[Ground-station] Ballon launch - lightweight inexpensive 10GHz transmit

[Michelle Thompson]

There was a talk at GRCon16 that this discussion reminded me of.

https://www.gnuradio.org/wp-content/uploads/2017/12/Wil-Myrick-GPS-Beamforming-with-Low-Cost-RTL-SDRs.pdf
https://www.youtube.com/watch?v=lhZ2uMJNdjw

Would this approach help us out in any way?

It doesn't seem like we should be as limited as we are on synchronizing
receive chains. This is high grade ore stuff that we should try and figure
out. I'm familiar with rake receivers in CDMA.

We have been assuming a synthesized tone to help with LNB/low-cost SDR
setups, which can be considered, I believe, to be something like the signal
of opportunity.

There's another paper on synchronizing HackRFs, but it requires a hardware
modification. The thing that made me remember Mr. Myrick was no need to cut
up the RTL-SDRs.

Needing the Jetson is a big deal, though. Can it be done with something
much less expensive, today? I'll try and see what's happened with Mr.
Myrick since this talk and report back.

-Michelle W5NYV



On Tue, May 15, 2018 at 9:47 AM, Zach Leffke via Ground-Station
<ground-station at lists.openresearch.institute> wrote:
>
> responses in line below.....
>
> 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 5/14/2018 3:45 PM, Phil Karn via Ground-Station wrote:
>
> On 5/14/18 15:21, Zach Leffke via Ground-Station wrote:
>
> Quick reply....haven't ingested everything yet.
>
> For Transmit, 10 GHz: (not practical for balloon due to weight/cooling,
> but maybe as a reference, or at least inspiration)
>
> Is this inherent to the design? There's no way to build a smaller and
> lighter version of the transmitter? What's the mass?
>
> Pretty sure it mostly has to do with the aluminum case its mounted
into....possibly for thermal reasons.....
>
> Mass of the upconverter is 310g / ~0.68 lbs.
> Mass of the heatsink (that we used) is 800g / ~1.76 lbs.
>
> The heatsink could probably replaced by something 'smarter'.  The version
we used is pretty traditional style fin design, relying on air flow to move
the hot air away (not much air to move at altitude).  I've seen nifty
'radiator' designs for balloons that move heat to the 'non sun' side of the
payload to radiate the heat away towards 'colder' sky, but those were
pretty sophisticated systems that had yaw control.  The heatsink was also
intended for use with Kuhne's HPAs, so was probably overkill in this
application.  The upconverter doesn't get nearly as hot as the PAs do, so
something 'minimal' might be sufficient for a couple hours of
flight......or no heatsink and duty cycling if that fits the overall conop.
>
> I'm pretty sure someone on this list could come up with their own design
and fabricate it that is 'purpose built' for this type of thing and places
a premium on efficiency / weight......not me though.
>
> I'd be very interested in how the phasing of multiple LNBs plays out.
>
> I'm not up on what you/they are trying to do here, but are you
> considering locking all of the converters to a common frequency source
> and processing each feed independently with a SDR to do software beam
> forming and steering?
>
> I think this is the next major step for ham radio now that basic SDR
> technology has become widespread. I've been thinking of doing it on HF
> since even large directional HF antennas have relatively few elements
> compared to, say, 70cm. But if we can do it on X band, great.
>
> For me the problem with SDRs and phased array systems seems to be a limit
on the number of synchronized receive chains.  I have a complete bias (that
I acknowledge and have accepted :-) ) towards Ettus products.  For me the
hurdle I run into with phasing things with their stuff is that at best you
can get 4 synchronized channels, at a cost of about $15k (X310 + two TwinRX
daughtercards), enough to experiment with monopulse designs (for receive
only).  They have a new product called the N310 (~$10k) that has 4 TX and 4
RX channels, but there is this 'nifty' feature of the UHD driver that
causes a random phase offset that has to be calibrated out each time you
change frequency or start a flowgraph (even if on the same frequency).  The
N310 is pretty new though, so there may be solutions that I just haven't
heard of yet.  For the X310 + dual TwinRX, I've 'heard tell' that the phase
issues have to be calibrated out only once (look up table style) and then
each time you restart the flowgraph you are good to go, but have yet to
play that game myself.
>
> I've heard of nifty ways to gang RTL-SDRs together by sharing the TCXO
from one to many....but again never tried it.
>
>
>
> -Zach
>
> 73, Phil
>
>
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