Virginia Coast Reserve LTER Wireless Network
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The Virginia Coast Reserve Long-term Ecological Research project (VCR/LTER) uses a hybrid network, incorporating serial spread-spectrum radios, Wi-Fi and proprietary 900-MHz access points to service a wide variety of science needs, from low-bandwidth applications such as wells and meteorological stations to bandwidth intensive applications such as web cameras and flux towers. The two largest challenges are providing adequate and reliable power in remote field environments, and ensuring high-quality radio links, often over long distances. For the former, the VCR/LTER depends almost completely on solar panels. Ideally, these power installations are over-engineered to assure adequate power during the darkest and stormiest parts of the year. Matching the radio with the bandwidth and distance also helps keep power needs to a minimum. Thus, extremely low-power serial radios are used to transfer data from wells, over a distance of several kilometers (requiring only a 5-watt solar panel), but higher-powered and higher-speed Wi-Fi radios are used to connect web cameras over distances up to 20 km (requiring 55-watt solar panels or higher). Use of timers to cut off radios when they are not needed, is also a critical part of saving power.
For obtaining high-quality radio links, nothing beats elevation! The VCR/LTER is fortunate in that Hog Island features an abandoned Navy lookout tower from World-War II, and the flag tower from an old Coast-Guard station. These towers provide clear line-of-sight between one another and to a wide array of research locations. The figure shows the Coast-Guard flag tower repurposed for the mounting of directional and omnidirectional Wi-Fi antennas, a 900 MHz omnidirectional antenna, a webcam and a wind monitor. The shed roof supports the solar panels and the shed itself contains the power systems (solar controller, battery and timer), 900 MHz serial radio, a Ethernet switch, a serial-to-Ethernet converter (for the serial radio), a webcam server and a data logger for the wind data, . It is coupled via an electrical conduit with a junction box at the top of the tower that contains power and data busses, an Ethernet switch and a Wi-Fi radio. A DC-AC inverter powers the web camera, which requires 24 volt AC power. All other power is 12v DC.
Data are transferred across the wireless network to a local computer located at the ABCRC laboratory and periodically transferred across the commodity Internet to the University of Virginia. This keeps the “chatter” associated with the data downloads on the local network, without impacting performance of the (slow) connection to the wider world.
The high-wind/salt water environment places a high premium on hardening of equipment against water damage. Ideally, even radios designed for external use are enclosed and all exposed cable connections are “potted” using one layer of electrical tape wrapped clockwise, covered with a layer of unvulcanized rubber tape, wrapped counter clockwise, and covered yet again with another layer of high-quality electrical tape wound clockwise, with each layer extending beyond the ends of the other.