# Pole bias adjustment

Pole bias adjustment may be required to correct small errors introduced when the reference point of the tilt sensor being used is not aligned with the point of measurement. The point of measurement is the pole tip (when the IMU is aligned) or the antenna phase center (GNSS‑only mode).

When using IMU tilt compensation, Trimble recommends using an undamaged carbon fiber range pole in excellent condition. The quick release should also be in optimal condition with no damage to the mating surface between the receiver and the quick release.

The pole bias adjustment corrects errors introduced when using a pole that may have become damaged during normal use and is no longer perfectly straight, or if the pole tip is no longer true and perfectly aligned with the center of the pole. Pole bias adjustment should be performed in optimal RTK environment with a good IMU alignment.

When to perform the pole bias adjustment

Trimble recommends that you perform the pole bias adjustment:

- When the receiver is using a pole and quick release in sub‑optimal condition.
- Each time you change to a different sub‑optimal pole.

The pole bias adjustment affects IMU tilt compensation measurements only. In GNSS‑only mode, make sure the pole is straight, has a calibrated physical spirit level vial and an accurately calibrated GNSS eBubble.

If a pole bias adjustment has already been performed with the current receiver, the software displays a Pole bias adjustment applied message when you start an RTK survey with IMU tilt compensation enabled. To dismiss the message:

- If you are using the same pole and quick release, and receiver as previously used, tap OK to use the current adjustment.
- If you always use the same pole, quick release, and receiver, tap Ignore to use the current adjustment and not show the message again when starting a survey with the same receiver. The message will appear if a new adjustment is applied.
- If you are using a different sub‑optimal pole or quick release, tap Adjust to perform a new pole bias adjustment.

- If you are using a different pole that is in excellent condition, tap Adjust then press Clear to clear the current pole bias adjustment from the receiver.

Before performing the pole bias adjustment

To set up the receiver:

1. Attach the receiver to the pole.

   If the receiver is an SPS986 Trimble recommends that you remove the quick release from the pole and mount the receiver directly to the pole to eliminate any play between the pole and the receiver.

2. Turn on the receiver and align the IMU well. The more movement involving changes of direction during the alignment process, the better the quality of the alignment.
3. Set up the receiver on a well defined point, with or without a bipod. The pole tip cannot move during the routine so it is best to place it on a control point or other stable and indented point where the tip of the rod can positively rest throughout the routine.
4. Determine if you need to run the routine by checking the horizontal accuracy of the receiver and pole pairing as described below.

To check the horizontal accuracy of IMU tilt compensation

1. Make sure the IMU is aligned and the pole tip is on a stable point that prevents pole tip movement.
2. Holding the receiver approximately level, take a single Topo point measurement facing North, East, South, and West.
3. Measure the distance between the opposite points (for example, North and South) to get an estimate of the horizontal accuracy of the receiver (use the Cogo menu to compute an inverse between them). If the distance between the two points is outside of the horizontal tolerance needed for the task, Trimble recommends that you run the pole bias adjustment.

To perform the pole bias adjustment

The pole bias adjustment takes one set of measurements while facing one direction, and then a second set of measurements after rotating the receiver 180 degrees. It then calculates the corrections to correct for any errors introduced by the pole.

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

1. To open the Pole bias adjustment screen, do one of the following:
   - Tap Adjust in the Pole bias adjustment applied message.
   - Tap and select Instrument / Tilt sensor options. Tap the Calib. softkey to open the Sensor calibration screen. In the Pole bias group box, tap Adjust.

2. Follow the instructions for each step very carefully. Tap Start.

   If the adjustment routine fails to run when you tap Start, for example if you get an out-of-tilt warning when you know the receiver is level, tap the Reset button. This button removes any values calculated during a previous routine and may decrease your horizontal accuracy. Once the reset is complete, immediately run the pole bias adjustment.

3. If the IMU is not aligned, you are prompted to align it. Because the pole bias adjustment requires the pole tip to be stable on the ground, you must keep the pole tip stationary on the ground while you tilt the pole in different directions to realign the IMU.

4. Phase one of the adjustment begins when you tap Start. Keep the pole vertical and stationary and the pole tip in the same location while the measurements are recorded. If you are not using a bipod make sure you hold the receiver as stable as possible.

   During the routine, values are constantly checked to ensure accurate measurements. If they go out of tolerance, the measurements stop. Some of these checks include:

   - The receiver must be kept at the same rotation/heading.
   - The receiver must be kept approximately level.
   - The receiver must stay aligned.
   - The precision values must stay in tolerance of 0.021 m horizontal, 0.030 m vertical. You cannot change these precision values, and if you are not in a survey they are not displayed.

5. When phase one is complete, rotate the receiver 180° without moving the pole tip.

   Once rotated within tolerance and level, phase two begins automatically.

   At the end of the routine, the calculated correction values are displayed. Trimble recommends applying the values if they are above 5 mm when using a 2 m pole.

   If the computed adjustment is more than 10 mm different to the previous adjustment or more than 10 mm from zero, a message warns that the adjustment appears to be excessive, which indicates a sub‑optimal pole setup. If you accept the large adjustment, you are prompted to perform a Calibrate to IMU eBubble calibration, as this will improve the GNSS‑only position results using the GNSS eBubble with a sub‑optimal pole setup.

6. Tap Yes to apply the correction values.

The IMU loses alignment when the pole bias correction is applied. To use IMU tilt compensation you must realign the IMU. See Aligning the IMU.