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That Miraculous Device: Onboard Radio
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Bunsen Offline
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Post: #11
RE: That Miraculous Device: Onboard Radio
(12-22-2014 09:38 AM)Newt Wrote:  As for the wavelength, even a quarter wave at 433Mhz is 17cm. Our longest side is 14cm. Is there a good reason to go with 433Mhz as opposed to say, 750Mhz, where a quarter wave equals 10cm?
Licensing. If this is to be an amateur-operated project (i.e. not being conducted for commercial purposes or as part of anyone's employment), then we can operate both the space and ground stations under ham radio licenses. Those only permit operation in certain bands, including 145.800-146.000 MHz, 435.000-438.000 MHz (exclusive to satellites), and 902-928 MHz (shared with other applications, but possibly usable). The measuring tape wrapped around the satellite has been used by a heck of a lot of cubesats now, and it seems to work well while being dirt cheap. 435 MHz is a nice middle ground between the awkwardly long antennas and high background noise of 146 MHz and the large number of consumer devices crowding 915 MHz.

Quote:Where are we getting these figures for communications windows? The ISS's orbit seems like roughly what we are guessing the sat's orbit to be like, but six windows, seven to ten minutes long, per day seems an unfounded assumption to me. Even with an ideally placed ground station, comm windows will be of varying lengths, ten minutes being the maximum of a spectrum that will range from zero to that figure. Based on this webpage, we maybe can expect three passes per day (at ten degrees over the horizon), depending on locations. Of course, if we have more stations, we have more windows, maybe that was what you were meaning.
From Ham Radio Deluxe's satellite tracker (a version from back when it was free). Notably, I'm at about 43° north latitude, which provides more frequent passes for an ISS-like orbit than a lower latitude would (the northern extreme of the ISS's 51.6° inclination orbit is within view here, so we get several orbits in a row with passes low over the northern horizon each day). I should have mentioned the impact of location earlier, sorry.

Quote:Could we compress images? Maybe send down half sized images, or compressed JPEG images first, and then request the ones that look most worthwhile, and more if we have more time, or if a particular sample starts doing something interesting later and we want to trace its history. Sending all the images would be best, but might be a bit redundant, and slow.
Yeah, I think "Needs more JPEG" is a critical part of the solution here, along with potentially reducing how often images are taken. Once per hour probably makes for a great timelapse video, but is once every two hours going to be much less informative? Dunno, but ground testing with the moss can find out. Storing more frequent images for later download is also helpful, particularly if we end up with an orbit that lasts a lot longer than the interesting part of the moss's life.

Quote:I brought up electrical inefficiency before and it seemed a bit dismissed, but I think that we will need to carefully look into how much heat will be generated by all the systems and how fast it might be radiated into space. It might be the case that all our radios and other electronics heat the thing up plenty, or more than that. But that probably belongs in a different thread too. For now I think 60% sounds consistent with what I have come across,and am happy to go with it before we get better information about exactly how the radio will work.
Thermal analysis is definitely required for a satellite with a temperature-sensitive payload. Heat transport is... less intuitive when you get rid of convection and just have conduction and radiation. It's also hard to do with any precision, especially without software that costs $OMG/license. The payload design is going to need a lot of attention, and nice wide performance margins, to keep the moss comfortable.
12-22-2014 11:00 AM
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MBobrik Offline
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Post: #12
RE: That Miraculous Device: Onboard Radio
Happy holidays to all.
Being essentially stuck with the camera till I can get a macro lens, I spent a little time playing with the excel sheet. Reducing sat power to 3 watt, adding some more signal losses, reducing ground station gain, adding some more reserve margin, and then reverse -engineering the sheet to get a max bitrate - angle over horizon relationship, and started playing with optimizing tracking station latitude to get maximum average bits per day and came to some astonishing numbers - at optimal 48.21 deg latitude, if we can vary the bitrate continuously as the sat approaches from very low to high, we can squeeze through as much as 30 megabytes (not bits !) per day.

[Image: CKyPfCQ.png]

EDIT:image size reduced by 2

Which is for our purposes far more than enough.The snag is of course we have to have a GMSK modulator and demodulator that can dial the bitrate and filter bandwidth continuously. Either that, or I was over-optimistic again in filling the sheet.

