THE GLOBAL POSITIONING SYSTEM (GPS) AS AN ICT IN AGRICULTURE

AN INTRODUCTION TO THE GPS AND HOW IT WORKS

While originally a military project, GPS is considered a dual-use technology, meaning it has significant military and civilian applications. The GPS has become a widely deployed and useful tool for use in agriculture. You may have heard how the use of GPS is revolutionizing the agriculture industry from finding your way to the middle of a corn field and accurate field guidance without foam markers, to critical row-crop driving and precise elevation mapping. But what is it? Even the development and implementation of precision agriculture or site-specific farming has been made possible by combining the Global Positioning System (GPS) and geographic information systems (GIS). These technologies enable the coupling of real-time data collection with accurate position information, leading to the efficient manipulation and analysis of large amounts of data. GPS-based applications in precision farming are being used for farm planning, field mapping, soil sampling, tractor guidance, crop scouting, variable rate applications, and yield mapping. GPS allows farmers to work during low visibility field conditions such as rain, dust, fog, and darkness. In the past, it was difficult for farmers to correlate production techniques and crop yields with land variability. This limited their ability to develop the most effective soil/plant treatment strategies that could have enhanced their production. Many of the new innovations rely on the integration of on-board computers, data collection sensors, and GPS time and position reference systems.

The Global Positioning System (GPS) is a space-based global navigation satellite system (GNSS) that provides location and time information in all weather, anywhere on or near the Earth, where there is an unobstructed line of sight to four or more GPS satellites. It is maintained by the United States government and is freely accessible by anyone with a GPS receiver with some technical limitations which are only removed for military users. GPS was created and realized by the U.S. Department of Defense (USDOD) and was originally run with 24 satellites. It became fully operational in 1994.In addition to GPS, other systems are in use or under development. The Russian GLObal NAvigation Satellite System (GLONASS) was in use by only the Russian military, until it was made fully available to civilians in 2007. There are also the planned European Union Galileo positioning system, Chinese Compass navigation system, and Indian Regional Navigational Satellite System.

A GPS determines the absolute location on Earth as coordinates from satellite signals with a GPS System consisting of:

• 24 - 32satellites orbiting the Earth;
• A control station that monitors the satellites;
• A  hand-held receiver.

A GPS system can be divided in to the three main categories: space segment, control segment and user segment.

1. Space segment consists of satellites that are placed in altitude of 20,200 from the earth. There are 24 to 32 satellites that send all the GPS signals to the earth.
2. The control system consists of monitoring stations that are located in the world. These stations monitor the satellites and that helps to calculate the orbits and ephemeris data.
3. The last segment, user segment, means the GPS receivers that are used to receive GPS data from the satellites. The GPS receivers used to be separate devices, but now most of them are integrated into different devices. GPS receivers are now even integrated in many mobile phones. The GPS receivers can receive and process data from many satellites simultaneously. 

There are 24 – 32 GPS satellites orbiting the planet in six orbital planes; the more satellites a reference station can track simultaneously, the more accurate the spatial data it provides.

HOW THE GLOBAL POSITIONING SYSTEM WORKS.

The principles of how GPS works are understood through the concepts of triangulation and that distance is measured by how long it takes a signal to get from one point to another. GPS uses the principle of triangulation to three or more satellites. The satellites that are travelling around the world twice a day send radio signals that contain originated time of the message, the ephemeris data than can be used to calculate the position of the satellite in orbit and also the almanac, the status of all the satellites in the system. These almanac data can be stored in the receiver too as the satellites rotate in the same orbit, but because of the gravity of the moon and the sun, satellites can change the orbits slightly from time to time. That’s why the up-to-date almanac data is also coming with the GPS signals. To measure its own location, the GPS receiver should know at least the location of three satellites and the distance from each of them to the receiver. The receiver calculates the time that it has taken the signal to travel between the satellite and the receiver. And then assuming that the signal travelled in a straight line, as radio signals travel in the same speed as the speed of light in free space, the receiver multiplies the time by the speed of light to measure the distance between each satellite and the receiver. (Light and Radio waves both being Electromagnetic waves travel at the same speed, which is the speed of light, in a vacuum. When propagated through a medium though, they are slowed down according to permeability and permittivity of the medium: C = 1/sqrt(Permeability * Permittivity). Air has nearly normal permeability and permittivity so in practice they do travel at nearly the speed of light). 

If the receiver knows the distance to at least three satellites in a given time, and if we assume there are no errors in the measuring, it can use three dimensional trilateration to measure its location. Simple explanation for trilateration is that you can calculate the coordinates of a given position if you know the distance from it to another three known positions in two dimensional space. In dimensional trilateration you need distance at least to four different known locations. In this case, you can draw spheres that have each satellite’s location as the centre and the distance between that satellite and the receiver as the radius. The intersections of these spheres can give the location of the receiver. But when you have only three known satellites, there can be two places that intersect. So to get the correct place, the receiver uses the earth as another sphere as the given position should be on the earth. So that gives the correct location of the receiver.

Even though the method looks very simple, the little difference in the clocks of the receiver and the satellites can cause a huge difference of the calculated location and actual location, because the speed of the light is quite high and multiplication with it can make a small error a huge one. To avoid that, the receiver and the satellites have to have synchronized clocks up to nanoseconds accuracy. Using atomic clocks can solve that problem, but since they are very expensive, using them in every receiver will not be very usable as it can increase the price of the receiver beyond the day today user limits. GPS systems therefore use the following method to avoid that problem: The receiver gets the signals from four or more satellites and the spheres that associate with each satellite have to intersect in single position. If not it’s because of the clock error of the receiver. So the receiver can calculate the error as it’s proportional to the distance. Then it can correct the error and calculate the exact location. That’s why the receiver has to get the signals from at least four satellites to calculate the position of the receiver accurately (although three are enough to calculate the location). An important aspect of GPS technology to keep in mind is that it is not two dimensional, but three dimensional. All too often we think of maps and GPS as something that provides points and coordinates for a position on the surface. GPS systems certainly perform this function, but they provide a measure of vertical position as well as horizontal. The implications of centimetre-level accuracy for GPS are amazing.

Let us leave the technical part of the GPS and as agriculturalists look more into practical applications of the GPS in agriculture. The purpose of this blog is to provide you with a basic understanding of how GPS technology works and how it can be implemented in agriculture. This will help you make good decisions about your own involvement in this new field. You will gain a basic understanding of GPS technology and terminology, but I will not delve into the electronics of GPS receivers, or the mathematics of geomatics... But first a mention of types of GPS units, how to use a GPS and much more GPS-wise. Catch me on my next blog