Nice thought provoking question, Joanne. Here's an off-the-cuff idea. I haven't tried to work out any of the numbers, it's just some idle musing.
It has occurred to me that we are in the midst of a minor revolution in amateur radio (which will undoubtedly be followed by a more major revolution). In the last 10 years, cheap computing and sound cards have caused a great deal of experimentation with "sound card modes" (the minor revolution), and will ultimately lead to the major revolution (widespread use of SDR). We have a kind of flexibility that we couldn't think of even a few years ago, a flexibility that we gain from Moore's law.
So here's an idea: let's do away with the need for Doppler correction entirely. It's not like we don't have decent orbital elements for the satellites that we use. It's not like our ground stations don't have accurate timing information available to them. Even if we didn't, we could still output a (coded?) carrier that our soundcard modem could lock onto, and then transmit relative to that frequency.
Given the relatively limited amount of power that we are likely to have in a cubesat, the question then becomes what is the best way to use that power? It seems unlikely that any kind of linear transponder will allow more than just a couple of users meaningful access. I'd suggest it might make more sense to do some kind of digital transponder. I'm imagining a satellite which monitors a chunk of spectrum roughly the size of a current SSB signal, say 2.4khz. Imagine that space was divided into (say) 10 channels, each 240hz wide. We could easily fit a PSK63 signal (or a similar FSK signal, pick your poison) in that space. You could use a bent pipe crossband transponder, or potentially do a simplex repeater (say the sat listens for 10 seconds, then re-echoes for 10 seconds) on the same uplink frequency. If you are a downlink station, you know what you sent, and can tell if your signal got collided with, and if so, you can switch to another of the 10 slots. In the mean time, you can easily monitor all of the other slots as well, and try to pick an unoccupied one. While it might be difficult for a power-efficient controller to actually _decode_ each of the 10 channels, it probably could determine which channels are busy itself by monitoring power in each of the channels. Maybe we can fill unused slots with telemetry? Or can we actually get enough DSP power into a cubesat to decode 10 channels of PSK (or some similar protocol), which would help a lot (the bird only transmits stuff, and what it transmits is free from noise/errors). If not 10 channels, then how 'bout 5? 2? Even one? Then, we basically have a simple digipeater, which can obviously be done, given the existance of 1200 baud modems based upon PIC microcontrollers.
Just some lunacy...
Mark KF6KYI
What I think would be a good cubesat is just an SSTV transmitter...
Robert WB5MZO
From: kf6kyi@gmail.com To: amsat-bb@amsat.org Date: Mon, 24 Nov 2008 20:56:44 -0800 Subject: [amsat-bb] bbsat ideas...
Nice thought provoking question, Joanne. Here's an off-the-cuff idea. I haven't tried to work out any of the numbers, it's just some idle musing.
It has occurred to me that we are in the midst of a minor revolution in amateur radio (which will undoubtedly be followed by a more major revolution). In the last 10 years, cheap computing and sound cards have caused a great deal of experimentation with "sound card modes" (the minor revolution), and will ultimately lead to the major revolution (widespread use of SDR). We have a kind of flexibility that we couldn't think of even a few years ago, a flexibility that we gain from Moore's law.
So here's an idea: let's do away with the need for Doppler correction entirely. It's not like we don't have decent orbital elements for the satellites that we use. It's not like our ground stations don't have accurate timing information available to them. Even if we didn't, we could still output a (coded?) carrier that our soundcard modem could lock onto, and then transmit relative to that frequency.
Given the relatively limited amount of power that we are likely to have in a cubesat, the question then becomes what is the best way to use that power? It seems unlikely that any kind of linear transponder will allow more than just a couple of users meaningful access. I'd suggest it might make more sense to do some kind of digital transponder. I'm imagining a satellite which monitors a chunk of spectrum roughly the size of a current SSB signal, say 2.4khz. Imagine that space was divided into (say) 10 channels, each 240hz wide. We could easily fit a PSK63 signal (or a similar FSK signal, pick your poison) in that space. You could use a bent pipe crossband transponder, or potentially do a simplex repeater (say the sat listens for 10 seconds, then re-echoes for 10 seconds) on the same uplink frequency. If you are a downlink station, you know what you sent, and can tell if your signal got collided with, and if so, you can switch to another of the 10 slots. In the mean time, you can easily monitor all of the other slots as well, and try to pick an unoccupied one. While it might be difficult for a power-efficient controller to actually _decode_ each of the 10 channels, it probably could determine which channels are busy itself by monitoring power in each of the channels. Maybe we can fill unused slots with telemetry? Or can we actually get enough DSP power into a cubesat to decode 10 channels of PSK (or some similar protocol), which would help a lot (the bird only transmits stuff, and what it transmits is free from noise/errors). If not 10 channels, then how 'bout 5? 2? Even one? Then, we basically have a simple digipeater, which can obviously be done, given the existance of 1200 baud modems based upon PIC microcontrollers.
