Turn off AGC when receiving BPSK-1000
I forgot to offer some advice when receiving the ARISSat-1 BPSK-1000 telemetry beacon: turn off your receiver AGC if at all possible. If you can only choose between fast and slow, pick slow. If this causes a large variation in audio level, reduce the gain to avoid clipping on the peaks. A sound card A/D is 16 bits so you have plenty of dynamic range; don't be afraid to use it.
Ideally the background noise level should be constant with the signal going up and down.
This greatly helps the demodulator and decoder to distinguish signal from noise. The error correction uses the Viterbi algorithm, and one of its big features is the ability to distinguish between "strong" and "weak" bits; a strong '1' or '0' is considered less likely to be in error than a weak '1' or '0'. The decoder can even accept "I don't know" for a limited number of bits.
The decoder can still fix errors in strong bits. But it can fix more of them in the weak bits and still more in the "I don't knows" (known technically as "erasures").
This is especially important when the signal fades deeply, as it often does with ARISSat-1. With the AGC off, the audio signal level falls during a fade and the decoder can recognize it as a burst of erasures or near-erasures.
As with many questions in life, "I don't know" or "I think it's X but I'm not sure" are better answers than being sure of the wrong answer.
73, Phil
Hi Phil, KA9Q
Since modern VHF/UHF receivers do not supply AGC switching ON/OFF I use and old HF Drake R-4C receiver tuned from 28 to 30 MHz with a 144/146 MHz receiving converter in front of it having a 28/30 MHz IF
In general the old HF receivers supply AGC slow-medium-fast and OFF
The above setup is usefull as well for Noise Figure measurements.
If a old HF receiver and a 2 meters converter are not available it is possible to use a modern receiver reducing the RF gain belove the AGC threshold level.
73" de
i8CVS Domenico
----- Original Message ----- From: "Phil Karn" [email protected] To: "Amsat - BBs" [email protected] Sent: Tuesday, August 16, 2011 11:31 PM Subject: [amsat-bb] Turn off AGC when receiving BPSK-1000
I forgot to offer some advice when receiving the ARISSat-1 BPSK-1000 telemetry beacon: turn off your receiver AGC if at all possible. If you can only choose between fast and slow, pick slow. If this causes a large variation in audio level, reduce the gain to avoid clipping on the peaks. A sound card A/D is 16 bits so you have plenty of dynamic range; don't be afraid to use it.
Ideally the background noise level should be constant with the signal going up and down.
This greatly helps the demodulator and decoder to distinguish signal from noise. The error correction uses the Viterbi algorithm, and one of its big features is the ability to distinguish between "strong" and "weak" bits; a strong '1' or '0' is considered less likely to be in error than a weak '1' or '0'. The decoder can even accept "I don't know" for a limited number of bits.
The decoder can still fix errors in strong bits. But it can fix more of them in the weak bits and still more in the "I don't knows" (known technically as "erasures").
This is especially important when the signal fades deeply, as it often does with ARISSat-1. With the AGC off, the audio signal level falls during a fade and the decoder can recognize it as a burst of erasures or near-erasures.
As with many questions in life, "I don't know" or "I think it's X but I'm not sure" are better answers than being sure of the wrong answer.
73, Phil _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Hi Phil,
This is a great reminder. Thus far, my data has been collected with the TS-2000x using a "mid" setting for AGC; and with the HDSDR software set to "AGC Med" when using/playing back Funcube Dongle data. I've set both to OFF now, since it's possible
I wonder if anybody has experimented with AGC settings and the HDSDR decoding? Might be worth running a recording through a few times at various settings..hrm.
In any event, Phil...THANK YOU for making this code real. I have seen it print data when the signal was visibly "in the dirt" which is impressive and fun to see.
73!
Mark N8MH
At 02:31 PM 8/16/2011 -0700, Phil Karn wrote:
I forgot to offer some advice when receiving the ARISSat-1 BPSK-1000 telemetry beacon: turn off your receiver AGC if at all possible. If you can only choose between fast and slow, pick slow. If this causes a large variation in audio level, reduce the gain to avoid clipping on the peaks. A sound card A/D is 16 bits so you have plenty of dynamic range; don't be afraid to use it.
