Many thanks to each of you who have replied to my inquiry.
VE7LDH: I am curious what got you to thinking about Doppler frequency tracking on the BPSK signals. Did you consider a specific approach on how? Had it occurred to you that it might have application beyond BPSK or AO-16? If you would rather not post the reply, you may contact me directly.
KD6OZH: Your reference to pilot carrier technique allowed me to find some interesting web reading, mostly IEEE and patent stuff. QEX had nothing of interest and the AMSAT journal does not seem to be available prior to 1995. Pilot carriers seem to have been proven to be workable, but not advantageous in the long run. I do not have the expertise to demonstrate, but I suspect that a pilot carrier required a minimum power to acquire and maintain lock on the signal and that the more modulation power being sent, the more pilot carrier power needed to maintain the lock. I will comment more below, but with a Costas Loop, the more power in the modulation information, the easier it is to acquire and maintain lock. As a result, a pilot carrier consumes power without benefit, while at the same time, as you suggest, accurate satellite data allows the shift to be predicted. Tuning the radio becomes something that can be learned by the "handy" or built into the radio. From my limited experience it seems intimidating and more than a touch expensive. Nothing suggests that DSB will work, or that it might become popular if it did work.
If DSB had ever been considered, it seems to have been cast aside pretty quickly. That may have been due to the double bandwidth of DSB. I was somewhat puzzled that AO-16 had used DSB downlink. I can only assume that the weight penalty for an SSB transmitter was traded for what was intended to be single channel BPSK. If the downlink design had been multiple BPSK, it would have required a more powerful transmitter and DSB bandwidth would have been a bigger problem.
KD6BD and ZL1TYF: I am glad that I sparked an exchange of ideas.
My first thought concerning CTCSS is that it shouldn't be necessary. The time for the Costas Loop to lock should be no worse than the time for a VOX circuit to key a transmitter. That is an opinion based on faith in the concept and the building skills of someone much better than I. But I suggest that it would do no harm. At least no more harm to the equipment than someone using very long AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHs! between words. The Costas loop would be able to maintain lock. It fact it seems to me that the level of CTCSS should be specifically set low, to barely maintain lock. The engergy to acquire the signal should come from speech, particularly intelligent speech rather than someone whistling into the mike.
As for a standardized tone, and again this comes from my personal bias that a tone should not be required to make the system work, I fail to see the purpose of standardizing the tone. The Costas Loop will not care what tone is used. It could even be a BPSK8 signal at 50 Hz with your callsign ID and latitude / longitude co-ordinates. I would not suggest using a 1KHz tone unless you expect everyone listening to have a sharp filter, or just listen to it. Adjust the tone for "minimum lock maintenance signal strength with no audio" to reduce the annoyance.
You asked about multiple overlaping uplinks. I am not sure that I understand. If we are talking about AO-16 with an FM uplink, is this really a problem? If this was a translating repeater ( sorry if I used the wrong term, I go back before OSCAR, but I am pretty new to this stuff. It took me several hours to find a clear reference to TCA. Once I did it was obvious that is the point that has zero doppler shift and maximum doppler rate... ) then two signals in the uplink passband would result in two signals in the downlink. With an FM receiver providing audio to a DSB transmitter, you are going to hear what the FM receiver locks onto. If that is one strong signal, or the interference between two weak signals that cannot maintain lock, you are going to get one signal to downlink. Here is a slightly ficticious example: I live in Kansas and tune my FM radio to a frequency unused in the local area, but that is used in both Denver and Dallas. Most of the time, I don't hear any music from the radio. If a meteor streaks over Cheyenne, I hear Denver until the meteor trail fades. If a meteor comes in over Austin, I hear Dallas. If the meteor comes overhead, I hear the overlap of Denver and Dallas which is probably unintelligible.
Using the measured downlink Doppler shift to derive the proper uplink shift. By theory, the shift is proportional to frequency. So multiply the downlink shift by the ratio of the uplink frequency to the downlink frequency. For AO-16, it is a waste of effort since the uplink is FM. If on the other hand you had a DSB uplink that you were monitoring to correct your shift through a translating repeater, you would a much more complex situation that would depend not only on the uplink and downlink frequencies, but the inverting or non-inverting type of repeater... The control function math is getting me confused so I think you would need to break the loop somewhere to prevent instability or oscillation. What my years of flight control development tell me is to design the system from the ground up with each transmitter fixed and just compensate at each receiver.
If AO-16 is manifesting a residual "pilot carrier" it may be an indication of an imbalance in what I can only presume is a double balanced mixer. On the other hand if the uplink FM receiver does not have the functionality of a squelch, there may be a "real" DSB downlink of the audio received by the uplink. The difference would be obvious to a synchronous detector.
Thank you again for your input.
