Performance of two low-cost GPS receivers for ground speed measurement under varying speed conditions

A global positioning system (GPS) receiver is an important sensor used in modern farming, particularly in precision agriculture to determine geographic location and ground speed. The aim of this study was to investigate the effectiveness of two low-cost GPS receivers for measuring ground speed under varying speed conditions on four different dates. A rotary shaft encoder on an auxiliary wheel mounted on an agricultural tractor was used as a reference. A significant time lag between the rotary encoder speed and the GPS speed was found over a range of speeds. It was observed that the GPS speed lagged the encoder speed in both increasing and decreasing speeds. The average time lag was found to be between 3.6 and 5.2 s for Receiver 1 while Receiver 2 had a time lag from 1.7 to 2.5 s. A significant difference was found between the two receivers for increasing and decreasing speeds in terms of time lags (P < 0.05). A post correction by shifting the GPS speed increased the accuracy of the speed measurement capability of both receivers resulting in an average correlation coefficient of 0.30–0.90 for Receiver 1 and 0.65–0.98 for Receiver 2. Effect of USB-to-COM converter was also studied and it did not have a significant effect on time delay. The GPS receivers provided reliable data during constant speed operating conditions; however, caution should be exercised in varying speed conditions when using low-cost GPS receivers. Also, the companies that produce GPS-based speed sensors should supply technical specifications related to velocity estimates during acceleration and deceleration.     

Implement lateral position accuracy under RTK-GPS tractor guidance

The accuracy of global positioning system receivers can be improved by differential correction systems (DGPS), which deliver sub-meter accuracy. Higher-accuracy, of about 1 cm on-the-go, is delivered by units generally referred to as real-time kinematic (RTK) DGPS systems. RTK-DGPS systems are presently used by commercial companies for automatic guidance of tractors in row-crop operations. Since high-end DGPS systems with high-accuracy are generally very expensive, it is necessary that the commercial benefit be maximized and that any related errors will be minimized. In the present study, the deviations from a predetermined route of a three-point hitch implement mounted on a RTK-DGPS based automatically guided tractor were recorded and were used to validate that the implement deviation is strongly dependent on the distance from the tractor rear axle. The recorded deviations were analyzed for paved and rough surfaces; the latter caused substantially greater deviations. Based on the above hypothesis, a possible way to improve the performance by reducing the deviations at a point on the implement is suggested.     

Error Sources Affecting Variable Rate Application of Nitrogen Fertilizer

The accuracy and interaction among global positioning system (GPS) horizontal accuracy, differential GPS (DGPS) sampling frequencies and machine delay times of a hypothetical variable rate applicator for nitrogen (N) fertilizer application based on an application map in Florida citrus were studied. Parameters studied included: five GPS horizontal accuracy levels, two levels of DGPS horizontal accuracy, two DGPS sampling frequencies and two machine delay times. Two integrated models were developed which documented the effects of the parameters. Machine delay time was the most important factor and GPS horizontal accuracy was the second most important.     

Centimetre-precision guidance of moving implements in the open field: a simulation based on GPS measurements

High quality and sustainable agricultural production in the open field will be supported by centimetre-precision guidance of agricultural implements. In this paper, a complete guidance system is proposed and simulated using real sensor data obtained from a real time kinematic (RTK) differential global positioning system (DGPS). This type of the satellite navigation system became available commercially in 1997 and is claimed to reach cm accuracy. It uses real time kinematics to improve the accuracy of a position fix by phase comparison of the carrier signal. The simulation primarily aims to investigate the accuracy of the guidance system that can be obtained with RTK DGPS. Using real DGPS measurement errors the simulated system showed a tracking error less than 0.005 m when the PID-controller had settled.     

