It's been two years since Google first disclosed Project Loon, and while the company continues to keep most details of the project secret, the technology and challenges behind it are slowly coming into focus.
Loon is an ambitious attempt to bring the Internet to the roughly 5 billion people on the planet who are out of range of existing networks. The project involves suspending cellular access points under high-altitude balloons to provide Internet access to those on the ground, an idea that sounds elegantly simple but was anything but.
A series of recent presentations and talks by Google X employees have revealed some of the technical and commercial challenges the company faced in realizing Loon, and in nearing its target cost of $10,000 per balloon.
The balloons travel in air currents at an altitude of around 60,000 feet. That's close enough to Earth to maintain a direct connection to smartphones but high enough to avoid aircraft. It's also in a part of the atmosphere criss-crossed by high-altitude winds, so Google can steer the balloons by moving them up and down to catch air currents moving in different directions.
But at that altitude, the balloon and the electronics are battling frigid temperatures of around -65 degrees Celsius.
Batteries and other components don't work well in the cold, so all the electronics sit in a large Styrofoam-insulated container to keep them warm. Most are kept switched on even when they're not needed, so the components stay warm. Perhaps counter-intuitively, that means better battery life, because bringing cold components back to life requires more energy than just leaving them ticking over.
The cold also makes the nylon the balloons are made from turn brittle and causes lubricants to break down -- challenges to Google's goal of keeping each balloon in the air for at least 100 days. The balloons are bathed in strong ultra-violet and cosmic radiation, adding to the challenges, and endure extreme pressure changes as the helium inside expands and contracts as the balloons float in and out of sunlight.
When Loon was first announced, high-altitude balloons typically could stay aloft for no more than a couple of days before puncturing, and some thought Google's endurance goal was crazy. But the company is now regularly meeting its 100 day target, and one balloon stayed aloft for 187 days.
For the communications, the initial plan was to send proprietary Wi-Fi signals to fixed antennas on the ground, but that was soon switched to cellular LTE signals. This brought the advantage of being able to deliver a signal directly to a smartphone, and means Loon can operate in a part of the spectrum with much less interference than the Wi-Fi bands.
Most of Google's current balloons carry radios that operate in the 2.2GHz and 2.6GHz LTE bands -- chosen because those bands provide coverage across the U.S., Europe and Asia. The signal from each balloon covers an area with a radius of 40 kilometers, and Google is preparing to do tests in the 700MHz LTE band to cover an area four times as large.
The balloons Internet connection comes from access points on the ground, and because a balloon wont always be within range of one, the signal is passed from one balloon to the next until an access point is in range. This balloon-to-balloon network can run at speeds of up to 4Gbps while the downlink speed to handsets on the ground is around 30Mbps at best.
Keeping track of the balloons is perhaps one of the easiest challenges, because it involves sifting massive amounts of data and modeling wind patterns -- the kind of things Google's data scientists are adept at doing.
Balloons stay in communication with Google through the Iridium satellite network. They send out location and other data at intervals ranging from every second to every couple of hours. But the balloons actually log thousands of data points every second that are recorded and stored for later analysis, so any part of the flight of any balloon can be analyzed.
Flight tests are currently taking place in New Zealand, Argentina, Chile, and the state of New Mexico, and one of the most recent innovations has been an automatic balloon launch system.
For Loon to cover the world, tens of thousands of balloons will need to be in the sky at any one time, and with a 100-day life span per balloon, that means hundreds of launches each day to keep the network up and running. At that level of activity, a system that relies on humans would have trouble keeping up.
But for all that's known about the technology behind Loon, the biggest unanswered question is perhaps the most interesting: when will it be commercially available?
Google declined an interview request for this story. It has said it won't offer Loon service to consumers itself but rather is partnering with cellular operators around the globe, essentially leasing out the balloons as they pass over areas that need coverage. Under this business model, Google is freed from the hassle of dealing with individual subscribers and can operate under the carriers' existing wireless licenses.
Unfortunately, there's still no clear indication of when commercial service will be available. Google says it want's to bring the cost per balloon down to about $10,000, and it's not there yet, but it is getting close.
"We're well within 10X of that," said an engineer.
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 firstname.lastname@example.org