James
n5gui
James,
Thanks for your email, which I have not included below, to save bandwidth.
You make a number of interesting points, and I might write more at a later time.
Regarding your point that there was little need to have good Doppler control of the uplink, as its FM:
In general terms, this is largely true, and for a 2m uplink, little or no tuning is required on AO-51/Echo etc. However, a number of earlier emails spoke of the more critical need for AO-16 as it appears to have tight filters compared to a typical FM uplink. Careful control of Doppler for the uplink would allow a more full use of the entire filter bandwidth, rather than using FM-Narrow, and having weak audio.
I still like the idea of attempting CTCSS, so may try this myself at some point. I have the sometimes great advantage that sat passes here may have only zero, one or two stations on, so it becomes possible to test or try things that might be considered unacceptable usage or sharing of the resource in many other locations around the world. If the sat makes a pass to my East, I can be close to 100% sure I would be the only station, so could test mostly anything I wanted that would not be harmful to the sat. (The downside is that its also impossible to make a contact too.)
Not related to AO-16, but something I intended to test on FO-29 at some point, was "two-station Doppler correction". Its not something I got around to, but the plan was to get the exact locations of both myself and the other station, and feed both sets of data into the Doppler corrections. The result could be that the other station would use a FIXED uplink and FIXED downlink for the entire pass, and the other station would do both. This would be ideal for someone to have their "first go" at working a contact via an SSB passband sat. HOWEVER, the signals would walk all over the passband on the sat, so would be totally unacceptable except for short tests, and when very few stations are in the footprint of the bird, as is mostly the case here in New Zealand. Sadly, its too late for FO-29, but it is still an idea I'd like to test. Again, I must repeat: This is NOT for use in busy parts of the world, were many stations are in the footprint, as it creates a more extreme use of the bandwidth.
Regards, Jim, ZL1TYF Wellington New Zealand
-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of James Whitfield Sent: Wednesday, 6 February 2008 1:58 p.m. To: amsat-bb@amsat.org Subject: [amsat-bb] Re: Automatic doppler tracking of DSB
Many thanks to each of you who have replied to my inquiry.
<<< Contents Removed >>>
---------- From: "Jim Towler"
In general terms, this is largely true, and for a 2m uplink, little or no tuning is required on AO-51/Echo etc. However, a number of earlier emails spoke of the more critical need for AO-16 as it appears to have tight filters compared to a typical FM uplink. Careful control of Doppler for the uplink would allow a more full use of the entire filter bandwidth, rather than using FM-Narrow, and having weak audio.
Hmmmmm. I would like to claim that I am a victim of the news article about the test mode for AO-16 on the ARRL website, but that would be admitting that I am gullable enough to believe the FM uplink did not need Doppler shift compensation. That it works at all on a system that was intended to be digital is a blessing.
I still like the idea of attempting CTCSS....
I think it is a great idea. I can imagine a comparison of three modes using it. The first is to use CTCSS in the uplink to model what was previously tried with SSB and pilot carrier. That is of course on the assumption that the pilot carrier did not need to be zero beat with the SSB signal. If you can find out about their experiements you might find suggested frequency and power levels to use.
The second mode would be to use the Costas Loop to compensate for Doppler shift and synchronously detect the DSB signal. That would give you comparison of the effectiveness of the Doppler tracking and maybe compare the noise benefit of the synchronous detector.
The third mode would be Costas Loop without CTCSS to find out if it really helps, and how much. As I indicated previously, I am guessing that the system will acquire lock on voice power peaks, so it may not require much CTCSS power to maintain the system lock. It may well be that the power required to maintain lock depends on the rate of change in the Doppler shift. Said another way, it may require more power to hold the signal at the time of closest approach than when the Bird is near the horizon, comming or going.
Not related to AO-16, but something I intended to test on FO-29 at some point, was "two-station Doppler correction"....
It is an interesting set of control systems, one for transmit and an independent one for receive. The transmit system would seem to be open loop and based on the known position of the other station and path of the Bird. The receive system could be either open or closed loop, perhaps both with the ability for the operator to select the "better" channel. It might also be possible to feed the receive side closed loop parameters into the transmit system. I will have to think a little more, but there does not seem to be any way to close the transmit loop.
Best Wishes
James n5gui
Success!!!!
Well at least sort of. I received an informative message from PE4WJ, Wouter Jan Ubbels, who is part of the team working on Delfi-C3, which is scheduled to launch in April. As part of that project he has made available a sound card based program, called Warbler, to demodulate the 1200 baud telemetry downlink. It uses a Costas Loop to demodulate and for Doppler correction. He suggests that the program could be expanded, or at least adapted for voice DSB signals. While he is busy with Delfi-C3, he seems interested in our discussions. Perhaps we can come up with some ideas that are worth trying to add to future versions of Warbler.