DGPS数据分析及定位精度研究

本文进行了DGPS数据分析及定位精度研究。通过试验分析表明，Ag132GPS接收机接收到的卫星信号数可以满足使用卫星数量的要求。采用伪距差分纠偏后，Ag132GPS接收机的DGPS单点定位精度可以达到0．232m，满足亚米级的定位精度。     

Evaluation of Sub-Meter and 2 to 5 Meter Accuracy GPS Receivers to Develop Digital Elevation Models

The digital elevation model (DEM) is considered by many to be an important base map for a precision farming GIS (Geographic Information System). Previous work has shown that dual-frequency survey grade GPS (Global Positioning System) receivers are capable of rapidly producing accurate positional data from a moving vehicle from which DEM's can be developed. However, this type of GPS receiver is expensive and somewhat difficult to use properly. This paper presents the results of a study to evaluate the potential of using single frequency sub-meter and 2–5 meter horizontal accuracy GPS receivers to enable the farmer to collect multiple passes of GPS data during normal field operations from which DEM's can be developed. The results show that when using a single frequency sub-meter GPS receiver: (1) it is possible to develop a DEM with standard deviation of the elevation accuracy on the order of 0.12–0.14 m, (2) in order to collect data with this level of elevation accuracy, data should only be collected when the GDOP (Geometric Dilution Of Precision) is less than 5.0, (3) at least 10 passes of data with appropriate data averaging is required to produce this level of elevation accuracy. The results also indicate that the vertical error associated with a 2–5 meter horizontal accuracy GPS receiver is such that it is not recommended for use in collecting data to develop DEM's in this application.     

Validation of GNSS under 500,000 V Direct Current (DC) transmission lines

The use of Global Navigation Satellite Systems (GNSS) is common among agricultural users and enables the producer to optimize crop production within soil variant fields to provide better farming practices. Many agricultural navigation systems are dependent on real time GNSS navigation solutions to aid and control farm machinery. Direct Current (DC) and Alternating Current (AC) transmission lines overhead are often suspected to create interference with GNSS equipment preventing farmers from utilizing their GNSS supported equipment. This paper provides evidence that only non-impeding effects on the receiver or incoming signals, in the form of cycle slips, were measured or detected from either the overhead lines and/or their corresponding support towers. No effect on code measurements was detected. The latter effect is due to reflection or brief masking by the towers. Tests were conducted under a set of three transmission lines, two 500 kV DC lines and one 230 kV AC line. Several GNSS receivers and processing methods, including real time and post-processed data, are used to measure and process data to study the position accuracy, dilution of precision, number of satellites tracked, code and phase errors, location and number of carrier phase cycle slips, carrier-to-noise density and L1–L2 carrier divergence. One commercial Real Time Kinematic (RTK) survey system was also used to verify the 450 MHz data link was operational.     

A harvest area measurement system based on ultrasonic sensors and DGPS for yield map correction

Unknown crop width entering into the header and the delay time caused by the uncertain start and stop of cutting are the two main error sources in a yield map. A harvest area measurement system (HAMS) is presented in this article. The system has ultrasonic sensors mounted on both sides of the harvest header to detect the presence of crop, which was used to start or stop data recording, as well as measure the cutting width. A high-precision Differential Global Positioning System (DGPS) receiver was used to measure the travelled distance. Field tests were conducted to evaluate the performance of the system. Results showed that: Firstly, the developed HAMS can be used to reduce the area error and the data collected by the HAMS can be used to correct the yield data. In a yield map, the area error reached 6.89% relative to the actual area calculated based on the DGPS tracks. The travelled distance error accounted for about 1.08% and the cutting width error accounted for the other 5.81%. However, the error of the area measured by the HAMS decreased to 0.95%. The position offset of yield points could be calculated and the correction coefficient at each sampling point was determined. Secondly, ultrasonic sensors could replace the header position sensors in most yield monitoring systems, as ultrasonic sensors can detect the presence of the crop, which can be used to start or stop data recording. Finally, the HAMS also provides a potential solution to realize online correction of yield data. The time delay estimated by the HAMS between cutting and sensing was 3–6 s at the start of cutting, and was 1–7 s at the end of cutting. An online correction model of yield data was proposed.     

