20
UM001
3.2
NavState Subsystem
The NavState subsystem generates a continuous reliable stream of low-latency, low-jitter state outputs
at a rate fixed to the IMU sample rate. The state outputs include any output such as attitude, position,
and velocity, which are not directly measureable by the IMU and hence must be estimated by the onboard
Kalman filters. The NavState runs immediately after, and in sync with the IMU subsystem, at a rate
divisible into the IMU Rate. This rate is referred to as the NavState Rate (default 800 Hz). The NavState
decouples the rate at which the state outputs are made available to the user from the rate at which they
are being estimated by the onboard Kalman filters. This is very important for many applications which
depend on low-latency, low-jitter attitude, position, and velocity measurements as inputs to their control
loops. The NavState guarantees the output of new updated state information at a rate fixed to the IMU
Rate with very low latency and output jitter. The NavState also provides the ability for the VN-100 to
output estimated states at rates faster than the rate of the onboard Kalman filters, which may be affected
by system load and input measurements availability.
3.2.1
NavState Measurements
The measurements shown below are calculated by the NavState subsystem and are made available at the
NavState Rate (default 800 Hz).
NavState Outputs
Attitude
(Yaw, Pitch, Roll, Quaternion, DCM)
Position
(LLA, ECEF)
Velocity
(NED, ECEF, Body)
Delta Angle (Available at full IMU rate)
Delta Velocity (Available at full IMU rate)
3.3
NavFilter Subsystem
The NavFilter subsystem consists of the INS Kalman filter, the Vector Processing Engine (VPE), and its
collection of other Kalman filters and calculations that run at a lower rate than the NavState. Most high
level states such as the estimated attitude, position, and velocity are passed from the NavFilter to the
NavState, and as such are made available to the user at the NavState rate. There are a handful of outputs
however that will only update at the rate of the NavFilter, some of which are listed below.
NavFilter Outputs
Attitude Uncertainty
Position & Velocity Uncertainty
Gyro & Accel Filter Biases
Mag & Accel Disturbance Estimation
Onboard Magnetic Hard & Soft Iron Estimation
World Magnetic & Gravity Model
3.3.1
INS Kalman Filter
The INS Kalman filter consists of an Extended Kalman filter which nominally runs at the NavFilter rate
(default 200 Hz). The INS Kalman filter uses the accelerometer, gyro, GPS, and (at startup) the
magnetometer to simultaneously estimate the full quaternion based attitude solution, the position and