Ideally the background noise level should be constant with the signal going up and down.
This greatly helps the demodulator and decoder to distinguish signal from noise. The error correction uses the Viterbi algorithm, and one of its big features is the ability to distinguish between "strong" and "weak" bits; a strong '1' or '0' is considered less likely to be in error than a weak '1' or '0'. The decoder can even accept "I don't know" for a limited number of bits.
The decoder can still fix errors in strong bits. But it can fix more of them in the weak bits and still more in the "I don't knows" (known technically as "erasures").
This is especially important when the signal fades deeply, as it often does with ARISSat-1. With the AGC off, the audio signal level falls during a fade and the decoder can recognize it as a burst of erasures or near-erasures.
As with many questions in life, "I don't know" or "I think it's X but I'm not sure" are better answers than being sure of the wrong answer.
73, Phil _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Hi Mark, Phil,
That's interesting. I have collected all my passes on the TS2000 with the AGC on and set to the longest setting. This is mainly because I often record the signal level every 0.5 seconds during a pass which requires the AGC to be on and the longest setting irons out any short fades. I will turn the AGC off and look at the statistics over a few passes but given that with the AGC on I get almost all the available frames I don't expect to see much difference. It will be interesting to see.
73 Alan ZL2BX
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Mark L. Hammond Sent: Wednesday, 17 August 2011 20:36 To: Phil Karn; Amsat - BBs Subject: [amsat-bb] Re: Turn off AGC when receiving BPSK-1000
Hi Phil,
This is a great reminder. Thus far, my data has been collected with the TS-2000x using a "mid" setting for AGC; and with the HDSDR software set to "AGC Med" when using/playing back Funcube Dongle data. I've set both to OFF now, since it's possible
I wonder if anybody has experimented with AGC settings and the HDSDR decoding? Might be worth running a recording through a few times at various settings..hrm.
In any event, Phil...THANK YOU for making this code real. I have seen it print data when the signal was visibly "in the dirt" which is impressive and fun to see.
73!
Mark N8MH
At 02:31 PM 8/16/2011 -0700, Phil Karn wrote:
I forgot to offer some advice when receiving the ARISSat-1 BPSK-1000 telemetry beacon: turn off your receiver AGC if at all possible. If you can only choose between fast and slow, pick slow. If this causes a large variation in audio level, reduce the gain to avoid clipping on the peaks. A sound card A/D is 16 bits so you have plenty of dynamic range; don't be afraid to use it.
Ideally the background noise level should be constant with the signal going up and down.
This greatly helps the demodulator and decoder to distinguish signal from noise. The error correction uses the Viterbi algorithm, and one of its big features is the ability to distinguish between "strong" and "weak" bits; a strong '1' or '0' is considered less likely to be in error than a weak '1' or '0'. The decoder can even accept "I don't know" for a limited number of bits.
The decoder can still fix errors in strong bits. But it can fix more of them in the weak bits and still more in the "I don't knows" (known technically as "erasures").
This is especially important when the signal fades deeply, as it often does with ARISSat-1. With the AGC off, the audio signal level falls during a fade and the decoder can recognize it as a burst of erasures or near-erasures.
As with many questions in life, "I don't know" or "I think it's X but I'm not sure" are better answers than being sure of the wrong answer.
73, Phil _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
_______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb ----- No virus found in this message. Checked by AVG - www.avg.com Version: 10.0.1392 / Virus Database: 1520/3841 - Release Date: 08/17/11
I have no option to turn off AGC with the FT-736R. I had been running on the Medium setting (my default) and have switched to Slow now. I'm not sure to say because it's rather subjective just watching, but I think it has slightly improved capture especially at AOS and LOS as well as when I have de-sensing due to my other VHF stations so rudely beaconing their APRS and WL2K info without regard to the science going on! ;-)
Jerry N0JY
On 8/17/2011 6:24 PM, Alan Cresswell wrote:
Hi Mark, Phil,
That's interesting. I have collected all my passes on the TS2000 with the AGC on and set to the longest setting. This is mainly because I often record the signal level every 0.5 seconds during a pass which requires the AGC to be on and the longest setting irons out any short fades. I will turn the AGC off and look at the statistics over a few passes but given that with the AGC on I get almost all the available frames I don't expect to see much difference. It will be interesting to see.