I had originally thought that the idea should have been considered before, and I am pleased to know that it is actually in use. The fact that it is in use by the digital "community" but I could not seem to find any reference to it, is a yet another example of a communications problem that has spanned my three decade engineering career: different technical fields have such specialized languages that the members of each cannot recognize when they are talking about similar things. I should have recognized that I needed better understanding of just why AO-16 has a DSB transmitter. It makes a lot more sense after I read an article by G3RUH called "the shape of bits to come" which Wouter suggested. I still have a lot to learn, but I am starting to comprehend the following quote from the article:
Binary data DSB modulating an RF carrier" and "binary phase shift keying (BPSK)" are exactly the same thing.
What I might add to try to clarify the statement for those, like me, for whom it was not immediately obvious is that "Binary Data" in this context comes in the form of +1 and -1 instead of 1 and 0 . For some of us it makes the difference between BPSK and OOK.
If I am understanding Warbler, it brings in the 1200 baud BPSK, which "looks" like DSB with 2400 Hz bandwidth, into a sound card. The signal is tuned so that the "carrier" would be between 1400 and 1800 Hz. Once the Costas Loop is locked, it will then compensate for 400 Hz of Doppler shift. If the "carrier" drops below 1400 or above 1800, the extremes of DSB signal would get attenuated by the passband of an SSB receiver, which is assumed to be used to pick up the signals from satellite.
Modifications for voice DSB, would start by the use of a wider receiver, perhaps an AM mode instead of SSB. Assume for the moment that we have a receiver audio bandwidth of 10 KHz. If the audio used to modulate the DSB transmitter is limited to 3 KHz, then the DSB signal will be 6 KHz. Set the Costas Loop to track "carrier" from 3.3 KHz to 7.0 KHz then you get a 3700 Hz Doppler correction range. The sound card would then process the upper and lower sidebands into the original speech which would then be produced by the sound card. You could whittle that down some by communication quality audio trimmed to 2.7 or even 2.5 KHz. The Doppler range increases to 4300 and 4700 Hz. A wider receiver band would also be a benefit, limited by the high frequency limit of the sound card. And of course a wider receiver lets in more noise. The additional noise suggests digital processing.
Wouter also suggested that software defined radio could be used instead of the scheme outlined above. SDR is the concept that I had in mind. Perhaps a simple system that has the I and Q channels fed into the L and R sound card inputs, Costas Loop and additonal processing in the computer, sound card or serial output to control the VCO. The bandwidth would be limited by the sound card audio inputs, but the Doppler tracking correction would then be limited only by the range of the VCO and its update rate.
I think this is worth the time to experiment. The situation with AO-16 certainly makes it easy to get voice DSB signals on the downlink. One only needs FM transmissions. It could be used on a translating repeater, but I would suggest that it would be courtesy to not try it on a crowded area. If DSB gets a bad reputation for "hogging" limited bandwidth, it will never get fair consideration for current or future satellites. DSB does not use as much spectrum as FM, and I hope that it can demonstrate a benefit for Doppler compensation. It has yet to prove its value the amateur satellite community. I think it will be fun to try.
I certainly hope that I have presented some ideas that encourage thinking, learning, and experimentation.
Best Wishes.
James n5gui
The reason that people looked at SSB with a pilot carrier is that DSB has a 3 dB penalty compared to SSB.
73,
John KD6OZH
----- Original Message ----- From: "James Whitfield" n5gui@cox.net To: amsat-bb@amsat.org Sent: Wednesday, February 06, 2008 00:57 UTC Subject: [amsat-bb] Re: Automatic doppler tracking of DSB
Many thanks to each of you who have replied to my inquiry.
VE7LDH: I am curious what got you to thinking about Doppler frequency tracking on the BPSK signals. Did you consider a specific approach on how? Had it occurred to you that it might have application beyond BPSK or AO-16? If you would rather not post the reply, you may contact me directly.
KD6OZH: Your reference to pilot carrier technique allowed me to find some interesting web reading, mostly IEEE and patent stuff. QEX had nothing of interest and the AMSAT journal does not seem to be available prior to 1995. Pilot carriers seem to have been proven to be workable, but not advantageous in the long run. I do not have the expertise to demonstrate, but I suspect that a pilot carrier required a minimum power to acquire and maintain lock on the signal and that the more modulation power being sent, the more pilot carrier power needed to maintain the lock. I will comment more below, but with a Costas Loop, the more power in the modulation information, the easier it is to acquire and maintain lock. As a result, a pilot carrier consumes power without benefit, while at the same time, as you suggest, accurate satellite data allows the shift to be predicted. Tuning the radio becomes something that can be learned by the "handy" or built into the radio. From my limit! ed experience it seems intimidating and more than a touch expensive. Nothing suggests that DSB will work, or that it might become popular if it did work.
