[Ground-station] Balloon Launch - experiments?
Zach Leffke
zleffke at vt.edu
Sun Apr 22 21:10:59 PDT 2018
Last note for today....I promise.
We've flown the Adafruit Ultimate GPS on multiple flights in the recent
past and they are good up to at least 100k ft. Breakout available
here: https://www.adafruit.com/product/746. It also has a 1 PPS
output. (don't forget the external antenna)
In our case we kept things super simple and married the GPS to 'gpsd'
and then wrote a simple little standalone python script that launched on
boot that logged the NMEA strings to a file. Allowed for 1 Hz
timestamped balloon positions.
As a byproduct......having the GPS based timestamp in a log that also
had the 'OS' time stamp (from the python 'datetime' module) allowed us
to figure out the time offset between the computer clock and true time
(OS time was wildy wrong without NTP or an RTC to keep it on track
through power cycles and when disconnected from internet). We're
experimenting with updating the computer clock automatically by tying
gpsd and ntpd together...along with the 1 PPS reference from the above
GPS.....but not there yet.
Above is relevant to reversing out the filename timestamps for the IQ
recordings (that would come from the OS and be wrong) so that you could
get the true UTC time for the recordings........
I'm done for tonight!
-Zach
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 11:57 PM, Zach Leffke via Ground-Station wrote:
>
> To Bruce's point, the ground based reference would eliminate the need
> for a freq reference onboard the payload. Would definitely save on
> SWaP that way.
>
> Would just have to make sure it has enough juice to be heard on the
> payload. Geometry is also a major factor......particularly with
> respect to antenna patterns. Depending on winds aloft, antenna
> patterns, antenna pointing on the balloon/ground, and expected travel
> of the payload, it might be worth having a few beacons along the
> expected path of the balloon to make sure at least one is always 'in
> view' of the payload that's nice and strong. Also, if planning to
> 'scan' the rx freq (maybe top/bottom halves of the 5 GHz uplink band?)
> then having multiple beacons on different freqs on the ground could be
> useful.
>
> Co-locating the beacons on chase vehicles (assumption is that folks
> are chasing, probably via APRS) might simplify things a bit. CW with
> callsigns to identify/differentiate between the beacons......and since
> the vehicles are moving, probably want ground recordings of the GPS
> coordinates of the beacons.
>
> Many 'dimensions' to the tradeoff.....but overall, a bit more
> complexity on the ground is probably worth the savings on SWaP on the
> payload. Or if SWaP and the parts are available, might as well use it
> to reduce complexity on the ground.
>
>
> Also, FAA requirements need to be checked by someone.....its been
> awhile since I've done one of these flights......I think I remember
> something about a 6 pound limit (there is a density requirement
> too...can't fly a 6 pound cannon ball). Also no cordage with greater
> than 50lb test. I believe there is a secondary cut down requirement
> as well (in case the balloon doesn't pop and you hit equilibrium).
>
>
> Fun Stuff!
>
> -Zach
>
> 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 11:17 PM, Michelle Thompson wrote:
>> I'll be working at Kerry Banke's tomorrow on 10GHz and will run it by
>> him as well.
>>
>> We have almost all the elements on hand, including an odroid with GNU
>> Radio.
>>
>> I can print an antenna if needed.
>>
>> -mdt
>>
>> On Sun, Apr 22, 2018, 20:12 Bruce Perens via Ground-Station
>> <ground-station at lists.openresearch.institute> wrote:
>>
>> I have been using Odroid rather than Raspberry Pi cor just that
>> reason. Pi has poor IO bandwidth.
>>
>> If frequency precision is why you'd like an oxco, would a ground
>> beacon that is recorded provide enough?
>>
>>
>> On Sun, Apr 22, 2018, 19:53 Zach Leffke via Ground-Station
>> <ground-station at lists.openresearch.institute> wrote:
>>
>> 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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