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Arab J Sci Eng (2014) 39:237–245 DOI 10.1007/s13369-013-0861-1

RESEARCH ARTICLE - EARTH SCIENCES

Testing of GPS Accuracy for Precision Forestry Applications Huichun Zhang · Jiaqiang Zheng · Gary Dorr · Hongping Zhou · Yufeng Ge

Received: 11 June 2012 / Accepted: 15 October 2012 / Published online: 3 November 2013 © King Fahd University of Petroleum and Minerals 2013

Abstract The use of existing geospatial technologies such as GIS and global positioning systems (GPS) to precision forestry is booming. The objective of this study was to examine the applicability of GPS and radio beacon differential global positioning system (RBN DGPS) for forestry machinery positioning. This would be an important step toward the realization of site-specific chemical application in China. A Trimble AG132 GPS receiver was mounted on a moveable platform and real-time differential GPS signals were obtained from the Yangtze River Beacon Station. The experiments were conducted on campus of Nanjing Forestry University, China. GPS data were collected and downloaded into a laptop computer via a RS-232 serial port and customer-developed data-acquisition software. Two tests were conducted: (1) a stationary test (where the mobile platform was maintained at a fixed location) in an open sky and forest condition to examine the precision (repeatability) of GPS signals, and (2) a mobile test (where the platform was moving) to examine the dynamic conformity degree. Analysis of variance was used to clarify the effects of positioning mode and stand area on the precision of GPS. In the stationary test, the variability of X and Y coordinates is smaller in RBN DGPS mode than that in the non-differential mode; and the performance

of GPS in terms of position dilution of precision, horizontal dilution of precision, vertical dilution of precision, time dilution of precision values, effective positioning rate and the number of visible satellites was superior when used in RBN DGPS mode. In the mobile test, the desired traces of the moveable platform (circles) were much better followed by the GPS locations recorded in the RBN DGPS mode. Compared with non-differential mode, RBN DGPS is more suitable for precision forestry positioning requirements. RBN DGPS improves accuracy both in open sky and forest condition, and the service is available to end users in China with no additional costs. . Keywords Precision forestry · RBN DGPS · Non-differential · Precision

H. Zhang (B) · J. Zheng · H. Zhou College of Mechanical and Electronic Engineering, Nanjing Forestry University, 159 Longpan Road, Nanjing 210037, People’s Republic of China e-mail: [email protected]; [email protected] H. Zhang · G. Dorr Faculty of Science, The University of Queensland, Brisbane, Australia Y. Ge Department of Biological and Agricultural Engineering, Texas A&M University, College Station TX, USA

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1 Introduction Traditional methods for forestry production are labor intensive, inefficient and often require a large amount of resources. Nowadays, with the growing awareness of environmental, economic and societal concerns, people are adopting the notion of “precision” into their forestry practice. Here, the word “Precision” implies a high degree of mechanization and widespread use of geospatial technologies [1]. Precision forestry has been defined as “analyzing the spatial– temporal variability based on natural biology and environmental resources (for instance, soil properties, topography, soil moisture content, microclimate, outbreak of diseases and insect pests, etc.). This information is used to plan and conduct site-specific forest management operations to minimize resource inputs, minimize environmental impacts and to maximize forest outputs [2]. Now precision forestry is used in almost every aspect of forestry production from soil management to yield modeling. In particular there has been a focus on precision pesticide and fertilizer applications, because offtarget spray deposition and drift will cause significant environmental problems. It is, therefore, essential that chemicals are applied in a responsible manner and with efficient methods. The rapid adoption of global positioning systems (GPS) has had a large impact on forestry operations. The prominent attributes of GPS make it a versatile tool in precision forestry [3]. The latest developments in GPS technology, including miniaturization, improved accuracies, and shorter time to fix, mean that the systems have great potential for applications in the field of precision forestry, especially in pesticide applications. Positioning precision is one of the most important parameters of GPS, and error is an important factor restricting position precision. Given the geometry between the GPS satellite constellation and terrestrial-based receivers, GPS accuracy is affected by a number of factors including satellite position, noise in the radio signal, multipath error, atmospheric conditions that block or attenuate satellite signals, imperfect time measurements, variations in the satellite almanac that describe the orbital patterns of satellites, and natural barriers to the signal [4–7].