If DSB had ever been considered, it seems to have been cast aside pretty quickly. That may have been due to the double bandwidth of DSB. I was somewhat puzzled that AO-16 had used DSB downlink. I can only assume that the weight penalty for an SSB transmitter was traded for what was intended to be single channel BPSK. If the downlink design had been multiple BPSK, it would have required a more powerful transmitter and DSB bandwidth would have been a bigger problem.
KD6BD and ZL1TYF: I am glad that I sparked an exchange of ideas.
My first thought concerning CTCSS is that it shouldn't be necessary. The time for the Costas Loop to lock should be no worse than the time for a VOX circuit to key a transmitter. That is an opinion based on faith in the concept and the building skills of someone much better than I. But I suggest that it would do no harm. At least no more harm to the equipment than someone using very long AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHs! between words. The Costas loop would be able to maintain lock. It fact it seems to me that the level of CTCSS should be specifically set low, to barely maintain lock. The engergy to acquire the signal should come from speech, particularly intelligent speech rather than someone whistling into the mike.
As for a standardized tone, and again this comes from my personal bias that a tone should not be required to make the system work, I fail to see the purpose of standardizing the tone. The Costas Loop will not care what tone is used. It could even be a BPSK8 signal at 50 Hz with your callsign ID and latitude / longitude co-ordinates. I would not suggest using a 1KHz tone unless you expect everyone listening to have a sharp filter, or just listen to it. Adjust the tone for "minimum lock maintenance signal strength with no audio" to reduce the annoyance.
You asked about multiple overlaping uplinks. I am not sure that I understand. If we are talking about AO-16 with an FM uplink, is this really a problem? If this was a translating repeater ( sorry if I used the wrong term, I go back before OSCAR, but I am pretty new to this stuff. It took me several hours to find a clear reference to TCA. Once I did it was obvious that is the point that has zero doppler shift and maximum doppler rate... ) then two signals in the uplink passband would result in two signals in the downlink. With an FM receiver providing audio to a DSB transmitter, you are going to hear what the FM receiver locks onto. If that is one strong signal, or the interference between two weak signals that cannot maintain lock, you are going to get one signal to downlink. Here is a slightly ficticious example: I live in Kansas and tune my FM radio to a frequency unused in the local area, but that is used in both Denver and Dallas. Most of the time, I don't hear! any music from the radio. If a meteor streaks over Cheyenne, I hear Denver until the meteor trail fades. If a meteor comes in over Austin, I hear Dallas. If the meteor comes overhead, I hear the overlap of Denver and Dallas which is probably unintelligible.
Using the measured downlink Doppler shift to derive the proper uplink shift. By theory, the shift is proportional to frequency. So multiply the downlink shift by the ratio of the uplink frequency to the downlink frequency. For AO-16, it is a waste of effort since the uplink is FM. If on the other hand you had a DSB uplink that you were monitoring to correct your shift through a translating repeater, you would a much more complex situation that would depend not only on the uplink and downlink frequencies, but the inverting or non-inverting type of repeater... The control function math is getting me confused so I think you would need to break the loop somewhere to prevent instability or oscillation. What my years of flight control development tell me is to design the system from the ground up with each transmitter fixed and just compensate at each receiver.
If AO-16 is manifesting a residual "pilot carrier" it may be an indication of an imbalance in what I can only presume is a double balanced mixer. On the other hand if the uplink FM receiver does not have the functionality of a squelch, there may be a "real" DSB downlink of the audio received by the uplink. The difference would be obvious to a synchronous detector.
Thank you again for your input.
James
n5gui
Sent via AMSAT-BB@amsat.org. 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
James N5GUI wrote:
I am curious what got you to thinking about Doppler frequency tracking on the BPSK signals. Did you consider a specific approach on how? Had it occurred to you that it might have application beyond BPSK or AO-16?
I was thinking of a particular application, and a particular way of solving a particular problem. In particular (!), I was thinking of how I might make a very simple, generally idiot- proof way of receiving AO-16. No megabucks on a 736 and a computer, which is how I was doing it at school. The signal is BPSK. Use it, rather than the clunky open-loop tracking AMSAT usually uses. BPSK has a certain elegance, and I wanted a more elegant solution.
Of course such things have wider applications. I'm not completely stupid...
Along with this was the notion of a phased array antenna that would place its main lobe at the optimal angle for receiving satellites. The amount of time the satellites spend directly overhead is small, while signal levels at the horizon are low. Optimum is somewhere in between.
Laura Halliday VE7LDH Que les nuages soient notre Grid: CN89mg pied a terre... ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte _________________________________________________________________
participants (4)
-
James Whitfield -
Jim Towler -
John B. Stephensen -
laura halliday