Abstract: Aircraft gyro (20, 22) signals .theta. and .phi., representative of pitch and roll attitude, respectively, and accelerometer (24, 26, 28) signals a.sub.x, a.sub.y, a.sub.z, representative of longitudinal, lateral, and normal acceleration, respectively, are processed (30, 32, 34, 36, 38, 40) to produce signals a.sub.GSLO, a.sub.GSLA, and a.sub.GSN, representative of the longitudinal, lateral, and normal groundspeed rate components, respectively. The component signals are summed (50) to produce a signal representative of total aircraft groundspeed rate.