73 Alan ZL2BX
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On 8/17/11 4:24 PM, Alan Cresswell wrote:
That's interesting. I have collected all my passes on the TS2000 with the AGC on and set to the longest setting. This is mainly because I often record the signal level every 0.5 seconds during a pass which requires the AGC to be on and the longest setting irons out any short fades.
The main thing is that the gain remain constant, or nearly so, during each fade so that the random series of 0's and 1's produced during the fade on thermal noise is mostly seen as 'weak' 0's and 'weak' 1's. Increasing the gain during a fade makes those random bits seem stronger and more certain when they're still just random bits. That can make it harder for the Viterbi decoder to correct them as errors.
A Viterbi decoder works much like a network routing algorithm that looks for the cheapest path to a destination. It finds the best path through a 'trellis', a pattern of links corresponding to all possible state transitions in the convolutional encoder that produced the transmitted signal. Out of every possible path the decoder finds the one that most closely matches the received sequence and declares it as the one that was most likely sent. It can still be wrong, and when it does it usually emits a burst of several dozen errors until the decoder gets back on the right path. In BPSK-1000, this error burst causes the HDLC decoder to abort the current frame or discard it with a CRC error.
The Viterbi decoder tallies up the 'cost' of each link in a path to find its total 'path cost'. If a particular link assumes that a '0' was sent, then receiving a strong '0' is the best possible match so that results in the lowest possible cost for that link. A weak '0' gives a greater cost, a weak '1' an even greater cost, and a strong '1' gives the highest possible cost.
Even if one link in a path has a high cost, the complete path will still be chosen as the winner if all the other paths are worse. When this happens, typically all the other links in the path will closely match the received sequence.
So you really want to avoid classifying bits as 'strong' when you know the signal is gone and the bits can't possibly be right. That means not boosting the gain (and the noise level) during a fade. The noise level should be kept constant.
-Phil
On 8/17/11 1:36 PM, Mark L. Hammond wrote:
In any event, Phil...THANK YOU for making this code real. I have seen it print data when the signal was visibly "in the dirt" which is impressive and fun to see.
You're most welcome. It was a lot of work mainly because there were so many options in the design that were hard to nail down. Nobody really knew how fast the fading would be, or how long the data frames should or would be, or how fast the telemetry values would change, or the frequency and phase response of all the SSB receivers people would use to receive the telemetry, or the accuracy of the A/D clocks in their computers, or the type and speed of their CPUs, or the experience level of the operators and whether they'd used automatic or manual tuning.
I did know that there would be fading, probably very deep. I knew the orbit would be that of the ISS so I knew the passes would be short with fierce Doppler. I knew that the beacon would operate in a broadcast mode so latency wasn't a major concern.
So I went for a conservative, robust design. I didn't try to maximize the data rate or minimize latency as I might have on a Pacsat being used for interactive and store-and-forward user communications. Those things might have made the signal much less robust especially by impairing its fade resistance.
I think my emphasis on fade resistance has definitely paid off, but I'm less happy with its tolerance (or lack thereof) of various frequency errors, from Doppler correction to off-frequency A/D clocks in computer sound interfaces. But that's one of the reason we fly these things, to get that kind of experience for the next time.
Another thing this mission (and many previous amateur satellites) shows is that the one thing we really, really need on our small satellites is a good attitude determination and control system. One would make *so* many problems just go away:
We could mount microwave antennas on a nadir-facing surface and provide consistent, predictable, strong, wideband signals to ground stations during a pass.
We could mount our solar panels on rotating booms to track the sun and generate far more power from a given number of (very expensive) cells than we now get by hedging our bets and covering every outside surface.
We could predict and control spacecraft heat flows and temperatures far more easily.
We'd know where our cameras are pointing and we could take pictures of predetermined targets.
The problem consists of two parts: attitude determination and attitude control. For determination I keep thinking that we should be able to do a lot with small, light and inexpensive CCD cameras. With proper light baffling it should be possible to see stars even in the daytime, and onboard software with a star chart could figure out which they are.
