A Low-Power Portable Post-Processed DGPS Package for Precise Position-Logging of Sheep on Hill Pastures

Gwyn Roberts, Aled Williams, J. David Last, Peter D. Penning and S. Mark Rutter

Abstract:	As a result of the explosion at the Chernobyl nuclear plant, significant quantities of airborne caesium-137 were transported into Sweden, the UK, Iceland, Greenland and even the north-eastern US. Rainfall washed the radioactive material into the soil from where it is absorbed by the roots of plants. Sheep eating the affected plants threaten to transfer the radiation into the human food chain. Consequently, the sale of all meat with radiation levels exceeding 1000 Bq/kg from sheep in the upland areas of North Wales and the Lake district of the UK is still banned, seven years after the incident. A research program into the correlation between levels of radioactivity and grass types in these areas has led to the need for an instrument that can monitor the positions of sheep very precisely. The specification calls for fixes at 1 minute intervals, with an accuracy of better than 5 m. An unusual approach to post- processed differential GPS has been shown to provide the optimal solution to the requirement. Detailed statistical analysis has demonstrated that, given the full GPS constellation, the target performance can be achieved within tight constrains of weight, size and power consumption. The sheep-borne unit employs a low-cost OEM GPS’ receiver board. Position and ephemeris data are recorded on a memory card for recovery and post-processing. Further, the same type of low-cost receiver has been shown to serve as a satisfactory reference station, providing accuracies in the l-5 m range without the need for a survey-grade receiver. The design of the sheep-borne unit has been optimised for minimal power consumption. In addition to recording data on the animal to avoid the need for data transmission or reception, the GPS receiver is switched off between samples. Its operation is also controlled by a sensor that removes power from the unit when the sheep is lying down. Key features of the system that are described in detail include: 0 a compact, lightweight, low-cost portable data logging package l a design optimised for minimal power consumption l an endurance and data capacity of 10 days with one-minute sampling l DGPS post-processing
Published in:	Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993) September 22 - 24, 1993 Salt Palace Convention Center Salt Lake City, UT
Pages:	1141 - 1148
Cite this article:	Roberts, Gwyn, Williams, Aled, Last, J. David, Penning, Peter D., Rutter, S. Mark, "A Low-Power Portable Post-Processed DGPS Package for Precise Position-Logging of Sheep on Hill Pastures," Proceedings of the 6th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1993), Salt Lake City, UT, September 1993, pp. 1141-1148.
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