VLBI !!!

[David Lonard]

Finally after considerable effort, false starts and lessons learned from regular interferometry, I have been able to detect astronomical fringes through VLBI. Some of my very early analysis of this data is attached and may be somewhat esoteric, so feel free to ask any questions.

This was done at 141 +/- 5 MHz over a 3000 meter baseline with two LimeSDR radios. For those keeping up with my efforts, you know that I've focused on using a digital FX correlator with the eventual goal of being able to do VLBI. I continue to argue that FX correlators are the way to go for interferometry, being easier to implement than earlier generation analog approaches. Surprisingly, I have not seen any other amateur radio astronomers show any FX correlator data of their own.

I've had a few other amateurs ask to coordinate observations with me, but due to differences in receivers, frequencies, etc. this never came to pass. Anyways, the baselines would have been so large (hundreds to thousands of km) that getting fringes would be nearly impossible anyways. Baselines larger than 10 km will be very tough in my opinion. This is due to the ever increasing fringe rates as baseline increases, leading to some serious challenges that will require sophisticated sky modeling and fringe stopping (see attached).

Anyways, the S/N of the data in the attached is still somewhat marginal, so I plan to try another observation with some Yagi antennas and better fringe stopping sky modeling. I think that ultimately, satisfying S/N will be achievable.

David

[Rolando Paz]

Hi David.

It's amazing what can be accomplished!  Congratulations, I still have a lot to learn :)

Regards

Rolando

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[David Lonard]

Rolando,

Thanks. VLBI has been quite a journey and there will be a lot more work to do in the digital domain. 

David

[David Lonard]

I tuned up a pair of commercial 3 element Yagis that I had for 2 meters to make them compatible for 137 MHz to 147 MHz interferometry.

The attached 4nec2 file has the parameters for the design. Should be about 4 dB better than QHA antennas and with better performance across the passband.

[Jim Sky]

Congratulation David on an achievement that has eluded amateur radio astronomers until now (that I know of anyway).

[David Lonard]

Thanks Jim. 

David

[Michiel Klaassen]

Hi David again,
I only saw your experiment just now; very well done.
VLBI; is what I also wanted to do together with you.
Coming days I will do a small scale experiment also.

I have a question.
It is difficult to start the capturing of streams at the same time.
To synchronize the streams, can I also do a correlation sequence.
I just want to shift one data stream in time and then correlate them.
When maximum correlation is reached it indicates the reference starting point for further multiplications.
What do you think.
Regards,
Michiel
parac.eu

Op vrijdag 11 februari 2022 om 21:50:01 UTC schreef David Lonard:

[David Lonard]

Hello Michiel,

Thanks for the encouraging remarks. I would still be interested in trying to do some VLBI with you after you try your smaller scale experiment and would interested to know what frequency band you would like to try.

Synchronization is not that hard as long as there are common strong satellite sources for both receivers to use as a reference. I tried a 30 km baseline at 50 MHz for instance, but without good references, I was unable to align signals, but at 140 MHz, many LEO satellites exist that I can use. From Europe, I think it will be challenging to find a common reference signal. Maybe a geosynchronous L or C band visible from both locations would work, but at these frequencies, astronomical sources are very weak and small dishes can only see the strongest sources at these frequencies. Another challenge is that we would need to observe a very compact source that won't get resolved out. For these and other reasons I would pick the 130-150 MHz band as a practical choice for amateur VLBI. 

My thoughts are that for transatlantic VLBI, a good reference would be to use signals from the GRAVES radar reflected off the moon at 143.050 MHz. The narrowband, pulsing illumination of the moon by GRAVES should make synchronization relatively easy. I will check some of my latest interferometer data to see if I can detect the GRAVES narrow band signal with my 3 element beam that would be wide enough to simultaneously capture the moon and other astronomical sources.

For synchronization, I use a two step approach where I first do a rough alignment with a Python program that looks at power spectrograms from the two stations' IQ data streams. Stream delays of up to 7 seconds exist from startup between my two LimeSDRs on two separate computers. After the rough alignment is done, I do a fine alignment looking at a phase spectrogram of the cross correlated visibilities. So far, using Orbcomm and other LEO satellites, I note the tilting of the phase across the FX correlator passband and add or remove samples from one of the streams until the phase shift is constant across the passband. This web link provides the math theory behind the approach:

https://www.dsprelated.com/showarticle/26.php

Once some high quality data streams have been captured from receivers in two locations, I think the big challenge will be to figure out how to come up with a sky model to 'fringe stop' the data. Even at 3km/140 MHz (1500 wavelengths) fringes come very fast and the sinc(x) function of a wideband interferometer requires that the FX correlator have fine frequency resolution (~100 Hz or 10,000 frequency bins with 0.05 time resolution). In other words, some computational heft will be required to crunch a large FX correlator visibility dataset. 

I think a key pre-requisite for VLBI is to verify that fringes can be detected from a given antenna pair in a 'regular interferometer' configuration first with good S/N. Of course, a good GPSDO is also needed that has an OCXO and not a TCXO reference. For instance, with two Trimble Thunderbolt GPSDOs on an oscilloscope, I can see that they can hold 10 MHz signals in synch over ~100 second time frames. Short term stability is more important than long term stability. I think that will be good enough for VLBI at 130 to 150 MHz at any baseline length with adjustments/corrections made in the digital domain in post-processing.

David

[Brett Dawson]

Hi David,

Congratulations on your recent VLBI work at VHF.  Your really are setting the bar high for the rest of us that are struggling to get basic systems performing well!

You made a comment in the attached paper which I thought I'd comment on.  You said, 'A digital FX correlator used here - The only practical way to achieve VLBI. Why isn’t every amateur using this approach?'  I think you meant this sincerely, and so will give you my view in the hope that it might lead to further discussion and others being able following your lead.  This relates to using FX correlators for single site interferometry rather than VLBI.

