TD News, Number 3, 2003 - Internet Web Site: 'http://www.fs.fed.us/t-d' Send an e-mail request for username and password to: 't-d@fs.fed.us'

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A Flexible VHF Telemetry System for Remote Sites

The Missoula center has developed a system that permits digital data to be transmitted over Forest Service VHF radio frequencies. This system uses the existing infrastructure of Forest Service FM radios, broadcasting on the least-used channels. It will allow data to be retrieved automatically from remote forest sites.

In July of 2002, the Pacific Southwest Research Station in Fresno, CA, asked MTDC to find a way to retrieve research data from remote sites in the Sierra National Forest. The research station was about to begin a 15-year study monitoring conditions in the Kings River watershed using four weather stations and eight stream-flow monitors. Visiting each site periodically to retrieve the necessary data would be very time consuming. This would be especially true in winter when most of the sites are accessible only by snowmobile.

MTDC investigated the options, addressing three problems: location, compatibility, and cost. The center immediately ruled out UHF spread-spectrum communications because of the line-of-sight and distance limitations. Satellite modems were eliminated because the volume of data to be transferred made the cost prohibitive. In addition, the data loggers transmitted data using a format that was incompatible with the satellite modems. Another problem was that different brands of data loggers had unique protocols for initiating a data download. The communications package and its support software needed to operate with all types of data loggers.

The research station had access to a VHF repeater maintained by the Sierra National Forest. One channel on the repeater received very little use. The research station’s data could be downloaded during the early morning hours using that channel. A modem from Teledesign Systems of San Jose, CA, solved the problem of shipping data over a channel designed for voice communication. This modem has several features that allow it to work with radios, such as generating a push-to-talk signal to key the transmitter.

The problem of selecting which data loggers would communicate with the host transmitter and managing the power consumption of the modems was solved by using dual-tone, multifrequency (DTMF) signals, such as those used by touch-tone telephones. A Bendix-King VHF-FM GPH5102 radio is used for communication at each site. This system could be readily adapted to other Forest Service applications where data needs to be retrieved from remote locations. The initial costs are between $1,400 and $1,800 per site. Depending on the terrain between the host station or repeater and the remote sites, the 5-watt Bendix-King radios can transmit signals up to 50 miles.

Flexible VHF Telemetry System.

For more information on this remote data retrieval system, contact Ted Etter, project leader (phone: 406–329–3980; fax: 406–329–3719; e-mail: tetter@fs.fed.us).


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