ments adjacent to the structures along with differential leveling,” said EBMUD Survey Supervisor Steven J. Martin, LS. “After the advent of real time kinematic (RTK) GPS surveying in the 1990s, staff had taken to running the sight lines with RTK; however, with the differential leveling, it was still a labor-intensive approach that took over a week to complete.” The old monitoring scheme technically met dam safety requirements but only provided semiannual data on dam crest displacements, leav- ing significant room for improvement in spatial and temporal resolu- tion. “EBMUD is committed to automating processes where it makes sense,” Martin said. “In this case, due to the remote location and criti- cal nature of the infrastructure, it made a lot of sense.” Designing for better data Consulting with Cory Baldwin, president of sensemetrics, a San Diego firm specializing in networked sensor applications, EBMUD designed and proposed a sophisticated monitoring system based on a total of 31 Leica GMX901+ GPS sensors, four Leica GM10 GNSS reference stations, a radio network consisting of 900 Mhz mesh radios, 2.4 Ghz repeaters, and two radio towers, feeding into the Leica GNSS Spider and GeoMoS software solutions. The use of Leica Geosystems receivers is important, according to Baldwin. “The GMX901+s are purpose-built for remote monitoring applications, with non-exposed, built-in antennas,” he said. “They were my first choice here because other vendors don’t really have a good option for monitoring in this environment.” Three of the GNSS reference station receivers are solar powered and one is powered by a 120v AC feed. All are securely fastened inside enclosures installed near the Leica AR20 antennas, which are mounted on concrete pedestals. The network is largely autonomous, needing only occasional attention. Data flow is through 900 Mhz and 2.4 Ghz spread spectrum radios into an existing microwave telemetry link to EBMUD’s business intranet at its Oakland headquarters, where a serv- er runs the Leica Spider and GeoMos software necessary to process the GPS data and results. The results are then presented through software customized by sensemetrics. Five of the Leica GMX901+ sensors as well as four seismographs are installed on the Pardee Dam, connected via fiber optic cable directly to the microwave business intranet. These instruments continuously monitor dam movement and report remotely. Two of the Leica GM10 reference stations were installed near and on either side of Pardee Dam and are connected to the fiber optic line by 2.4 Ghz radio connections. Downstream from Pardee Dam, the Camanche Reservoir site consists of one large earth-filled dam and six dikes. Camanche Reservoir is pri- marily used to control releases to downstream agencies and maintain flows for the salmon. Twenty-six Leica GMX901+ sensors are installed around the reservoir, and these also monitor and report continuously. The State of California Division of Safety of Dams (DSOD) requires semiannual monitoring surveys. The new system provides more ac- curate information more rapidly, reduces staff time spent on monitor-
Monitoring sensors and seismographs are installed on the Pardee Dam and connected via fiber optic cable directly to the microwave business intranet. These instruments continuously monitor dam movement and report remotely. Photo: © EBMUD, used with permission
What makes a good monitoring GNSS receiver? The Leica Geosystems GMX901+ GPS sensor was a good solution for the EBMUD monitoring network for several reasons. A relatively low- cost L1 GPS-only solution purpose-built for monitoring, it features: • low power consumption; • integrated, non-exposed antenna; • highly accurate and precise ability to monitor small movements; • durable and robust housing designed to withstand extreme tem- perature variations, vibrations, and constant exposure; and • seamless connection to dedicated monitoring network software, in this case Leica GeoMoS and Leica GNSS Spider.
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csengineermag.com
april 2018
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