I am trying to find out if there has been any development of a frequency tracking system to compensate for doppler shift, or at least find a person or organization with which to discuss concepts. I tried to contact AMSAT about this more than a week ago, but there has been no response. I have tried to contact AMSAT again, and decided to try this bulletin board as well.
It started when I read that AO-16 had been placed in a test mode with an FM uplink and DSB downlink. I remembered a diagram of a double sideband suppressed carrier ( DSB ) demodulation technique that used a variation of a phased locked loop to recover the frequency and phase of the original carrier. I realized that it should be able to track the "original carrier" through the doppler shift caused by the relative motion between the satellite and the receiving station.
I decided to find out more about the concept. I was not able to find any reference to the technique having been used with satellites, but I believe the system is called a Costas Receiver, or perhaps is refered to as a Costas Loop.
If someone knows if this was used for doppler frequency tracking, please let me know where I can get more information. If there is a person or organization that would be better suited to my inquiry, I would like to know that as well.
Thank you.
James Whitfield n5gui
James N5GUI asked about Doppler tracking of DSB signals...
I investigated this in the context of the original BPSK data downlink on AO-16, didn't get too far with it. No technical issues, but I started a Real Job (tm) shortly afterwards. :-(
If you think there is something interesting here, why don't you go ahead and develop it? You don't need AMSAT's permission to be creative!
Laura Halliday VE7LDH Que les nuages soient notre Grid: CN89mg pied a terre... ICBM: 49 16.05 N 122 56.92 W - Hospital/Shafte _________________________________________________________________
AMSAT has only one full-time employee who is non-technical and the technical volunteers are usually working full-time elsewhere so responses can take some time. I've seen a description of the Costas loop in many engineering texts but have never seen it used for recovery of a suppressed carrier on a satellite downlink. There was some research on using pilot carriers for Doppler correction of SSB signals published in either QEX or the AMSAT Journal in the 1980's but it never caught on. I think that it was just easier to use the Keplerian elements to predict Doppler shift and do the correction open loop. The loop bandwidth for the Costas loop would also have to be fairly narrow so it may not work very well during the middle of a pass for LEO satellite.
73,
John KD6OZH
----- Original Message ----- From: "James Whitfield" [email protected] To: [email protected] Sent: Friday, February 01, 2008 16:46 UTC Subject: [amsat-bb] Automatic doppler tracking of DSB
I am trying to find out if there has been any development of a frequency tracking system to compensate for doppler shift, or at least find a person or organization with which to discuss concepts. I tried to contact AMSAT about this more than a week ago, but there has been no response. I have tried to contact AMSAT again, and decided to try this bulletin board as well.
It started when I read that AO-16 had been placed in a test mode with an FM uplink and DSB downlink. I remembered a diagram of a double sideband suppressed carrier ( DSB ) demodulation technique that used a variation of a phased locked loop to recover the frequency and phase of the original carrier. I realized that it should be able to track the "original carrier" through the doppler shift caused by the relative motion between the satellite and the receiving station.
I decided to find out more about the concept. I was not able to find any reference to the technique having been used with satellites, but I believe the system is called a Costas Receiver, or perhaps is refered to as a Costas Loop.
If someone knows if this was used for doppler frequency tracking, please let me know where I can get more information. If there is a person or organization that would be better suited to my inquiry, I would like to know that as well.
Thank you.
James Whitfield n5gui _______________________________________________ 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
It started when I read that AO-16 had been placed in a test mode with an FM uplink and DSB downlink. I remembered a diagram of a double sideband suppressed carrier ( DSB ) demodulation technique that used a variation of a phased locked loop to recover the frequency and phase of the original carrier. I realized that it should be able to track the "original carrier" through the doppler shift caused by the relative motion between the satellite and the receiving station.
I think this can be made to work, although maybe not as well as you'd like.
Given a receiver passband wide enough for the full DSB bandwidth, plus some additional "wiggle room" to chase the signal if the demodulator should temporary fall out of lock, it probably can be done PROVIDED the modulation of the DSB signal never goes to zero for any extended period of time (especially around TCA). Since people will be taking turns using the transponder, that last requirement may be difficult to achieve.
The reason Doppler tracking of AO-16's digital transponder worked so well is because the downlink was ALWAYS modulated with BPSK, even if it was AX.25 NULLS.
Such is not the case with analog audio. :-(
73, de John, KD2BD
Visit John on the Web at:
http://kd2bd.ham.org/ . . . .
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Hi John,
What if someone turns on a CTCSS subtone for their FM uplink?
