Importance Of Lever Arms

For the SPAN system to provide optimum performance, the lever arm of the IMU to the antenna should be measured as accurately as possible. The accuracy of the lever arm measurement impacts the accuracy of the INS solution PVA output. The type of the GNSS positioning used is also a factor. For example, an application using single point GNSS requires less accurate lever arm measurements compared to an application using RTK requiring centimetre level accuracy.

In general, the lever arm measurements should be measured to a better accuracy than the desired INS PVA output accuracy. The accuracy of the GNSS position is also a factor in the overall accuracy. For example, if the desired INS accuracy level is to be within 3 cm, and RTK positioning with an accuracy of 2 cm is used, then the lever arms must be measured to an accuracy level better than 1 cm. This results is an overall accuracy level of 3 cm (2 cm for RTK and 1 cm for INS).

The location of the IMU with respect to the primary antenna is also important. The IMU should be placed as close as possible to the primary antenna for the best performance. This is not always practical so longer lever arm lengths are okay. However as the lever arm distance increases, the accuracy of the lever arm measurement becomes more important.

Finally, when entering the lever arm into the SPAN system via the SETINSTRANSLATION command, the standard deviations of the lever arm measurement are also entered. This is an estimate of how accurate the lever arms were measured. These standard deviations should be entered with correct accuracies for their measurements. An incorrect lever arm standard deviation, either pessimistic or optimistic, might have negative impacts on SPAN performance.

Measuring the Lever Arm

One method of measuring the lever arm is to use a total station and perform a survey between the IMU and the antenna. This method is ideal when the IMU and antenna are in a permanent location in the vehicle and will not be removed for an extended period. A survey will normally provide very accurate lever arm measurements so this is the best option when using RTK.

Performing a survey with a total station can be time consuming and expensive, so a simpler method is to use a measuring tape and a plumb bob. The vehicle should be level when measuring is done. The first measurement should be the Z axis starting at the antenna to the surface where the IMU is mounted. This measurement should be done with both the plumb bob and the measuring tape. Once this measurement is completed, the X and Y axis measurements can be done with only the measuring tape.