# GNSS eBubble calibration

This topic applies to Trimble receivers with built-in tilt sensors, including inertial measurement unit (IMU) or a magnetometer tilt sensor.

The GNSS eBubble uses the accelerometers in the receiver to provide an electronic representation of the degree of tilt of the receiver.

The GNSS eBubble operates independently of any IMU sensors in the receiver. For receivers that support IMU tilt compensation, the GNSS eBubble appears in the software only when the receiver is operating in GNSS‑only mode.

GNSS eBubble calibration aligns the accelerometers in the receiver to the physical sensor used to measure tilt:

- When connected to a receiver that supports IMU tilt compensation, you can calibrate the GNSS eBubble to one of the following:
  - Select Calibrate to vial if you have a well-calibrated physical spirit level vial to calibrate to, and the pole setup is known to be straight and optimal.
  - Select Calibrate to IMU if you do not have a well-calibrated physical spirit level vial to calibrate to, or if the pole being used is in less than excellent condition (for example the pole is not perfectly straight or the pole tip is misaligned). Trimble recommends using the Calibrate to IMU option when the pole setup requires a pole bias adjustment. Perform the Calibrate to IMU eBubble calibration immediately after you have applied a new pole bias adjustment.
- When connected to a receiver that does not support IMU tilt compensation, the only GNSS eBubble calibration option is Calibrate to vial.

When to calibrate the GNSS eBubble

The GNSS eBubble calibration takes 30 seconds to complete. You must perform the GNSS eBubble calibration:

- The first time you use the receiver. (Or the first time you use the receiver in GNSS‑only mode, if you are using a receiver that supports IMU tilt compensation.)

- When the previous calibration expires.
- After completing a pole bias adjustment.
- If the GNSS receiver suffers severe abuse such as a pole drop.
- If the temperature inside the receiver is more than 30° Celsius different to when the eBubble calibration was performed, the calibration is invalidated.
- If the Trimble Access software detects the GNSS eBubble is not calibrated and displays the warning message Calibration required to use eBubble tilt functions. Calibrate now?.

Before performing the GNSS eBubble calibration

Take great care when calibrating the eBubble to make sure the most accurate tilt information is available at all times, including:

- Bubble reference: Calibrate the GNSS eBubble against a properly calibrated reference, such as the physical vial. If the receiver has a built-in IMU, you can use the IMU as the reference. The accuracy of the eBubble depends totally on the accuracy of the reference used to calibrate it.
- Pole stability: When calibrating the GNSS eBubble, the pole the GNSS receiver is on should be as vertical and as stable as possible. In practice this means using at least a bipod to hold the pole as still as possible.
- Pole straightness: The straightness of the pole affects the tilt measured by the sensors in the GNSS receiver. If you change poles, and the poles are not both in excellent condition, you should recalibrate the GNSS eBubble. When using IMU tilt compensation, you should perform a pole bias adjustment after changing poles and then recalibrate the GNSS eBubble.

To calibrate the eBubble

Calibration routines should not be left incomplete. You should not need to navigate to a different screen during the calibration, but if you choose to navigate to a different screen, Trimble recommends that you first complete the calibration process, or tap Cancel to cancel the calibration.

1. Set up the receiver so that the pole the GNSS receiver is on is as vertical and as stable as possible, and the receiver has a clear view of the sky.

   If the receiver supports IMU tilt compensation, then to calibrate to the IMU, IMU tilt compensation must be enabled, and the IMU must be aligned.

2. Make sure the receiver LED panel is facing you.
3. Tap and select Instrument / Tilt sensor options.
4. Tap the Calib. softkey to open the Sensor calibration screen.
5. In the GNSS eBubble group box, select the physical reference that the eBubble will be calibrated against:
   - Select Calibrate to vial if you have a well-calibrated physical spirit level vial to calibrate to, and the pole setup is known to be straight and optimal.
   - Select Calibrate to IMU if you do not have a well-calibrated physical spirit level vial to calibrate to, or if the pole being used is in less than excellent condition (for example the pole is not perfectly straight or the pole tip is misaligned). Trimble recommends using the Calibrate to IMU option when the pole setup requires a pole bias adjustment. Perform the Calibrate to IMU eBubble calibration immediately after you have applied a new pole bias adjustment.

   When connected to a receiver that does not support IMU tilt compensation, the only GNSS eBubble calibration option is Calibrate to vial.

6. Tap Calibrate.
7. If calibrating to the vial, use the vial to make sure the pole is vertical. If calibrating to the IMU, use the IMU eBubble to make sure the pole is vertical. Hold the pole still and stable. Tap Start.

8. Keep the pole stable and vertical until the progress bar completes.

   Once complete, the software returns to the Sensor calibration screen.

9. If the receiver has a built-in magnetometer, you must recalibrate the magnetometer because calibrating the eBubble invalidates the magnetometer alignment. See Magnetometer calibration.

10. To close the Sensor calibration screen, tap Accept.

The details of the calibration are stored in the job. To review them, tap ≡ and then select Job data / Review job.