Monday, June 17, 2013

Project Loon

Google just announced project Loon (Google Blog, AFP article, and Wired article).  This is a scheme to provide internet access to folks in the southern hemisphere from high altitude balloons.  Short take: they can steer the balloon, which is potentially the required gamechanger, the balloon is too big for their stated payload, and it'll need to get bigger if they want to build a global WiFi hotspot.

This is Google's second try at the southern hemisphere problem.  In 2008 they helped start O3b Networks, which is attempting to launch a low earth orbit satellite constellation that would provide broadband access.

I'll start with a small point.  Something is odd about the size of the balloon.  The stated payload is 10 kg and the stated balloon is 15 meters in diameter, 3 mil thick polyethylene, and operates at 60,000 feet.  That's too big a balloon for that payload.

The stated balloon envelope will weigh about 20 kg.  So, they'd need about 30 kg of lift, which is about 30 m3 of helium at standard temperature and pressure.  But at 60,000 feet, the ambient density is around 100 g/m3 and helium density is around 14 g/m3.  So that's around 360 m3 of volume.  Google's 15 meter balloon holds 1767 m3 and is sized to pick up something a lot bigger.

So, I'm guessing they actually want to lift something more like 100 to 150 kg.  The helium to fill that is around $1300, the balloon is another $1500 in quantity, and the payload will be around $2k (but see below).

I wondered if they'd lose helium fast.  It turns out, no.  According to this table, the helium permeability of polyethylene is 5.3e-8 cm^2/sec. The 'Loon probably has 235 m^2 of 75-micron-thick polyethylene, and so leaks 1.4 m^3/day. I'm surprised at how small that is... sounds like they could stay up for a year if the balloon is fully inflated and residence was limited by leakage. My guess is that time aloft will be limited by ultraviolet breakdown of the polyethylene and by wind shear occasionally ripping the things apart.

Assuming that ground terminals require a line of sight to the 'Loon at least 10 degrees above the horizon, each 'Loon covers a circle 200 km in diameter.  Uniformly covering the whole earth would take 16,300 balloons.  But they aren't going to cover the whole earth, and they had better not cover it uniformly.

The first big rollout, if it ever happens, will be over the Hadley cell between 30 and 60 degrees south of the equator, and will cover primarily Australia, New Zealand, Argentina, South Africa, Chile, and Uruguay.  They'll need around 3000 balloons to get complete coverage.  The second big rollout will be over the Hadley cell between the equator and 30 degrees south, will require 4000 balloons, and will primarily cover Indonesia and Brazil.  There will be major objections to these overflights in all the affected countries, and I'm not sure how Google will try to overcome those.  But covering 1 billion people for $15m is pretty awesome.

The next logical rollout would be over the Hadley cell between the equator and 30 N.  Once again, it would take 4000 balloons and cover India, the Philippines, most of central Africa, everything from Columbia to central Mexico.  I just can't imagine this ever being accepted by governments with the ability to shoot down unwanted overflying balloons.  The next Hadley cell north gets China and the US and is even less politically feasible.

The 'Loon has a proprietary link to the ground stations.  Why not WiFi?  The problem is that a single 'Loon will often cover 100,000 cellphones, each with a WiFi endpoint.  Without some way to separate all those signals, the 'Loon will just see noise.  A proprietary link allows Google to throttle the number of simultaneously transmitting terminals, and add coding gain, to get enough signal to noise ratio to communicate over dozens of km.  Basically, the proprietary link is a way to shake the tree and see how governments react to a potentially uncensorable global ISP.

