Ground displacement measurement by radio interferometric ranging for landslide early warning

In this paper, we propose a novel approach to measuring ground surface displacement at high accuracy by radio interferometric ranging (RIR) with extremely low-cost devices. A major challenge to RIR in accurate measurement is multipath in mountainous environments. We propose a novel RIR scheme called Downslope Interferometric Phase (DIP) to decouple the effect of reflective path from that of line-of-sight path. With a two-path channel model, we find that the displacement errors in DIP fluctuate periodically with respect to measurement frequencies. Based on such a revelation, we propose to employ multiple carrier frequencies from large enough bandwidth to mitigate multipath. Experiments both in open spaces and dense multipath environments show that millimeter level displacement can be detected by DIP within several minutes. With better performance and much lower cost than carrier phase GPS receivers, DIP provides a promising method to build landslide monitoring and early warning systems covering vast areas at reasonable cost.