This page explains how to calibrate the external magnetometer embedded into the GPS antenna.

Page Summary

SKYMATE Configuration Manual

Access

To access Calibration page, connect the autopilot to your computer and launch SkyControl. In SkyControl click on the UAV icon and then on the left-hand side panel click on the Compass tab.

On-Site Calibration

All SKYMATE sensors are factory calibrated. However, magnetometers are affected by local magnetic anomalies and therefore may need to be calibrated on-site. Calibration is important as it reduces heading errors and leads to better navigational solutions. Thus, reducing undesired behaviors such as toilet-bowl and fly-away. SkyControl offers a set of tools to help operators easily calibrate the external magnetometer embedded into the GPS antenna whenever needed and easily manage calibration parameters.

Factory Calibration Parameters

On-site calibration does not remove or delete factory calibration parameters. The on-site calibration is simply added up to factory calibration. Factory calibration can be retrieved once on-site calibration is reset. This is explained in the subsequent calibration steps.

Calibration validation

Calibration validation consists in validating whether current calibration parameters are still valid or not. Whenever calibration parameters are no longer valid or sufficient, a new on-site calibration must be carried out by using the calibration tool of SkyControl. Four check-points are provided by SkyControl to validate whether current calibration is sufficient or not:

Date and Location Checkup: Once SKYMATE is connected to SkyControl, a pop-up message will request the operator to perform heading check. When the pop-up message is clicked, the operator will be directed to the Heading Check tab. If an on-site calibration is present, the date and location of this calibration are displayed. Otherwise, nothing will be displayed. Operators may choose to re-calibrate the magnetometer if they judge that the calibration is too old and/or performed at a distant location with respect to their current one.
Magnetometer ID Consistency: Calibration parameters are saved in SKYMATE. If calibration is performed with one GPS antenna and later on, the antenna is replaced with another one, SkyControl will issue a warning to inform the operator that calibration parameters are not valid anymore and that they will be simply ignored. The invalid calibration parameters will not be erased but will be ignored. In this case, operators must keep in mind that calibration parameters are out of date and that another calibration for the new magnetometer may be necessary.
Norm Consistency: SKYMATE continuously monitors the norm of the magnetometer measurements. This is very useful to detect anomalies in the magnetic field whenever the UAV is operated near electromagnetic objects. Two levels of norm error are considered in SKYMATE. If the first level is exceeded, SkyControl will issue a warning to recommend calibration. If the second level is exceeded, SkyControl will issue an error to require calibration.
Heading Check: This check must be carried out by the operator to confirm that UAS heading is correct. To check heading correctness, click on the icon in SkyControl and go to Compass tab in the left-hand side then choose Heading Check. The heading check can be done with the following steps:
1. Point the aircraft by referring to a landmark on the field. For example, point the aircraft toward a building or, parallel or perpendicular to a road. This landmark must be also visible on the map.
2. Once the aircraft is pointed toward the chosen landmark, click on the button 'UAV Positioned'. The map, the aircraft and a white line will appear.
3. Check the map, the direction of white and orange lines must reflect the aircraft heading. For example, if on the field the aircraft points toward a building, then on the map the white and orange lines must also point toward the same building.
4. After the heading is validated in the previous step, click on the button 'Heading Validated'. Blue, red and yellow lines will appear.
5. Rotate the aircraft 90 degrees clockwise. On the map, the orange line must coincide or be close to the blue line (within an interval of +/-5 degrees).
6. Rotate the aircraft another 90 degrees clockwise. On the map, the orange line must coincide or be close to the red line (within an interval of +/-5 degrees).
7. Rotate the aircraft another 90 degrees clockwise. On the map, the orange line must coincide or be close to the yellow line (within an interval of +/-5 degrees).
  
  To restart the process, click on 'Done' button. An on-site calibration should be carried out if any of heading checks fails.

Calibration performing

To perform calibration, the operator must follow these steps:

On the test field, keep a distance of around 2 m from metallic objects, such as cars. If the calibration is to be performed on a table, ensure that the table is metal-free.
Launch SkyControl on your Laptop.
Prepare the UAS and be sure that the GPS antenna is connected.
For security reasons and for operator's safety, disconnect UAS motors. Install the battery on the UAS and then connect it to the power monitor.
Connect to SkyControl via DataLink (you may also connect by using the USB cable, but, it is not recommended as it complicates calibration).
Once the connection between the UAS and SkyControl is established, you can check if any on-site calibration is already present. To do so, go to "All Params" section and then to MagCalibration (Calibration | On-Site Parameters) part. Non-zeros values in the 4th to 9th fields indicate that an on-site calibration is already present.
In SkyControl, click on the icon in SkyControl and go to Compass tab in the left-hand side then choose Calibration. The following page is obtained. On the bottom of the page, you will find the number of collected points, the percentage of maximum norm error before calibration (in red) and after calibration (in green). The operators can also find at the bottom of this page, the calibration parameters (scale factor, misalignment and bias errors) that are currently present in the autopilot (in the right-hand column) as well as the calculated calibration parameters (in the left-hand column).

