Google's ambitious efforts to bring balloon and aircraft-borne connectivity to underserved areas of the globe are pushing past some key milestones, and the company expects a public launch in a few years.

Both projects have captured the imagination of many for their ability to beam Internet signals from platforms high up in the sky to areas without cellular networks, but represent significant engineering challenges for Google -- just the kind of thing the company likes, said Sundar Pichai, a senior vice president at Google, speaking at Mobile World Congress in Barcelona.

The oldest and perhaps best known of the two projects, Project Loon, seeks to use balloons flying around 20 kilometers (65,000 feet) above the Earth to deliver Internet signals. The company's first experiments used a proprietary Wi-Fi signal but that's since changed to LTE cellular signals.

When Google first began launching the balloons two years ago, it couldn't manage to keep them up for more than about five days at a time, but now they are in the sky for as long as six months, delivering LTE signals directly to handsets on the ground. The range it can achieve with each balloon has quadrupled, he said.

"We think the model is really beginning to work," he said.

Google is working on tests of the technology with Vodafone in New Zealand, Telstra in Australia and Telefonica in Latin America.

A newer project called Titan is at the stage where Loon was a couple of years ago, said Pichai.

Titan relies on solar-powered airplanes that can stay aloft for long periods. Unlike the balloons, which will largely be propelled and steered by winds, the airplanes will have a greater degree of movement, he said.

"The planes can supplement in areas where we need extra capacity, such as during disaster relief," he said.

Combining the balloons and planes will result in a floating mesh of cell towers, delivering Internet connections to many areas of Earth, said Pichai. Or at least, that's the plan.

The planes are scheduled to make their first test flights in the next few months and Google will be working with cellular carriers around the world on their testing.

"You'll see a lot of progress in the next few years, that's the time frame," said Pichai when asked when the technology will move beyond testing.

He said it won't necessarily be limited to remote areas and could see use in developed nations.

"Even in places like the U.S., we've struggled with peak-demand situations," he said.

Google has two other projects that are bringing wired connectivity to urban areas. Google Fiber continues its expansion in the U.S. this year and Pichai said that Project Link, which has brought a high-speed telecommunications backbone to Kampala in Uganda will be expanded to "many more" countries in Africa this year.

Martyn Williams covers mobile telecoms, Silicon Valley and general technology breaking news for The IDG News Service. Follow Martyn on Twitter at @martyn_williams. Martyn's e-mail address is martyn_williams@idg.com