Fig. 1

The angle of each lower leg is compared to the critical angle \(\left( {{\uptheta }_{{\text{c}}} } \right)\) to determine uprightness. Accelerometer measurements (a_x, a_y, and a_z) and gyroscope (i.e. angular rate) measurements (p, q, and r) are in the local coordinate frame—x, y, and z. Roll \(\left( \phi \right)\) and pitch \(\left( {\uptheta } \right)\) are measured using the fixed global coordinate frame—X, Y, and Z