San Francisco: Google Inc.’s expected unveiling of a rival to the iPhone is part of its careful plan to try to do what few other technology companies have done before: retain leadership as computing shifts from one generation to the next.

New entrant: The Nexus One. NYT

As people increasingly rely on powerful mobile phones instead of PCs to access the Web, their surfing habits are bound to change. What’s more, online advertising could lose its role as the Web’s primary economic engine, putting Google’s leadership role into question.

Top Google executives, including Eric E. Schmidt, the chief executive, have long said that the mobile Internet was Google’s biggest opportunity for new growth. They orchestrated a string of acquisitions of companies with mobile-related technology.

Google also invested far more aggressively than its competitors in mapping technologies and services tied to a user’s location, which are likely to become the vital underpinnings of new advertising systems on global positioning system-equipped mobile phones.

The expected unveiling on Tuesday of the Nexus One, a thin, touch-screen handset built to Google’s specifications and made by Taiwanese company HTC Corp., is a challenge to a newly minted industry power: Apple Inc., whose iPhone dominates the high end of the smartphone market. While the iPhone sends millions of people to Google’s search and other services, some of the company’s applications, like Google Voice, have not been allowed to run on the phone.

Analysts say that with Nexus One, which Google plans to sell to consumers directly, the company is trying to free itself from Apple’s growing influence. It also wants to broaden the appeal of Android’s technology. The phone is expected to be sold unlocked, allowing consumers to buy service plans separately.