A Farmers Guide to Precision Farming

Although the principles of precision farming have been known for some time, it is only since the arrival of spot beam DGPS services that they have become a practical reality in Europe.

They are now permitting a completely new approach to farm management that offers important commercial and environmental benefits.

Positioning accuracy to within a metre now makes it possible for a suitably equipped combine to continuously monitor crop yield as it harvests an individual field, relating growth levels to specific points within the field. Systematic soil samples taken using DGPS positioning and the same yield data immediately after harvesting can identify the reasons for any variations. When this information is downloaded into a computer controlled fertiliser spreader, DGPS can ensure that it only applies its chemicals to those parts of the field that need them. This can create significant cost savings while also reducing the environmental problems associated with the run-off of surplus chemicals.

The reliability and accuracy of Differential GPS has now reached a standard that offers farmers possibilities that are only limited by their imagination. Asset management, plotting land boundaries, forestry management and vehicle tracking now become simple and straightforward.

The technology now exists to make automated ploughing a practical reality and many predict that it is only a matter of time before satellites are regarded as indispensable farm tools.

Differential GPS explained

When the US Government established the Global Positioning System (GPS), it launched 24 satellites that changed the world. For the first time they made it possible to use a small receiver that can tell you your location, wherever you might be on the planet.

Effortless navigation brought obvious advantages to seafarers and travellers in remote parts of the world, and also brought benefits to

a host of other users ranging from surveyors, transport companies, emergency services and, thanks to the introduction of Differential GPS, to farmer too.

Navigation for all

To understand the advantages that Differential GPS technology brought to modern farming, it is worth taking a little time to first explain how GPS works.

The original idea of the US Department of Defence was to establish a constellation of satellites orbiting some 23,000 km above the surface of the Earth. Each of these transmitted signals that could be used to navigate the warships, missiles and other tools of the Department's trade.

It was quickly appreciated, although the positioning signals transmitted by the GPS satellites could be received by anyone whether they were driving a tank, a supertanker or a combine harvester. The US Government accepted this reality and was happy to make the service available free of charge. Anyone could make use of this multi-billion dollar technology; the only condition was just to have a GPS receiver, which, nowadays, can easily be bought for €200 or less.

The GPS receiver contains a tiny computer that works by listening for the unique signals transmitted by each of the satellites. The receiver's price is largely governed by the number of channels it contains. Because each channel is used to listen to an individual satellite, the more channels it has the more positioning signals it can receive and the better its performance.

The GPS receiver instantly knows which of the 24 orbiting satellites it is listening to but more significantly, its internal clock knows when that satellite's signal was transmitted. It also knows where that satellite should be in its orbit so by measuring the time that it takes the signal to travel from the satellite to the GPS receiver and can calculate the distance between them. If the GPS receiver can listen to three or more satellite signals, its supercharged little brain can instantly perform the trigonometry necessary to measure its distance from each and calculate its position. The process is, in essence, the same as using a compass to take cross bearings from a church spire and headland to plot a position on a two dimensional map. However, because satellites are moving in three dimensions, a 3^rd bearing is needed to provide the final measurement that will fix the receiver's location on the surface of the Earth or, in the case of aircraft, somewhere above it.

If the GPS receiver can listen to a 4^th satellite, it can perform some even cleverer mathematics that synchronise its internal clock with the universal time being used by the very precise atomic clocks aboard the satellites. Because all of the receiver's measurements are based on the time being taken for a satellite's signal to reach it, this synchronisation is essential. The mathematics involved can, however, be heavy going for the casual enquirer and are best avoided here. For the purpose of this article it is probably simplest to take this point on trust, accept that it happens and move on.

Accurate but not accurate enough

If a GPS receiver was being used in a perfect vacuum, the time taken for the satellite's radio signals to reach it would correspond exactly with the speed of light - 186,000 miles per second. Unfortunately because the signals must pass through the Earth's atmosphere, they are subjected to a number of factors that can slow them down. The signals are transmitted in the high frequency L-band, which is highly resistant to interference, but charged particles in the ionosphere and water vapour in the troposphere can play their part in an unpredictable way. Once on the Earth, the signals may bounce off other objects or landscape features to cause local multipath errors. Although the signal that reaches the GPS unit's antenna directly will be the most accurate, echoes of the same signal received from buildings, mountains or other objects will blur its accuracy in the same way that they can cause the more familiar ghosting sometimes experienced on televisions.

Other potential sources of error may also exist; these can be caused by the satellite itself not being exactly where it should be in its orbit and cause what are known as ephemeris errors. The GPS receiver may also be less than perfect and cause internal errors of its own. Most significantly, however, are errors caused by a deliberate distortion known as Selective Availability (SA).

