[Ground-station] Balloon Launch - experiments?

Zach Leffke zleffke at vt.edu
Sun Apr 22 21:45:39 PDT 2018 

What are you talking about Kent......you're a ham!

I'd wager a dollar or two that YOU can build a CP antenna at 5.66GHz!

hint hint........nudge nudge.........there's a particular 
need........and a member of the list with a particular capability.......

:-)


Assuming of course that we actually need CP on the payload....that was 
just a thought..........if not though and we are linear on the payload, 
and we have linear beacons on the ground......cross pol issues could 
arise with the changing geometry/attitude of the payload and we wouldn't 
know on the ground which way to 'twist' the beacon antenna......


Though that brings up another idea.....................

completely different idea.....slightly more advanced..........fly a B210 
with dual synchronized receive capability......each rx chain connected 
to its own linear antenna and the two antennas mounted 90 deg 
offset...........Each receive chain would dump IQ to its own capture 
file.  With a reference beacon on the ground that could be used to 
calibrate out the random UHD phase offset that happens each time the 
flowgraph is started.......we could get all 4 polarizations.  Each 
receive chain would dump IQ to its own capture file.........and with the 
phase offset calibrated out from the ground beacon........could combine 
the two signals in software with +/- 90 deg offset to generate RHCP/LHCP 
in post processing.

Net result.......would get half as much record time (or would need 
larger SSDs).....but would get 4 polarizations worth of collect / RF 
survey of the uplink band........

The antennas would have to be more than just a pair of dipoles though 
right?  Axial ratio would degrade off nadir vector assuming the 'crossed 
dipole' is pointed at nadir.  Same goes for an eggbeater.......so not 
sure what the right elements would be for this......maybe the inverse 
idea.......a quadrafilar helix on each rx chain......one RHCP/one 
LHCP.......combined in software to produce H and V?  I think the QFHA 
maintain better circularity at the 'horizon' (90 deg off nadir)......but 
then someone would have to build two CP antennas.........


That was really my last comment for the night.......fun stuff....but I 
need to go to bed!

-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 4/23/2018 12:21 AM, KENT BRITAIN wrote:
> Good luck on the circular polarization.
>
> I have tested nearly 2000 antennas at various conferences with several 
> hundred
> that were suppose to be Circular Polarized.   Only about 2 in 10 meet 
> the minimal
> definition 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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