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Re: GRC max sample rate

From: Koyel Das (Vehere)
Subject: Re: GRC max sample rate
Date: Mon, 27 Jul 2020 05:50:52 +0000

Hi Marcus,

Many thanks for your reply. I saw in simulation adding a DC offset in the phase difference between two channels has no impact in the Delay- Rate spectrum. It is giving the same spectrum without this offset. The delays (td1-td2) are same in the delay spectrum with and without offset. Possibly a DC offset is not counted for while generating the DR spectrum by 2D FFT. Need to think what’s going on.


From: Marcus D. Leech <patchvonbraun@gmail.com>
Sent: Monday, July 27, 2020 1:19:51 AM
To: Koyel Das (Vehere) <koyel.das@vehere.com>; discuss-gnuradio@gnu.org <discuss-gnuradio@gnu.org>
Subject: Re: GRC max sample rate
On 07/26/2020 03:15 PM, Koyel Das (Vehere) wrote:
Hi Marcus,

Good that you replied my email. Since you work on Radio Astronomy as I heard from Neel, I want to ask you something. By using USRP I saw that without any special configuration the phases of any two channels of USRP 2955 are phase coherent and the phase difference value is constant for a stationary source in one run of the application. However, for phase aligned operation that is instrumental phase difference between two channels to be zero we need LO sharing and time commands. So suppose I don’t phase align the system and find delay- rate spectrum then what impact the instrumental phase offset between two channels will have on the delays of sources in the DR spectrum. Will the offset add an additional delay to every source by a fixed amount or this will have some random effects on the delays of the sources in DR spectrum. You may be knowing but still, in delay-rate, rate stands for Doppler shift.


Thinking about it for 30 seconds, I don't think it would have any impact on perceived doppler shift--it's a one-time phase diference only.  So distance
  estimates would be affected, but not doppler or doppler derivative, I think.

In radio astronomy interferometry, if the phases aren't aligned, then the "phase center" of the array isn't where you think it is, and that can
  lead to sky-position estimates being a bit wrong.  If your science goals don't include high-quality position estimates, then as long as the
  two sides "march together" (that is, are long-term phase-coherent, without necessarily having a zero phase offset), then you can still
  do science.

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