For attitude control, I think control moment gyros are the way to go. (They're somewhat different from momentum wheels in that they operate at constant speed.) This is largely a mechanical problem: designing flywheels and motors that are small, lightweight, can store a lot of angular momentum, draw minimal power, and be precisely moved around to control the direction and magnitude of the overall spacecraft angular momentum vector. We'd still need magnetorquing coils to dump excess momentum, but the cm gyros would provide quick and accurate control of spacecraft attitude.
-Phil
--- On Thu, 18/8/11, Phil Karn [email protected] wrote:
For attitude control, I think control moment gyros are the way to go. (They're somewhat different from momentum wheels in that they operate at constant speed.) This is largely a mechanical problem: designing flywheels and motors that are small, lightweight, can store a lot of angular momentum, draw minimal power, and be precisely moved around to control the direction and magnitude of the overall spacecraft angular momentum vector. We'd still need magnetorquing coils to dump excess momentum, but the cm gyros would provide quick and accurate control of spacecraft attitude.
There is discussion of the means of stabilsation and the use of 2.4 and 5 GHz on a future satellite in the Positioning and Experiments sections of the Forum at http://tinyurl.com/RadioSkaf
73 Trevor M5AKA
[Using attitude control]...
We could mount microwave antennas on a nadir-facing surface and provide consistent, predictable, strong, wideband signals to ground stations during a pass.
The problem with LEO satellites is that a nadir facing antenna does give great gain directly overhead ground stations, but only for about the center 2 minutes of only the one direct overhead pass a day. The problem with facing antennas "down" on a LEO satellite is that 90% of the users are not "under it", but to the side of it.
For example, lets say that we put relatively high gain antennas facing down giving about a 45 degree antenna pattern (say around 10 dB). Now looking at the total time that ARISSat is above 45 degrees, turns out to be about 3 minutes a day or less than 10% of all the time it is in view to any one ground station.
But as you say, it is a great advantage if the objective is to provide a 2 minute comm. window to anyone on the planet once a day, then such a design does give as much as 16 dB or so advantage over an omni antenna on a satellite.
Anyway, just a thought. Bob, WB4APR
From what I have read that is published, the DOD Colony II 3u cubesats are 3 axis controlled and pointable to within a degree or something close to that. As Bob pointed out this isn't a big help for antenna pointing for multiuser LEO sats, but when you read the power production the Colony II sats have, you'll see the application in a HEO orbit. I believe a 3u in HEO with this ability would be an affordable, usuable satellite for AMSAT to launch if we could duplicate the Colony sats abilities.
73, Drew KO4MA/p at the corner of No and Where, Oklahoma
Sent from my iPhone
On Aug 18, 2011, at 7:47 AM, "Bob Bruninga" [email protected] wrote:
[Using attitude control]...
We could mount microwave antennas on a nadir-facing surface and provide consistent, predictable, strong, wideband signals to ground stations during a pass.
The problem with LEO satellites is that a nadir facing antenna does give great gain directly overhead ground stations, but only for about the center 2 minutes of only the one direct overhead pass a day. The problem with facing antennas "down" on a LEO satellite is that 90% of the users are not "under it", but to the side of it.
For example, lets say that we put relatively high gain antennas facing down giving about a 45 degree antenna pattern (say around 10 dB). Now looking at the total time that ARISSat is above 45 degrees, turns out to be about 3 minutes a day or less than 10% of all the time it is in view to any one ground station.
But as you say, it is a great advantage if the objective is to provide a 2 minute comm. window to anyone on the planet once a day, then such a design does give as much as 16 dB or so advantage over an omni antenna on a satellite.
Anyway, just a thought. Bob, WB4APR
Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
--- On Thu, 18/8/11, Andrew Glasbrenner [email protected] wrote:
published, the DOD Colony II 3u cubesats are 3 axis controlled and pointable to within a degree or something close to that.
UK company Clyde Space also doing work in this area, see
CubeSat High Resolution Camera http://www.southgatearc.org/news/august2011/cubesat_high_resolution_camera.h...
