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<p>Last note for today....I promise.</p>
<p>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: <a class="moz-txt-link-freetext" href="https://www.adafruit.com/product/746">https://www.adafruit.com/product/746</a>. It also
has a 1 PPS output. (don't forget the external antenna)<br>
</p>
<p>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.</p>
<p>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.</p>
<p>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........</p>
<p><br>
</p>
<p>I'm done for tonight!<br>
</p>
<p>-Zach<br>
</p>
<pre class="moz-signature" cols="72">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</pre>
<div class="moz-cite-prefix">On 4/22/2018 11:57 PM, Zach Leffke via
Ground-Station wrote:<br>
</div>
<blockquote type="cite"
cite="mid:57a7a26b-1bba-10f5-59b4-6d6ce8d08959@vt.edu">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<p>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. <br>
</p>
<p>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.</p>
<p>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.<br>
</p>
<p>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.<br>
</p>
<p><br>
</p>
<p>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).</p>
<p><br>
</p>
<p>Fun Stuff!</p>
<p>-Zach<br>
</p>
<pre class="moz-signature" cols="72">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</pre>
<div class="moz-cite-prefix">On 4/22/2018 11:17 PM, Michelle
Thompson wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CACvjz2XP-Jv0n6Gznd7KJBUEYorB5ENozZj0Sec3JUV6CeeSmw@mail.gmail.com">I'll
be working at Kerry Banke's tomorrow on 10GHz and will run it by
him as well.
<div><br>
</div>
<div>We have almost all the elements on hand, including an
odroid with GNU Radio. </div>
<div><br>
</div>
<div>I can print an antenna if needed. </div>
<div><br>
</div>
<div>-mdt<br>
<br>
<div class="gmail_quote">
<div dir="ltr">On Sun, Apr 22, 2018, 20:12 Bruce Perens via
Ground-Station <a class="moz-txt-link-rfc2396E"
href="mailto:ground-station@lists.openresearch.institute"
moz-do-not-send="true"><ground-station@lists.openresearch.institute></a>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex"
id="gmail_block_quote0">I have been using Odroid rather
than Raspberry Pi cor just that reason. Pi has poor IO
bandwidth.
<div><br>
</div>
<div>If frequency precision is why you'd like an oxco,
would a ground beacon that is recorded provide enough?</div>
<div><br>
<br>
<div class="gmail_quote">
<div dir="ltr">On Sun, Apr 22, 2018, 19:53 Zach Leffke
via Ground-Station <a class="moz-txt-link-rfc2396E"
href="mailto:ground-station@lists.openresearch.institute"
moz-do-not-send="true"><ground-station@lists.openresearch.institute></a>
wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">
<div text="#000000" bgcolor="#FFFFFF">
<p>I like this idea......it has elements of a
couple projects that I've worked on in the last
year or so.<br>
</p>
<p>I just did similar math from a slightly
different angle......</p>
<p>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.<br>
</p>
<p>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.</p>
<p>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).<br>
</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>Any ideas on antenna types? patch array? horn?
conical spiral? circular or linear (I vote
circular on the balloon, pointed at Nadir)?<br>
</p>
<p><br>
</p>
<p>This sounds like a perfect summer project for
some of our undergrads!<br>
</p>
<p>-Zach, KJ4QLP<br>
</p>
<pre class="m_-8715101577330520922m_5315733742041589355moz-signature" cols="72">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</pre>
</div>
<div text="#000000" bgcolor="#FFFFFF">
<div
class="m_-8715101577330520922m_5315733742041589355moz-cite-prefix">On
4/22/2018 1:20 AM, Phil Karn via Ground-Station
wrote:<br>
</div>
<blockquote type="cite">
<pre>On 4/20/18 10:07, Douglas Quagliana via Ground-Station wrote:
</pre>
<blockquote type="cite">
<pre>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.
</pre>
</blockquote>
<pre>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
</pre>
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