If XM Satellite Radio and Sirius Satellite Radio succeed at winning regulatory approval of their $11.4 billion merger, the technical integration of the two systems could take years to complete.

While a major technical hurdle involves millions of incompatible radio receivers, engineers will also have to sort through the integration of two very different satellite constellations. XM delivers its programming via two Boeing 702 satellites in geostationary orbit 22,300 miles above the equator. While the orbit gives them a fixed position in the sky, the satellites' low sight angles with the continental U.S. require XM to use about 800 ground "repeaters" to keep the signal flowing when an automobile briefly moves out of a spacecraft's line of sight.

By contrast, Sirius' highly-elliptical orbit offers a more direct angle of coverage, so far fewer repeaters are needed. But the elliptical track puts each satellite over the U.S. only 16 hours per day, requiring the use of a third spacecraft to ensure continuous radio service.

Spokesmen for both companies declined to answer most queries about the technical integration, saying details were still being worked out. But XM's system clearly has an advantage in terms of longevity (See related chart, page 8).

XM was forced to orbit two new satellites in 2005 and 2006 to replace spacecraft whose lives were shortened by faulty solar panels (the original two spacecraft remain in orbit as backups). The new satellites are designed to operate at least 15 years, meaning they should function until at least 2020-21 unless new technical problems arise. By contrast, Sirius' three satellites have been in orbit since 2000 and are projected to begin reaching the end of their service lives in 2013.

The merger plan - which is expected to face intense regulatory scrutiny in Washington - also could affect Space Systems/Loral if it is approved. Both companies have one new geostationary satellite on order from the satellite builder. According to a high-level engineer close to both XM and Sirius, if the merger goes through management might ask Space Systems/Loral to terminate work on one of the spacecraft.

Engineers from both companies, under a mandate from the U.S. Federal Communications Commission, have been working on development of a new radio that could receive signals from both satellite systems. And from a radio spectrum point of view there's little problem in merging the two systems, since they're located on contiguous blocs of the S-band spectrum. "You just make the radio tune across a bigger space," says a communications industry source.

But that won't do any good for the radios currently in use by XM's 7.6 million subscribers and Sirius' 6 million customers. Because those listeners are likely to balk at replacing their radios - many of which came factory installed when they bought their cars or trucks - executives at a merged company would likely phase in any changes over a long period of time, says Tom Taylor, editor of Inside Radio, an industry trade publication. "Your current receiver will continue pumping out programming for a good while," he predicts.

Dara Panahy, a lawyer with Milbank, says that in defining its next-generation system a combined XM-Sirius would also have to decide whether to remain radio-only or add video and broadband services to its content offerings.