Sunday, April 11, 2010

GPS Tracking Beacon

In my previous post I showed the status of the air frame for our high altitude balloon (Serenity). I have now completed the air frame. I added the attachment points in the corners and porthole covers (including 1 camera porthole, 1 control panel, and 1 antenna boom).

I also finished the on-board GPS tracker. I've seen other groups use cell phones for this. While that certainly is the simplest and lightest solution (and possibly the cheapest if you can find good deals), I think it's inadequate. Mainly because cell phones stop working at high altitudes. Well, technically, they work but the cell towers can't figure out how to hand-off so the phone gets no signal. So you don't get real-time tracking. You have to wait until it comes back down and hope it descends in a coverage area. I've seen where some teams completely lose track of and never retrieve the payload. I'm not willing to take that chance.

Instead, I went with a standard HAM radio solution. This type of tracker has been in use for many years and seems to be the most reliable. Again, I'm following the advise and designs from L. Paul Verhage's Near Space ebook, along with additional personal research.

From the photo:

1) Garmin eTrex GPS receiver (ebay $43)
2) Byonics TinyTrak3 TNC ( $33)
3) Alinco DJ-S11 handheld 2M radio (ebay $35)
4) Audio beacon
5) Control panel
6) Serenity crew

The GPS receiver (1) was chosen for its ability to continue to work at high altitudes. Also, it's plentiful and easy to find on ebay. I actually bought 2. One for the balloon, the other for the chase vehicle.

The TinyTrak3 TNC (2) acts like a modem to translate and modulate the GPS serial signal into audio tones to be sent over the radio.

The handheld radio (3) will be tuned into 144.390 MHz, the standard North America APRS frequency. A HAM radio license is required to operate this radio and transmit on the APRS frequency. I just happened to get my license a few years ago. How convenient. (I also bought 2 of these. The second going in the chase vehicle. I'll cover the chase vehicle electronics and tracking equipment in another post.)

I did have to replace the retractable antenna on this radio. I can't have the antenna inside the airframe. The thermal blanket layers will block the radio signals to the ground. Also, most people agree that the antenna on this radio is poorly designed and inadequate. So I'll be mounting a 40 inch dipole antenna to the external boom.

The audio beacon (4) emits an ear piercing 2-tone beeping that's sure to be heard as we get closer to it. It's mainly a backup should the GPS beacon fail, or if we're close-by but just can't seem to find it.

The control panel (5) has power switches for the TinyTrak3 TNC and audio beacon as well as status lights for the TNC.

Yes, the crew (6) will be flying. Serenity just wouldn't be complete without her captain and crew. I wish the rest of the crew was available too, but only Mal, River, and Jayne are available in these small PVC figures.

Part 1: Take Me Out Into The Black
Part 3: More High Altitude Preparations
Part 4: Launch - Breaking Atmo

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