# IMU tilt compensation

This topic applies to the Trimble receivers which have a built-in IMU sensor, such as the R780 and R12i.

Using a Trimble receiver with IMU tilt compensation allows points to be measured or staked out while the survey rod is tilted or tipped. This enables accurate measurements to be taken without having to level the antenna, allowing for faster, more efficient work in the field.

The inertial measurement unit (IMU) in the receiver uses information from acceleration sensors (accelerometers) and rotational sensors (gyroscopes) as well as GNSS to continuously determine its position, rotation, and degree of tilt, and correct for any amount of tilt. With IMU tilt compensation, the pole can be tilted at any angle and the software is able to calculate the tilt angle and the tilt distance to determine the position of the pole tip on the ground.

When enabled, IMU tilt compensation is "always on" and can be used for any measurement method except observed control point. When measuring an observed control point, the receiver automatically switches to GNSS-only mode, and the GNSS eBubble appears automatically if it is enabled.

IMU tilt compensation offers a completely different way of working because you can:

- Measure accurate points quickly while standing or walking without having to level the pole.
- Concentrate on where the pole tip needs to go, which is especially useful during stakeout.
- Easily survey hard-to-reach locations such as building corners and pipe inverts.
- No longer worry about movement of the pole when measuring, because the receiver automatically corrects for “pole wobble” when the pole tip is stationary.

Because performance is unaffected by magnetic interference, IMU tilt compensation can be used in environments susceptible to magnetic disturbance such as around vehicles, heavy machinery, or steel reinforced buildings.

In situations where IMU tilt compensation may not be able to be used, such as in very difficult RTK environments, you can manually switch to GNSS-only mode. To do this, tap the receiver icon in the status bar to view the GNSS functions screen and then tap IMU tilt compensation to toggle on/toggle off GNSS‑only mode.

Check out the R12i with Trimble Access playlist on the Trimble Access YouTube channel to see how you can get the most out of your R12i receiver using IMU tilt compensation.

Available survey types

IMU tilt compensation can be used in an RTK or RTX survey.

Correction methods available with IMU tilt compensation:

- RTK surveys with any type of real-time data link (internet, radio)
- RTX surveys (satellite or internet)

When using IMU tilt compensation, xFill can be used to bridge communication outages during an RTK survey, but not during an RTX survey.

When measuring or staking out points using IMU tilt compensation, make very sure the entered antenna height and measurement method is correct. Alignment reliability and pole tip position reliability, especially during movement of the antenna while the pole tip is stationary, relies completely on the antenna height being correct. Residual error in horizontal position caused by antenna motion while measuring when the pole tip is stationary cannot be removed by changing the antenna height after measuring the point.

Enabling IMU tilt compensation

Enable IMU tilt compensation in the Rover options screen of the survey style to enable "always on" tilt compensation using the internal IMU sensors while roving, navigating, or when measuring points using any measurement method except observed control point. See IMU tilt survey style settings.

Enable eBubble functions in the survey style so that you can use the GNSS eBubble to help you keep the receiver's integrated antenna level when measuring a point if you are working in GNSS‑only mode. The GNSS eBubble is not shown when the IMU is aligned.

IMU alignment

To use IMU tilt compensation, the IMU in the receiver must be aligned. Align the IMU after you start the survey, or during the survey when alignment is lost. The alignment process is simple and straightforward, and mimics normal use of the receiver. In good RTK environments the IMU reliably realigns itself automatically during natural pole movement. See Aligning the IMU.

When the IMU is aligned, the Position screen shows the position of the pole tip. This applies during and outside of a survey.

Sensor calibration

Once the IMU is aligned, IMU tilt compensation can be used “out of the box” without further calibration of the receiver. A number of calibration routines are available to calibrate the sensors in the receiver for normal maintenance. Calibrations should be performed as required. In particular, Trimble recommends performing the pole bias adjustment whenever you are using a different pole that is not in excellent condition.

When using a receiver that has IMU-based tilt compensation, the following sensor calibration routines are available:

- GNSS eBubble calibration
- Pole bias adjustment
- IMU bias calibration

Calibrations should be performed as required. In summary, Trimble recommends that you:

- Perform an eBubble calibration if the GNSS eBubble does not seem to align with the level reference you are using.
- Perform a pole bias adjustment whenever you are using a different sub-optimal pole or quick release.
- Perform an IMU bias calibration rarely, and only when the Excessive IMU bias warning appears.

In general, the sensor calibration routines are independent of one another. However, in a well-used pole (or with a badly calibrated vial), the vial may not be exactly perpendicular to the axis from the APC to the pole tip, and the IMU reference point may not be exactly in line with the pole tip. After completing a pole bias adjustment you should consider calibrating the GNSS eBubble to the IMU.

For more information, refer to the section for each calibration.

IMU status

During a survey using a receiver with IMU-based tilt compensation, the GNSS survey mode shown in the status line is:

- RTK+IMU in an RTK survey
- RTX+IMU in an RTX survey

When IMU tilt compensation is enabled, the receiver icon in the status bar is shown as:

The IMU alignment status is shown next to the receiver icon. A green check mark indicates the IMU is aligned . A red cross indicates the IMU is not aligned .

