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"vernier engine throttle settings were controlled using a proportional-plus-derivative
feedback loop closed around the measured vs. desired velocity magnitude as
a function of altitude, represented by the [~parabolic] descent contour"
Surveyor Spacecraft Automatic Landing System
Sam W. Thurman
AAS 04-062
Fifty Years of Orbit Determination: Development of Modern Astrodynamics Methods
Jerome R. Vetter

3 guys, 1500-2000 hours to first product

CrazyFlie: Tiny Open Source Quadrotor -