73 Trevor M5AKA
Op 18-8-2011 16:08, Andrew Glasbrenner schreef:
From what I have read that is published, the DOD Colony II 3u cubesats are 3 axis controlled and pointable to within a degree or something close to that. As Bob pointed out this isn't a big help for antenna pointing for multiuser LEO sats, but when you read the power production the Colony II sats have, you'll see the application in a HEO orbit. I believe a 3u in HEO with this ability would be an affordable, usuable satellite for AMSAT to launch if we could duplicate the Colony sats abilities.
73, Drew KO4MA/p at the corner of No and Where, Oklahoma
Sent from my iPhone
On Aug 18, 2011, at 7:47 AM, "Bob Bruninga"[email protected] wrote:
[Using attitude control]...
We could mount microwave antennas on a nadir-facing surface and provide consistent, predictable, strong, wideband signals to ground stations during a pass.
The problem with LEO satellites is that a nadir facing antenna does give great gain directly overhead ground stations, but only for about the center 2 minutes of only the one direct overhead pass a day. The problem with facing antennas "down" on a LEO satellite is that 90% of the users are not "under it", but to the side of it.
For example, lets say that we put relatively high gain antennas facing down giving about a 45 degree antenna pattern (say around 10 dB). Now looking at the total time that ARISSat is above 45 degrees, turns out to be about 3 minutes a day or less than 10% of all the time it is in view to any one ground station.
But as you say, it is a great advantage if the objective is to provide a 2 minute comm. window to anyone on the planet once a day, then such a design does give as much as 16 dB or so advantage over an omni antenna on a satellite.
Anyway, just a thought. Bob, WB4APR
Has anyone ever considered piggybacking on a geosat in a simular way the rs11 to 13 did? Sure you would need a very good lobbyist to get a ride with a multi milion dollar broadcast sat and it would only see one continent but it will give that continent 24/7 coverage, using 3 cm as downlink it would give pleny bandwith for all kinds of experiments and if you can use the host sat's powerbus there would also be no power restrictions.
73 Andre PE1RDW
Hello Andre,
Has anyone ever considered piggybacking on a geosat in a simular way the rs11 to 13 did? Sure you would need a very good lobbyist to get a ride with a multi milion dollar broadcast sat and it would only see one continent but it will give that continent 24/7 coverage, using 3 cm as downlink it would give pleny bandwith for all kinds of experiments and if you can use the host sat's powerbus there would also be no power restrictions.
Yeah, we still have dreams! At various times it had been called AMSAT-Eagle, Phase IV Lite, C-C Rider, and other things. You'll notice from the dates on these papers how long we've had this dream of a millions dollar rideshare with a millions dollar satellite ...
http://www.arrl.org/news/amsat-announces-plans-for-2008 http://home.comcast.net/~k9jkm/Education_AMSAT_Eagle.pdf http://home.comcast.net/~k9jkm/CQVHF_Eagle_ACP_Emcomm.pdf
Microwave downlinks were initially proposed here ... http://www.cnssys.com/files/amsat/cc_amsat.pdf http://www.cnssys.com/files/amsat/cc-revisited.pdf http://www.cnssys.com/files/amsat/SDX_and_Future_AMSAT_Missions.pdf
-- 73 de JoAnne K9JKM [email protected]
On 8/18/11 11:03 AM, JoAnne Maenpaa wrote:
Yeah, we still have dreams! At various times it had been called AMSAT-Eagle, Phase IV Lite, C-C Rider, and other things. You'll notice from the dates on these papers how long we've had this dream of a millions dollar rideshare with a millions dollar satellite ...
Yes, and the fact that none of these piggyback payloads have ever come to fruition, while we do continue to get the occasional ad-hoc small satellite deployment opportunity, suggests that we need our own attitude determination and control system if at all possible.
The most likely opportunity to piggyback a payload on a controlled platform with its own power supply would be the ISS itself. Although they've already got plenty of comm systems, one might pitch this as yet another backup comm system. It wouldn't be terribly hard to network the ground stations so that a conversation could be maintained as the ISS moves from one to the next. To simplify the implementation, provide good voice quality and a backup data capability, the system would have to be completely digital.
