The INS APP supports all of the features and operating modes of the VG/AHRS APP, and it includes additional capability of interfacing with an external GPS receiver and associated software running on the processor, for the computation of navigation information as well as orientation information. The APP name, GPS/INS APP, stands for Inertial Navigation System, and it is indicative of the navigation reference functionality that APP provides by outputting inertially-aided navigation information (Latitude, Longitude, and Altitude), inertially-aided 3-axis velocity information, as well as heading, roll, and pitch measurements, in addition to digital IMU data.
At a fixed N rate, the INSx81ZA continuously maintains the digital IMU data; the dynamic roll, pitch, and heading data; as well as the navigation data. As shown in the software block the calibrated IMU data is passed into an “Integration to Orientation” block. The “Integration to Orientation” block integrates body frame sensed angular rate to orientation at a fixed N times per second. For improved accuracy and to avoid singularities when dealing with the cosine rotation matrix, a quaternion formulation is used in the algorithm to provide attitude propagation. 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 the diagram, 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 INS APP 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 in INS Apps 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 AHRS/VG APP, the algorithm has two major phases of operation. Immediately after power-up, the INS APP 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 INS APP 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 INS APP operates with lower levels of feedback (also referred to as EKF gain) from the GPS, accelerometers, and magnetometers.
For proper operation, the INS APP relies on magnetic field readings from its internal 3-axis magnetometer. The INS APP must be installed correctly and calibrated for hard-iron and soft iron effects to avoid any system performance degradation. See section 3.4.1 for information and tips regarding installation and calibration and why magnetic calibration is necessary. Please review this section of the manual before proceeding to use the INS APP
For optimal performance the INS APP utilizes GPS readings from an external GPS receiver. The GPS receiver requires proper antennae installation for operation. See section 2.1.4 for information and tips regarding antenna installation.
The OpenIMU300Rx (RI and RA) do not provide inputs for GPS or other external sensors, so they can’t support the INS App.