Just some lunacy...
Mark KF6KYI
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I think an SSTV beacon sat would be really cool.
On Nov 24, 2008, at 11:20 PM, Rocky Jones orbitjet@hotmail.com wrote:
What I think would be a good cubesat is just an SSTV transmitter...
Robert WB5MZO
From: kf6kyi@gmail.com To: amsat-bb@amsat.org Date: Mon, 24 Nov 2008 20:56:44 -0800 Subject: [amsat-bb] bbsat ideas...
Nice thought provoking question, Joanne. Here's an off-the-cuff idea. I haven't tried to work out any of the numbers, it's just some idle musing.
It has occurred to me that we are in the midst of a minor revolution in amateur radio (which will undoubtedly be followed by a more major revolution). In the last 10 years, cheap computing and sound cards have caused a great deal of experimentation with "sound card modes" (the minor revolution), and will ultimately lead to the major revolution (widespread use of SDR). We have a kind of flexibility that we couldn't think of even a few years ago, a flexibility that we gain from Moore's law.
So here's an idea: let's do away with the need for Doppler correction entirely. It's not like we don't have decent orbital elements for the satellites that we use. It's not like our ground stations don't have accurate timing information available to them. Even if we didn't, we could still output a (coded?) carrier that our soundcard modem could lock onto, and then transmit relative to that frequency.
Given the relatively limited amount of power that we are likely to have in a cubesat, the question then becomes what is the best way to use that power? It seems unlikely that any kind of linear transponder will allow more than just a couple of users meaningful access. I'd suggest it might make more sense to do some kind of digital transponder. I'm imagining a satellite which monitors a chunk of spectrum roughly the size of a current SSB signal, say 2.4khz. Imagine that space was divided into (say) 10 channels, each 240hz wide. We could easily fit a PSK63 signal (or a similar FSK signal, pick your poison) in that space. You could use a bent pipe crossband transponder, or potentially do a simplex repeater (say the sat listens for 10 seconds, then re-echoes for 10 seconds) on the same uplink frequency. If you are a downlink station, you know what you sent, and can tell if your signal got collided with, and if so, you can switch to another of the 10 slots. In the mean time, you can easily monitor all of the other slots as well, and try to pick an unoccupied one. While it might be difficult for a power-efficient controller to actually _decode_ each of the 10 channels, it probably could determine which channels are busy itself by monitoring power in each of the channels. Maybe we can fill unused slots with telemetry? Or can we actually get enough DSP power into a cubesat to decode 10 channels of PSK (or some similar protocol), which would help a lot (the bird only transmits stuff, and what it transmits is free from noise/errors). If not 10 channels, then how 'bout 5? 2? Even one? Then, we basically have a simple digipeater, which can obviously be done, given the existance of 1200 baud modems based upon PIC microcontrollers.
Just some lunacy...
Mark KF6KYI
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I've heard seeds, but from what I remember, it has a predetermined picture that it broadcasts
On Nov 25, 2008, at 10:46 AM, "Andrew Glasbrenner" <glasbrenner@mindspring.com
wrote:
I think an SSTV beacon sat would be really cool.
Check out CO-66 aka SEEDS II
On Mon, 24 Nov 2008 20:56:44 -0800 Mark Vandewettering kf6kyi@gmail.com wrote:
... Given the relatively limited amount of power that we are likely to have in a cubesat, the question then becomes what is the best way to use that power? It seems unlikely that any kind of linear transponder will allow more than just a couple of users meaningful access. I'd suggest it might make more sense to do some kind of digital transponder. I'm imagining a satellite which monitors a chunk of spectrum roughly the size of a current SSB signal, say 2.4khz. Imagine that space was divided into (say) 10 channels, each 240hz wide. We could easily fit a PSK63 signal (or a similar FSK signal, pick your poison) in that space. You could use a bent pipe crossband transponder, or potentially do a simplex repeater (say the sat listens for 10 seconds, then re-echoes for 10 seconds) on the same uplink frequency. If you are a downlink station, you know what you sent, and can tell if your signal got collided with, and if so, you can switch to another of the 10 slots. In the mean time, you can easily monitor all of the other slots as well, and try to pick an unoccupied one. While it might be difficult for a power-efficient controller to actually _decode_ each of the 10 channels, it probably could determine which channels are busy itself by monitoring power in each of the channels. Maybe we can fill unused slots with telemetry? Or can we actually get enough DSP power into a cubesat to decode 10 channels of PSK (or some similar protocol), which would help a lot (the bird only transmits stuff, and what it transmits is free from noise/errors). If not 10 channels, then how 'bout 5? 2? Even one? Then, we basically have a simple digipeater, which can obviously be done, given the existance of 1200 baud modems based upon PIC microcontrollers.
