GNGTS 2017 - 36° Convegno Nazionale

180 GNGTS 2017 S essione 1.2 GNSS single-frequency devices at OGS: LZER0 a cost-effective prototype D. Zuliani 1 , M. Bertoni 1 , C. Ponton 1 , P. Fabris 1 , M. Severin 2 , G. Ferin 3 , G. Rossi 1 1 Centro di Ricerche Sismologiche CRS, Istituto Nazionale di Oceanografia e di Geofisica Sperimentale OGS, Trieste, Italy 2 SoluTOP sas di Marco Severin, Udine, Italy 3 Istituto tecnico “I.T.G. G. Marinoni”, Udine, Italy Low cost single frequency GNSS (Global Navigation Satellite Systems) receivers, usually developed for the mass market or for hobby use, recently (Eyo et al., 2014) gained more importance in the surface or near surface process monitoring. Some studies have investigated the accuracy of single-frequency GPS receivers for landslide monitoring (Eyo et al. , 2014; Squarzoni et al., 2005; Janssen et al., 2003). The big challenge in landslide monitoring is how to reduce the monitoring costs and the prospect of losing the equipment during a landslide event. Dual-frequency apparatus can track at least two signals, L1 and L2, from the GNSS satellites and they are commonly used for the mentioned monitoring applications (Zuliani et al., 2018) when their data are post-processed. Unfortunately they are expensive because of patents on L2. Single-frequency devices are less expensive because they just provide L1. With some limitations, respect to the dual-frequency models, they can be used anyway for positioning with sub centimeter accuracy in post-processing mode. The single-frequency limitations can be described inside the GNSS data processing method subject (O’Keefe 2016): both single and dual-frequency GNSS data elaboration can be carried out using different techniques, the most common is called Double-Differences DD (Kaplan et al., 1996; Hofmann-Wellenhof et al., 2001). DD, in a standard single-baseline method, needs a Master GNSS station installed on a stable structure with respect to the Rover unit that is constrained to the monitored structure (landslide, building, bridge, road). Both Master and Rover GNSS data are combined by DD to assess the final distance, or baseline, between Master and Rover. The Use of a single-frequency dataset forces the baseline to be less than 5 km with at least 1 hour of data (Heunecke et al., 2011) to reach a centimeter level accuracy. The influence of the baseline length on measurement accuracy is mainly due to the troposphere and ionosphere. Their effects can be reduced just using DD (both single and dual-frequency cases) only for the shortest baselines and when the GNSS sites are located at almost the same altitude (Malet et al., 2002). However Dual-frequency equipment’s con reach better performances in terms of baseline length: they provide both L1 and L2 signals to be combined before using DD to completely remove some effects such ionospheric errors. In this way the baseline limitation is extended from 5 km to different hundreds of km (Cina et al., 2000). With that difference in mind a cost-effective single-frequency GNSS receiver system can be used to monitor bridges, roads, dams, landslides when the supposed movements or deformations are within a sub centimeter level. Centro di Ricerche Sismologiche (CRS) of Istituto Nazionale di Oceanografia e di Geofisica Sperimentale OGS, since 2015, directs a network of single-frequency GNSS sites (Zuliani et al., 2016) to monitor a landslide near the Tolmezzo municipality (central Friuli, Cazzaso village). The system, called SENDAS (and now DEDALOS), achieves centimeter accuracy enough to detect the most important landslide slips (one main event in 2016 and one in 2017 as described in Fig. 1) but with a third of the dual-frequency device costs. The system can be accessed by a software Client that provides the position of each site with a minimum delay of 1 hour for near real-time monitoring. Even precise GNSS real-time positioning techniques, RTK (Hu et al., 2003), can benefit from cost-effective single-frequency devices. In real-time applications a GNSS receiver, the Rover can reach a centimeter precision in real-time if aided by RTK services such VRS, MAC or FKP (Landau et al., 2002). The Rover receives GNSS corrections (double-differences approach)