[amsat-bb] Community Survey Request -- crosslinks, multi-hop packet, and satellite DX

[Zach Leffke]

All,

Many thanks for the plethora of responses on and off list.  I think the 
simple answer that I was looking for is that generally yes there is 
interest from the community.

Below is my attempt to answer multiple questions/points from the 
numerous (and appreciated!) comments and questions (some overlap):

1.  Technically the universities involved are not competing, we are 
cooperating (weekly telecons, ICDs, etc. etc.).  Not 3 cubesats designed 
in a vacuum (hi hi) the magically working together.  Three designs that 
are done individually by the institutions, but with open lines of 
communication between the respective design teams and subsystem teams 
(turns out things are cheaper when you buy in bulk too!).  That being 
said, if you ever eavesdrop on those phone calls, sometimes it might 
sound like competition.......but not unexpected, good lesson in teamwork 
for the students (and faculty.....).

2.  combination of active and passive stabilization is *planned.*  I 
have yet to get the 'warm and fuzzy' feeling concerning the active 
stabilization bounced against the power budget.

3.  Concerning expected ranges..........good question, thats a bit what 
the experiment is all about.  We're hoping for a few tens of kilometers, 
100km is probably a stretch (OK so maybe not satellite DX per se).  One 
path I have the students going down is tradeoffs between crosslink radio 
'modes of operation.'  The RFM69HCW has power control, baud rate 
control, channel filter BW control, and RX gain control (which affects 
linearity and Noise Figure).  So maybe as things get further and 
further, FSK9600 becomes, FSK48000, then FSK2400, then FSK1200.  Maybe 
that is tied to a bit of power control where before we drop back in baud 
rate we have reached peak power output.  Likely not an 'automagic' 
decision made onboard, likely all three get commanded to the new modes 
of operation by ground.  We'll see..........which reminds me, the 
students owe me link budgets........

3.b  Also, as a super-rough back of the envelope, MO-76 links were 
closing with Arrow antennas on the ground when the slant range was on 
the order of around 800km.  Assume ~12 dB of gain from the arrow, and 
the loss of 6dB for doubling of slant range. So if we go from an Arrow 
to a 0 dBi antenna, we've cut our range by a quarter (two 6dB losses in 
the link) so looking at 200km. BUT MO-76 was low bit rate and various 
modulation schemes, some of which are very narrow (like CW).  So 
accounting for increased bandwidth, and polarization 
mismatches..........we'll be happy with a few 10s of km.

4.  #s 1, 2, and 3(b) are hinging in large part on simulations in AGI's 
STK.  First step for the undergrads:  learn how to use STK. Very complex 
program, very easy for garbage in garbage out situations.  But we have a 
good number of AOE types working on it.

5.  Polarization.  Looks like crossed dipoles all around at the moment, 
shared between command radio and crosslink radio. Definite problem.  
Especially when you consider the L/R flip depending on look angle.  So 
we're playing around with ideas about monopoles, dipoles, crossed 
dipoles, maybe one type on one bird, different type on another, etc. 
etc.  How does that tie into attitude control.  What about if we lose 
attitude control.  What is the balance between command radio 
requirements with the ground and crosslink requirements.  What about 
when the third bird deploys their drag brake and *speeds up* (one of 
those cool counter intuitive things about space, drag break slows them 
down, they drop altitude, which speeds them up relative to the other 
birds).  Lots of tradeoffs to consider.

6.  Ground networks.  Currently a lot of igates are out there monitoring 
UHF bands for ISS (since the 2m radio failure and switch to UHF).  Would 
be superrrrrrrr sweet if those stayed up and running to monitor the hops 
for this constellation.  However, it is likely going to be a year or 
more before the first launch opportunity (we turned down a chance for 
this August, way to soon for us).  We are still pretty early in the 
evolution of this project, so its possible ISS issues could be rectified 
(replacement installed), they switch back to 2m, and the UHF igates go 
bye bye.  So maybe there will be a follow up request in a year or so to 
knock the dust off those raspberry pis and RTL-SDRs and stick em back 
outside.  We'll see.....Fantastic idea....

