Sunday, October 18, 2009

Including GPS in the Mix

Processing new information brings new knowledge.

First things first, one absolute thing to avoid is putting the plane in the stall position. Listen to the sound of the wind just before the crash:

video

This includes aircraft speed and airfoil angle of attack (which is what happened in this case).

This set of experiences also includes aircraft dynamics such as roll and pitch tolerance during navigation maneuvers (using rudder for course correction puts the plane out of stable flight position), using ailerons for larger and faster turns (requires pitch and roll coordination) or climbing and descending (which was, surprisingly to me, more depended on aircraft speed then motor power or thrust).

And with GPS data we can visualize the flight and put it in to perspective:



The data indicates the flightpath was around 850 meters and it lasted for about 72 seconds. That puts the aircraft average speed at good 11 meters per second (40 km/h), which is consistent with the GPS measurements.

Next issue to solve is the GPS speed measurement spikes (accruing with the change of GPS accuracy or used used satellites in computing), which could be done with checking for accuracy changes and substituting with kalman filter values (to avoid needles calculations).

And the altitude offset (kind of important to know the correct altitude), caused by mathematical model used to calculate altitude from the center of the earth, with possible fix by manually adjusting for the WGS 84 model of Ellipsoid for planned flight path. Quick research shows that mathematical model of sea level in the Europe region is about 30 meters about actual sea level, combining that with (rule of thumb) 3 times the GPS accuracy margin, the GPS measurements came within map altitude. With enough altitude clearance probably good enough, but landing will take some more work!

Accelerometer and Orientometer sampling and smoothing is progressing well:


One thing left is to manage or wrap around the transitions in orientometer such as compass going from 350° to 10° trough North, which would be transition for 20° but is computed with kalman as transition for 340°.

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