What about a simple software dynamic control system with an accelerometer then? Basically you apply some thrust to the engine and look at the resulting acceler

What about a simple software dynamic control system with an accelerometer then? Basically you apply some thrust to the engine and look at the resulting acceleration with an accelerometer. Knowing the remaining length of the runway one can compute the acceleration that produces takeoff velocity at the end of the runway minus a safety margin, and then change engine thrust by the ratio between that desired acceleration and the measured acceleration. The remaining length of the runway can be computed by knowing the total length, initial position and integrating the accelerometer data, or by using GPS knowing the coordinates of the runway ends. This also automatically handles factors like wind, variable friction on the runway and miscalibrated engine thrust.