Re: [Discuss-gnuradio] PSK demodulator and Doppler

[mleech]

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 Rondeau wrote:

 

 

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

 

 

 

On Wed, Sep 3, 2014 at 4:53 PM, Mike Willis <address@hidden> wrote:

Hi Colin,

 

Not really, though there is an AX25 style header. Far from ideal 01111110 flags.

 

Mike

 

From: Colby Boyer [mailto:address@hidden]
Sent: 03 September 2014 03:11
To: Mike Willis
Cc: GNU Radio Discussion
Subject: Re: [Discuss-gnuradio] PSK demodulator and Doppler

 

On Tue, Sep 2, 2014 at 10:09 AM, Mike Willis <address@hidden> 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

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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.

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