The unit operates on 9 to 30 volts, using its own internal transformer. He also considered the Tron & Tron unit; this unit is smaller than the Trimble unit and draws less power but Keith liked dealing with the Trimble people. The Trimble electronics unit provided an RS232 serial port, printer port (DB9), power connection and antenna connection. He thought the unit was solid, and well built. The unit can be turned on via a logic level control through the power connection. During their work, he found that the unit was able to transmit to the satellite 98% of the time. Messages are in a minimum of 32 bytes which cost 20¢. It cost him approximately $6.50 to transmit 1kbyte; the normal rate is 1¢ per byte. He was able to negotiate some free time before the project began to help during the debugging and testing phase.
Inmarsat C supports an e-mail service and a modem (like a bullitin board) service. He chose to use the e-mail service. There is a latency of about 2 - 3 minutes in transmissions from the mobile unit to shore and about 20 minutes from the shore unit back to the mobile unit. He found that errors were not a problem; the system was robust. If a connection was made between the mobile unit and the satellile, and the unit said "message sent", that the message indeed was sent. There is an option to get an achnowledgement but he chose not to do it.
He used the procmail utility to handle the e-mail transmissions automatically. He had to handle the situation when the mobile unit was not turned on and logged into Inmarsat C so it could not received messages. He dealt with this by setting up a mail queue on a local machine ashore. Messages (e-mail) destined for the buoy would be received by the mail queue ashore, header text removed and saved in a file for later transmission. When the remote system (buoy) established a connection to the local machine, he would leave the remote system on for thirty (30) minutes to allow the local machine time to send out the stored messages. There is no penalty for leaving the unit on and "logged in" all the time, except for the 10 watt power drain. Since they were battery based, they could not afford to leave the unit left on.
Keith said that there are other choices that we could consider. For example, Inmarsat M, which operates at 2400 baud, is four times faster than Inmarsat C. Transmission costs are less, but the terminal costs are higher (see table). Also, error detection is now the sender's responsibility although using a PAD (digital modem) can be used to address this. Another alternative is the Westinghouse system which uses a bigger antenna, tuned for the latitude. Texas A & M used it on their buoys. Another contact is Woods Hole Instrument Systems for their experiences with data transmissions between shore and a buoy about a year ago. Jim Irish (WHOI) may be familiar with this project if we want to follow up.
Andy suggested I contact Keith von der Heydt at WHOI for details on how he used Inmarsat C to transmit data back from moorings.
| INMARSAT C | World wide communication access |
| MSAT | 100 miles offshore access only Canadian service provider |
| AMSC | 100 miles offshore access only American service provider Financial stability questionable |
| Iridium | Eventual world wide access Slow speed access (2400/4800 baud) Uses ~50 low orbit satellites Operational now in some areas See Jim Irish or Walter Paul (WHOI) for more details. |
Andy is working on several projects that might have tie-ins with our project. The SEANET Project is working on the ability to provide full network access to ships and buoys using whatever communications paths exist. There current test system is using the INMARSAT B system, which is a high end, high band-width and therefore high cost system designed to be used on UNOLS vessels. There next step (in the 2nd year) will be to implement the functionality using cell phones and the AMSC/MSAT systems. Beginning Auguest 10, they are doing a test run of the system in Woods Hole, a 10 day "cruise" from the Smith Building to the Smith Building.
The second project he called "Wiring the Sound Project". This project calls for providing data communication services for WHOI's buoy farm and Vineyard Sound using Spread Spectrum 900 MHz radio technology. Using this frequency does not require licensing and yet can provide 100kbit/second line-of-site transmissions.
His third project is a NOPP funded project with Brad Butmann (USGS) and Dan Frye (WHOI) to design and construct a low cost, expentable buoy that also transmits data. This project uses the idea of "data pipes", a data transmission abstraction, that attempts to hide the details of exactly how data are transmitted. A "data pipe" can thus be created using an IP (internet, TCP) connections, xmodem or even a serial interface.
The forth project is his GeoBrowser Project which attemps to offer Web access to on-line geographical data, stored (conceptually) as index cards.
I think all of these projects share the view that a web user interface is the best to use.
Andy offered to prepare a table of transmission options, including costs and capabilities. We are meeting again July 15th.
There is a comparable project going on in Europe. They are further along. They plan to use INMARSAT to handle the data transmissions.
He is considering teaming with "BOAT-TRACKS" as they are the sole source for handling vessel tracking data for NMFS. However, this system is not cost effective. He thought "BOAT-TRACKS" uses GE America GSTAR-1 for data transmissions. In Canada, they use CANCOM, and in Europe, they use EUTELSAT.
For additional background information see Real-time Data Telemetry - Background.