Wednesday, January 30, 2013

Thousands of drones

Someone recently asked me if aerial survey could be accomplished with a swarm of tiny drone aircraft.  Imagine foot-long planes made of styrofoam, carrying the camera, IMU, and GPS of a cellphone, minus the display, with a far larger battery for propulsion.  How can this fail to be a cheaper way to survey large areas?

I've worked on a drone program before, but that was with a gas-powered UAV weighing 25 kg empty with a 2 kg payload and a range of around 1100 km over an 6 hour flight, costing in excess of $50k.  Operational costs were expected to be dominated by the cost of crashing.  I can say confidently that these are not a cheap way to survey anything.

In researching this blog post, I learned about solar-electric RC airplanes, which are a completely different kettle of fish.  The Aphelion, above, was built for under $500 (materials only) in Professor Mark Drela's MIT lab, and can fly at 5-6 m/s for as long as the sun is 6.5 degrees above the horizon, which is something like 8 to 12 hours a day.  That puts the daily range around 150 km.

Crucially, the whole airplane weighs 1300 grams distributed over a 3.3 meter wingspan.  My guess is that a Cessna might survive an in-flight impact with one of these, and maybe even something a bit heavier.

A survey drone version would be ruggedized for e.g. 1000 flights MTBF, would carry a 200 g camera payload and a 50g battery, would fly at 12 m/s, and cover 300 km during a day's flight.  Higher speeds lead to more coverage and better survivability in higher winds, but also smaller wings and less solar power.  The battery allows flight with intermittent solar coverage, due to clouds and building blockages.

This range is not great.  If a drone maintained 10 m/s from 9AM to 3PM, it would cover 216 km.  For comparison, a King Air B200, which is something of a standard twin-turboprop airplane for aerial survey, flies at 140 to 150 m/s and can cover 1700 km in a morning.

These drones are going to require an ultralight camera -- basically a cellphone camera.  The camera scales surprisingly well, primarily because you are comparing cellphone sensors to huge custom CCDs used in e.g. Leica ADS80 aerial cameras, and of course the cellphone sensors are far better because they have far more engineering investment.  Nowadays I would use an Aptina MT9F002 14-megapixel sensor with a 10mm lens, which will deliver 2 inch resolution from 1000 feet up.  This will deliver a swath of over 200 meters.  Because the drone will bounce around a fair bit, we'll need a very generous overlap between the imagery captured in one flight line and the next, so the flight pitch will be something like 150 meters.  For comparison, an ADS80 camera in a B200 flies with a 1 kilometer flight pitch when shooting this resolution.

Due to the close range and excellent sensor there is no need for a gimbal for the drone camera.  I'd use a 400 microsecond exposure time.  Combine that with the 140 microradian pixel field of view, and the camera can tolerate rolls and pitches of 25 to 50 degrees per second without excessive motion blur.  I think even a little drone with very low wing loading can deliver that most of the time.  If we shoot 94% forward overlap between frames (1 frame per second), which is possible because we are going so slowly, then if we lose a few shots due to motion blur we can afford to just drop them.  We'll collect about 100 GB of data in 6 hours at that frame rate, which will be storable on a microSD card next year.

Now we come to the very bad news for this idea.  The combination of low speed and small flight pitch means that we'll need something like 100 drones to replace a ADS80 in a B200.  If you talk to R/C folks, you will find that they spend at least as much time fixing their aircraft as they do flying them.  So those 100 drones are going to require an army of people to launch, recover, and repair them.

The bottom line is that using drones instead of manned aircraft still seems like a bad idea... if you can use a manned aircraft.  But here is how survey drones might turn out to be a really wonderful idea:  If you can make the drone light enough that it just won't hurt anyone when it crashes, then you might be able to get permission to fly down city streets.  THAT would rock, because you'd simultaneously solve a bunch of problems that a bunch of folks (Nokia/Navteq, TomTom/TeleAltas, Google) are spending hundreds of millions of dollars a year on today:
  • You would be collecting street level imagery without having to wait for traffic.  Much, much faster.
  • The native perspective of the cameras would be the one that people actually want.  This is just my intuition, but my sense from working on Street View is that it was too damn slow and difficult to navigate.  I think people want a raised perspective, but just not raised too much.
  • You would be collecting the sides of buildings at high resolution.  Right now there is a major gap between street-level resolution (Google's Street View is 3mm resolution) and aerial resolution (Google's 45 degree view is 100mm resolution).  That gap means that the sides of buildings don't match up well, which is a problem if, like Apple and Google, you want to deliver great looking 3D models of buildings.
  • Aerial obliques don't actually work in places like New York because the buildings are too tall compared to the street width.  However, if you fly between the buildings, this is no longer a problem!  This is a major issue now that over half the world's population (likely the wealthier half) live in cities.
I think it's a big stretch to imagine getting permission to fly your UAVs down city streets, and technically it would be a big challenge just to plan the routes... you'd probably end up needing to survey the city from high altitude first.  It sure is fun to think about, though.

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