GPS Data via Audio Link

[Hell-e-Guy]

Hi Guys,

I am trying to help a friend send GPS data from his plane to the ground via the microphone input that's on the 2.4GHz wireless camera.
The GPS data is 4800 baud.
I think the required circuit will be a level shifter from TTL to +/-0.5V.
and the reverse on the receiver side.

Is this possible?

Hell-e

[Miami Mike]

Amateur Radio operators have a popular system called APRS (http://www.google.com/search?&q=APRS) that you could adapt.

[dalbert02]

The easiest way I know of is use a Tiny Trak III http://byonics.com/tinytrak/ and APRS as Mike suggested.
-dave

[tomp]

I doubt this will work. The microphone input is designed for audio signals and you have a digital signal. One way to convert the digital signal into audio is with a modem IC. That's what modems do: take a computer signal (digital) and convert them into audio so you could send the information over phone lines (and vice-versa!).

Texas Instruments used to make a modem IC that I used (TCM3105), but they've been obsolete for years. Didn't do 4800 baud anyway. A quick check of my bookmarks came up with CML Microcircuits:
http://www.cmlmicro.com

Check out their CMX469A and FX/MX469. Both will do 4800 baud.

Good luck!

tom

[Miami Mike]

That TinyTrak unit ought to work just fine. It's meant to connect to the microphone input of an amateur radio transmitter, but you can connect it to the microphone input of your camera instead.

[Hell-e-Guy]

Thanks!
I know a simple voltage conversion of the digital signal will probably not work.
Does anyone know what conversion the TinyTrak does.

Hell-e

[Miami Mike]

Does anyone know what conversion the TinyTrak does.It converts GPS data to a packet radio (http://en.wikipedia.org/wiki/Packet_radio) format used by ham radio operators. You'll need a terminal node controller (http://www.google.com/search?hl=en&lr=&safe=off&rls=GGGL%2CGGGL%3A2006-16%2CGGGL%3Aen&q=terminal+node+controller&btnG=Search) or "TNC" on the receiving end to decode the data, and an APRS program to map the position. There should be plenty of free APRS programs out there.

Since you won't actually be using any ham radio frequencies, it won't be necessary for you to have a ham license.

[dalbert02]

AGW Tracker and AGW Packet engine is free software based TNC / tracker.
http://www.elcom.gr/sv2agw/tracker.htm
-dave

[Hell-e-Guy]

Hi,

The TinyTrak modulates the signal and can change protocol and baud rate.
What hardware decodes & demodulates the signal on the receiving side?

Hell-e-guy

[dalbert02]

All you have to do is hook up the TT3 to your GPS and to the audio channel of your video transmitter. On the ground, connect the audio out of your reciever to the 'audio in/mic' of your sound card on your laptop. Run AGW packet engine to demodulate the packets and extract the raw GPS sentences. Run AGW tracker to track your position on a moving map. It is real easy. The hardest part is setting the volume of your 'audio in' so that the sound is not distorted and the packets are demodulated correctly. On one laptop I had to turn the volume all the way up to get it to work. I was using a Sound Blaster Audigy PC card. On the other laptop, it had a mic in that was very sensitive to the line level out of the reciever, and I had to turn the volume all the way down. It took me a little while to figure this out becuase I could hear the modulation over the speakers, but no data would come in. A good way to test everything is to take the 'audio out' of the TT3 and plug it directly to the 'audio in' of your laptop, removing the wireless part. Once that is working, then go wireless. You might need to make a cable, but that is easy too. You can get the parts from radio shack for like $5.
-dave

[Miami Mike]

What hardware decodes & demodulates the signal on the receiving side?I suggested a TNC, which is a separate device that you'd have to buy, and which has some capabilities you won't need because it's meant for 2-way communication, but dalbert02 is recommending a cheaper software solution that uses your sound card. His way sounds better if you can make it work.

[Hell-e-Guy]

Thanks for all the info guys!

I see that the TT3 has two modes of operation 1200 & 300 baud depends on the
frequency range.
What do you think the baud rate should be for a 2.4GHz link via the audio link?

Hell-e

[Miami Mike]

The 300 baud option is due to bandwidth limits on the lower frequency amateur radio bands. You'll want to use the fastest rate available.

Keep in mind that you may run into obstacles that we didn't anticipate, since as far as I know, you're the first to try this. For example, I don't know how often you'll be able to get the TinyTrak unit to send its data, since it's meant to be used on frequencies that are shared with others. Speaking for myself, I can't guarantee that this will work for you, but I think we've given you the most promising course to follow with your project.

[dalbert02]

Well, I can tell you it works! It is not difficult to set up at all. You can send your data as fast as once a second. I use 1200 baud with no problems. And you won't be the first, check out the UAV forums :rolleyes: (Not sure what Mike was thinking there). If you encounter any hitches, I'd be more than happy to help you. And Byonics offers awesome tech support if you need it, check out his testimonies!
-dave

[Miami Mike]

(Not sure what Mike was thinking there)Well EXCUSE ME! I thought up the idea of adapting APRS on my own. I don't read the UAV forums.

[Hell-e-Guy]

Thanks!
As usual I'm learning a lot from this forum.
I'll order the TT3 SMT version.
APRS....I would have never thought of it as I was in liquid form when it was invented.

Hell-e

[dalbert02]

If you have any problems feel free to pm me. I have the Black Widow board camera, 1w Tx, RC-CAM Inspire OSD, FMA copilot and TT3 all in a Kadet Senior (electic of course :) ) with a diversity Rx on the ground. I also have an AP4 autopilot that I am configuring now with a Geko GPS.

Mike - relax dude. :o

-dave

[Wulffy]

I may be mistaken, but I think that there is info in the Catania Island Thread that is directly related to using the TinyTrak to modulate an audio carrier with GPS data. That thread is here: http://www.rcgroups.com/forums/showthread.php?t=631757

Good luck!

-t

[RENATOA]

Does any of the above works with modern 5fps 9600 baud GPS?
I don't dare to ask 38400 :)

Thank you