Just some lunacy...
Mark KF6KYI
Mark, I fully support your lunacy. In fact, I was having something similar in mind when I first saw the SuperBrowser in DM780 by Simon Brown HB9DRV. So, half of the software is already there running on the desktop :-) Personally, I think it should be a full duplex real-time repeater (not just a bent pipe).
Moreover, if using SDR it would be easy to switch between various digital modes, i.e. one week PSK, next week SSTV and so on.
I'm really not worried about computing power. Already today you can get the processing power of a laptop in the size of a PDA.
If somebody wants to work on with this idea I'd love to get involved. I might even be able to help establish some launch opportunities in the 2011-2012 time frame.
73 Alex OZ9AEC
On Nov 25, 2008, at 11:39 AM, Alexandru Csete wrote:
On Mon, 24 Nov 2008 20:56:44 -0800 Mark Vandewettering kf6kyi@gmail.com wrote:
... Given the relatively limited amount of power that we are likely to have in a cubesat, the question then becomes what is the best way to use that power? It seems unlikely that any kind of linear transponder will allow more than just a couple of users meaningful access. I'd suggest it might make more sense to do some kind of digital transponder. I'm imagining a satellite which monitors a chunk of spectrum roughly the size of a current SSB signal, say 2.4khz. Imagine that space was divided into (say) 10 channels, each 240hz wide. We could easily fit a PSK63 signal (or a similar FSK signal, pick your poison) in that space. You could use a bent pipe crossband transponder, or potentially do a simplex repeater (say the sat listens for 10 seconds, then re-echoes for 10 seconds) on the same uplink frequency. If you are a downlink station, you know what you sent, and can tell if your signal got collided with, and if so, you can switch to another of the 10 slots. In the mean time, you can easily monitor all of the other slots as well, and try to pick an unoccupied one. While it might be difficult for a power-efficient controller to actually _decode_ each of the 10 channels, it probably could determine which channels are busy itself by monitoring power in each of the channels. Maybe we can fill unused slots with telemetry? Or can we actually get enough DSP power into a cubesat to decode 10 channels of PSK (or some similar protocol), which would help a lot (the bird only transmits stuff, and what it transmits is free from noise/errors). If not 10 channels, then how 'bout 5? 2? Even one? Then, we basically have a simple digipeater, which can obviously be done, given the existance of 1200 baud modems based upon PIC microcontrollers.
Just some lunacy...
Mark KF6KYI
Mark, I fully support your lunacy. In fact, I was having something similar in mind when I first saw the SuperBrowser in DM780 by Simon Brown HB9DRV. So, half of the software is already there running on the desktop :-) Personally, I think it should be a full duplex real-time repeater (not just a bent pipe).
Moreover, if using SDR it would be easy to switch between various digital modes, i.e. one week PSK, next week SSTV and so on.
I'm really not worried about computing power. Already today you can get the processing power of a laptop in the size of a PDA.
It's not the size that is worrisome to me. It's the power budget. The normal guideline for cubesats is 1W total power. Every milliwatt you use on DSP and the like is power you don't get to use for the downlink. It's not hard to chew up your budget with cpus. For instance, an ATMEGA168 (the chip used on the arduino) is 250 microamps at 1.7v and 1Mhz clock rate. It can go even lower in sleep mode. Yet these chips are actually quite capable of doing a reasonable amount of work. A dsPIC chip might go from a few milliwatts to nearly 100mw when churning out 20 MIPS. It's not clear to me how much compute power we need, but if we are in the 100mw range, we are spending 10% of our power budget.
(Note: I know nothing about the viability of any of these chips in the space environment. I picked a couple of chips that I know something about as typical examples of what we can get on the power/curve. I also tend to think in terms of software, since I'm a software engineer by training. Hence, software defined radios have some appeal. It might be more reasonable to consider a less computerized approach.)
If somebody wants to work on with this idea I'd love to get involved. I might even be able to help establish some launch opportunities in the 2011-2012 time frame.
73 Alex OZ9AEC
participants (5)
-
Alexandru Csete -
Andrew Glasbrenner -
Andrew Koenig -
Mark Vandewettering -
Rocky Jones