Global Positioning System (GPS).

Global Positioning System (GPS) is a technology that can locate positions or navigate the user to a location. Most vehicle guidance systems in agriculture use GPS to determine the position, speed, and heading of the vehicle, and steer the vehicle in the proper direction. The GPS system uses signals emitted from a constellation of 24 satellites orbiting the Earth to determine the geographic position of the receiver on the earth’s surface. The satellites emit signals in two frequency bands, referred to as L1 and L2, to improve the accuracy of the position signals. The nominal accuracy of GPS systems is 10 to 20 meters with single-band receivers and 5 to 10 meters with dual-band receivers. The accuracy of the GPS signal is affected by atmospheric con­ditions, obstructions, reflections, and the visibility of satellites. When the receiver has a clear view of the horizon and can see many satellites, the quality of the position fix is good and the receiver can accurately determine its position. However, if the weather is bad, and there are obstructions that block the signal or cause reflections, such as trees and build­ings, the position signal becomes much less reliable. Similarly, if there are few satellites in view, or if they are low on the horizon, the position signal also becomes less reliable and has larger error.

GPS units consist of a receiver, antenna, display screen, and/or lightbar for tracking. These units can be handheld, carried as a backpack, mounted in mobile equipment, or an integral part of a desktop computer depending on desired application and accuracy.

Differential GPS (DGPS) uses a base station or a special signal to supply a correction value to the receiver’s data. This combination can provide accuracy from 30 feet to inches depending on type of equipment. Base stations use post-processing of data. Differential signals for real-time applications include Omnistar (worldwide L-band satellite signal), Landstar (L-band satellite signal), Coast Guard beacon (where available), WAAS (Wide Area Augmentation System), and RTK (Real-Time Kinematic).

There are some techniques that can be used to minimize some of the errors in the positioning signal. Differential GPS (DGPS) receivers use an existing GPS receiver at a known static location on the ground, called a base station, to correct for errors in the position of the mobile receiver. The difference between the actual location of the static receiver and the measured location of the static receiver (the error) is calculated and broadcast on a radio to the mobile GPS receiver. The mobile receiver then subtracts the error from the measured location of the mobile receiver, correcting for errors caused by the visibility of the satellites and the atmospheric conditions. As the distance from the base station increases, the accuracy of the differential correction decreases, since the signals at the base station will be slightly different from the signals seen by the mobile GPS receiver.

Setting up a local base station for differential correction doubles the cost of the GPS system and poses additional hassle for the user. Several systems have been set up to provide differential correction signals without requiring a local base station. In North America, the most common forms of differential correction are the Coast Guard Beacon, fee-based satellite correction, and the Wide Area Augmentation System (WAAS). The Coast Guard Beacon was developed by the United States Coast Guard to improve the accu­racy of GPS receivers used to guide ships through navigable waterways. Although the Coast Guard Beacon signal is freely available, it is only available near navigable water­ways, and the accuracy of the signal degrades as distance from the waterway increases. Several subscription-based differential correction services are available from providers, including Omnistar. The subscription-based services are widely available and have good accuracy, but they do require an annual fee. Another system used for differential correc­tion in North America (WAAS) was developed by the Federal Aviation Administration to provide accurate and reliable differential correction to aircraft using GPS to aid in guiding the aircraft and landing. WAAS operates similarly to the fee-based satellite correction systems, but requires no subscription fees. Similar systems are being developed in other parts of the world, Euro Geostationary Navigation Overlay Service (EGNOS) in Europe and Multi-Functional Satellite Augmentation System (MSAS) in Asia. Typical accuracies for DGPS systems are less than one meter.

Other types of GPS systems used for highly accurate navigation are Real-Time Kine­matic (RTK) GPS systems. With RTK-GPS systems, a base station must be used with a radio to transmit the data to the GPS receiver. The RTK-GPS receivers monitor the carrier phase of the GPS signals to help improve the accuracy. The accuracy of RTK-GPS systems is typically within 1 cm, provided that the mobile receiver is within several miles of the base station. Unfortunately, RTK-GPS receivers do require a base station and they are expensive.

In addition to position, GPS receivers can also give accurate information on heading and speed. GPS receivers can output heading and velocity information that is much more accurate than what can be calculated based on the difference between the last two posi­tions. Most overlook the usefulness of the heading and speed information, but it is helpful in developing guidance systems for vehicles.

If multiple antennas are used on the vehicle, additional information can be determined with regard to the pitch, roll, and yaw angles of the vehicle. These systems, called vector GPS systems, are often used on aircraft. Stanford has also used a vector GPS system to guide a John Deere tractor. Since the vector systems provide pitch, roll, and yaw infor­mation, they can be used to compensate for the error in vehicle position due to the tilt of the vehicle. Although these systems give additional useful information, they are more expensive since they require additional antennas.

If only one antenna is used, inertial sensors or tilt sensors can be used to correct for the position of the antenna when the vehicle is operating through uneven terrain. Tilt sensors use accelerometers to measure the tilt angle of the vehicle by measuring the change in direction of the pull of gravity on the vehicle. Once the pitch and roll of the vehicle are know, the GPS location can be transformed into the vehicle coordinate system to deter­mine the exact location of the vehicle.

GPS systems vary in accuracy and cost when used with different guidance strategies, including operator assist guidance systems, such as light bars, to completely autonomous vehicle systems. GPS systems that use freely available differential correction signals are accurate enough to use with the operator-assisted guidance systems. The fee-based differential correction signals provide enough accuracy to guide semi-autonomous vehicles for planting and spraying applications. However, RTK-GPS systems are often required when centimeter-level accuracy is needed for such operations as cultivation.

Дата добавления: 2015-11-26