To Undertand the Difference Between Alignment and Calibration

by HanaHana

1. What is System Alignment?

The inertial navigation system (INS) calculates the navigation results by integrating the measured values ​​of the inertial sensors, that is, each iteration of the navigation equation needs to use the previous position, speed, attitude and other navigation parameters as its initial values. Therefore, before using INS to provide navigation results, it must be initialized.

The initial position and velocity information of the INS needs to be provided externally. In the GNSS/INS integrated navigation system, the initial position and velocity information is provided by the GNSS receiver. In addition, the INS can also be initialized by placing the INS near a known location point measured in advance. At this time, the INS can know its position in the outside world and its existing velocity through external information, but it is still not certain that when it measures its acceleration in the X-axis, or in the Y-axis when the rotation axis rotates, where is the acceleration moving toward the outside world, or where the axis rotates. Therefore, the INS has to initialize the attitude.

The process of attitude initialization is called alignment, because this process is to physically align the IMU with the local navigation system axially, or to clearly know the attitude of the inertial device in the local navigation system. The initial attitude information can be provided through external information, or it can be determined by the INS itself through the sensitive gravity vector and the earth's rotation vector.

In a word, every time the device is powered on, it needs to go through the alignment process before it can work normally.



2. What is RBV Calibration?

In the GNSS/INS integrated navigation system, when the INS is aligned, we can know the INS attitude in the local navigation system. At the same time, we can also obtain the position and velocity results in the local navigation system through GNSS reciever.

Since the axis of the IMU Body system and the carrier (vehicle) system cannot completely coincide when installing the GNSS/INS receiver, the orientation of the INS system cannot be directly used as the orientation of the carrier to achieve motion constraints. Instead, we need to set first the RBV (Rotation from IMU Body to Vehicle) parameter to rotate the relative relationship between the INS and the carrier's attitude so that the INS system and the carrier (vehicle) system's axial direction overlap as much as possible to achieve motion constraints.

Since the installation cannot guarantee absolute accuracy, there will be an offset of several degrees between the input RBV value and the actual installation. The RBV error mainly affects the attitude of the vehicle body in the navigation information, and also affects the navigation center and azimuth to a certain extent due to the lever arm configuration. The impact is mainly reflected in the LAND MODEL (Check with SETINSPROFILE command). When the satellite signal is good, there will be no obvious impact; when the satellites signal is lost, the error divergence is particularly obvious. At that time, the constraint deviation caused by the RBV error in the NHC processing will be increasing with time. If you are not using LAND MODEL, there is no significant difference in navigation results.

To get RBV calibrated, you need to meet the following conditions:

1. Drive in a straight line (no turns);

2. Driving speed reaches 5m/s (minimum speed);

3. Flat road (the carrier cannot be inclined);

4. Drive in the forward direction (reverse driving is not allowed)

Among them, you need to pay attention: the first two conditions will be automatically detected during calibration process, and the receiver will use the data that meets the requirements for calibration; the latter two cannot be automatically detected, and must be performed in accordance with the requirements during calibration, otherwise the RBV calibration result may be inaccurate.

In a word, generally when the receiver is not moved, the RBV calibration is only required in the first-time installation to improve the accuracy. If you are unable to get RBV calibrated, just try to ensure that the receiver installation attitude is consistent with the vehicle frame, in this case it can work normally as well, but the accuracy may not be as good as the calibrated.



3. System Alignment Process

Alignment generally goes through the following process:

A. Power-on, alignment not activated: ins_inactive

B. Waiting for the initial position: waiting_initialpos

C. Waiting azimuth: waiting_azimuth

D. Aligning: ins_aligning

E. Rough alignment completed: ins_alignment_complete

F. Fine alignment stage, rtk corrects the alignment result, after fine alignment converges: ins_solution_good

In the working process, you may also see the following status: 

A. Large precision variance: ins_high_variance

B. Satellite results are not available: ins_solution_free

Among them, ins_solution_good is the optimal status and can be used with confidence, ins_alignment_complete is the second, and the accuracy is slightly worse, and can be used as appropriate (for details, please refer to the output standard deviations), other status are intermediate processes or unavailable status.



4. RBV Calibration Process

1. Ensure that X1 enters RTK fixed solution

2. Drive the vehicle

3. Configure RBV calibration information output and frequency: log comX inscalstatus onchanged

4. Enter the command to start calibration: inscalibrate rbv new

5. Status information during calibration:,

ins_converging: large error

insufficient_speed: insufficient speed

high_rotation: too large turns

calibrating: calibrating,

calibrated: calibration completed

6. Calibration completed: the output status is calibrated, and the message output stops

7. Save configuration: saveconfig

8. Reboot: reboot

9. Query INS configuration (log insconfig) to ensure that the configuration is correct


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