From my perspective as a relative new comer to amateur RA I'd (eventually) like to use a software correlator for my interferometer experiments.  I have looked at doing this, but there are a several reasons why I haven't gone in that direction so far.  Firstly, I think that interferometry is a relatively complex pursuit and I feel that starting with a basic analogue system where system performance and cause and effect are probably easier understood is helpful to the uninitiated (i.e. me!).  My ambition has been to get my analogue systems working predictably and reliably before moving to digital techniques.  Secondly, digital techniques for amateur radio astronomy interferometry are not readily available off-the-shelf.  The blockers I see are (i) being able to write code (I can but my skills are limited),  (ii) being able to use GNU radio (again I can but need more exposure), and (iii) understanding interferometry to a much greater depth than what is required to get an analogue system working. 

I have also balked at the possibility that amateur digital interferometry systems may be less sensitive than analogue systems and not having a similar minded group to be able to collaborate with - and by collaborate I mean share code/GNU radio flowgraphs, ask questions, and get help with other detailed information needed to get started and improve.

To summarise the above ramble, I would like to work on getting more of us using digital techniques.  Ideally this would start with using SDR hardware like RTLSDR dongles, SDR Play receivers and similar affordable units which many people will already have at hand.  As individuals skills improve, some will want to upgrade to more capable hardware.  A Windows based software approach would probably suit most, however with assistance, Linux could work as well.

Cheers,

Brett

[David Lonard]

Brett,

Thanks for your comments. Regarding my advocacy for using a digital FX correlator, I would like to make a few points. I agree that interferometry can be a complex approach and for amateurs, it is a good idea to try reduce the barrier-to-entry so that at least as a first step, some sort of detection of astronomical radio sources is obtainable. For a single-site, traditional interferometer, I agree that an analog interferometer can get the job done, so long as the baseline is kept fairly small and its fractional bandwidth is also kept small. Additional effort is needed if one wishes to obtain the sine and cosine products to produce a complex visibility. For those with strong skills in analog circuit construction, this is a good path to take, but will be hard for many amateurs without RF and electronics skills. Analog interferometers are also better suited to interferometry at C band and higher, where the fractional bandwidth of the interferometer is smaller. At the higher frequencies, a wide bandwidth can be used, provided RFI doesn't fall within the passband.

I have written code for an FX correlator that I have published on this site for anyone to use provided one has a computer with a Nvidia GPU and a LimeSDR and it bypasses GNU Radio and will work on either Linux or Windows machines. Regarding the use of the SDRs that you mentioned, they are harder to use than an analog system because they have only a single channel and/or low bandwidth.  Combining streams from two radios is indeed a big challenge and will require some familiarity with Python. This is why I would stress that the easiest turn-key approach is to purchase a dual-channel LimeSDR, as it can do coherent two-channel reception right out of the box with my published FX correlator software (requires minimal fiddling with Python). Another route would be to use a USRP B210 or a BladeRF 2.0 SDR that are similar to the LimeSDR.

FX correlators are vastly more sensitive at low frequencies than analog-based correlators due to their ability to process much larger bandwidths and their ability to excise RFI, making it still the easiest path to interferometry in my opinion. At longer baselines, signal delays produce a sinc(x) effect that causes 'fringe washing' that single channel 'monochromatic' analog interferometers can not overcome. This is apparent in the FX phase spectrogram images that I published. In short, FX correlators open up additional capabilities that are very interesting.

For VLBI, digital methods are a firm requirement of course, so any future VLBI collaboration with other amateurs will require a move to digital techniques.

Best regards,

David

[Michiel Klaassen]

Hi David,

Thank you for your extensive response.

I tried to copy your 12GHz setup; see picture.

Done some measurements; bottom line; analog coupling of the two antennas give some result; digital not.

We are preparing to travel to Portugal again (April 10) and stay there for 6 months. So I had to disassemble my setup without good results.

I tried with two Goobay lnb's. I removed the x-tal and replaced them by a connection to a Leo Bodnar GPS disciplined osc.

Even with the analog combiner I get no smooth fringe curves. When I only plot 1 channel I do get a normal sun drifting curve.

I did not couple the oscillators of the two sdr dongles yet.

So, Lots of things to do.

In Portugal I will try to capture masers on 6.7 and 22GHz and pulsars again.

If there is spare space in the car I will take the dishes also.

Again all by myself; no one from the local technical school is interested.

Best regards,

Michiel

 parac.eu

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[David Lonard]

Michiel,

For my 12 GHz work, I am using a pair of modified Outernet LNBs that take a 25 MHz reference and find that they work well. With my LimeSDR, I can see very stable phase relationships when aimed at geosynchronous satellites so I would check them first to determine if the LNBs are really coherent. The input level for the 25 MHz reference can be a little tricky also, so another sanity check is to use a satellite station receiver box to see if each LNB can detect a broadcast from a target satellite.  I did have a reference with a lot of phase noise such that the LNB didn't properly lock its PLL circuit. It still gave nice sun noise figures, but was basically acting like a total noise receiver with no valid modulation.

David

[Brett Dawson]

Hi David,

I appreciate your response and thoughts and I'll take a look at your FX correlator software. 

Will it work as-is with USRP B210 or BladeRF 2.0 SDR?  It seems that there is often a long wait time on LimeSDR.

Cheers,

Brett

[David Lonard]

I haven't tried either the USRP B210 or the BladeRF 2.0 SDR myself, but they should work. I have seen quite a few demonstrations of coherent channel use with the B210, so it is likely the safest overall choice.

I would also consider a computer that has a GPU. A computer with a mid level GPU such as a Nvidia RTX2060 will help speed up the real-time FX correlator.

David