Bunsen could you send me your filled sheet so that I may compare the values and find where they differ ?
(This post was last modified: 12-27-2014 08:33 AM by MBobrik.)
12-26-2014 01:23 AM
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Newt Offline
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Post: #13
RE: That Miraculous Device: Onboard Radio
That image, is huge.

Otherwise, that makes sense if we could do it. Could you post your filled in sheet, too, MBobrik? varying the bitrate was something I had briefly looked into before but am reading more into now.

Thank you for the lengthy and well put together response, Bunsen. It seems more and more like getting around to doing a proper ground test will provide a good number of answers that we are currently just guessing at, and consequently not really knowing.
12-27-2014 07:46 AM
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MBobrik Offline
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Post: #14
RE: That Miraculous Device: Onboard Radio
My xls. But I've reverse engineered it, and in my code used to compute the above values and graphs, I used yet more pessimistic values. However because it was destructive reverse engineering I lost the exact numbers except the final constant in my code Smile
(This post was last modified: 12-27-2014 08:34 AM by MBobrik.)
12-27-2014 08:31 AM
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NERVAfan Offline
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Post: #15
RE: That Miraculous Device: Onboard Radio
(12-22-2014 11:00 AM)Bunsen Wrote:  
(12-22-2014 09:38 AM)Newt Wrote:  ]I brought up electrical inefficiency before and it seemed a bit dismissed, but I think that we will need to carefully look into how much heat will be generated by all the systems and how fast it might be radiated into space. It might be the case that all our radios and other electronics heat the thing up plenty, or more than that. But that probably belongs in a different thread too. For now I think 60% sounds consistent with what I have come across,and am happy to go with it before we get better information about exactly how the radio will work.
Thermal analysis is definitely required for a satellite with a temperature-sensitive payload. Heat transport is... less intuitive when you get rid of convection and just have conduction and radiation. It's also hard to do with any precision, especially without software that costs $OMG/license. The payload design is going to need a lot of attention, and nice wide performance margins, to keep the moss comfortable.

What should we do to start tackling the thermal problem? Is there any free software that is good for approximations?
12-27-2014 05:36 PM
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MBobrik Offline
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Post: #16
RE: That Miraculous Device: Onboard Radio
(12-27-2014 05:36 PM)NERVAfan Wrote:  What should we do to start tackling the thermal problem? Is there any free software that is good for approximations?
I think we should just keep the waste heat from the transmitter more or less equal to the heating power required, so that all we have to do during the transmission is to switch the heating off.
12-27-2014 09:28 PM
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Bunsen Offline
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Post: #17
RE: That Miraculous Device: Onboard Radio
(12-26-2014 01:23 AM)MBobrik Wrote:  Bunsen could you send me your filled sheet so that I may compare the values and find where they differ ?

Yep, here's mine: https://www.dropbox.com/s/5fumu7zc3wmlw5...H.xls?dl=0 (numbers may not be identical to what I had discussed before; I hadn't saved that copy and I probably made some different off-the-cuff guesstimates this time)

Don't bet on being able to adapt the link rate to changing conditions, at least not without some significant software work. Cell phones and wifi do that, but those technologies only look simple and cheap when they're being produced in billion-unit quantities. I could see some approaches for simplified versions, but getting the two ends coordinated is nontrivial. Even when the signal is strong enough to provide plenty of margin over average noise, there will be bursts of interference that knock out the odd packet here and there, which Murphy demands will happen during link speed negotiation.

You might be able to set something up where the ground station's receive preamp output could be split to, say, 4 cheap receivers each configured for a different bit rate. The ground station could request changes in downlink bit rate through a fixed-speed uplink, but it would keep receiving even if the speed change request fails. High link rates should automatically fall back to lower speeds after a timeout, unless periodic "keep going fast" packets are received from the ground. It still bumps the complexity up quite a bit, but mostly on the ground end.

As for thermal concerns, I think we'd be better off isolating the moss chamber as well as possible and using a low-power heater inside the thermally isolated section, then mounting the radio so as to provide good radiative cooling. I'd suggest placing the radio at the end of the satellite opposite the solar panels, so it's facing either cold, dark space or a moderately warm Earth. It would minimize the change in thermal characteristics during transmissions, and make for short wiring between the radio and antennas (you might even be able to mount the antennas straight on the radio PCB).