Precision values shown account for the number of GNSS satellites, current DOP, the quality of the IMU alignment and the tilt of the receiver. When the IMU is aligned, precision values displayed are at the pole tip. If IMU tilt compensation is enabled but the IMU is not aligned, no precision values are shown. Generally, the more the receiver is tilted, the larger the precision values become.

When IMU tilt compensation is disabled the receiver operates in GNSS-only mode, and precisions are calculated at the Antenna Phase Center.

In the map, the GNSS cursor indicates the IMU status. When the IMU is aligned, the cursor indicates the direction the receiver is facing.

GNSS cursor | Indicates
--- | ---
| IMU tilt compensation is enabled and the IMU is aligned. The arrowhead displays the direction the receiver is facing in relation to North or the Reference azimuth, depending on your map orientation settings. You must be facing the LED panel of the receiver for the GNSS cursor to be orientated correctly.
| IMU tilt compensation is not enabled, or IMU tilt compensation is enabled but the IMU is not aligned. The software does not know the direction the receiver is orientated.

Measurement methods

Measuring a point using IMU tilt compensation does not require a specific measurement method. When IMU tilt compensation is enabled and the IMU is properly aligned, most measurement methods can be used to measure a tilt compensated point including:

- Topo point
- Continuous topo
- Rapid point
- Measure to surface

- Horizontal tilt offset

  Measuring a horizontal tilt offset is useful to measure locations that cannot be occupied by the tip of the pole, for example when measuring the center of a tree or post.

- Observed control point

  The receiver automatically switches to GNSS-only mode because a vertical pole is required

Measuring points

When measuring points when the IMU is aligned, you do not need to level the pole before measuring. The tilted measuring mode icon in the status bar indicates the point can be measured without leveling the pole and without having to hold it very still.

When Auto-measure is enabled, the software starts measuring occupations as soon as the pole tip is stable on the point to be measured. When Auto-store is enabled, the point is automatically stored when the required occupation time and precisions have been reached. Simply pick up the pole and move to the next point.

Observed control points

When measuring an observed control point, the Trimble Access software automatically switches to GNSS-only mode so the point can be measured in static mode. The eBubble automatically appears, unless you have previously chosen to hide it for that measurement method. Use the GNSS eBubble to level the receiver before measuring.

In GNSS-only mode, the status bar shows RTK and the static measuring mode icon in the status bar indicates the pole should be vertical before you measure the point.

Once you have measured the observed control point, if you then select the topo point method and the IMU is still aligned then the software returns to using IMU tilt compensation. The GNSS eBubble automatically disappears, the status bar shows RTK+IMU and the tilted measuring mode icon in the status bar indicates that the point can be measured without leveling the pole and without having to hold it very still.

You can seamlessly switch between point measurement methods that use IMU tilt compensation and the observed control point method (RTK only) without having to realign the IMU as long as the IMU alignment is maintained throughout the measurements. If IMU alignment is lost while in GNSS-only mode then you must realign the IMU before you can measure a point using IMU tilt compensation.

Continuous topo points

When measuring points in continuous mode with IMU tilt compensation, you do not need to hold the receiver level while measuring. The tilted continuous mode icon in the status bar indicates points can be measured without leveling the receiver. You should closely follow the feature you are measuring with the tip of the pole. Stop and Go continuous points are stored when the software detects the pole tip has stopped.

Stakeout

Using IMU tilt compensation in stakeout provides large productivity gains, as you do not need to level the pole while moving it to minimize the stakeout deltas. Simply move the pole tip to minimize the deltas. IMU tilt compensation also allows the stakeout navigation feature to know the direction you are facing when you are stationary, which is an advantage when close to the point being staked.

You must be facing the LED panel of the receiver for stakeout navigation features to provide the correct information.

Stored IMU tilt information

When points are measured using IMU tilt compensation, device orientation information is stored with the point, including the tilt angle, tilt distance, azimuth, and the state of the IMU. This information can be viewed in the Store point form, or the Review job or Point manager screens.

When reviewing a point measured using IMU tilt compensation, the following additional information is provided.

Device orientation

Field | Description
--- | ---
Tilt angle | The tilt of the receiver based on the IMU.
Tilt distance | The horizontal distance from the position of the pole tip to the position of the APC projected vertically to the ground.
σ Tilt | Estimated tilt error (sigma tilt).
Azimuth | The azimuth (direction) of the tilt.
σ Azimuth | Estimated azimuth error (sigma azimuth).
IMU state | Shows that the IMU was aligned when measuring.

Occupation warnings

Field | Description
--- | ---
Poor IMU alignment | A value of Yes may be seen during the measurement if the IMU temporarily loses alignment and then regains it during the measurement.
Excess movement | With IMU tilt compensation the pole tip has moved during measurement. For GNSS‑only mode, the APC has moved during measurement.
Poor precision | The precision estimates have exceeded the configured tolerances. With IMU tilt compensation the precision is calculated at the pole tip position. For GNSS‑only mode, the precision is calculated at the APC position.
Position compromised | This can happen while static if the position moves by more than the 3-sigma precision estimate. With IMU tilt compensation this is the pole tip position. For GNSS‑only mode this is the APC position.