The oldest instrument created to help man with location and navigation was the compass, in the 11th century. This Chinese creation was a milestone for maritime navigation. Another very important moment for the development of navigation techniques was during the Second World War, when man greatly expanded his control over radio waves, developing radio navigation, but the location systems at that time still did not allow obtaining global positioning and changes in relief or electronic interference impaired its accuracy.
After the creation of several technologies, the first satellite positioning systems with global coverage emerged in the 1970s, called GNSS (Global Navigation Satellite Systems – Global Navigation Satellite Systems). The first of these was Navstar GPS, or just GPS (Global Positioning System - Global Positioning System), American system, which because it is the first and most famous, we mistakenly use the term GPS to describe any GNSS. In addition to the American system, also in the 1970s, the Russian system called Glonass appeared (currently operational) and in the 2000s, Galileo, from the European Union, and the Chinese BeiDou, both in the final phase of development.
The GNSS is generally made up of three segments: 1) space segment, made up of several satellites in orbit around the Earth; 2) ground control stations for monitoring satellite orbits and their internal (atomic) clocks, correcting any discrepancies at least twice a day; and 3) user segment, which is made up of receivers used for the most diverse functions. These receivers have the function of decoding information from satellites and, through calculations, providing the user's location. Highly widespread, GNSS equipment is widely used in agriculture for navigation to a specific point in the field, remote management of machines, autopilot and countless other functions.
In the case of the GPS system, satellites emit radio waves called “L1” and “L2” bands, with frequencies of 1.575,42MHz and 1.227,60MHz, respectively. The L1 band carries the “Coarse Acquisition” codes (Coarse Acquisition – C/A) and “Precise” (Precise - P), while L2 only has the code “Precise” (P). Both bands are modulated in binary form, containing information from the satellite and the exact moment they were emitted, determined by their internal atomic clocks. The receiver, with this information, calculates the time (t) that the wave took to reach its receiving antenna. Knowing the speed of these waves (c), that of light, we can calculate the distance (d) between the receiver and the satellite (d = c * t).
With the distance and position of the satellite in space, an imaginary sphere of radius R1 is formed, as seen in Figure 1, which the receiver can be in relation to the satellite. With a second satellite there is another sphere of radius R2 and then the intersection of spheres R1 and R2 forms a circle of possible position of the receiver. Adding a third satellite, its respective sphere R3 intersects the circle, generating two possible points. Finally, with a fourth satellite and its R4 sphere, a single position is found where the user should be, thus determining their longitude, latitude and altitude.
It is worth noting that the further apart the satellites are, the better the quality of the triangulation performed. The precision associated with such distancing is called DOP (Dilution of Precision – Dilution of Precision), which can be expressed in different ways, such as HDOP which concerns the dilution of horizontal precision, VDOP relating to vertical precision and PDOP, corresponding to precision in three dimensions.
The accuracy of the location depends on the receiver, the quality of its manufacturing, the digital code used, which can only be (C/A) or (C/A) together with (P), in addition to its use, military or civil. There are also factors that cause errors in positioning, such as loss or degradation of the signal, errors in clocks and deviation of satellites from their orbits. The Ionosphere and Troposphere, layers of the Atmosphere, have electrically charged particles and water particles, respectively. When the radio wave encounters these, its speed is reduced, leading to positioning errors.
The error in the clocks causes an error in the calculation of the time traveled by the wave, generating an incorrect satellite-receiver distance. The gravitational forces of the Moon and the Sun “pull” the satellites, deviating them from their planned orbit; These factors cause an error in the positioning of, according to the American government, 3m horizontal and 5m vertical for C/A receivers, disregarding errors on the part of the receiver itself. Finally, buildings and mountains reflect the waves and cause the most varied positioning errors, however for agriculture they are not usually a problem, but trees, rivers and lakes are.
In several agricultural operations, such as directing machines, common receivers (C/A code), with errors of 3m (horizontal) in positioning, are undesirable. For this reason, more precise receivers are used that use the two frequencies, L1(C/A and P) and L2(P), guaranteeing submetric precision and, if subjected to differential signal correction, they can reach accuracy in the order of 2cm to 3cm. Differential correction is usually used in autopilot systems, mainly ensuring that the machine passes in the correct location, avoiding problems such as changing the spacing between sowing lines, trampling on planting lines, application failures, among others.
The most famous correction system is RTK (Real-Time Kinematic), which uses a mobile antenna (called “base”), placed at known coordinates (geodetic landmark or mobile station), which tunes to the same satellites as the receiver and compares the known coordinates with those calculated by the GNSS, sending the data from error to the receiver (called the rover), which increases its accuracy in estimating its position on the ground. Generally, this correction information is transmitted via radio communication link, which has a distance limitation of 20km in direct sight.
However, to reduce communication problems due to rugged terrain or greater distances between base and rover, it is possible to use radio signal repeaters. However, an important caveat is that the further the base is from the rover, the lower the efficiency of the correction system will be, since atmospheric and climatic conditions can vary significantly between the position of the base and the rover, as well as different satellites being tuned. for them.
A similar signal correction system can use geostationary satellites to transmit position correction information instead of a radio link, a technology known generically as SBAS (Satellite based augmentation system). This type of correction is generally provided by companies that sell GNSS receivers and may or may not charge a fee to release the correction code at the different levels available. The main advantage, in this case, is that the user (farmer) does not need to worry about maintaining the RTK reference bases.
GNSS is an essential component of commercially available agricultural machinery steering technologies, i.e. light bar and autopilot. Light bars appeared in 1995 in agricultural aviation, mainly for the application of agrochemicals. This system consists of a GNSS receiver that uses the definition of a working line based on a first pass and the machine's working width, thus defining parallel lines to be followed by the machine operator. To do this, the lights on an LED bar (hence the name light bar) light up as the machine deviates from the programmed lines, indicating to the operator the direction to take to correct the trajectory. This system is widely used in broadcast fertilizers and sprayers.
More accurate technology than the light bar is the directing of machines through the automatic pilot system, as it does not depend on the operator to direct the machine during operation in the field. This control can be done by electro-hydraulic actuators directly on the wheels, by a motor on the steering column or even by friction, where a motor placed against the steering wheel rotates it. With this, the machine operator can monitor the machine's other functions and adjustments, generally taking control only during headland maneuvers.
In the context of precision agriculture, GNSS plays a fundamental role. An example of this is monitoring crop productivity. In conjunction with sensors coupled to harvesters, GNSS makes it possible to georeference crop production throughout areas, allowing the creation of productivity maps such as the one in Figure 2. This information is important for identifying factors limiting plant production and can play a fundamental role in localized management decision-making following the precepts of precision agriculture.
Another major use of GNSS in precision agriculture is georeferenced sampling of plants, pests or soil. In other words, the collection of samples, generally in a grid, illustrated in Figure 3, which have a location defined by GNSS, enabling, in the case of soil sampling, the generation of soil fertility maps and, thus, fertilization in varied doses.
João Vítor Fiolo Pozzuto, Joaquim Pedro de Lima, Thiago Luis Brasco, Lucas Rios do Amaral, FEAGRI/UNICAMP
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