2. Positioning in Precision Farming

Positioning is the key element in many precision farming systems. In most cases x–y (longitude and latitude) coordinates are sufficient. For some cases and for more sophisticated requirements, z (elevation) may be of interest. A categorization of accuracy requirements into four different classes can be seen in Table 3.7.

Satellite Navigation Systems. With the installation of satellite navigation systems during the late 1980s, military and civilian industries acquired access to worldwide cost-free location data available continuously independent of daylight and weather conditions. These systems often are generically termed GPS after the most popular systems.

Positioning accuracy requirements

Table 3.7.

Required Accuracy	Task	Example
	- Navigation	* Targeting of fields (machinery ring, contractor) * Targeting of storage area (forestry)
	- Job execution - Information - Documentation	* Local field operations such as yields monitoring, fertilizing, plant protection, soil sampling, action in protected areas * Automated data acquisition
	- Tractor guidance	* Gap and overlap control (fertilizing, spraying) * Harvesting without skips
	- implement (tool) guidance	* Mechanical weed control

Component and Method of Operation. The two currently available systems GPS (USA) and GLONASS (Russia) have similar characteristics (Table 3.8)

Configurations of gps and glonass

Table 3.8.

Name	GPS - NAVSTAR	GLONASS
	Global Positioning Systems – NAVigation System by Time and Range	GLObal NAvigation Satellite System
Ownership	USA: Department of Defense	Russia: Department of Defense
Satellites	24	24
Orbits	6	3
Altitude (km)	20,183	19,100
Coordinate system	WGS84	SGS85
Approximate accuracy for civilians (2drms) (m)	100	35

GPS and GLONASS each consist of three segments (Fig. 3.2):

^• The monitoring and control segment, which maintains the overall control of the system and is operated secretly by the system owner

^• The space segment consisting of positioning satellites (currently 24) with a lifetime of up to 7 years in “blocks” of equal configuration and performance

^• The user segment on land, at sea, and in the air with an unrestricted number of receivers.

Location is determined in each individual GPS or GLONASS receiver based upon at least three satellite signals (satellite position and time of signal generation). The location of the receiver is determined by the known satellite locations and the measurements of the ranges between the satellites and the receiver.

Figure 3.2. Global Positioning System (GPS).

GPS and GLONASS have inaccuracies that depend upon atmospheric effects and the instantaneous geometric configurations of the satellites. In addition, the owner of GPS purposely degrades the civilian accuracy to 100 m, which is insufficient for agriculture. This inaccuracy must be corrected for GPS to be useful.

Differential Corrections. The accuracy of GPS location sensing can be improved by using an additional receiver at a fixed known position (e.g., the DGPS base station in Fig. 3.2). The additional receiver compares the GPS indication of its position to its known location to determine the instantaneous magnitude and direction of the GPS error. Assuming the same error is present for the receiver at the unknown location of the moving farm equipment, the GPS indication of that receiver can be corrected easily for the error. Different methods can be used for correction (Table 3.9).

Correction methods and agricultural operations suitability

Table 3.9.

Method	Mapping Operations	Control Operations
Real-time: - Own base station - Commercial correction	Yes Yes	Yes Yes
Post-processing	Yes	No

Real-time correction signals are necessary for operations in which agricultural equip­ment is controlled based upon position, such as fertilizer or pesticide application. The GPS error correction is transmitted from the stationary receiver to the mobile (rover) receiver on the agricultural equipment. The correction can be supplied by the rover user’s own base station, although it should be close to the rover due to governmental restrictions on the power of private radio links. Alternatively, a commercial or governmental correc­tion service may be used. Less financial investment and user maintenance is required with a correction service, although there may be a usage charge from the correction provider.

Post processing corrects the position data after the data has been collected and trans­ferred to an office computer. No radio link is needed, and higher accuracy might be achievable. However, because accurate position information is not available until after the operation, control operations cannot use this technology. In addition, corrections must be obtained and used to process the uncorrected location data.

Other Location Systems. Other non satellite location-sensing systems may be of regional or farm-specific in­terest (Table 3.10).

Due to the large investment in sender installations, maintenance requirements, and the fulfillment of legal constraints, these positioning systems are concentrated in small areas or fulfill specific purposes in which they are competitive with satellite systems.