Node-density independent localization

This paper presents an enhanced version of a novel radio interferometric positioning technique for node localization in wireless sensor networks that provides both high accuracy and long range simultaneously. The ranging method utilizes two transmitters emitting radio signals at almost the same frequencies. The relative location is estimated by measuring the relative phase offset of the generated interference signal at two receivers. Here, we analyze how the selection of carrier frequencies affects the precision and maximum range. Furthermore, we describe how the interplay of RF multi-path and ground reflections degrades the ranging accuracy. To address these problems, we introduce a technique that continuously refines the range estimates as it converges to the localization solution. Finally, we present the results of a field experiment where our prototype achieved 4 cm average localization accuracy for a quasi-random deployment of 16 COTS motes covering the area of two football fields. The maximum range measured was 170 m, four times the observed communication range. Consequently, node deployment density is no longer constrained by the localization technique, but rather by the communication range.

1. INTRODUCTION

We have recently proposed a novel approach to sensor node localization, the Radio Interferometric Positioning System (RIPS) [4]. RIPS creates a low-frequency interference signal by one pair of nodes transmitting simultaneously at close frequencies. The relative phase offset at a pair of receiver nodes is used to determine a distance measure between the transmitting and receiving nodes. Unlike traditional ranging approaches, which determine the pairwise distance between two sensor nodes, RIPS measures , a distance aggregate called the “q-range” involving four nodes: two transmitters and two receivers . We reported a localization accuracy of 3 cm in a 16-node setup covering an area of 324 m². We estimated the maximum range of RIPS on Mica2 motes to be 160 m, but this was not experimentally verified.