executed in 0.02 second or 50Hz, this frequency is chosen
because it's considerably matched to actuator servo
frequency.
The absolute attitude determination is implemented using
TRIAD algorithm, because this algorithm is considered
computationally efficient (no recursive computation) and
combines the gravity acceleration and magnetic filled
elegantly. To be able to correctly get the gravity
acceleration external information of speed and dynamic
acceleration from GPS receiver is needed, so this algorithm
is executed every 0.25 second or 4 Hz.
So in overall, we have attitude update every 0.02 second
(50Hz) and the attitude integration error is zeroed every
0.25 second (4Hz). By combining those 2 algorithms we
can have a stable AHRS for considerably long term
operation time, including in high G turn (centrifugal or
coordinated turn). The fusion algorithm is Kalman Filter.
4.1.4.3 Kalman Filter Formulation
Time varying Kalman filter is used to combine the high
update rate rotation rate integration (state update) and low
update rate absolute attitude determination (measurement
update). Here is the AHRS state space formulation