The invention relates generally to automatic guidance systems and more specifically to global positioning system based sensor for vehicle steering control.
Movable machinery such as agricultural equipment, open-pit mining machines and airplane crop dusters and the like all benefit from accurate global positioning system (GPS) high precision survey products, and others. However, in existing satellite positioning systems (SATPS) for guided parallel swathing for precision farming, mining, and the like, the actual curvature of terrain may not be taken into account. This results in a less than precise production because of the less than precise parallel swathing. Indeed, in order to provide parallel swaths through a field (in farming for example), the guidance system collects positions of the vehicle as it moves across the field. When the vehicle commences the next pass through the field, the guidance system offsets the collected positions for the previous pass by the width of the equipment complement (i.e. swath width). The next set of swath positions are used to provide guidance to the operator as he drives vehicle through the field.
The current vehicle location as compared to the desired swath location is provided to the operator. The SATPS provides the 3-D location of signal reception (for instance, the 3-D location of the antenna). If only 3-D coordinates are collected, the next swath computations assume a flat terrain offset. However, the position of interest is often not the same as where the satellite receiver (SR) is located since the SR is placed in the location for good signal reception. For example, for a tractor towing an implement, the best location for the SR may be on top of the tractor cab will experience large attitude excursions as the tractor swaths the field, but the position of interest (POI) for providing guidance to the tractor operator may be the position on the ground below the operator. If the tractor is on a flat terrain, determining this POI is a simple adjustment to account for the antenna height.
However, if the tractor is on an inclined terrain with a variable tilt, which is often the case, the SATPS alone cannot determine the terrain tilt so the POI also cannot be determined. This results in a guidance error because the POI is approximated by the point of reception (POR), and this approximation worsens as the terrain inclination increases. This results in cross track position excursions relative to the vehicle ground track which would contaminate any attempt to parallel a defined field line or swath. On inclined terrain, this error can be minimized by collecting vehicle tilt configuration along each current pass or the previous pass. The swath offset thus becomes a vector taking the terrain inclination into account with the assumption that from the first swath to the next one the terrain inclination does not change too much. It can therefore be seen that there is a need for a better navigation/guidance system for use with a ground-based vehicle.
Various navigation systems for ground-based vehicles have been employed but each includes particular disadvantages. Systems using Doppler radar will encounter errors with the radar and latency. Similarly, gyroscopes, which may provide heading, roll, or pitch measurements, may be deployed as part of an inertial navigation package, encounter drift errors and biases. Gyroscopes have good short-term characteristics but undesirable long-term characteristics, especially those gyroscopes of lower cost such as those based on a vibrating resonator. Similarly, inertial systems employing gyroscopes and accelerometers have good short-term characteristics but also suffer from drift. Various systems include navigating utilizing GPS, however, these systems also exhibit disadvantages. Existing GPS position computations include may include lag times, which may be especially troublesome when, for example, GPS velocity is used to derive vehicle heading. As a result, the position (or heading) solution provided by a GPS receiver tells a user where the vehicle was a moment ago, not in real time. Existing GPS systems do not provide high quality heading information at slower vehicle speeds. Therefore, what is needed is a low cost sensor system to facilitate vehicle swath navigation.