Re: [Discuss-gnuradio] PSK demodulator and Doppler

[Tom Rondeau]

On Wed, Sep 3, 2014 at 5:12 AM, Vanush Vaswani <address@hidden> wrote:

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

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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