[Ground-station] Balloon Launch - experiments?

[KENT BRITAIN]

Good luck on the circular polarization.
I have tested nearly 2000 antennas at various conferences with several hundredthat were suppose to be Circular Polarized.   Only about 2 in 10 meet the minimaldefinition of CP.    Hams can't build CP Antennas!!!!
Kent WA5VJB
PS  Only about 1 Helix in 10 were CP.   Just because it has a spiral doesn't make it CP.


      From: Zach Leffke via Ground-Station <ground-station at lists.openresearch.institute>
 To: Phil Karn <karn at ka9q.net>; ground-station at lists.openresearch.institute 
 Sent: Sunday, April 22, 2018 9:53 PM
 Subject: Re: [Ground-station] Balloon Launch - experiments?
   
 I like this idea......it has elements of a couple projects that I've worked on in the last year or so.
  I just did similar math from a slightly different angle...... 10 Msamp/sec complex capture (so 10 MHz of spectrum, using GNU Radio Complex datatype) at 64 bits per sample yields roughly 1.86 hours of record time on a 500 GB SSD......about perfect for this type of balloon flight.......go up, burst, come back down.  The recording could be triggered at a specific time or altitude to optimize the collect.
  I'd recommend the XU4 over the RPi as it has two USB 3.0 ports, one for a B200 mini, and one for an external USB3.0 SSD.  also its got a beefier processor and more RAM, better to keep up with the write speeds. A B200 mini is tiny and relatively cheap, though it would probably need a 10 MHz reference of some sort (could be a GPSDO, but would need to make sure the GPS works at altitude......standalone OCXO might be a better solution).  It also has a similar RF front end to the Astrod SDR (an ADXXXX whatever whatever, I can't remember off the top of my head, but same family at least).
  For the 2017 Eclipse experiment (recording satellite downlinks, looking for Scintillation effects due to the eclipse) I used a very similar setup to this and used a simple script to control the flowgraph recording....one minute on, one minute off.  I ended up with hours of 1 minute IQ recordings started at every odd minute.  Something like that could be used here, with the possibility of tweaking parameters between each recording (like stepping the USRP gain in 10dB increments each time, or possibly changing center freq).  Log files or the filename itself could be used to keep track of which IQ capture had what settings. Marc Franco has designed and I believe built some 5 GHz pre-amp prototypes for the uplink band...might be good to give them a test. Ground based 'reference beacons' might be useful/fun.  Maybe sending out CW at known center freqs at known power levels to give a reference to hunt for in the collects. Any ideas on antenna types?  patch array? horn? conical spiral? circular or linear (I vote circular on the balloon, pointed at Nadir)?
  
  This sounds like a perfect summer project for some of our undergrads!
  -Zach, KJ4QLP
  Research Associate
Aerospace Systems Lab
Ted & Karyn Hume Center for National Security & Technology
Virginia Polytechnic Institute & State University
Work Phone: 540-231-4174
Cell Phone: 540-808-6305 On 4/22/2018 1:20 AM, Phil Karn via Ground-Station wrote:
  
 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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