Sirius' debut comes eight months after that of its only competitor in the nascent industry, XM Satellite Radio Holdings, but a month ahead of its own schedule.

That stepped-up launch has left the companies making the receivers for Sirius struggling to keep up. Retail inventories are still thin and the initial sales promotion will be muted, Sirius executives said.

"We think if we're too loud too early and there's not enough product on the shelf, we'd be driving our customers in, and our competitor would have more product available than we do," said Stan Kozlowski, Sirius' senior vice president of retail distribution.

Sirius had already launched its three satellites and was broadcasting live demonstrations at major conventions before XM had a single satellite off the ground.

But in the space of eight months last year, XM went from nowhere to a national service, while Sirius was mired with its own issues, like funding, radio availability and the replacement of its chief executive.

Now with Sirius poised to take market share, some analysts have grown cautiously optimistic.

In a research note to clients, Bear Stearns analyst Robert Peck said Sirius' suppliers--chipset and radio manufacturers--were ramping up production capacity, while retailers were stocking the receivers, placing a strong bet on the ultimate success of the service.

Sirius radios will be available in more than 3,500 stores nationwide at launch and up to 9,000 stores by year's end, with 60,000 radio units already in store inventories and 300,000 units expected by the end of December.

But the company has moderated its promotions in the early going, as it works to boost product availability and give its manufacturing partners, like Japan's Kenwood, time to catch up with the moved-up launch.

The launch comes after a number of missed start-up plans. Last October the company said it would not succeed in its goal to launch at the end of 2001. Before that, it had discussed plans as late as the summer of 2000 for a late 2000 launch.

To mark the final occasion, Sirius is offering people who purchase one-year subscriptions their first three months for free. A standard monthly subscription to the service is $12.95.

While that is higher than XM's $9.99 monthly fee, Sirius' music channels are all commercial-free, whereas most XM music channels carry a few commercials every hour. Sirius has repeatedly emphasized its "commercial-free" airtime as one of its biggest selling points.

Kozlowski also said radios will be available with "plug-and-play" capacity--so that they can be used in the home as well as in the car--though he said the company does not plan to put any significant emphasis on in-home use until late this year and early next year.

Shares in both XM and Sirius have been under heavy pressure this year, in part because of general trends among technology stocks and in part because of investor concerns that both will need new funding sooner rather than later to keep them operating through expected break-even points in 2005 or 2006.

XM's shares are down more than 60 percent this year, while Sirius' shares are down closer to 70 percent. Sirius' shares now trade for less than $5, well off a high of more than $60 in February 2000.

Some financial analysts show a preference for XM. Of the 12 brokers who rate both companies, four rate XM a "strong buy," with only one "hold" and one "sell." In contrast, four also rate Sirius a "strong buy," but five rate it a "hold," with one "sell" and one "underperform."

Kozlowski said part of the reasoning in moving up the launch--despite the effect it had on forcing manufacturing partners to catch up--was to prove to investors that the company can get its service going.

"We've had a lot of false starts and things," Kozlowski said. "Sirius Satellite Radio has gone through a lot of scheduled grand-opening days, lots of scheduled start-of-service days."

But he also acknowledged relief that the launch is finally ready. "I've been living for this day," he said.

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