Re: [Discuss-gnuradio] PSK demodulator and Doppler

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
On 03/09/14 15:30, mleech@ripnet.com wrote:
>
>
> 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 <willis.mj@gmail.com> wrote:
>>
>> Hi Colin,
>>
>> Not really, though there is an AX25 style header. Far from ideal 01111110 flags.
>>
>> Mike
>>
>> FROM: Colby Boyer [mailto:colby.boyer@gmail.com]
>> 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 <willis.mj@gmail.com> 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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