7.  RFM69 is a cool little radio, but there are definitely better 
options for crosslink radios.  We settled on this one due to the MO-76 
flight heritage from the RFM22 family, the availability of interrupt 
outputs (6 different programmeable ones) that we could use for the time 
of flight measurements, as well as the ease of integration (C++ code and 
arduino sketches exist on github).  Our first idea (before we settled on 
packet) was inspired heavily by the AO-73/FO-29 (and now EO-79) 
crosslinks, so we were looking at what could be done with linear 
transponders (and more traditional ranging techniques).  Also back then 
we were considering PSK modulation formats for the digital data we 
needed to move around, along with heavy FEC (again AO-73 inspirations), 
but in the end we settled on the RFM69 (mainly once we found out 1U per 
bird, so the power budget got thrown out the window).  For future flight 
opportunities though (Assuming the massive success of this one and the 
crazy influx of cash that is sure to ensue for follow up missions, hi 
hi), we'll definitely throw all the options back on the table, AFSK, 
FSK, PSK, linears, etc.

8.  There are probably a few more comments/questions I'm forgetting to 
address, I apologize for that.  But again many thanks to all for the 
very positive feedback and great recommendations.


-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 4/2/2017 9:49 PM, Greg D wrote:
> Interesting proposal, with 3 universities "competing".  Images of Battle
> Bots in Space coming to mind...
>
> Given that UHF is less efficient for distance contacts (at least by the
> physics), and that you'll be running the linking radios at a low power
> level, what sort of distance between the satellites do you expect to
> maintain contact?  I'm presuming that the cubes are not going to be
> actively stabilized, and that the antennas will be more-or-less
> omnidirectional.
>
> Which brings up the last thought...  As the satellites spin, their
> respective antenna polarizations will rotate with them.  How will the
> satellites deal with cross polarization effects (20-ish dB loss)?  Given
> a random polarization between any two satellites, plus the 70cm path
> loss penalty, the chances of getting through all 3 birds once they
> separate could be vanishingly small, no?
>
> Still, we're getting quite a flury of reports recently of FunCube
> telemetry via FO-29, so it is certainly possible for this to work.
> Suggestion might be to figure out why FunCube to FO-29 is working, and
> make sure that is part of your design.
>
> Good luck!
>
> Greg  KO6TH
>
>
> Zach Leffke wrote:
>> Hello fellow satellite nuts!
>>
>> This email is to humbly request the opinions of those in the Amateur
>> Satellite Community about the idea of setting new satellite DX
>> records.  Nothing formal, you can email on list or off list as you
>> think appropriate.
>>
>> So here is the idea:  What if there were a constellation of three
>> satellites (3 1Us), built by students (undergrads) at three different
>> universities, but launched together and deployed from the same
>> deployer that had crosslink packet communications built into the
>> design.  Then, what if those birds were licensed via the Amateur Radio
>> route (instead of Experimental as is the norm for most University
>> birds, with some notable exceptions of course) allowing the entire
>> global amateur satellite community to use the crosslink capability in
>> an attempt to set new satellite DX records?
>>
>> Now, there is a primary science mission (pseudo-range determination as
>> the constellation separates, different mass and drag profiles for each
>> bird) so during the work week, science happens, but on the weekends,
>> the constellation is made available to the community for crosslink
>> packet comms.  Additionally, if your setup has enough G/T to monitor
>> the lower power crosslink comms, and with a little bit of technical
>> tweaking to your ground station (you would need GPS based PPS to
>> measure propagation time, maybe ~$100 invested), you could participate
>> in Space to Earth pseudo-range measurements that would contribute to a
>> secondary orbit determination goal.  To be clear, the up and down
>> linking for the satellite DX attempts would be standard FSK9600,
>> AX.25, so as long as your station can do that, you can use the
>> crosslink path for multi-hop comms.  Its only the S-to-E pseudo range
>> measurement that would require a bit of additional HW.
>>
>>
>> Does this sound interesting?  Would you as an operator be interested
>> in multi-hop satellite packet comms?  Do you think this type of
>> operating schedule is acceptable (weekends) to justify Amateur Radio
>> licensing instead of Experimental?  Would you be interested in
>> contributing to the science mission (S-to-E pseudo-range
>> measurement)?  Would you track the telemetry downlinks and forward
>> that data on to the mission data warehouse?
>>
>>
>> Any and all opinions, good or bad, are welcome.  Again, on or off list
>> as you see appropriate is fine with me.  My goal with this is to get a
>> sense of what the community's opinion on this topic is and if the
>> constellation were available for use, how much interest there would be
>> in this type of operation.
>>
>>
>> Thanks in Advance!
>>
>>
>> Sincerely,
>>
>> Zach, KJ4QLP
>>
>>