## 1 Introduction
A low-cost approach requires the following components: (1) L1 GNSS board; (2) L1 GNSS antenna; (3) reference data, i.e., RTCM (Radio Technical Commission for Maritime Services) differential correction data from L1 local CORS, e.g., via NTRIP (Networked Transport of RTCM via Internet Protocol) or from L1/L2 base station; (4) PC, running an RTK GNSS software for positioning [11]. Procedures for evaluating the results of L1 RTK-capable GNSS receivers were carried out for the first time by [PERSON] and [PERSON] [11]. These receivers were able to calculate 3D positions with a level of accuracy of one centimetre [12]. The use of local L1 reference information is the most cost-effective approach for low-cost RTK GNSS applications [11]. Following the implementation of GNSS in agriculture, a slow change of applied technologies, based on the geo-referenced measurement of within-field crop and soil parameters, is in progress, allowing for more effective crop input management. This new approach, which is achieving great success worldwide, is known as precision agriculture or precision farming. The low-cost implementation of these technologies can increase their use and make spatially variable crop input management more effective [4]. Even though precision agriculture was implemented for the first time in US at the beginning of 1980s, it is currently widespread only in large areas of US, Germany, Denmark, Netherlands and United Kingdom [13]. Moreover, precision agriculture is spreading slowly in Italy (where it is currently implemented in only 1% ca. of cultivated area), above all due to farmers' resistance to investment in manpower and capital without knowing if this will provide a significantly higher profit [14]. Different studies [15; 16; 17] have proved the economic benefits of precision agricultural technologies. The capital to be invested for implementing precision agriculture is variable from \(\copyright\)2500 to \(\copyright\)50,000. The implementation of precision agriculture requires the investment of fixed capital, for buying suitable agricultural machines and/or parts of them (e.g., aided guidance system, satellite antenna, GNSS receiver, monitor) and related software [18]. This increases the depreciation charge and, therefore, the fixed costs. However, precision agriculture influences the cost of using agricultural machines by causing the reduction of variable costs proportional to their use: fuels, oil, maintenance and insurance. This reduction of the cost of using agricultural machines is variable, depending on the technologies implemented and the regularity or irregularity of fields and cultivated crops. The first step for implementing precision agriculture is buying a GNSS receiver, which should be selected depending on farm needs. In fact, sometimes a low-cost GNSS receiver can provide the farmer with a higher profit, i.e., if an agricultural implement with a high working width is used for soil tillage or an agricultural machine is used for a crop operation requiring a maximum error of 0.3 m (e.g., fertiliser application). On the contrary, when a tractor is used for high-accuracy crop operations (e.g., soil tillage of row crops, strip tillage, pesticide spraying, hoeing) a survey-grade RTK-capable GNSS receiver is needed. A positioning accuracy level of 10-30 mm is needed within precision agriculture, for the control and guidance of agricultural machines [11]. GNSS receivers are included in the aided guidance systems of agricultural machines. For instance, a study carried out on corn crop showed that the cost of using agricultural machines decreases 29.37 \(\copyright\)/ha when replacing manual tractor guidance with an automatic one [18]. It is possible to distinguish between three types of aided guidance systems of agricultural machines: manual; semiautomatic; automatic. In the manual guidance system, the steering wheel is operated by the driver by relying on the information provided by a LED light bar or a monitor placed on the dashboard and indicating the corrections to be carried out (turn left or right).