Satellite Systems

The main problem satellite systems solve is getting high-bandwidth access to places without a high-bandwidth infrastructure (Conemara) . It's unlikely that a satellite system could compete with Digital Subscriber Line (DSL) or fiber to the home or office -- if you can get those services. Still, if you're in a rural area of the Ireland -- or in a low-population area in any country -- you may not be able to get such services. But Satellites will deliver them, enabling not only high-speed Internet browsing but all forms of high-speed networking, including such things as videoconferencing, collaborative work sharing, and telemedicine.

But bandwidth is only half the story. The other half is latency -- the amount of time for your data to get from point A to point B. GEO satellites park some 22,300 miles above the equator: 0.24 second -- an eon to computers -- of round trip away. With that kind of latency built into the system (not counting whatever latency is added by the various gateways and translations the data must go through), a telephone conversation is an annoying, awkward mess. And any kind of interactive application has to be nonlatency-sensitive.

Seeing as we cant send signals faster than the speed of light (Yet) there's a simple solution: Move the satellites closer to earth. That's just what systems such as Teledesic, Alcatel's Skybridge, and Motorola's Celestri will do. With low earth orbits (LEOs) under 1000 miles, these systems offer latency that's barely apparent: hundredths of a second.

Of course, it's not that simple. While GEOs are a well-known technology (TV broadcasts, for example, have been using them for decades), LEOs are new and face new challenges. Perhaps the biggest one is that you need a lot of them to get total global coverage. At one point, Teledesic planned a constellation of more than 800 satellites, for example (that number recently dropped to 288 when it signed an agreement to work with Boeing). Until recently, the concept of launching dozens or hundreds of multimillion-dollar satellites was a pipe dream.

And then there is the problem of tracking the sattlites from the users terminal. A technology called a phased-array antenna solves the antenna problem. Unlike a satellite dish, which mechanically tracks satellite locations, phased-array antennas are self-aiming boxes consisting of many smaller antennas. They can track several satellites using the slightly different signals received by the array of antennas --without physically moving, reducing wear and tear and cost among other advantages.

The problem of keeping a link active when your satellite disappears every half hour is solved by keeping at least two satellites in view at all times (many LEOs will keep three or more in view). The antenna array is aware of all the satellites' positions and starts a new link before it severs the one to the setting satellite. This is "make before break" in satellite parlance.

If the system is a bent pipe (no satellite to satellite network)system, such as Alcatel's Skybridge, the satellites don't have to be very smart (Cheeper). The LEO satellite over Dublin will beam the signal down to a ground station, which will route the signal over landlines to a ground station near New York. That station will feed the signal up to the LEO satellite over New York, which will in turn bounce it down to the user there.

According to Motorola "Bent pipes are not good. There are too many hops from sky to earth." And that means dreaded latency -- defeating the whole reason LEOs are supposed to be better than GEOs. Instead, some systems, including Teledesic and Celestri, use satellite-to-satellite routing. The Teledesic constellation communicates in the 40-50-GHz band. Celestri uses lasers for its links.

The downside is, of course, that each satellite has to have more communications and tracking hardware – more intelligence -- and therefore a higher price than a bent-pipe system. Also, the performance gain over a bent pipe is not tremendous -- a few hundredths of a second.

In spite of the concerns of latency, GEOs and LEOs will likely coexist. Guy Christensen, of Leslie Taylor and Associates, sums up the markets based on whether the system is a GEO, with its inherent 0.24-second delay, or a low-latency LEO. LEOs will be good for high-speed networking, teleconferencing, and telemedicine – interactive applications. GEOs will be better for information downloading and video distribution -- broadcasting and multicasting.

One of the systems I looked at is considering offering the best of both worlds: a hybrid solution. Motorola's Celestri plans a LEO constellation of 63 satellites (initially) coupled with one GEO satellite over the U.S. Motorola has the rights to eight more GEO orbital slots if it needs them. The LEO constellation and the GEO satellites will be able to communicate directly through a satellite-to-satellite network.

Once you get beyond the latency and bandwidth issues (which is what the satellite creators spend a lot of time arguing over), there is another challenge: security. If your data is being packaged up and broadcast into space, can't anybody with a scanner just tune in? In theory, the answer is yes. But the access technologies that these systems use -- combinations of code division multiple access (CDMA), time division multiple access (TDMA), frequency division multiple access (FDMA) -- make that at least as difficult as it will be to intercept a digital cellular signal. On top of that, many of the networks will offer some kind of internal security systems.