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The Alaska Forest Inventory and Analysis Program uses recreational-grade GPS to georeference field plots [8,9]. The 12-channel, single-frequency receivers provided nominal accuracies of 2–7 m in clear-sky conditions with good satellite visibility. However, they could be considerably less accurate (>20 m error) under typical forest conditions and poor satellite geometry [10]. In a test carried out under dense forest canopy in western Oregon, it was reported that an average error of 7.2 m was obtained from a Garmin V recreational grade unit, with errors exceeding 22 m at several points [11]. The accuracy of recreational-grade GPS was 3–7 m across all sites. For survey-grade units, accuracies were influenced by forest type and baseline length, with lower errors observed with open stands and shorter baseline length [12]. Non-differential GPS technology determines position based on just one GPS receiver. It is mainly used for navigation and positioning of vehicles. Differential global positioning system (DGPS) is a method of correction requiring at least two GPS receivers; one placed at the base station whose geographic coordinates are precisely known and another placed in the field and referred to as a rover receiver. Correction values from the base station can then be used to improve the positioning of the roving receiver. Non-differential positioning is easy to operate and low in costs, but its low accuracy can substantially limit its reliability in many applications [13]. To overcome or mitigate these factors, other technologies such as DGPS, wide-area augmentation system (WAAS), and realtime kinematic (RTK) systems have been developed. Highaccuracy real-time GPS positioning techniques are usually differential techniques. These allow users to measure positions in real time and errors less than a few centimeters are possible. Usually DGPS technology can provide more precise positioning in forest environments than typical handheld low-cost GPS receivers [14]. Such a level of accuracy can be reached using RTK techniques after the removal or mitigation of different error sources. Karsky [15] showed that recreational-grade unit that was enabled to receive correction signals from a WAAS satellite improved the error range from 20 m down to 1.5 m. Differential positioning may include pseudorangeposition difference or carrier phase difference (CPD), depending on the method of sending signals between the base station and mobile station. Pseudorange-position difference systems such as radio beacon differential global positioning system (RBN DGPS) are widely applied and the positioning precision can reach meter level. Positioning accuracy of CPD, also called RTK, can reach centimeter level and meet the requirement of the dynamic application to achieve a high level of accuracy [16]. Beacon and satellite differential services differ in the following aspects: range and area of coverage, annual subscription cost, initial equipment cost and accuracy [17].

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Following the recent advances of information technology into precision forestry, GPS has played an important role in advancing traditional forest industries and improving societal and economic efficiency in China. GPS has found an increasingly wide utilization in many facets of forestry systems. These include woodland adaptive evaluation, silvicultural planning [18], forest breeding and planting [19], forest resource investigation [13], forest growth monitoring [20], yield forecasting [21], pesticide application [22,23], forest harvesting [24], forest fire fighting [25], and the design of transportation systems [26]. This has enabled forest production to maintain high productivity and high efficiency with low cost and less environmental pollution. Many developed countries have already achieved widespread adoption of GPS applications in precision forestry. Over the past 10 years, DGPS has begun to be used successfully in forestry applications in North America and Australia. Taking the forestry aviation industry as an example, the immediate cost savings and reduction in human exposure to pesticides through eliminating flaggers makes this technology very attractive [27,28]. In China, however, precision forestry is still in the beginning stage. This is partly due to poor infrastructures for geospatial technologies and the lack of effective extension programs to quickly deliver research to forest workers. The high cost of owning DGPS and flying aircraft has prohibited the application of DGPS technology. There is, therefore, an enormous potential and market yet to be explored. The Department of Transportation in China has established a large DGPS network in the coastal areas. The network, consisting of 21 stations which are distributed uniformly in coastal areas, provides an all-weather beacon DGPS service. These signals are typically available to determine corrected positions for 300–400 km in the sea and 200– 300 km over coastal area. The advantages of using RBN DGPS from coastal stations include continuous, steady and powerful signals that are free of charge [29]. The main objective of this study was to examine the applicability of using the RBN DGPS technology instead of establishing base stations for forestry machinery positioning. This would subsequently facilitate site-specific chemical application. The aims of these experiments were to (1) quantify the precision of non-differential mode relative to the RBN DGPS mode in both open areas and forest locations, and (2) to investigate the dynamic conformity degree of non-differential and RBN DGPS modes. Moving within certain regions or along certain paths is a common operation in precision forestry and this has a requirement for dynamic accuracy, therefore dynamic conformity degree can check the conformity between the expected track and actual track [23].

