# IMU integrity monitoring

The receiver firmware constantly monitors the IMU sensors for data quality and indicates the current quality status in the IMU bias group box in the Sensor calibration screen.

The IMU integrity monitoring field can contain the following values:

- IMU OK
- IMU error detected
- IMU excessive bias detected

IMU error detection

If the IMU integrity monitoring feature detects the IMU sensors have become temporarily saturated due to an impact such as a pole drop, Trimble Access displays the warning message IMU error detected. When this happens, you must restart the receiver to reset the sensors.

Actions to deal with the warning are provided with the warning message. To immediately restart the receiver, tap Restart. To continue surveying without IMU tilt compensation, tap Disable IMU and proceed with using the receiver in GNSS‑only mode.

If the IMU error detected message persists after restarting the receiver, contact your Trimble distributor for further advice.

IMU excessive bias detection

If poor quality data is detected, such as excessive IMU bias, Trimble Access displays the warning message Excessive IMU biases detected. Perform IMU bias calibration or disable IMU tilt compensation. You should perform an IMU bias calibration only when you have received this error message.

Actions to deal with the warning are provided with the warning message. To perform the IMU bias calibration when the warning appears, tap Calibrate. To continue surveying without IMU tilt compensation, tap Disable IMU and proceed with using the receiver in GNSS‑only mode.

Causes of excessive IMU bias

Excessive IMU bias can be caused by any of the following:

- The receiver may have been dropped or suffered from some other form of physical abuse.
- The receiver has experienced a large temperature variation since the last time an IMU bias calibration was carried out, or the temperature is very different (many tens of degrees Celcius) from the time of the previous calibration.
- The internal biases inside the IMU increase as the sensors age over a long period of time.

To perform an IMU bias calibration

The IMU bias calibration should only be performed if the Excessive IMU biases detected warning message appears. The IMU bias calibration procedure enables the receiver firmware to measure and correct for excessive IMU biases. This affects the fundamental operation of the IMU sensor and so must be performed with utmost care, at approximately the average temperature that the receiver will operate in, and following the on-screen instructions as closely as possible.

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. Remove the radio antenna and quick release from the receiver.
2. To open the IMU bias calibration screen, do one of the following:
   - Tap Calibrate in the warning message Excessive IMU biases detected.
   - Tap and select Instrument / Tilt sensor options. Tap the Calib. softkey and then in the IMU bias group box, tap Calibrate.

3. Place the receiver on a very stable surface free of all vibration and any movement (it does not need to be level). Tap Start.

   When the progress bar for the first step has completed, instructions and a picture of the receiver laid on the side appears and the eBubble is shown. For the remaining steps, the eBubble will operate as if the instructions are being followed and the side of the receiver facing upward is to be leveled.

4. Lay the receiver on the side with the battery door facing up and the LED panel facing towards you. Level the side with the battery door using the eBubble. When the battery door side of the receiver is level, hold the receiver as still as possible while keeping the eBubble centered. The progress bar will start when the receiver is leveled appropriately, and will continue as long as the eBubble remains level. If the eBubble goes out of level, the progress suspends until the eBubble is leveled correctly again, then continues from where it was paused.
5. As the progress bar for each step completes, a new set of instructions and a new guide picture appears. Follow the instructions for each step very carefully, holding the receiver as still as possible for each step. The receiver automatically starts the process when the receiver is leveled in the correct pose, and automatically progresses to the next step when each step is completed satisfactorily. If the receiver detects a step has already been completed satisfactorily then that step will be skipped in the process.
6. A confirmation message appears when the process is completed. Tap OK to set the new IMU bias correction in the receiver. An Excessive bias calibration record is written to the job.