Performance Comparison of a Low- Cost Mapping Grade Global Positioning Systems (GPS) Receiver and Consumer Grade GPS Receiver under Dense Forest Canopy

corrections to collected data (Leick 2004). The locations of GPS base stations are established using very accurate measurements and base stations continually compare their known locations to positions generated by satellite signals. Any difference between the known and satellite-derived location of the base station is regarded as a positional error and can be used to estimate a correction factor for field-collected GPS data. Through differential correction techniques, survey grade GPS receivers are capable of generating location measurements that are within 1 cm of true position, provided that satellite signal geometry is consistently strong and available over periods of time. However, forest environments feature canopy cover, vegetation, and topography that often preclude the efficient use of survey grade GPS receivers (Wing and Kellogg 2004). Operator skill also is necessary for both hardware and software applications of survey grade GPS receivers and forest conditions often are not suitable for the delicate nature of the equipment. In addition, many measurement applications do not require accuracies that are within 1 cm of true position. Mapping grade GPS receivers represent the middle ground between survey and consumer grade receivers, although prices still may be prohibitive to potential consumers. Costs of mapping grade GPS receivers vary from $2,000 to $12,000 depending on the manufacturer and model. Previous studies have found mapping grade GPS receivers to be capable of acceptable measurement accuracies when working under forest canopies. Bolstad et al. (2005) tested several mapping grade GPS receivers below hardwood canopies in Minnesota and found average errors between 3.0 and 4.8 m for uncorrected measurements, 2.9 m for real-time differentially corrected measurements, and average errors between 2.5 and 4.0 m for postprocessed differentially corrected data. Johnson and Barton (2004) reported nondifferentially corrected mapping grade errors of 20–30 m under a partial hardwood forest canopy in New Hampshire. Naesset and Jonmeister (2002) found positional errors between 2.2 (20-minute observation time) and 5.6 m (2minute observation time) in dense spruce forests in Norway for differentially corrected GPS measurements. Liu (2002) determined an average positional error of 4.0 m under thick hardwood canopies using uncorrected mapping grade GPS data but did not report a study location. The 4.0-m average was based on 17 observations with the average of 180 readings being used to create a location for each observation. Sigrist et al. (1999) determined a differentially corrected root mean square error of 5.1 m under a white pine (Pinus strobus) canopy in north central Indiana based on a 3-hour acquisition time for a single point. Consumer grade GPS receivers are available for several hundred dollars or less and have been found to collect measurements with accuracies that are acceptable for many forestry applications. Wing et al. (2005) found that consumer grade GPS receivers were capable of accuracies within 10 m under closed canopies and 7 m under young forest canopies in western Oregon. Bolstad et al. (2005) tested a consumer GPS receiver under heavy forest canopy (more than 70% sky obstruction) in Minnesota and found average errors of 6.5 and 7.1 m. Karsky at al. (2001) reported average errors of 3–24 m under medium canopy in Montana. Although consumer grade GPS receivers are affordable for many users, they are limited in a number of characteristics. Limitations typical of consumer grade GPS receivers include not being able to set minimum standards for satellite geometry for data collection, a data storage limitation of 500 coordinate pairs, and an inability to differentially correct data after field data collection without third-party software. Differential correction allows errors caused by atmospheric conditions to be addressed and reduced; atmospheric interference is expected to increase in future years. In addition, many consumer grade GPS receivers do not allow users to automatically conduct point averaging and the software that accompanies most consumer GPS is limited in scope. A mapping grade GPS receiver called the SXBlue (GENEQ, Montreal, Quebec, Canada) has become available for about $2,000 and makes use of Bluetooth wireless technology to communicate with a digital data logger. The SXBlue is intended to take advantage of Space-Based Augmentation Systems (SBAS) that are capable of providing conventional real-time differential corrections to GPS receivers as they collect data. Conventional real-time differential uses the more accessible coarse/acquisition satellite signals rather than phase code signals. Although phase code signals have a greater potential for more accurate GPS measurements, continuous and uninterrupted satellite signals are required, conditions which often are not attainable under forest canopy. A SBAS derives separate measurement correction factors (rather than a single factor) for several potential sources of GPS error including atmospheric interference of signals, timing intervals used to estimate satellite signal range (distance), and the tracking of satellite orbital patterns. In the United Sates, there is currently one operational SBAS. This is the US Federal Aviation administration’s Wide Area Augmentation System (WAAS), which featured two operational WAAS satellites during the study period. The satellites operate in geosynchronous orbits with equatorial locations over the Pacific Ocean and northern Brazil. A line of sight between a GPS receiver and a WAAS satellite is necessary for satellite signal reception. Forest canopy, structures, and landforms can effectively block signal reception. In addition, GPS measurement reliability decreases as distance between a GPS receiver and the WAAS satellites increases. Only a single WAAS satellite signal is necessary for a GPS receiver to apply real-time correction factors but reception from two WAAS satellite signals is preferred because a second provides a backup should reception from one satellite become unavailable. In the United States, only western states had the potential to receive signals from both operational WAAS satellites during the study period. Two additional WAAS satellites are anticipated in 2006. Other SBAS include the European Geostationary Navigation Overlay System and the Japanese Multi-Functional Transport Satellite-based Augmentation System. The SXBlue GPS receiver configuration is intended to be compatible with these international SBAS in addition to WAAS. The SXBlue GPS receiver also features a navigation system (COAST Technology) that is designed to allow accurate GPS measurements during times when satellite reception becomes degraded or lost, such as what might occur if data are being collected under a dense forest canopy. The navigation system uses algorithms that are intended to continue applying differential corrections even if real-time connectivity to WAAS is lost. More specifically, errors that would be expected from atmospheric, satellite, and ephemeris conditions are predicted and removed from recorded measurements. For the navigation feature to work, successful reception of differentially corrected satellite signals (WAAS) for up to 5 minutes must occur first. After successful reception, the 10 Journal of Forestry • January/February 2007 GPS receiver is supposed to allow up to 30–40 minutes of data collection before additional signal reception is necessary. Our objectives were to compare the accuracy and reliability of a relatively low-cost mapping grade GPS receiver operating in two different data collection modes with a consumer GPS receiver collecting measurements under a dense forest canopy. The mapping grade GPS receiver measurements were collected in autonomous mode and also through real-time differential corrections as supplied by WAAS for comparison. We synchronized measurement times for both GPS receivers and used a point averaging benchmark (the average of 60 points collected at one-second intervals) that should present field-collection efficiencies for those who collect measurements under canopy cover. In addition, we were interested in determining whether the SXBlue’s navigation system capabilities would enable us to collect accurate GPS measurements when satellite signal reception was not initially available.