[Ground-station] Balloon Launch - experiments?

[Phil Karn]

On 4/20/18 10:07, Douglas Quagliana via Ground-Station wrote:
> I would like to suggest sending up a software defined radio that can
> recording on (and around) the proposed uplink frequencies.  It should
> sample at the highest sampling rate possible to capture the widest
> bandwidth possible into the recording. 

Let's see... what sample rate would fill a 256GB thumb drive during a
typical 2 hour balloon flight?

256e9 bytes / 4 bytes/sample = 64 gigasamples assuming 16 bit complex
samples. Over 2 hours that would be a sample rate of 64e9/7200 = 8.89
megasamples/sec. I.e., we could collect 8+ megahertz of spectrum,
depending on the anti-alias filters. You'd have to make sure that the
drive can write continuously at that rate. A SSD might be necessary.

Plenty for the 2m/70cm satellite subbands or for the entire 2m band, but
not the entire 70cm band. It would be fun going through these recordings
with my 'radio' program.

Most latex weather balloons maintain a remarkably constant ascent rate
of 1,000 fpm (5 m/s) and burst at 100,000-120,000' (30.5-36.5 km). Ergo
the ascent time would be 100-120 minutes or from 1 hr 40 minutes to 2
hr. The ascent is somewhat turbulent until the turbopause, then things
usually smooth out in the stratosphere. At altitude it's positively
serene until the balloon bursts. Then all hell breaks loose.

This assumes hydrogen. Pretty much everybody uses it now since helium is
getting scarce and bloody expensive. Treat it with respect and you'll
have no problems.

The descent by parachute usually takes about 30 minutes. Descent is VERY
quick at first because of the thinner air (1% of surface density at 32.6
km) but then slows as it descends into exponentially denser air.
Descents are pretty violent due to turbulence (especially if the remains
of the balloon are still attached) so it might not be worth collecting
data on the way down since you're seeing much the same area anyway. It's
easy to get sick watching HDTV of a descent on a large screen...

Phil


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