Time synchronization for wireless sensors by using low-cost GPS module

Abstract

This study presents a feasibility study on low-cost GPS modules for time synchronization of wireless sensor nodes. A low-cost GPS module provides 1) Pulse-per-second (PPS) signal which is a short pulse generated at the exact beginning of each second and 2) NMEA sentences containing absolute date and time information in addition to 3D location of the module. PPS and NMEA sentences can be combined with internal clock-counter of a Micro Processor Unit (MPU) to get an accurate timestamp for each measurement made. Three experiments were carried out to investigate 1) long-term reliability of a low-cost GPS module for timestamping purpose, 2) accuracy of PPS signals itself and 3) accuracy of proposed time-stamping method. An Arduino Mega 2560 board with a 10MHz oven-controlled crystal oscillator (OCXO) was used in the experiment. It was found that the module was operated reliable for a week and accuracy of PPS signals were ±300 nano-sec, and maximum time-stamping error was within ±1 μsec which is very promising. Furthermore, a possible extension of the proposed system to time synchronization problem inside a building where no GPS signal available was investigated by utilizing the OCXO.