If set high enough, it might make it thru the audio filters, so could be tracked with a PLL at the receive end. (But not so high it's a problem for human ears :-)
1) Would this work?
2) Will it cause any problems to AO-16, by raising the duty-cycle of the DSB downlink? (Clearly this would be a bad idea if it might cause harm or unacceptable power-budget issues. Mind you, it can't be too much worse than the original BPSK can it?)
3) If this is a good idea, does we need a standard tone?
4) How would it cope with multiple overlapping uplinks?
If someone was able to automate the downlink Doppler correction, that can then feed right back into the uplink correction too.
It would be interesting to them compare the corrections against that calculated from the Keps.
I'd love to hear ideas re above, but most critical is to be sure it's not harmful to the sat in any way.
I think everyone should hold off testing this until some feedback has gone a round or two here first.
Regards, Jim, ZL1TYF.
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of John Magliacane Sent: Saturday, 2 February 2008 11:31 a.m. To: [email protected] Subject: [amsat-bb] Re: Automatic doppler tracking of DSB
It started when I read that AO-16 had been placed in a test mode with an FM uplink and DSB downlink. I remembered a diagram of a double sideband suppressed carrier ( DSB ) demodulation technique that used a variation of a phased locked loop to recover the frequency and phase of the original carrier. I realized that it should be able to track the "original carrier" through the doppler shift caused by the relative motion between the satellite and the receiving station.
I think this can be made to work, although maybe not as well as you'd like.
Given a receiver passband wide enough for the full DSB bandwidth, plus some additional "wiggle room" to chase the signal if the demodulator should temporary fall out of lock, it probably can be done PROVIDED the modulation of the DSB signal never goes to zero for any extended period of time (especially around TCA). Since people will be taking turns using the transponder, that last requirement may be difficult to achieve.
The reason Doppler tracking of AO-16's digital transponder worked so well is because the downlink was ALWAYS modulated with BPSK, even if it was AX.25 NULLS.
Such is not the case with analog audio. :-(
73, de John, KD2BD
Visit John on the Web at:
Hi Jim.
Hi John,
What if someone turns on a CTCSS subtone for their FM uplink?
If set high enough, it might make it thru the audio filters, so could be tracked with a PLL at the receive end. (But not so high it's a problem for human ears :-)
- Would this work?
I believe it could be made to work. However, during the absence of any CTCSSed uplink, your PLL would lose lock and your Doppler tracking function would stop.
- Will it cause any problems to AO-16, by raising the duty-cycle
of the DSB downlink? (Clearly this would be a bad idea if it might
cause harm or
unacceptable power-budget issues. Mind you, it can't be too much
worse
than the original BPSK can it?)
The downlink was modulated continuously with BPSK during Pacsat service, but the state of the batteries were almost certainly better then than they are now.
However, in its present configuration, the downlink is modulated by white noise if there is no signal present on the uplink, although the peak-to-average power ratio is probably higher with noise than it was with BPSK (a pure guess on my part).
- If this is a good idea, does we need a standard tone?
A standard tone would be required.
- How would it cope with multiple overlapping uplinks?
That would be a problem, as would any period where there is an absence of an uplink.
If someone was able to automate the downlink Doppler correction, that can then feed right back into the uplink correction too.
Very true.
It would be interesting to them compare the corrections against that calculated from the Keps.
I'd love to hear ideas re above, but most critical is to be sure it's not harmful to the sat in any way.
I have only had the chance to tune into AO-16 once during a time when I happened to catch Jim, WD0E, testing the satellite before it was open for general use, and I thought I heard some residual carrier in the downlink. That "pilot carrier" (if present) could be used for Doppler tracking purposes as well. The problem of finding and keeping track of a very narrowband carrier would be almost as difficult as tracking a CTCSS tone.
(I say "almost" because in the case of using a DSB demodulator, it might be a little easier to find and track a DSB carrier (produced by a CTCSS pilot tone) because of its redundancy than a single pilot tone caused by a residual suppressed carrier. The residual carrier, however, will be farther removed from voice frequencies than any CTCSS tone.)
Considering all of the above, I might be inclined to try a Costas or Squaring Loop along with a DSB demodulator for "closed loop" Doppler correction. With any luck, maybe the loop can acquire and track the white noise that is present when no uplink signals are present.
Another experiment you might try is to see how a receiver with an FM center tuning discriminator meter reacts to AO-16's downlink with and without signals present on the uplink.
73, de John, KD2BD
Visit John on the Web at:
http://kd2bd.ham.org/ . . . .
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participants (5)
-
James Whitfield -
Jim Towler -
John B. Stephensen -
John Magliacane -
laura halliday