# PSK demodulator and Doppler **URL:** \ **Category:** GNU Radio\ **Created:** [September 2, 2014, 7:11pm UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848 "2014-09-02T19:11:20Z")\ **Posts on this page:** 8\ **Page:** 1
**Author:** ![addis\_a](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/a/c4cdca/32.png) [@addis\_a](https://www.ruby-forum.com/u/addis_a)\ **Post date:** [September 2, 2014, 7:11pm UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848/1 "2014-09-02T19:11:20Z")
I am trying to develop a satellite ground station using the PSK demodulator block. This works fine when tuned accurately. However, with low satellites there is quite a bit of Doppler at VHF / UHF and there is also some frequency drift with satellite temperature as it enters or comes out of eclipse. This is a problem as the signals are relatively narrow in bandwidth compared to the Doppler and drift. I am wondering how to track this Doppler in Gnuradio. I have tried a PLL block and while this works it isn’t quite right unless the signal is very strong. It can also get fooled by one of the many spurious signals encountered on the bands. To some extent the Doppler can be predicted and compensated for, but only when the orbital parameters are known accurately. Even a few seconds error at TCA can make quite a difference. Has anyone solved this one? Mike ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [September 3, 2014, 4:12am UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848/2 "2014-09-03T04:12:39Z")
On Tue, Sep 2, 2014 at 10:09 AM, Mike W. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Mike > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) Is there a preamble/training sequence you can search for? If so, you can use that to get the initial frequency offset estimate to correct and then use the PLL to track the fine phase correction. ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [September 3, 2014, 8:54am UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848/3 "2014-09-03T08:54:16Z")
Hi Colin, Not really, though there is an AX25 style header. Far from ideal 01111110 flags. Mike From: Colby B. [[mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)] Sent: 03 September 2014 03:11 To: Mike W. Cc: GNU Radio D.ion Subject: Re: [Discuss-gnuradio] PSK demodulator and Doppler On Tue, Sep 2, 2014 at 10:09 AM, Mike W. \ wrote: I am trying to develop a satellite ground station using the PSK demodulator block. This works fine when tuned accurately. However, with low satellites there is quite a bit of Doppler at VHF / UHF and there is also some frequency drift with satellite temperature as it enters or comes out of eclipse. This is a problem as the signals are relatively narrow in bandwidth compared to the Doppler and drift. I am wondering how to track this Doppler in Gnuradio. I have tried a PLL block and while this works it isn’t quite right unless the signal is very strong. It can also get fooled by one of the many spurious signals encountered on the bands. To some extent the Doppler can be predicted and compensated for, but only when the orbital parameters are known accurately. Even a few seconds error at TCA can make quite a difference. Has anyone solved this one? Mike * * * Discuss-gnuradio mailing list removed\_email\_address@domain.invalid [mailto:removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) [https://lists.gnu.org/mailman/listinfo/discuss-gnuradio](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) Is there a preamble/training sequence you can search for? If so, you can use that to get the initial frequency offset estimate to correct and then use the PLL to track the fine phase correction. ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [September 3, 2014, 11:13am UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848/4 "2014-09-03T11:13:41Z")
My solution involved the frequency translating block and a third party application. As part of the initial channel filter, I set the the “Center Frequency” attribute to -40000+fftshift\*rx\_shift\_factor (assuming my LO tuned to +40 KHz from the carrier). The satellite tracking program, PREDICT, provides a Doppler shift calculation assuming a 100 MHz carrier. rx\_shift\_factor is the satellite carrier frequency divided by 100 MHz. I then polled PREDICT, which provides a UDP server interface and then in Python called set\_fftshift method on the flowgraph object generated by gnuradio-companion. In turn, this would update the taps of the FIR filter to provide a desired frequency shift. As for temperature drift, I assumed that with a strong enough signal, the inbuilt Costas loop would be able to bring the signal back to baseband. The drawback of this solution is that it’s not fully contained within gnuradio-companion, and like you said, requires the orbital elements to be known. One solution that uses the PLL block is here: [https://www.cgran.org/wiki/ESTCube-1Receiver](https://www.cgran.org/wiki/ESTCube-1Receiver) ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [September 3, 2014, 3:05pm UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848/5 "2014-09-03T15:05:16Z")
On Wed, Sep 3, 2014 at 5:12 AM, Vanush V. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > As for temperature drift, I assumed that with a strong enough signal, the > inbuilt Costas loop would be able to bring the signal back to baseband. > > The drawback of this solution is that it’s not fully contained within > gnuradio-companion, and like you said, requires the orbital elements to be > known. One solution that uses the PLL block is here: > [https://www.cgran.org/wiki/ESTCube-1Receiver](https://www.cgran.org/wiki/ESTCube-1Receiver) It might not be completely contained in GRC, but it’s a clever solution. Another thing to look into is the fll\_band\_edge block. This will only work, however, if the signal is pulse-shaped. Specifically with an RRC filter by design, but we can modify it for other pulse shapes as well. This only does coarse frequency correction, but it’s lock in bandwidth is much wider than that of the Costas loop block. You’d still want to follow this up with the Costas loop for fine frequency and phase tracking. One thing, too, is that this block is fairly expensive computationally. It’s calculating two FIR filters, which should be replaced by fast convolution filters. And I think that we could even combine this into a single filter operation. Tom ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [September 3, 2014, 4:32pm UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848/6 "2014-09-03T16:32:28Z")
Using predict is a similar solution to what I do in simple\_ra for “fringe stopping”, except that it’s all contained within the same chunk of Python, using a “helper” function that’s tied to a every-5-seconds polling loop. The helper function calculates the required phase rotation, given a number of input parameters relating to the observation. On 2014-09-03 09:03, Tom R. wrote: > Hi Colin, > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [1] > > Is there a preamble/training sequence you can search for? If so, you can use > that to get the initial frequency offset estimate to correct and then use the PLL > to track the fine phase correction. > > * * * > > Discuss-gnuradio mailing list > [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) > [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [1] * * * Discuss-gnuradio mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [1] * * * Discuss-gnuradio mailing list [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) [Discuss-gnuradio Info Page](https://lists.gnu.org/mailman/listinfo/discuss-gnuradio) [1] Links: ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [September 4, 2014, 12:23am UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848/7 "2014-09-04T00:23:28Z")
Just curious, any estimate on how much temperature drift in PPM you are getting? Thanks, Lou KD4HSO Mike W. wrote > …and there is also some > frequency drift with satellite temperature as it enters or comes out of > eclipse. – View this message in context: [http://gnuradio.4.n7.nabble.com/PSK-demodulator-and-Doppler-tp50185p50210.html](http://gnuradio.4.n7.nabble.com/PSK-demodulator-and-Doppler-tp50185p50210.html) Sent from the GnuRadio mailing list archive at [Nabble.com](http://Nabble.com). ---
**Author:** ![vh1](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/278dde/32.png) [@vh1](https://www.ruby-forum.com/u/vh1)\ **Post date:** [September 3, 2014, 5:57pm UTC](https://www.ruby-forum.com/t/psk-demodulator-and-doppler/231848/8 "2014-09-03T17:57:54Z")
I use predict, the xmlrpc server, and it bit of glue code for twiddling the FIR filter’s offset for my Funcube and NOAA receivers to account for doppler. Allows me to have tighter BPFs (well LPF’s really), although does mean I need to remember to update the TLE’s from time to time! Biggest problem I have is Funcube-1’s beacon has thermal drift as she comes out of eclipse that goes the opposite way…Been trying to use a Freq\_Det block to help, but not had the time to experiment recently Iain