Indian Summer

It’s the 26th September, and the regular English “Indian Summer” has arrived: clear blue skies, no wind, and 18.2°C in the shade, 22.2°C in the sun.  Time to do some more testing on GPS to see it’s viability to use it as part of auto-pilot control i.e. flight plans defined by a series of GPS waypoints.

Measured by me, a tape-measure and a magnetic compass, the flight was about 13m long heading almost exactly (magnetic) west.  Here’s the raw data I got from GPS with a few bytes zeroed out so you can’t track me down!

latitude, longitude, altitude, satellites, epx, epy
51.000968, -1.000573, 85.323000, 9, 22.995000, 58.705000
51.000765, -1.000710, 81.490000, 9, 22.995000, 58.705000
51.000803, -1.000683, 81.943000, 9, 22.995000, 58.705000
51.000803, -1.000683, 81.943000, 6, 22.995000, 58.705000
51.000816, -1.000683, 82.107000, 6, 22.995000, 58.705000
51.000823, -1.000683, 81.487000, 6, 22.995000, 58.705000
51.000841, -1.000684, 82.436000, 9, 22.995000, 58.705000
51.000854, -1.000684, 82.601000, 9, 22.995000, 58.705000
51.000860, -1.000684, 83.385000, 9, 22.995000, 58.705000
51.000872, -1.000685, 83.549000, 9, 22.995000, 58.705000
51.000878, -1.000685, 84.334000, 9, 22.995000, 58.705000
51.000883, -1.000685, 85.118000, 9, 22.995000, 58.705000
51.000890, -1.000700, 84.518000, 9, 22.995000, 58.705000
51.000896, -1.000700, 85.303000, 9, 22.995000, 58.705000
51.000901, -1.000700, 86.087000, 9, 22.995000, 58.705000
51.000901, -1.000700, 86.087000, 9, 22.995000, 58.705000
51.000907, -1.000700, 86.871000, 9, 22.995000, 58.705000
51.000907, -1.000714, 86.892000, 9, 22.995000, 58.705000
51.000907, -1.000714, 86.892000, 9, 22.995000, 58.705000
51.000919, -1.000715, 87.056000, 9, 22.995000, 58.705000
51.000919, -1.000715, 87.056000, 9, 22.995000, 58.705000
51.000919, -1.000715, 87.056000, 9, 22.995000, 58.705000
51.000919, -1.000729, 87.076000, 9, 22.995000, 58.705000
51.000925, -1.000729, 87.861000, 9, 22.995000, 58.705000
51.000925, -1.000729, 87.861000, 9, 22.995000, 58.705000
51.000925, -1.000729, 87.861000, 9, 22.995000, 58.705000
51.000919, -1.000744, 87.096000, 9, 22.995000, 58.705000
51.000919, -1.000744, 87.096000, 9, 22.995000, 58.705000
51.000919, -1.000744, 87.096000, 9, 22.995000, 58.705000
51.000913, -1.000744, 86.312000, 9, 22.995000, 58.705000
51.000913, -1.000758, 86.332000, 9, 22.995000, 58.705000

It’s a 33 seconds flight, and there a 31 GPS samples.  There’s 9 satellites throughout barring a couple of samples.  Ignoring the first two samples, this data suggests latitude changes by (51.000917 – 51.000803)° or +12.23 meters (north) once converted to radians and multiplied by the average radius of the earth of 6,371,000 meters.  Longitude isn’t quite as simple to calculate as you need to know the latitude to work out the longitude distance; just trust me to say it’s -5.17 meters (west).  Alternatively, here’s the graph:

GPS drive flight tracking

GPS drive flight tracking

And if you haven’t spotted it, there’s a problem:  GPS says the flight was 13.5m* long at -22° or roughly NNW.  The distance is right, but the direction is out by 68° according to the magnetic compass’s direction of west  I checked with my iPhone compass and it agrees. I can’t explain this; it’s worked fine in the past, and to the best of my knowledge, satellites round here haven’t been shifted.  More digging required…

*Just out of interest, the ground video tracking measured the flight length as 8 meters – I really do need to fix the arbitrary scaling I’m using.

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