Several DGPS-based navigational aids have recently become available to producers.  These systems are designed as steering aids for equipment operators applying products such as fertilizers and pesticides in equidistant parallel or curved swaths.  DGPS-based navigational aids generally work by projecting an optimal travel path based on the previously completed swath, using DGPS to measure vehicle position in real-time, and providing the operator with visual feed-back indicative of deviation from the optimum.

In response to questions raised by Tennessee row-crop producers, University of Tennessee Sensors and Controls Lab personnel initiated a project in 2001 to evaluate DGPS-based navigational aids.  The overall goal of the study was to investigate effectiveness of these systems for parallel swath-width vehicle control.  Specifically, this objective was formulated as follows:  "Using experienced agricultural equipment operators, evaluate the accuracy, ease-of-use, and potential economic benefits resulting from the minimization of under- and over-applications using selected DGPS-based navigational aids."  To accomplish this objective, tests were designed to compare three, commercial, DGPS-based row-guidance systems for parallel-swathing applications using a conventional foam marker as the standard for evaluation.

Tests were conducted at two locations using 60- and 80-ft swaths.  GPS navigational aids were equipped with either one- or two-row LED arrays (light bars) or an anti-glare LCD graphics display.  Two of the systems were mounted inside the vehicle and one unit was mounted on the vehicle hood, all according to manufacturers' instructions.  Each guidance system was used with the same high-quality RTK-GPS system.  Twenty-one sprayer operators participated.  Both private and commercial operators were included.  Operator experience varied, in terms of both years experience and acreage sprayed annually.  Operators maneuvered the parallel swaths at two application speeds.  Each operator drove the course using the foam marker as a standard, and with two of the three DGPS-based systems.  A variety of performance measures were calculated for each operator, including areas of overlap and skip.

Examination of results revealed that test runs driven using foam markers had larger average overlap and skip values than those driven with the DGPS-based navigational aids.  These differences were statistically significant for overlap, but not for skip.  No statistically significant differences in performance were detected between the three tested DGPS-assisted systems.  These results suggest drivers were more accurate using DGPS-assisted guidance systems than using a traditional foam marker system.  Driver performance at high and low speeds did not statistically differ; therefore, speed did not affect performance of the guidance systems, nor did it affect operator ability to properly maneuver the vehicle.

Since a high-quality RTK-GPS system was used in place of the standard DGPS receiver in these tests, results should be interpreted carefully.  Test results suggest that operator performance improves when an accurate guidance system with a well-designed, heads-up display replaces the traditional foam marker system.  However, positional errors inherent to standard DGPS technology will degrade the accuracy of such a system.  Effects of position errors are not included in test results presented here.  

Morrow, T.F.  2002.  Evaluation of DGPS row guidance systems, analyzing operator feedback methods based on accuracy and operator insights.  M.S. thesis, The University of Tennessee, Knoxville, Tenn.

Cotton Incorporated

John Wilkerson, Ph.D. 
The University of Tennessee
2506 E. J. Chapman Drive
Knoxville, TN  37996-4531
PHONE:  (865) 974-7266
wilkerj@utk.edu