DMU380ZA Series
User’s Manual
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Doc# 7430-3810 Rev. 02
Page 24
Following the integration to orientation block, the body frame accelerometer signals are
rotated into the NED level frame and are integrated to velocity. At this point, the data is
blended with GPS position data, and output as a complete navigation solution.
As shown in Figure 4, the Integration to Orientation and the Integration to Velocity signal
processing blocks receive drift corrections from the Extended Kalman Filter (EKF) drift
correction module. The drift correction 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 gyro and
accelerometers are performed.
The INS380ZA 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 gyro bias correction and provides stabilized angle outputs.
The EKF tracking states are reduced to angles and gyro 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 only EKF formulation. The UTC packet synchronization will drift due to
internal clock drift.
The status of GPS signal acquisition can be monitored from the hardwareStatus BIT as
discussed in Section 3.5.3 INS380ZA Built in Test. From a cold start, it typically takes 40
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 processor performs time-triggered trajectory propagation at 100Hz and will
synchronize the sensor sampling with the GPS UTC (Universal Coordinated Time)
second boundary when available.
As with the AHRS380ZA and VG380ZA, the algorithm has two major phases of
operation. Immediately after power-up, the INS380ZA 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 INS380ZA 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 INS380ZA operates with lower levels of feedback (also
referred to as EKF gain) from the GPS, accelerometers, and magnetometers.
Digital data is output over the UART port at a selectable fixed rate (100, 50, 25, 20, 10, 5
or 2 Hz) or on as requested basis using the GP, ‘Get Packet’ command.
In addition to the
angle mode packets of the AHRS380ZA and scaled sensor packets of the IMU380ZA, the
INS380ZA
has 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. See Sections 6
and 7 of the manual for full packet descriptions. All data is also available on the SPI
output port registers. Please refer to section 5 for a complete description of the SPI port
functionality.
IMPORTANT
For proper operation, the INS380ZA relies on magnetic field readings from its internal 3-
axis magnetometer. The INS380ZA must be installed correctly and calibrated for hard-
