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