System too accurate for its own good

When the US Department of Defence (DoD) established the GPS system it was certainly not intended to provide a free and easy way for the United States' enemies to launch weapons against it. Although the DoD wanted the system to be used for peaceful purposes by anyone, it was recognised that the surest way of preventing its use for weapons guidance was to reduce its accuracy. This has been achieved by a process known as Selective Availability (SA).

This is process whereby the DoD introduces some 'noise' into the satellites signal, which affects the accuracy of its message. These distortions did not worry the US military and its allies who used special P-code receivers that could decode the effects of Selective Availability and provide a positioning accuracy of around 15 metres as a matter of routine.

The application of SA is currently switched off. Under these conditions the civilian receivers were found to be as accurate as their more sophisticated military counterparts. When atmospheric conditions are favourable, systems are functioning correctly, a standard GPS receiver with a clear view of the sky can provide positioning accurate to within a few metres.

Unfortunately the user has no way of knowing when such conditions might prevail and must prudently assume that the position being given could be inaccurate by up to 20 metres. This is not a problem for a tanker skipper trying to find Antwerp but is useless for finding a buried pipeline or for plotting crop yield in the latest precision farming processes.

Correcting the errors with DGPS

The answer to the problem of GPS inaccuracy lies with Differential GPS. This was devised as a simple system for correcting the errors simply by measuring them. With the exception of multipath errors and the technical shortcomings of individual receivers, the factors affecting the accuracy of GPS positioning will be common throughout an area that may cover thousands of square kilometres. All GPS users in the region will be listening to the same satellites with the same SA and the signals will be passing through much the same distorting layers of atmosphere.

By sitting a GPS receiver on a very precisely surveyed location, it becomes a simple matter to identify the extent of any errors in the satellite signals.

The true location of such a reference receiver, or reference station as it is known, will be surveyed to within a few millimetres using a highly accurate international scientific reference framework. If the GPS unit receives satellite signals telling it that it is, say, 25.73 metres due east of where it actually knows itself to be, it is simply a case of telling all the other GPS receivers in the area that if they bring their position 25.73 metres due west they will have their exact location.

In practice the process is more complicated as the reference station will be listening to the signals from every satellite that it can see above the horizon. It will compare the time taken for each signal to reach it with the time that it knows the signal should have taken. It is these time differentials that are provided to the mobile receiver - hence the name Differential GPS. Because the mobile receiver may have fewer channels or be in a location masked by hills or other objects, it may not be listening to as many satellites as the reference station.

This means that it will have to pick the information it needs from the packet supplied and match it to the satellites that it is listening to.

Spreading the word

The only way that a mobile GPS receiver can receive corrections is by some form of radio link. The nature of this link will vary according to the DGPS service being used and can include terrestrial radio beacons or communications satellites.

The corrections themselves are usually transmitted in a format known as the RTCM SC-104 protocol. This is an internationally agreed standard that enables any mobile GPS receiver equipped with the appropriate box to receive, understand and apply the corrections.

The International Association of Lighthouse Authorities (IALA) has also established its own protocol, which provides differential corrections from its own network of radio beacons to suitably equipped ships free of charge.

When a user has the option of choosing between access to a free beacon differential service such as the IALA system or to a commercial DGPS system, the decision is usually governed by the level of positioning quality and service support required. The IALA type beacon services represent an invaluable boost to marine safety through being universally available to anyone with the appropriate receiver. Commercial DGPS providers exist to meet the demand of users requiring a service that continuously monitors the quality of satellite signals and correction messages and who may also require technical support to be sure of a higher standard of positioning accuracy at any time of the day or night.

The problems associated with terrestrial radio beacons are more pronounced due to the properties of the MF frequency. This is more susceptible to electromagnetic noise and weather conditions such as thunderstorms, atmospheric distortion, multipathing, masking and the receiver simply being beyond the range of the radio transmissions. Because of the curvature of the Earth, these may typically only be received up to 200 km away.

Satellite links

Communication satellites are now providing a solution to this problem. Instead of the correction messages being sent from a beacon at ground level, they are uplinked to a satellite in geostationary orbit above the earth.

The satellite then re-transmits the corrections, which may be received by users anywhere within a vast area of the Earth's surface. Together, they cover the Earth’s entire surface with the exception of the north and south poles, which are masked by the curvature of the planet. These signals are relatively low powered and require a big antenna comparable to a satellite TV receiver.

This is not a problem for large, relatively slow moving users such as ships or offshore oil drilling rigs. They are, however, totally unsuitable for use on farm vehicles.

Since 1995 a new type of communications satellite has become available that transmits telephone, television and other services via powerful spot beams. These are focused on specific geographical areas and are now being used by services such as LandSTAR-DGPS and SkyFix to transmit differential corrections. Because of their power, the signals can be received by users equipped with a combined GPS and DGPS antenna barely the size of a saucer.