Tilt Compensation (at the Rod)
A recent thread spurred a side conversation regarding Tilt Compensation for pole tilt for a GNSS receiver. I’m most familiar with the Javad GNSS tilt compensation capabilities, limitations, and processes, but I’d invite any others with knowledge of other systems to feel free to add their views as well.
The tilt compensation for the Triumph-LS and other Javad systems incorporates a combination of two types of sensors: accelerometers and a magnetometer. The accelerometers determine tilt using the force due to gravity (acceleration) as measured in three different axes. The tilt then is known for the receiver, but to find the tilt direction relative to the user’s coordinate system, the orientation of the receiver relative to the Earth must be known. This is done using a magnetometer to determine magnetic North. The compass direction is further refined using the Worldwide Magnetic Model to compensate for magnetic declination. With this known, the tilt of the receiver can be determined relative to North. One thing we observed early on in testing was that the tilt sensors may not necessarily be precisely placed perpendicular to the receiver housing, or perhaps the threads may not be precisely perpendicular to the receiver, or perhaps the pole threads may not be precisely in line with the pole. These potential misalignments are very small in my experience and are typically less than 1?ø but must be accounted for by comparing the computed tilt from the accelerometers to a well calibrated bubble vial on a pole. Once this calibration is made, the tilt compensation is usually good for less than 0.2?ø, which at normal pole heights is less than 0.02′ or sub-centimeter. The accuracy of the tilt may degrade as the magnitude of the tilt increases. In the field, the main concern for the operator when using a properly calibrated tilt sensor is potential magnetic interference. If the direction of the tilt is not known accurately, then the correction will be wrong and likely produce even more error. So taking a leaning shot next to a vehicle, pipe post, metal building, etc. can be problematic.
As I understand, the Leica GS18T uses a different method to determine the direction of the tilt: gyroscopes. The gyroscopes are zeroed by comparing its measured acceleration to the movement of the receiver (when fixed) as determined by RTK. The direction is then carried forward by the gyros based on previous headings from the RTK which is impervious to magnetic interference, since a magnetometer is not used for determining orientation. The downside to gyros is that they drift over time. As more time passes from the last calibration to the RTK heading, the less accurate the gyro determined orientation becomes. From the product videos I’ve seen from Leica, they’ve limited the application of the gyro heading to 60 seconds, after which a tilt measurement cannot be made. In open environments that require a tilt measurement due to obstruction, this limitation may not be a problem, however, in difficult canopy, this approach could be an issue. The receiver requires an RTK fix and motion to calibrate the gyros and, once on point may must be measured in less than 60 seconds. It’s difficult to maintain a fix under canopy already, but to do so while in motion for the gyros to zero may be impossible. Furthermore, if the calibration were somehow valid, it may be that the receiver cannot fix before the calibration timer expires.
Perhaps the best approach would be some hybrid that relies on a combination of both methods, and perhaps Leica does this already. For the surveyor it is important to know the capabilities and limitations of the technology we use. I like using tilt compensation, but I only turn it on when I need it and I usually have some sort of method for checking the results after the fact. If I can use a conventional level bubble I will leave the tilt compensation turned off. One last addendum: for tilt compensation to properly work, the pole height must be known as the pole tip is calculated based on the tilt values, the orientation of the receiver and the length of the pole from the receiver.
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