

XM's satellites are engaged in a geostationary orbit directly above the equator. This type of orbit keeps the satellites in a fixed place in the sky, which means that signal reception is extremely predictable. If a car radio can pick up the transmission while parked beside an office tower, it should also be able to receive the signal in that position every day at any time.

The potential downfall is the angle at which XM satellites transmit their signal. Since the satellites are positioned above the equator, the signal comes in on a relatively flat plane relative to the surface of North America, which means there is a greater likelihood of buildings or trees interfering with the transmission. The signal from the satellite acts a lot like rays from the sun — when the sun is directly overhead, there are few shadows, but the lower the sun gets on the horizon, the longer the shadows thrown by objects in its path.

In the case of satellite radio, the "shadows" are areas where the signal is blocked. With geostationary orbiting satellite radio signals, the reception problem increases the further north you go with a receiver.

Sirius' satellites, on the other hand, have been deployed in what's called an "elliptical geosynchronous" orbit, which means they move in a rough figure eight pattern over North, Central, and South America. At any given time, there are at least two Sirius satellites hovering above North America, while the third goes silent and conserves power as it makes a round trip over South America.

The two satellites that are actively broadcasting travel up, across, and back down North America. This means that the direction from which a Sirius satellite signal is coming, relative to a fixed spot on the continent, is constantly in flux. It could come anywhere from almost directly overhead to a more severe angle.

The upshot is that the quality of reception at a fixed location may vary over the course of the satellites' orbits, but the angle at which the transmission is received should, in theory, almost always be closer to vertical than the signal received from an XM satellite — especially in northern regions.

Repeaters

No matter what kind of receiver you're using, neither XM nor Sirius's satellite radio systems can claim perfect coverage.

Satellite radio receivers are popular in cars because vehicles are typically outdoors and within direct sight of satellites - perfect conditions for satellite signal reception. The pocket-sized receivers don't need a dish, but they do have problems getting a strong signal directly from the satellite in some locations. Receivers also exist in some home stereo systems (which require an antenna positioned outside the house) as well as personal devices with form factors similar to MP3 players.

This has led to the construction of terrestrial repeaters. These are Earth-based towers that receive and rebroadcast satellite radio signals in regions with poor reception, such as populous areas located behind tall mountains and urban centres with lots of tall structures.

Typically, areas served by repeater towers have access to a more powerful signal that can penetrate denser materials, often allowing for strong signal reception inside buildings. Some satellite radio aficionados claim that repeaters cheat the mystique of picking up radio from space, but repeaters irrefutably make for more reliable service.

Receivers

The final piece of the puzzle is the receiver itself. Proprietary chipsets developed by both XM and Sirius have effectively eliminated the ability for average consumers to pirate satellite radio signals.

In order for a receiver to pick up a satellite radio transmission, it must be activated - just like a cellphone. Activation requires that a technician send a command to the satellite network to broadcast a coded transmission that will unlock a particular receiver. Should a subscription lapse or be cancelled, a signal will be sent to re-lock the receiver.

In the end, despite industry secrets, space radio isn't all that difficult to fathom. But at least one question remains: Will a satellite radio listener one day make the civilization-ending mistake of misinterpreting an alien attack fleet's transmission as the latest in experimental music?

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