[Ground-station] Balloon Launch - experiments?

[Phil Karn]

By the way, one of the pieces we'd need for a microwave balloon
experiment is already in ka9q-radio. The 'packet' module monitors a PCM
audio stream, looking for and decoding any AX.25 frames sent as 1200 bps
AFSK/FM.

Decoded frames are multicast back out, where they are picked up by the
'aprs' module. It looks for APRS frames from a specific station (direct
or digipeated), extracts position reports, and computes the azimuth,
elevation and range to that location from a specified point.

The idea is to automatically track the balloon during flight. Despite
its appallingly bad power efficiency, AFSK/FM is usually receivable with
omnidirectional antennas over line of sight balloon paths with
transmitter power of a few hundred mW (we typically use 300 mW
Radiometrix modules). So we'd know where it is and where to point our
antennas even without a working microwave downlink.

How much power and mass are we talking about for an airborne microwave
transmitter? Could there be more than one, pointed in different
directions? If you only need the RF to go in one direction, and if power
is a problem, you could use an IMU to determine orientation and switch
the transmitters on and off as needed.

In our experience, power is usually not a problem for 1.5 kg payloads on
2-3 hour flights. Lithium primary batteries can usually provide whatever
you need. In fact, heat buildup can be a factor if (as is often done)
you put everything in a styrofoam picnic cooler. A few years ago we flew
a WiFi transmitter carrying (among other things) a multicast digital
video stream. Right before launch we discovered that the high duty cycle
hammered all three of our GPSes. This was obviously intolerable, so I
was forced to kill the video feed.

The WiFi worked great for non-video data for the first 20 minutes or so
until the guy manually pointing the dish lost the balloon in the sun. We
never got it back, so at that point the WiFi transmitter became a space
heater. And it got pretty toasty in there despite the -40 to -50C
outdoor temperatures.

Some devices like GoPro cameras seem fine without any insulation (which
is good since styrofoam is opaque). I think this is because they're
pretty power hungry (they get noticeably warm). If a camera lost power
during flight it would probably freeze quickly.

Thermal design in a balloon payload is very complex. If anything, it's
probably harder than a satellite.

Phil

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