-Phil
fact that none of these piggyback payloads have ever come to fruition,
Yeah, it brings us back to the original question about "why don't you guys propose a satellite that hooks on a bigger satellite?" ... we can't say we haven't tried ... Synchronous Amateur Radio Transponder (SYNCART) was an AMSAT rideshare was proposed in 1971: http://www.amsat-dl.org/images/stories/satellites/syncart/sync.pdf
suggests that we need our own attitude determination and control system if at all possible.
This topic came up a couple of times at the 2009 Baltimore Symposium. Tom Clark, K3IO gave a talk about the physics we need to know about if we put a motor on our satellite to raise the orbit ourselves. Dan Schultz, N8FGV presented on electric thrust for small satellites. In both cases attitude determination and control was needed because we had to point the right direction before turning on the thrust. The bonus would be we would know where antenna(s) were pointing.
It wouldn't be terribly hard to network the ground stations so that a conversation could be maintained as the ISS moves from one to the next.
It could be like a handover during a cellular call? I proposed the ground stations could be tied together with a GEO-Eagle class bird, while the ISS was the "mobile": http://home.comcast.net/~k9jkm/Education_AMSAT_Eagle.pdf Lacking an Eagle we could substitute less glamorous but nicely functional terrestrial networks ;-)
To simplify the implementation, provide good voice quality and a backup data capability, the system would have to be completely digital.
Tim, AB0DO wrote a paper on this very topic: http://www.saloits.com/papers/AMSAT2008.pdf
Heck, with all these papers written about it ... now we just gotta build one!
-- 73 de JoAnne K9JKM [email protected]
On 8/18/2011 10:38 PM, Phil Karn wrote:
On 8/18/11 11:03 AM, JoAnne Maenpaa wrote:
Yeah, we still have dreams! At various times it had been called AMSAT-Eagle, Phase IV Lite, C-C Rider, and other things. You'll notice from the dates on these papers how long we've had this dream of a millions dollar rideshare with a millions dollar satellite ...
Yes, and the fact that none of these piggyback payloads have ever come to fruition, while we do continue to get the occasional ad-hoc small satellite deployment opportunity, suggests that we need our own attitude determination and control system if at all possible.
The most likely opportunity to piggyback a payload on a controlled platform with its own power supply would be the ISS itself. Although they've already got plenty of comm systems, one might pitch this as yet another backup comm system. It wouldn't be terribly hard to network the ground stations so that a conversation could be maintained as the ISS moves from one to the next. To simplify the implementation, provide good voice quality and a backup data capability, the system would have to be completely digital.
What kind of digital are you suggesting? Voice and data both? A digital path from anywhere on the planet to the appropriate ground station is easily doable with some "documentation" of the ground stations.
Gregg Wonderly
What kind of digital are you suggesting? Voice and data both?
The Eagle project at one point proposed an AMSAT Advanced Communications Package (ACP) microwave, digital-uplink (5650 MHz) and digital downlink (3400 MHz).
Realizing that microwave earth station design is beyond the scope of most hams the Eagle team proposed making an ACP-capable earth station within reach of most radio amateurs that was thought it could be distributed along the lines of the project kits offered by the Tucson Amateur Packet Radio group (TAPR).
Radio link budgets and on-air protocols were being studied so the average earth station could have used 24 inch diameter dishes similar to home satellite television. Well, that was a goal anyways.
This Advanced Communications Package has been called 'Project Namaste' and 'Microwave Engineering Project' over the years. There was a proposal that a portion of the transponder would be reserved for analog operations so there was some chance the gear you already had would be useful, perhaps with Transverter or receive converter. You would need the ACP package to take advantage of all the advanced features of the network.
The Doppler shift at microwave frequencies would be manageable on HEO or GEO. It would be quite a challenge on something moving across your range like the ISS does.
A digital path from anywhere on the planet to the appropriate ground station is easily doable with some "documentation" of the ground stations.
This was what was proposed back in 2008 at the Atlanta Symposium: http://www.saloits.com/papers/AMSAT2008.pdf
-- 73 de JoAnne K9JKM [email protected]
On 8/19/11 7:51 AM, Gregg Wonderly wrote:
What kind of digital are you suggesting? Voice and data both? A digital path from anywhere on the planet to the appropriate ground station is easily doable with some "documentation" of the ground stations.