Trying to roll the radio's thermal dissipation into the normal heater power would mean dumping an enormous amount of energy as heat -- more than 400cm^2 of solar panels will provide on average, I suspect. Also, the heaters won't run all the time; they're needed in the dark, but the whole satellite will be doing its best to overheat in the sun. Since we won't get to pick the orbital alignment during the mission (some months you get nighttime comm windows, some months they're during the day), it has to be designed to allow running the radio during the hot part of an orbit.
12-28-2014 05:42 AM
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NERVAfan Offline
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Post: #18
RE: That Miraculous Device: Onboard Radio
(12-28-2014 05:42 AM)Bunsen Wrote:  As for thermal concerns, I think we'd be better off isolating the moss chamber as well as possible and using a low-power heater inside the thermally isolated section, then mounting the radio so as to provide good radiative cooling.


That sounds good, but we were talking on the KSP Forums thread about making the magnetorquers do double-duty as heaters for the moss. How much mass/volume would a small heater in the moss chamber take up?

I also would like to find out how much "free" space will be in the satellite to use for insulation around the moss chamber.
12-28-2014 08:29 AM
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MBobrik Offline
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Post: #19
RE: That Miraculous Device: Onboard Radio
(12-28-2014 05:42 AM)Bunsen Wrote:  Yep, here's mine:

So I re-ran the calculation with the numbers from your except using GMSK modulation and I restricted the variable bitrate to only 4 discrete values. The result is that we can download on average 25 megabytes of data per day, which is kinda impressive.

[Image: gcLdpT6.png]

The optimal latitude is 49.060 ° which is kinda impressive too because it is just a few km north of my home Smile.

(12-28-2014 05:42 AM)Bunsen Wrote:  You might be able to set something up where the ground station's receive preamp output could be split to, say, 4 cheap receivers each configured for a different bit rate. The ground station could request changes in downlink bit rate through a fixed-speed uplink, but it would keep receiving even if the speed change request fails. High link rates should automatically fall back to lower speeds after a timeout, unless periodic "keep going fast" packets are received from the ground. It still bumps the complexity up quite a bit, but mostly on the ground end.

seems reasonably robust and simple to me.

(12-28-2014 05:42 AM)Bunsen Wrote:  As for thermal concerns, I think we'd be better off isolating the moss chamber as well as possible and using a low-power heater inside the thermally isolated section, then mounting the radio so as to provide good radiative cooling. I'd suggest placing the radio at the end of the satellite opposite the solar panels, so it's facing either cold, dark space or a moderately warm Earth. It would minimize the change in thermal characteristics during transmissions, and make for short wiring between the radio and antennas (you might even be able to mount the antennas straight on the radio PCB).

That is surely a possibility if we don't go the big pressurized container route. We will however have then to heat the sample AND the batteries separately.

(12-28-2014 05:42 AM)Bunsen Wrote:  Trying to roll the radio's thermal dissipation into the normal heater power would mean dumping an enormous amount of energy as heat -- more than 400cm^2 of solar panels will provide on average, I suspect.

Standard cubesat panels provide 2.1 W each and we will have five of them.
So we will have 10.5 watt power, or, averaged over dark and light 5.25 watt.

(12-28-2014 05:42 AM)Bunsen Wrote:  Also, the heaters won't run all the time; they're needed in the dark, but the whole satellite will be doing its best to overheat in the sun.

My original idea was the entire sat to be thermally insulated so it would receive only minimal amount of heat even when exposed to the sun. Thus heating at slightly reduced power would be necessary permanently.
Of course having the whole sat heated increases power requirements, but on the other hand, any waste heat from sat's systems would be used as heating, and we would get the maximum thermal inertia to smooth out any surges.

We will of course have to do some sort of evaluation which is more practical, and I believe K^2 wanted to do some more detailed computations of heat management, but he seems to be full busy with something else, he didn't even register on this forum yet.
(This post was last modified: 12-28-2014 10:06 AM by MBobrik.)
12-28-2014 09:20 AM
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Kanojo Offline
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Post: #20
RE: That Miraculous Device: Onboard Radio
Concerning base station, I know (knew) some guys who worked at Esrange in Sweden. Esrange do sounding rockets but are mostly involved in satellite management for 3rd parties.
http://www.sscspace.com/esrange-space-center-3
A couple of years ago they "gave away" a stratospheric balloon flight, so with luck they might have some good advice or even point us to someone with a good base station antenna.

I don't know if these people still work there (it was 2 or 3 years ago I talked to them). This might just be a dead end, but I'll let you know what I find.
01-13-2015 06:01 AM
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