This is how your GPS system works accurately

16-04 | Autonomous/semi-autosteering systems | Article

Checking the checkpoints ensures that a GPS system also does what it's meant to do: work accurately. Tips and tricks for maximum precision. - Photo: Mark Pasveer

Going through the different checkpoints ensures that a GPS system also does what it’s intended for: working accurately. Future Farming provides you with tips and tricks for maximum precision.

RTK-GPS works extremely accurately but can also exhibit deviations. Erik Jan Schenk from Dutch company RTKned (one of the founders and partners of GPS specialist SBG) and Jaap Nijboer from RTK-gps.nl (Dutch) mention which points require attention for precise operation with a GPS system.

It starts with properly adjusting your GPS system. Even if the system has always worked well, it can suddenly change. Checking all the data at the beginning of the season is usually enough to work smoothly for a long time.

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Whether it's a wheel sensor or a sensor on the implement: over time, deviations can occur. Regularly check the calibration. - Photo: Ronald Hissink

Accurate GPS system? Check 8 focal points

Check the cables for breaks or abrasions, ensure they are securely fastened, check for corrosion and the mounting of connectors, fuse holders.
Sensors measure the position of the steering wheels. Over time, the adjustment of the sensors can exhibit deviations. Check for secure fastening and any bending of the mounting. Controlled wheels do not move parallel to each other, so in a turn, the rotation of one wheel deviates a number of degrees from the other wheel. Therefore, the ideal position for the sensor is straight, in the middle of the steering axis.
The steering slide that operates the hydraulic steering cylinders must open to a certain percentage, often around 15%, before the cylinder actually moves. A new steering slide may experience some wear over time, so it may need to be open around 16-17% to steer the wheels. This seemingly subtle difference can cause deviations. Therefore, calibrate this when using a new tractor after a few months. After that, there is usually no need to worry about it anymore; this ‘breaking in’ process does not continue endlessly.
The ‘neutral position’ is a number in the GPS system that ensures the vehicle stays precisely on the same line when driving back and forth. Adjusting the neutral position may be necessary for an implement that does not follow symmetrically due to uneven load on the left and right sides of the center. An error in the neutral position can cause deviation from the ideal line. In practice, this is also a reason to try to maintain the seeding direction as much as possible when hoeing after seeding.
A nervous reacting GPS steering system is often the result of control settings. You can adjust how aggressively the steering responds to a deviation from the ideal line on the GPS system screen. The wheels should steer calmly – without swaying, but fast enough – on the line.
Built-in systems with an electric motor or a driven wheel on the steering also work with parameters that must be correct. Think of the number indicating how many times the wheel must rotate for a certain steering angle. A wheel on the steering must not slip.
The balls of the steering mechanism must be virtually free of play. A well-calibrated GPS system also causes less wear. The lifting mechanism must also have limited play.
Usually, the gyroscope is integrated with the antenna. Whether integrated or not, makes no difference: the gyroscope must be level when the tractor is level. While stationary on a flat concrete floor, read the value on the screen that the gyroscope gives for the horizontal position to see if the attachment needs correction. A change in the height of the antenna can also be a checkpoint, for example, if there is a clear height difference after a tire change. However, a difference of a few centimeters due to other tires or tire pressure is unlikely to cause a problem in practice.

Action and reaction

The data that the GPS system receives from the sensors must be accurate. And the signals that the system sends out must have the expected effect. Well-laid cables and connectors are essential for reliable signals. Problems can arise simply because something in the electronics or elsewhere in the total system is malfunctioning. Deviations that occur only occasionally are often difficult to trace, but structural errors can be prevented and detected even before the start of the season. The simplest test is to draw a chalk line once on a field path or a concrete slab with the tractor and check if the steering works as accurately as it should. Also, occasionally use a tape measure in the field to check the alignment.

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Asymmetric loading of the implement may cause it to "drift," requiring a correction of the neutral position to ensure accurate alignment. - Photo: Jaap Nijboer

Accuracy correction signal

The accuracy of RTK-GPS depends on the correction signal. When stationary, the positions that the system works with can be seen on the screen by the movement of a cloud of points. The point cloud should not be wider than 1.5 centimeters but sometimes shows differences of up to 8 centimeters. In practice, there is also some variation in the point cloud, so that differences can still occur with repeated operations. If the vehicle is stationary on the AB line, have a cup of coffee and see if the position on the line is still the same afterwards.

The correction signals have improved in recent years, but it still holds true that the accuracy decreases as the distance to the base station increases. Until a few years ago, the unwritten rule was that the deviation increased by 1 millimeter per kilometer distance. This is now 0.5 millimeters. Some GPS systems transmit the correction via mobile internet (MoveRTK), others via transmission towers. In terms of accuracy, one does not necessarily work better than the other. A transmission tower may transmit with a maximum of 1 watt of power. This means that the range is up to 8 to 10 kilometers. If the deviation becomes larger than desired, a repeater station, or even several, can help. Expanding with repeater stations has its limits. The maximum is about 50 kilometers.

As many satellites in view as possible

Back to the point cloud: the more satellites the GPS receiver has in view, the more accurate the position determination. Strictly theoretically, three satellites are sufficient, but at least twice as many are needed for the desired accuracy. Years ago, there were usually only five or six in view. Thanks to multiple networks (constellations), there are now considerably more, provided that the receiver is suitable and activated for those satellites. With satellites from GPS (US), Galileo (EU), Glonass (Russia), and BeiDou (China) in view, a GPS system can work with about fifty signals.

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The proper functioning of the GPS system can largely be determined by the data displayed on the screen. However, field verification remains essential. Occasionally, step off and use the tape measure! - Photo: Fred Libochant

Human action error-prone

Lastly, but not least, humans remain a possible source of interference. Whether consciously or not, someone can change a digital setting that causes a deviation. This error easily occurs due to changes in drivers. Regardless of whether the equipment is in top condition for maximum accuracy, working with RTK-GPS also requires a certain discipline. Especially for subsequent operations based on recorded GPS lines. Those who work according to the book can farm with an accuracy of a centimeter. On the other hand, a deviation of a few millimeters is inherent in the system. Remember that a correction only occurs if there is already a deviation!