
This thread is going to be an ongoing discussion of our project to get the daily Marine VHF "Boot Key Harbor Cruisers Net" up and available online. This initial post will discuss the background of the project, some technical details, and how we are (currently) planning to approach it. More posts will follow with current status, questions, and looking for help and or suggestions in dealing with the stumbling blocks we are sure to encounter. Please feel free to comment at your pleasure. Hopefully our experiences may be of help looking to do something similar elsewhere.
Background:
We are located just above the 7 Mile Bridge in the Florida Keys, and have a large "live-aboard" community here. Our numbers range from a hundred or so during the summer, up to 350+ boats in the height of the winter season. As a community we get together every morning on Marine VHF Channel 68 at 0900 local time for the Boot Key Harbor Cruisers Net. This controlled net is sort of like our local newspaper, where we share arrivals and departures, announcments of local events, general comments, questions, needing help, and so forth. While the net always starts promptly at 0900 365 days a year, its' length can vary from over an hour in the middle of the season, down to Charles (one of our net-controllers) record of 57 seconds last summer.
Unfortunately, if you don't happen to be on the boat or have a handheld with you when the net is on... too bad for you. Also some of our regular "snow birds" who return to paradise year after year would like to keep track of our version of How The Harbor Churns while they are back in their home ports for the summer. Talking with one of the other net-controllers we decided to think about putting the daily net up online. How hard could it be?
Some Technicalities, Goals, and Philosophy:
Net runs on Marine VHF Channel 68 (156.425Mhz Narrow Band FM)
Transmitter strength at participating stations 1W min 25W max
Area of the net coverage is around a 10 mile radius (max) Most stations within a 5 mile radius can participate even using a handheld radio, but reception of these stations can be spotty.
General philosophy is keep it simple stupid... Charles (the other guy who wants to see this fly) is a "tech-head" and has most every wifi device known to man. His idea is to use SDR-Touch to grab the net and stick it on an android device. Dump that to Twitter Periscope as a 'cast and there you go. Now this offends my sensibilities, my apparant need for control, and the Periscope stream dissapears after 24 hours. My thoughts on the issue are as follows, grab the vhf and demod it, convert to an mp3 file and archive it on a real server we control. Real-time is not critical, we have an archive copy, and I have a full time server available with the storage and bandwidth to work. Plus it is on a CDN and besides.... I need a hobby. So for now, thats the way we are gonna go. Android stream may follow.
Couple other considerations in this ball of worms. Power is an issue. Living out on the mooring ball my extension cord does not reach to shore. The batteries and the solar panels will keep up most of the time, but hours and hours on the laptop will put a dent in the system. Going to try to stick to native linux programs that are known to run on a Rasberry Pi 3. Headless, the pi uses almost nothing even running wide open. Which brings us to another question, what's gonna happen when I am not there to punch up the computer and get this system underway. I mean I am here most of the time, but the occasional out of town trip means that the system should start and stop on its own. Cron in linux makes the start simple, and some scripting should take care of pushing the recorded mp3 up to the server via SFTP but still not sure how to shut down the recording at the end of the net. Maybe no squeltch break for 45 sec or so? Remember the net starts at a fixed time, but isn't over till its over. Something to think about.
So the goal seems to be a linux based system up 24/7 but only records from 0858 till the net ends. Then squirts resulting mp3 file up to the net server which handles the heavy lifting of archiving, sorting, distributing, etc. Hope that if we can get this whole thing down to a small enough footprint (hardware) maybe the City Marina will let us hang it on their tower which would give us excellent coverage, a fat wifi connection for uploads and headless admin of the pi.
Status:
Been working on this for a bit and here is a basic status report. More detail to follow in next post. Ordered a RTL-SDR stick from RTL-SDR.com which arrive soon after their end of April restock at Amazon. Got it with the generic telescope antennas (Hey its only $5 or so) and a SMA to UHF type F connector to hook up to an old VHF quarter wave whip I had onboard. Turns out the whip was bork, so ordered another which arrived today.
Installed GnuRadio Gqrx and updated RTL-SDR tools, all on an old IBM T-42 Using the telescoping antenna stretched out to about the right length, was able to follow most of the net via Gqrx which is easier than the planned rtl_fm demodulator as the waterfall and tuning is visual. Switched over to a local FM broadcast station for further experimentation as it has enough signal strength to overcome antenna shortfall. Set up rtl_fm piped to sox via the command line and was able to record a decent mp3 file. If new loaded whip antenna performs as hoped, will start using VHF channel WX2 as experimental base as it broadcasts 24/7 and has the same freq and demod characteristics as channel 68.
Next Post:
Next post we will update the status, performance of the new antenna, and details of the linux command line strings to invoke rtl-fm | sox output, and anything else we happen to remember.
Comments and questions encouraged.
David
Onboard S/V Solitaire
Boot Key Harbor