Moog Crossbow GNAV540 - Manual (Page 32)

Page 32
GNAV540 User Manual
7430‐0808‐01 Rev. B
module uses data from the aiding sensors, when they are available, to correct the errors in the velocity, attitude, and
heading outputs. Additionally, when aiding sensors are available corrections to the rate gyroscopes and
accelerometers are performed.
The NAV Function blends GPS derived heading and accelerometer measurements into the EKF update depending on
the health and status of the associated sensors. If the GPS link is lost or poor, the Kalman Filter solution stops
tracking accelerometer bias, but the algorithm continues to apply gyroscope bias correction and provides stabilized
angle outputs. The EKF tracking states are reduced to angles and gyroscope bias only. The accelerometers will
continue to integrate velocity. However, accelerometer noise, bias, and attitude error will cause the velocity
estimates to start drifting within a few seconds. The attitude tracking performance will degrade, the heading will
freely drift, and the filter will revert to
the VG Function; if the magnetometer is running, then the filter will revert to
. The UTC packet the AHRS function. In either function, the EKF formulation will continue without GPS velocity
synchronization will drift due to internal clock drift.
The status of GPS signal acquisition can be monitored from the hardwareStatus BIT (refer to Chapter 11. Built In Test
(BIT)). From a cold start, it typically takes 40–90 seconds for GPS to lock. The actual lock time depends on the
antenna’s view of the sky and the number of satellites in view. The DSP performs time‐triggered trajectory
propagation at 100Hz and synchronizes the sensor sampling with the GPS UTC (Universal Coordinated Time) second
boundary when available.
As with the AHRS and VG Functions, the algorithm has two major phases of operation. Immediately after power‐up,
the unit uses the accelerometers and magnetometers to compute the initial roll, pitch and yaw angles. The roll and
pitch attitude will be initialized using the accelerometer’s reference of gravity, and yaw will be initialized using the
leveled magnetometers X and Y axis reference of the earth’s magnetic field. During the first 60 seconds of startup,
the unit should remain approximately motionless in order to properly initialize the rate sensor bias. The
initialization phase lasts approximately 60 seconds, and the initialization phase can be monitored in the
softwareStatus BIT transmitted by default in each measurement packet. After the initialization phase, the unit
operates with lower levels of feedback (also referred to as EKF gain) from the GPS, accelerometers, and
magnetometers.
The NAV Function provides additional output measurement packets including the default N1 Navigation Packet,
which outputs the Latitude, Longitude, Altitude, X,Y,Z velocities, accelerations, and roll angle, pitch angle, yaw angle,
and digital IMU data. For more information about packets, refer to Chapter 9. Communicating with the GNAV540
Unit and Chapter 10. Programming Guidelines.
NAV Advanced Settings
In addition to the configurable baud rate, packet rate, axis orientation, and sensor low‐pass filter settings, the NAV
unction provides additional advanced settings which are selectable for tailoring the unit to a specific application
equirements. The advanced settings are listed in
F
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Table 10 GNAV540 Advanced Settings below:
Table 10 GNAV540 Advanced Settings
Setting Default Value Comments
Baud Rate 38400 9600, 19200, 57600 also available
Packet Type N3 S0, S1, S2, A0, A1, A2, N0, N3, N4 also available
Packet Rate 25 Hz This setting sets the rate at which selected Packet Type, packets are output. If
polled mode is desired, then select Quiet.