Digital voice would be the easiest to support since the data rate is so modest. Low rate data (< 100 kb/s) wouldn't be much harder. All it takes is a stabilized platform with microwave antennas. Any ground station with an Internet connection could automatically link with the ISS and relay it to a central point (e.g., Houston) and then hand it off to the next ground station. One advantage we hams have always had over NASA itself are our numbers and geographical distribution. We obviously wouldn't be able to cover the large parts of the earth that are entirely water but we could still do a pretty good job with the rest.
What kind of codec makes the most sense to you? We have things like D-Star that have existing hardware (the codec exists and is documented). Many really seem to find it unusable since they have to pay for it. I find it odd that their time to reinvent the wheel is somehow free.
Are there any other answers, such as the GSM codec? Echolink uses that, and thus a path out of an echolink client to the ISS could be direct. I have a Java version of the echolink client that I wrote quite a few years back that could be used to investigate digital voice with other software codecs.
It would seem wise for the RF modulation scheme to have a reasonable FEC to try and minimize retransmission. What kinds of modulation schemes would be easy to put on board the ISS and potentially other craft that could be 100% hardware based to minimize the "moving" parts? For example are there any existing "FPGA" kind of device based SDR kits with "digital data modulation"? I've seen quite a few that are based on complete programs running on Windows or other OSes. We'll need something in hardened hardware I'd think.
Thoughts?
Gregg Wonderly
On 8/20/2011 9:10 PM, Phil Karn wrote:
On 8/19/11 7:51 AM, Gregg Wonderly wrote:
What kind of digital are you suggesting? Voice and data both? A digital path from anywhere on the planet to the appropriate ground station is easily doable with some "documentation" of the ground stations.
Digital voice would be the easiest to support since the data rate is so modest. Low rate data (< 100 kb/s) wouldn't be much harder. All it takes is a stabilized platform with microwave antennas. Any ground station with an Internet connection could automatically link with the ISS and relay it to a central point (e.g., Houston) and then hand it off to the next ground station. One advantage we hams have always had over NASA itself are our numbers and geographical distribution. We obviously wouldn't be able to cover the large parts of the earth that are entirely water but we could still do a pretty good job with the rest.
On 8/18/11 5:47 AM, Bob Bruninga wrote:
The problem with LEO satellites is that a nadir facing antenna does give great gain directly overhead ground stations, but only for about the center 2 minutes of only the one direct overhead pass a day. The problem with facing antennas "down" on a LEO satellite is that 90% of the users are not "under it", but to the side of it.
You don't have to use a narrow beam antenna with maximum gain on axis. You can always design it with a bowl-shaped pattern that increases gain toward the edges and lowers it in the middle.
The GPS L-band transmitting antennas are a good example; even at their much higher altitudes, the earth appears fairly large and a simple lobe would provide uneven illumination across the earth.
A stabilized satellite pointing an antenna at a fixed angle to the local vertical would eliminate the deep, slow, periodic fading that we see at present due to constant changes in polarization and/or the antenna lobes sweeping past earth. Without these fades, coherent demodulation would be practical, picking up several dB of performance even before you consider the considerable increase in average signal strength. On a microwave band with sufficient bandwidth, efficient coding would make possible much higher user data rates and/or operate with considerably less DC power. Data rates of several megabits/sec would be entirely practical from a small satellite to a small dish on the ground. That means HDTV in near real time, or a steady stream of high resolution still photographs.
-Phil
You don't have to use a narrow beam antenna with maximum gain on axis. You can always design it with a bowl-shaped pattern that increases gain toward the edges and lowers it in the middle.
That's why the quadrifilars work so well. I measured the pattern some time back, and the "beam width" is about 140 degrees....
Point them straight up and be satisfied with that part of a pass, or mount them at 40 degrees, and rotate AZ with a TV rotor for the entire pass. For 50 degree max passes point them at the center of the pass...
73, Dave, WB6LLO [email protected]
Disagree: I learn....
Pulling for P3E...
participants (12)
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Alan Cresswell -
Andre v Schaijk -
Andrew Glasbrenner -
Bob Bruninga -
Dave Guimont -
Gregg Wonderly -
i8cvs -
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Mark L. Hammond -
N0JY -
Phil Karn -
Trevor .