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Re: Looking for a way to plot V_LSR vs intensity in GNU Radio

From: Paul Boven
Subject: Re: Looking for a way to plot V_LSR vs intensity in GNU Radio
Date: Sat, 25 Jul 2020 09:52:14 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

Hi Ellie,

On 7/24/20 5:09 PM, Ellie White wrote:
My question is this: do any of you know of a method -- or a GNU Radio module -- compatible with GR version 3.8 that allows you to plot raw data from a USRP and get a plot of intensity versus velocity with respect to the local standard of rest (V_LSR) -- instead of intensity versus frequency, as given by the time sink block?

We solved this somewhat differently: I wrote some Python code that, given Ra/Dec (which we can get from the telescope) and date/time/position, calculates the V_LSR for that viewing direction. The calculation itself is done in Astropy, hence the need for Python3, and the reason we upgraded to GR 3.8 as soon as we could. One of our other volunteers wrote a Telescope class that can be queried for pointing direction and the like.

The frequency offset is calculated every second, and sent back into the flowchart using ZMQ. The ZMQ sink sends the tuning commands to a frequency-translating-fir block to shift the received frequency automatically. The advantage of using this method is that the frequency bins for the FFT integration stay at the same V_LSR during long observations, or observations carried out at another date/time, so you don't need to resample/interpolate to compare.

As we show the integrated spectrum using the Vector Sink anyway, it's very easy to have it calculate the x-axis labels in terms of doppler velocity, instead of frequency.

We took some extra effort that the doppler shift calculation can simply be included in the flowchart, and starts/stops with the flowchart.

Happy to send you some code, although some of it is specific to the Dwingeloo telescope.

Regards, Paul Boven.

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