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UPDATE: GAGA team hunts down grass-smoking ROBOT

Where in the world is our lawnmower?

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The rise, and fall, of GPS positioning

We ordered a GlobalSat MR-350 waterproof GPS receiver, which ironically had a terrible time reaching us from Expansys (though, to be fair, it was DHL's fault as they shipped it repeatedly to Edinburgh and then decided that the weather was too bad for the package to reach the Scottish Highlands – despite the fact that the only snow we'd had here during the big freeze last month wasn't enough to fill an ice tray).

Capture of DHL's disappointing delivery

Our GPS kit spends a few days knocking around Edinburgh before settling in Inverness for a few days to wait for the snow which would justify its delay

Irony aside, there are distinct problems with the Global Positioning System, which would appear to make it almost entirely incapable of meeting our needs.

GPS works by picking up the time from satellites, and using that time to measure how far away they are. The satellites carry the heroically accurate clocks, and the distances involved are great enough to allow comparative measurements – if only the speed of light weren't so damned variable.

The absolute constant, c, is only constant in a vacuum, and while the majority of the signal's journey to earth is completed within a vacuum, the last bit has to penetrate our soupy nitrogen/oxygen atmosphere where the speed of light (and thus radio) varies depending on the pressure - that is to say, GPS is horribly weather-dependent.

This can be addressed through the use of Differential GPS. Differential systems start with a known location, often a lighthouse or similar, and fit a decent GPS receiver there. The difference between the known location and the GPS fix at that location is then transmitted across the ground, at around 300kHz, so a GPS receiver which can pick up the Differential signal can modify its own results by the same amount to account for atmospheric distortion.

Sadly our test site lies bang between the two Scottish Differential transmitters, at Aberdeen and the northern tip of Lewis, around 160km from either. According to the US Federal Navigation Plan, Differential GPS can drift by 67cm for every 100km of distance, so at the very best we'd have an accuracy of around 1m, that is, if we could actually pick up the Differential signal at all – unfortunately it seems that the mountains between our test site and either transmitter make Differential GPS another dead end.

It is possible to create a local Differential signal, as the farmers of East Anglia have done, but East Anglia isn't significantly more hilly than the open sea, and is also a very long way from us, so some sort of local solution is necessary.

It occurred to us we could create our own differential: using two GPS receivers, one stationary and the other mounted on the robot, we could subtract the stationary from the moving to create a relative position, which is all we need.

The GSP receivers communicate using RS232. The exact protocol costs £400 to buy, but there are enough documented examples to enable basic functionality without needing the exact specs. A pair of MR-350s were connected up and monitored to test the idea, but to our horror we hadn't even started either of them moving when we were able to watch as they happily reported colliding, and eventually passing through each other, within minutes – all while absolutely stationary and standing more than two metres apart.

Clearly cheap GPS kit wasn't going to suffice, so we upgraded to a Garmin 10x-5Hz costing well over a ton, and watched to see how consistent it would be in picking up the signal.

Low-res image captured for analysis

Fortunately Garmin supports the industry-standard crocodile-clip-and-twisted-wire interface

But that too drifted by a handful of metres while being watched, and to be fair Garmin does state that even when a Differential signal is available one shouldn't expect accuracy better than three metres.

Garmin's accuracy in pictures

So we are left without a way of locating our robot, lost in a sea of lawn and unable to find its way home. We could, of course, concede that intelligence should be limited to those with flesh around their bones, and just build something which bounces around the garden, but that seems like cheating so we're opening the floor to suggestions for better navigation technologies.

Meanwhile we'll get on with finding the best way to hack through the growth. Several people pointed out that our 6,000 rpm motor wasn't up to the job of whisking plastic blades through the grass, so a 10,000 rpm motor was obtained and testing continues, but at the moment the winning technology is a bloody great chunk of metal with a sharpened edge:

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