Kris Kirby wrote: > On Sun, 22 Jul 2007, skipp025 wrote: >> Don't know how practical it would be... but (some) rotating dopplers >> make a tone that can pass through a linked radio system. If the >> antenna array reference was known you might be able to get a general >> direction back to x-desired location. s. > > If you use a diode-switched doppler system, the sidetone becomes > component of the signal, which one could detect on and correlate to a > particular direction. That tone, however, is related to antenna > switching speed, not direction. One could easily rig something to a > fixed dopper direction finder to generate a particular frequency based > on which antenna is selected and provideo an audible output that could > be directed to a repeater.
The problem with such systems is the real-estate necessary at the repeater site for them. The group here found many years ago that it was easier to get small places to put individual receivers directly at the airports, with single antennas (well, actually they have a 121.5 antenna and a UHF 1/4 wave to send alerts out to the repeater) than it would ever be to pay for enough antenna space at a repeater site for rotatable or electronically switched antenna solutions with multiple antennas. In fact, you typically don't have to pay for them at all, and they'll throw in the small amount of AC power you draw for free... unlike a commercial communications site! If you're hanging around aviation, there's always someone you know with a hangar that has power, and will let you put two tiny antennas on the roof, and run those to a locked NEMA box inside with your gadgets. And in many cases the FBO themselves and/or the Airport Authority were happy to provide a spot. We then augment the airport-based systems with one receiver way up high at the repeater site. It leads to things like this: - One alert monitor going off, it's probably at that airport. Someone drives there and DF's it, since you already know where you CAN hear it... the hard part's done for you. - One alert monitor going off, and the mountaintop receiver, it's probably at that airport and stronger. (GRIN) - More than one alert monitor and/or the mountain going off, it's likely airborne (and since we can listen to the audio from our monitors, you can listen to it fly off into the sunset, decreasing in signal strength everywhere... you can also usually tell what general direction it was going). - Only the mountaintop receiver hearing it... that's the most likely time it could be "real"... or just at an airport we don't have covered yet. (GRIN) Anyway, as soon as any of them are determined to be non-emergency, we command the remote receivers and the mountaintop one to OFF, since retransmission of the 121.5 or 243.0 signals into Part 97 territory is a no-no unless it's an emergency. Every ELT starts out an "emergency" and gets downgraded from there... (GRIN). There's some other nifty features that the ham building/designing the airport monitors has done with them over the years, too many to mention really... but some of them are: Anything "commandable" is handled via DTMF through the repeater and down to the little controllers at the airports. - Voice alerts along with DTMF alerts, where the DTMF alert can be decoded elsewhere, allowing for an amazing array of features, driven by a computer somewhere "else". Currently used for logging and e-mail alerts/pages. - CW ID's (of course) - Signal strength (RSSI) announcements during alerts. - The ability to do various things including listen to 121.5, or 243.0, turn on/off alerts for any band. - Measure and annouce AC input voltage and DC/battery backup voltage. - Alert on door opening of the NEMA boxes (tampering) and/or check on the door closure switch status. Etc etc etc... the list goes on. It's basically been a decade-long project for him, with various new releases of his controller and controller's code for doing all of the above and more. He's not interested in selling/supporting them outside our environment, however. In our environment he has complete control over the radios used, etc. Building a few of something you know is easy, building a "product" is a lot harder. You can see the daily alerts (that DTMF during alerts thing again) at the website, http://www.fredf.org on the left sidebar. Pick yesterday to see an ELT around 3AM at 2V2 (actually 2V2 may have changed identifiers recently, some changes needed in code...) and APA at around 11 AM. Pick July 19th to see one that flew into the Denver Metro area, and landed at APA. Heh... see how only the mountain (labeled "Repeater") only heard it until it landed? And the 18th of July for the one I hunted down over at Adam Aircraft at APA. It was in one of the brandy-new VLJ's under construction, the bird didn't even have a paint job on it. I LOVE it when those guys set one off, you get to see the new/pretty birds up close and personal! And a non-painted Adam bird, not even airworthy yet, is probably about as new as you'll ever get! :-) Probably the only perk of ELT hunting... is getting to see the pretty birds... and inform their poor owners that the ELT has been running over an hour... "Sorry you'll have to replace that battery, but nice airplane!" Nate WY0X