农机直线行走作业DGPS导航软件开发

对于大中型宽幅农业机组的田间行走作业,利用DGPS进行导航具有很多优点,本研究利用VB6.0编程语言,对农机直线行走作业DGPS导航涉及的GPS定位信息的获取、坐标投影变换、导航AB线的确定等编制了相应的程序,开发了应用软件。农业生产中可利用触摸屏计算机实现对机组田间直线行走作业导航。田间作业记录进行模拟试验表明,此软件可有效地实现导航功能,并同时具有作业行走记录和行走距离、作业面积实时显示的功能。     

Kinematic correction for a spatial offset between sensor and position data in on-the-go sensor applications

On-the-go data collection is the basis for many applications that require spatially referenced information at a sub-field scale. A spatial or temporal offset can occur between sensor and position measurements (mostly obtained with global navigation satellite systems such as the Global Positioning System, GPS), e.g. if the antenna of the navigation system cannot be placed on top of the sensor. To correct for such an offset, we present a kinematic model that accounts for the mechanisms underlying the offset. Our model considers a sensor on a cart or sledge towed by a drawing vehicle equipped with a positioning system. The model was applied to a soil electrical conductivity survey on a 135 ha site with over 100 000 measured positions. Application of the model halved a nugget effect in semivariograms, significantly (p < 0.01) reduced prediction errors and thus substantially improved map quality. The model allows offset correction between sensors and a positioning system mounted independently on agricultural machinery, even retrospectively from the geometric configuration of the positioning system-sensor combination. Thus we propose the model for use in research and practical applications of sensor based mapping.     

A yield mapping system for hand-harvested fruits based on RFID and GPS location technologies: field testing

It is proposed that radio frequency identification (RFID) technology be used to overcome the limitations of existing yield mapping systems for manual fresh fruit harvesting. Two methods are proposed for matching bins—containing harvested fruits—with corresponding pairs of trees. In the first method, a long-range RFID reader and a DGPS are mounted on an orchard tractor and passive low-cost RFID tags are attached to the bins. In the second method, the DGPS is not used and RFID tags are attached to individual trees as well as bins. An experimental evaluation of the accuracy and reliability of both methods was performed in an orchard. The first method failed in half of the trials because the tree canopies interfered with the GPS signal. The RFID reader miss ratio for the detection of the bins was 0.32% for both methods. However, the attachment of RFID tags on suitable tree branches (to achieve 100% detection), in the second method, is not a well-defined procedure; some trial is demanded to determine the best positions and orientations of the tree tags in order for the RFID reader to successfully detect them. The first method seems more promising if robust tractor location under foliage can be achieved.     

林冠与山地对GPS定位精度的影响及消减对策

为解决林冠与山地对RTDGPS定位精度影响的难题 ,作者在不同地类、不同地形、不同测程、不同林冠下进行RTDGPS复测实验 ,并获得数据 .通过数据统计分析 ,得出RTDGPS定位精度随上述因素变化的具体规律 .提出了应用高增益天线法、砍树剥枝法、升高天线高于树冠法、伪卫星法、GLONASS法、差分GPS +气压计法、延长观测时间法、插转台法等多种对策 ,在测区内高处设置插转台进行观测、选取适当的觇标高、选用适当的历元数等手段 ,通过增强GPS接收卫星的信号 ,提高定位精度 ,消减林冠、地形、测程等因素对RTDGPS定位效果的影响     

低成本农用GPS接收系统的研制及在面积测量中的应用

针对目前我国应用于精细农业的GPS接收系统成本高,不能接收虚拟参考站(VRS)差分信息的问题,采用低成本亚米级精度GPS接收模块,并结合ARM7微处理器、基于GPRS(general packet radio service)方式的无线通信模块、U盘存储模块、键盘和LCD设计研制了低成本GPS接收系统。该GPS接收系统可以通过GPRS方式接收北京VRS控制中心发送的差分改正信息,实现实时差分定位及长时间稳定的通信链路。定位信息通过串口输出(输出频率为1 Hz)和U盘存储2种方式实现数据的输出和保存,用户可利用本接收机进行定位和面积测量。面积测量实验结果表明,该接收机在不同大小、不同形状的面积测量中,其相对测量误差小于3%,满足精细农业中面积测量的要求。     

机载平台基于全球卫星定位系统的土壤湿度遥感

根据美国航天局2002年10月进行的基于地表面反射GPS信号的遥感土壤湿度试验数据进行了土壤湿度反演。结果证明,反射GPS信号对土壤湿度特性十分敏感,湿度较大的水体区和农田区反射率较大,干旱区最小,不同植被覆盖对于反射率的影响不同,并对其进行了修正。反演得到的该地区土壤湿度值的分布规律也于10月份相对干旱的实际天气条件相吻合。这一结果证明了利用该技术反演土壤湿度的可行性。     