Operators will be able to push the calibration parameters into SKYMATE when two conditions are fulfilled: first, the number of collected points is equal or greater than 200 and second, the collected points are well distributed over the space. SkyControl gives a tool to help operators track the distribution of the collected points over the space. Click on the '2D Views' button in the right-top corner of the page. The following six circles are obtained. During data collection, the quadrants of the circles will gradually fill up in yellow color and will turn into green when sufficient number of points is obtained. The second condition is attained when all six circles in the 2D view completely turn into green.
Click on the button "Clear Autopilot" to clear the existing on-site calibration parameters (this action will not remove the factory in-house calibration parameters).
Start collecting measurements by doing the following steps:
1. Hold the aircraft in level position with SKYMATE x-axis pointing in the same direction as you. Make a 360 degrees around yourself.
2. Pitch down the aircraft for 90 degrees so that SKYMATE x-axis points downwards. Make a 360 degrees around yourself.
3. Pitch the aircraft another 90 degrees so that the aircraft is upside down and SKYMATE x-axis points in the opposite direction as yours. Make a 360 degrees around yourself.
4. Pitch the aircraft another 90 degrees so that SKYMATE x-axis points upwards. Make a 360 degrees around yourself.
5. Hold the aircraft in level position with SKYMATE x-axis pointing in the same direction as you. Roll the aircraft for 90 degrees to the right. Make a 360 degrees around yourself.
6. Hold the aircraft in level position with SKYMATE x-axis pointing in the same direction as you. Roll the aircraft for 90 degrees to the left. Make a 360 degrees around yourself.
7. Repeat the above steps until the two conditions are fulfilled.
  
  These steps are supposed to fill the 2D circles in the order from left to right and from top to bottom. The following video shows how to rotate the UAS to collect data. During this step, collected data will be shown in red color and the operator can already judge fitting quality. In the same time, the temporary calibrated points will be shown in yellow color. In the absence of errors, all the red points will lay on the reference sphere. If this is not the case, the percentage of maximum norm error before calibration given in red in the bottom of the screen will provide the operator with a quantitative measurement of how far the collected data are from the reference sphere. At the same time, the scale factor, the misalignment and the bias errors will be calculated in real-time each time a new measurement is collected. The percentage of maximum norm error in green will reflect the improvement in fitting when the measurements are corrected for the calculated scale factor, misalignment and bias errors. It is expected that fitting error after calibration must be smaller than that before calibration.
  
  The following figure shows an example of collected measurements where 142 points are collected. The red points are the uncalibrated data and the yellow ones are temporary data given for calibration preview. The fitting error before calibration is around 20.8% whereas that after calibration it is about 10.2%. It is clear that the second condition on data distribution is fulfilled because all quadrants of the six circles are green. However, the number of collected points is less than 200. Therefore, one must collect more points before activating the push button to allow operators pushing calibration data into SKYMATE.
Once the two conditions are fulfilled, the push button will be activated. Click on the button to push calibration parameters into SKYMATE. The calibrated data will be shown in green. At this point, the operator may judge the calibration quality by inspecting if green points lay on the reference sphere or not and by comparing the fitting error percentage before and after calibration. The operator will also be asked to disconnect and to reboot the autopilot. Disconnect and reconnect the battery and you are ready to fly. The figure below shows in green the collected measurements when the calibration parameters are applied to them.
Redo the heading check procedure to validate calibration.

Unsatisfactory Calibration Results

If, for any reason, you are not satisfied with the calibration results, you can retry again by starting from Step 8.

Interrupting On-going Calibration

If you are in Step 9 and you are willing to interrupt the on-going calibration, you may click on the button "Restart Calibration". This will remove all previously collected data and will start to collect new ones.

Performing Back-to-Back Calibration

Normally, one calibration is sufficient. However, If you would like to further reduce magnetometer errors, you may perform a second calibration. In this case, restart Steps 9 and 10. This second calibration will not erase the previous one, it will be added to it. In this case, it is normal to see that the red points will be close to the reference sphere and that the fitting error percentage before calibration will be small since the collected data are now corrected with the previous calibration parameters.

Next step: Gains Configuration