SAN FRANCISCO - Apple Inc looks set to unveil a new iPhone on Tuesday after a 15-month hiatus, hoping to fend off hard-charging rivals running Google Inc's Android and safeguard its lead with the fifth incarnation of the iconic device.Newly appointed CEO Tim Cook will do the honors this time from Apple's headquarters, running the first major product launch in years without impresario Steve Jobs as he tries to take Apple to still-loftier heights.The iPhone 5 - expected to be faster, thinner and larger-screened - would arrive just in time for the crucial holiday shopping season. It should sustain Apple's smartphone momentum and help ward off a challenge from close runner-up Samsung Electronics.Apple launches are some of the hottest events on the tech calendar, scrutinized by fans, investors, the media and industry insiders alike. Tuesday's "Let's talk iPhone" show, starting at 1p.m. Eastern time, will mark Cook's unofficial debut since taking over from co-founder Jobs in August.Some shareholders and analysts expect a cameo appearance by Jobs, now chairman, though a no-show would not be a major disappointment.With the launch of the new phone, Apple faces the challenge of surpassing the popularity of its current iPhone 4, a 15-month-old model and bona fide hit with more than 20 million sold in the third quarter ended June 25.Some on Wall Street also expect the company to unveil a cheaper model tailor-made for the fast-growing Asian market, one of the few arenas where Apple can accelerate its growth."We expect improved processing power and camera capabilities, with a potential bump in RAM, though that may not be needed with iCloud," said William Power, analyst with Baird Equity Research."Various sources have suggested that Apple will debut more robust voice control functionality," he added.The new iPhone is also expected to come loaded with the updated iOS5 software that Apple unveiled in June. Some of the new features include improved notification functions and better Twitter integration.The iPhone - still the gold standard in the smartphone market - has been a huge success since it debuted in 2007, transforming it into one of the world's leading consumer electronics powerhouses.But it faces stiff competition from phones based on Google's Android operating system, which led the US market with a 43 percent share, Nielsen data shows. The iPhone came in second with 28%.Globally, according to IHS iSuppli, iPhone shipments climbed 9.1 percent in the second quarter while Nokia's plummeted more than 30 percent, handing the top spot to Apple with a market share of 18.4%. Samsung, whose shipments grew even faster, is coming on strong with a global market share of 17.8%.The iPhone 4 pricing starts at $199 and is available on the networks of AT&T Inc and Verizon Wireless, a venture of Verizon Communications Inc and Vodafone Group Plc. Many analysts expect Sprint will become the third U.S. operator to sell the iPhone when the next version is launched.