The better solution, the one I'm sure the Google engineers would love to implement, would be to put 802.11ac on the 'Loon.  802.11ac is the next-generation wireless Ethernet standard, and it will be on all smartphones in two years.  Crucially, the standard protocol requires the handsets give the access point the feedback necessary for the access point to use phased array antennas to form beams.  Those beams have two big consequences.
  • The beams let the 'Loon capture more of the energy transmitted from the handset.  A 6 m diameter phased array should capture enough energy from a cellphone at 40 km to enable 2-3 Mb/s transmission.  I don't know that 802.11ac has a LDPC code that lets it go that slowly, but if not, Google may be able to require that Android handsets implement an additional optional royalty-free code.
  • The beams let the 'Loon distinguish between tens of thousands of handsets simultaneously.  A 6 meter diameter phased array could implement cells on the ground 250 m diameter (right under the balloon) to 500 x 1000 m (far away from the balloon).  This density isn't going to let folks in Cape Town watch HDTV via YouTube, but it'll handle email and web surfing just fine.
It's interesting to think about the parallels between this idea and the 1990s idea of having a constellation of low earth orbit satellites providing broadband access.  At least five different schemes were proposed.  Iridium and Globalstar, both providing cellphone coverage, were actually built, and both went broke.  Teledesic, backed by Bill Gates, Paul Allen, and Saudi prince Alwaleed bin Talal, got canned before it launched it's constellation.  The problems with all these schemes were:
  1. The handsets were bulky and heavy because the satellites were 400 km up and 1000 km away.  'Loon cuts the range by more than an order of magnitude, which cuts the antenna size on both the access point (balloon) and handset by an order of magnitude.
  2. The constellations required custom handsets.  'Loon has this problem too but 802.11ac is a path to a solution.
  3. The access point hardware went on satellites which are expensive and hard to maintain.  The balloons should be easier to maintain... than satellites.  Though it's better than satellite constellations, I'd still count this issue against 'Loon.
  4. The orbital geometry meant that coverage was concentrated near the poles, where there aren't many people.  'Loon does not have this problem.
  5. The schemes required access point hardware sufficient to cover the entire earth, but only provided value where there were customers.  'Loon has this problem even worse than Teledesic.  The Earth as a whole is about 30% land, but the southern hemisphere is 19% land.  Worse yet, only 12% of the world's population lives in the southern hemisphere.  This problem can kill 'Loon.
There are currently two broadband LEO constellations in the works.  O3b, as mentioned above, and COMMstellation.  I don't see how either has fixed the problems that sunk Teledesic et al.

There has also been a bunch of work on broadband from High Altitude Platforms.  Balloons have been considered before, as have high altitude, long endurance aircraft, and interesting hybrids between the two.  In the late 90s, Angel Technologies was going to fly a Rutan-designed aircraft in the stratosphere, carrying a phased array broadband access point.  Aircraft can carry larger payloads (Halo could carry 1 ton), provide more power (Halo could provide 20 kilowatts), and can keep station over populated areas.  Stationkeeping fixes problem #5 above, and is a huge deal.

'Loon may have a new answer to problem #5.  They can steer.  Fast forward to 1:11 in this video.

Steering is a big difference from satellites.  Stratospheric winds will carry the 'Loons eastward around the globe, but if they can steer north-south while that is happening, they may be able to crowd the 'Loons over denser populations, and scoot across the oceans on the jet stream.  Steering could dramatically reduce the number of 'Loons needed.

Technology developments to watch for:

A solar power array that faces nearly sideways, steered toward the sun with fans or something.  Google is going to need this for the first big rollout, since the sun will not be more than 20 degrees above the horizon for most of the winter, and the grazing angle will kill their panel efficiency.

An ASIC that enables a 802.11ac access point to handle 1000 transceivers in less than 5 kilowatts.  They are going to need this chip for the last forseeable rollout, the one that interfaces directly to cellphones and turns Google into a global ISP for Brazil and Indonesia at least.  The ASIC is required because this generation 'Loon will be constrained by power.  Power requires big solar arrays and batteries to get through the night, and those require a bigger, stronger balloon.

A balloon-to-balloon optical link consisting of multiple small gimbal-stabilized telescopes, perhaps two inches in diameter, to relay the photons between 10 gigabit ethernet fiber optic transceivers.  I've wanted to see this technology for years, but the killer app hasn't shown up yet, because at ground level the weather would frequently disrupt any link.  At 60k feet that shouldn't be a problem.  They'll need this technology for that last big rollout.  I don't think radios are going to work well enough.

As a camera nerd, I can't help but note the temptation to put a 2 kg camera on each 'Loon. You'd get something like hourly coverage of everywhere at 12 inch resolution, and real-time video coverage of smaller targets (like traffic accidents) as well. There is a real market for that... SkyBox Imaging just got $90m in venture capital to address it.  This imagery, piped into Google Maps/Earth, would provide the live view of the entire earth that everyone already expects to be there.