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2 Materials and Methods 2.1 Study Area This study was conducted at the Nanjing Forestry University (118◦ 49 E, 32◦ 05 N), southeast China. About 75 ha on the campus was selected for the experiment. To minimize the effect of terrain on GPS signal reception, the selected area exhibited very little slope variation. The test area was composed of two parts: an open sky location and a plantation forest. The plantation forest had rich plant resources with the main tree species being metasequoia and liriodendron chinense. The stand density was 2,738 tree/ha, with average canopy density of 0.52 and a crown overlap percentage of 21.9 %. In this study, these experimental stands were referred to as either open or forest. Because the study area was in the Yangtze River Delta which is covered by China’s coastal beacon signals, RBN DGPS was available. 2.2 Hardware and Software Configuration A Trimble AgGPS132 GPS receiver was selected for this study. It is a real-time, 12-channel differential GPS receiver that can be operated in non-differential mode and also allows both beacon and satellite-based differential correction. It has been used worldwide in precision agriculture and forestry applications. The system operates in a geodetic coordinate system and allows users to set receiver parameters in the display and keypad of the front panel. The mobile system was composed of a GPS receiver, an antenna with magnetic mount, and a laptop computer (Fig. 1). The GPS receiver was connected to the laptop computer by a serial cable. The prototype receiver system was mounted on a remote-controlled

Fig. 1 The experiment configuration showing the GPS antenna and receiver, mobile platform, and laptop computer for controlling and data acquisition purposes

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Fig. 2 The module-level logic diagram of the GPS data collection program

GPS Serial Data

Serial Communication Module

Receiving Buffer of the Main Window

Control Module Display Module Database Handling Module

vehicle (1:8 TWIN FORCE, JingShang Company, Tokyo, Japan) for the field tests. Data can be displayed in the Trimble AgGPS132 receiver, but not stored. To solve this problem, data were collected and downloaded into a laptop computer via a RS-232 serial port and a data-acquisition program developed using VC++ (Microsoft Corporation, USA). The program extracted the global positioning system fix data (GPGGA), GPS dilution of precision and active satellites (GPGSA), and recommended minimum specific GPS/TRANSIT data (GPRMC) messages according to the National Marine Electronics Association standard protocol for the transmission of GPS data (NMEA0183). The program could also output the position, time (Universal Time Constant, UTC), speed over ground (SoG), GPS status, fix type, the number of visible satellites, identity of the satellite being used for the fix (SV used), position dilution of precision (PDOP), horizontal dilution of precision (HDOP), vertical dilution of precision (VDOP), time dilution of precision (TDOP), and other related information. This was stored into databases via open database connectivity (ODBC) real time in situ. The various functions of the complete GPS data collection program were complex and the parameters and structure characteristics of source codes were also different. If all functions were programmed together, it would reduce the program’s readability and eventually affect the system’s maintainability. Therefore, a modular programming method was adopted which contributed to the system integration and maintenance. It also improved the system reliability and saved time for subsequent development. Four major functional modules were included in the GPS data collection software: a serial communication module, a database handling module, a control module and a display module. The serial communication module received the GPS data from the serial port and sent it to a receiving buffer in the main window. The control module examined the receiving buffer at intervals, manipulated data which met specific criteria, stored it using the database handling module and simultaneously notified the display module to update records (Fig. 2). The RBN DGPS system consisted of two main components: the base stations and the mobile receiver. The base system was a beacon referenced station that China Coast Guard continuously operated for differential positioning. When

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“Beacon mode” is selected in the DGPS Source screen, the receiver automatically detected the two closest or two most powerful beacons. 2.3 Experimental Methods Two experiments were performed to compare GPS precision operation in the non-differential and RBN differential modes. In both experiments the interval of GPS data logging was set to 1 s for the receiver. Replicate tests were conducted for each positioning mode. A complete test included the following procedures: (1) standing over a point with the receiver, (2) beginning acquisition, (3) obtaining position correction until a predetermined number of points were collected, and (4) saving the collected data into the database for further processing. The elevation mask angle of the GPS receiver was set to 8 degree, PDOP