A sag monitoring device based on a cluster of code based GPS receivers

A relatively new technique of differential global positioning system (DGPS) has potential to measure sag on an overhead transmission line conductor. The technique relies on the information received from GPS satellites and carrier based receivers so that sag can be measured to an accuracy of approximately 1″. However, the overall cost of the carrier based system is extremely high. An alternative method based on using a cluster of cheap GPS receivers was explored for the first time in this paper. A single code based GPS receiver is considerably inaccurate to the extent that it is almost unusable for a vertical measurement such as sag. This paper describes two different approaches to improve the accuracy of data from a cluster of cheap GPS receivers so that they can be used for sag measurements. Kalman filter and bad data rejection techniques were adapted to reduce vertical error in measurement of sag. An error of ±4.5 m in the raw data was reduced to ±0.45 m after using Kalman filter and to ±0.37 m after using the bad data rejection techniques. Details of these techniques in the context of potential application for sag measurement are described in the paper.