GPS卫星定位导航系统飞播造林试验研究

采用美国国防部星座式全球生卫星定位导航系统和ＧＡＲＭＩＮＧＰＳ９５型接收机进行飞播林导航试验，结果表明，采用ＧＰＳ导航试验，播区平均有效落种率为８６．７％，超过了８５％的国家标准，飞行作业日减少３５％，节省飞播作业导航投工，投资７８％。     

Use of real-time extend GNSS for planting and inverting peanuts

Among the main techniques employed in precision agriculture, yield mapping and automatic guidance of agricultural machines are the best-known to farmers. The objective of this study was to evaluate, using statistical process control tools, the quality of automatic guidance using satellite signals, to reduce positioning errors and losses in peanut digging. The treatments consisted of the use of manual (operator guidance) and automatic (autopilot) guidance with RTX satellite signals in sowing and digging operations. The quality of the operation was evaluated after collection of 30 points spaced at 100 m for each quality indicator, which are the losses and the errors of alignment of the mechanised sets in sowing and digging operations. From the perspective of statistical control, manual guidance was shown to be compromised for the quality indicators of digging losses. Despite the instability in the sowing and digging operations, the use of automatic guidance proved to be accurate. The use of automatic guidance increases the precision and reduces overlaps (<?38 mm, as stipulated by the supplier) for sowing and digging. The manual sowing mean error between overlaps was stable; however, it did not remain constant over time.

     

Navigation system for agricultural machines: Nonlinear Model Predictive path tracking

This article presents a new kind of navigation system for agricultural machines. The focus is on trajectory control where a Nonlinear Model Predictive path tracking for tractor and trailer system is presented. The experiments of the proposed method are carried out by using real agricultural machines in real environments.The agricultural objective is to drive so that swaths are exactly side-by-side, without overlapping or gaps. Hence, the objective of this research was to control the lateral position of the towed implement and to keep it close to the adjacent driving line. The adjacent driving line was recognized locally by using a 2D laser scanner. The local measurement and global position information was merged with the help of an Extended Kalman Filter (EKF). The measurement of the heading by GPS was improved by using an inertial measurement unit and a separate EKF filter. The position of the implement was controlled by steering the tractor and by the use of a hydraulically controlled joint. Because there were two actuators which affected the position of the implement, the problem was a multivariable control problem. Nonlinear Model Predictive Control (NMPC) was used to accomplish the navigation task.The goal was to build a system, which is able to have at least the same accuracy as a human driver. The sufficient accuracy requirement was at most 10 cm lateral error at a speed of 12 km/h. The results presented in the article show that the goal was met and NMPC is a feasible method for accurate path tracking. Further investigation is, however, needed to adapt the method to other kinds of agricultural machines.     

Development and performance assessment of a DC electric variable-rate controller for use on grain drills

Site-specific crop management is a technology that modulates the application rate of field inputs such as seeds, fertilizers, and herbicides based on the needs of each management zone within a field. There are two methods available for changing the seeding rate in fluted-feed-roll type seed drills: (1) changing the active feed-roll length and/or, (2) changing the seed meter drive shaft speed. A possible method to develop a variable-rate seeder is to add a controller to a conventional grain drill which can change the speed of the seed meter drive shaft on-the-go. This was explored in the present study with the following main objectives: (a) to design a DC electric variable-rate controller to change a grain drill from a uniform to a variable-rate seeder, and (b) to determine the response time of the system. A motor control circuit was designed which used the output signals of two encoders as feedback. The system was consisted of: (1) a DC motor with a fixed-ratio gearbox, (2) encoders for sensing the rotational speeds of the grain drill drive wheel and the motor, (3) a GPS receiver, (4) a pulse-with-modulation (PWM) DC motor controller, and (5) a laptop. Dynamic tests were conducted at application rates of 87.5 (low) and 262.5 (high) kg ha⁻¹. Sigmoid equations were best fitted to the transition data from low-to-high and high-to-low seeding rates. Our findings showed that the response times of low-to-high and high-to-low transition rates were 7.4 and 5.2 s, respectively.