<p>Yes, we can do radio astronomy with heterogeneous, geographically distributed antennas.  The <a href="https://en.wikipedia.org/wiki/Very-long-baseline_interferometry" rel="noreferrer">VLBI</a> is an excellent example.
<a href="https://i.sstatic.net/HpKjr.png" rel="noreferrer"><img src="https://i.sstatic.net/HpKjr.png" alt="enter image description here" /></a></p>
<p>Of course, the antennas have to be similar in some ways for it to work:</p>
<ol>
<li>If the antennas are ground based, they need to be in the same hemisphere, or the Earth will prevent them from looking at the same target at the same time.</li>
<li>They must have extremely stable timing sources, since error or drift in the time tagging will result in dB losses for a detected signal and inaccuracies in angular resolution.</li>
<li>They must have accurate enough timing sources to allow synchronized collections.  The collection times need to be offset in time to account for path delays to the various antennas.</li>
<li>They must overlap in collection RF.  Any differences in center frequency and bandwidth will mean that some of the antennas will be picking up different signals than the other(s).  Practically this usually degrades the collection quality.</li>
<li>It's desirable to have similar antenna beamwidths or, again, the antennas won't all be picking up the same signals.</li>
<li>You need to synchronize the collectors and have accurate estimates for the distances between them.  The VLBI systems use methods described here: <a href="https://astronomy.stackexchange.com/questions/33840/how-are-the-atomic-clocks-synchronised-between-worldwide-vlbi-telescopes/40237#40237">How are the atomic clocks synchronised between worldwide VLBI telescopes?</a>,</li>
</ol>

CC BY-SA 4.0
2021-03-23T18:14:54.450Z
very informative! i have a better notion of how this works now. thank you
Joe Cooper
CC BY-SA 4.0
2021-03-23T18:25:36.017Z
You are quite welcome!  Back in the early 2000s we would collect data from disparate antennas on electronic tape and ship it to Green Bank, WV for correlation.  The internet was too slow at that time to accommodate the data volumes.  An astronomer might wait weeks or months to determine if their collection succeeded!
Connor Garcia
CC BY-SA 4.0
2021-03-20T22:57:58.417Z
2021-03-20T22:57:58.417Z
Connor Garcia
<p>I understand some radio telescopes are built as arrays of receivers. Could one build an array from a heterogeneous set of receivers in scattered locations? Suppose a large number of persons each operate a receiver, and data is gathered together; can it function as an array? Asking for a friend.</p>

CC BY-SA 4.0
2021-03-20T15:29:18.507Z
Not the same question, but [Radio calibration target for amateur radio telescope](https://astronomy.stackexchange.com/q/41906) discusses an amateur distributed radio telescope using different words. There is such a thing as [intensity interferometry](https://en.wikipedia.org/wiki/Intensity_interferometer) but generally one wants to do normal interferometry which means one needs to reconstruct the relative phases of the radio signals from each.
uhoh
CC BY-SA 4.0
2021-03-20T15:30:46.003Z
However, if you are looking at time-varying signals like millisecond pulsars, you could time the signals relative to a 1 pulse per second output of a standard hobby GPS receiver at each site and pinpoint the location of the pulsar. (a bit like how lightning detectors triangulate) Of course you could potentially do that just with one antenna and letting Earth's motion move you around, so that's not really what you're asking about.
uhoh
CC BY-SA 4.0
2021-03-20T15:37:28.287Z
So a good answer will address how you are going to record the signals digitally with a MHz to GHz bandwidth in such a precise way that you can recover the phases later. I guess you could use a high quality stereo audio board of a computer and include the 1 pulse per second "clicks" from a GPS in one channel and and the audio of an AM radio in the other, but the local oscillator of your radio also has to be very very stable, and you'll have a tiny bandwidth (e.g. 20 kHz) Ya, hardest part to doing at least something is stabilization and locking of your local oscillators against phase noise&drift
uhoh
<amateur-observing><radio-astronomy><radio-telescope><radio>
Can one build a distributed radio telescope?
Joe Cooper
# Can one build a distributed radio telescope?

(Asked by: username_0 on 2021-03-20)

I understand some radio telescopes are built as arrays of receivers. Could one build an array from a heterogeneous set of receivers in scattered locations? Suppose a large number of persons each operate a receiver, and data is gathered together; can it function as an array? Asking for a friend.

**Tags:** amateur-observing, radio-astronomy, radio-telescope, radio
> **Comments:**
> **username_1**: Not the same question, but [Radio calibration target for amateur radio telescope](https://astronomy.stackexchange.com/q/41906) discusses an amateur distributed radio telescope using different words. There is such a thing as [intensity interferometry](https://en.wikipedia.org/wiki/Intensity_interferometer) but generally one wants to do normal interferometry which means one needs to reconstruct the relative phases of the radio signals from each.  
> **username_1**: However, if you are looking at time-varying signals like millisecond pulsars, you could time the signals relative to a 1 pulse per second output of a standard hobby GPS receiver at each site and pinpoint the location of the pulsar. (a bit like how lightning detectors triangulate) Of course you could potentially do that just with one antenna and letting Earth's motion move you around, so that's not really what you're asking about.  
> **username_1**: So a good answer will address how you are going to record the signals digitally with a MHz to GHz bandwidth in such a precise way that you can recover the phases later. I guess you could use a high quality stereo audio board of a computer and include the 1 pulse per second "clicks" from a GPS in one channel and and the audio of an AM radio in the other, but the local oscillator of your radio also has to be very very stable, and you'll have a tiny bandwidth (e.g. 20 kHz) Ya, hardest part to doing at least something is stabilization and locking of your local oscillators against phase noise&drift  

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## 1 answers

### Answer by username_2 on 2021-03-20. Score: 9 | Accepted answer

Yes, we can do radio astronomy with heterogeneous, geographically distributed antennas. The [VLBI](https://en.wikipedia.org/wiki/Very-long-baseline_interferometry) is an excellent example.
[![enter image description here](https://i.sstatic.net/HpKjr.png)](https://i.sstatic.net/HpKjr.png)

Of course, the antennas have to be similar in some ways for it to work:

1. If the antennas are ground based, they need to be in the same hemisphere, or the Earth will prevent them from looking at the same target at the same time.
2. They must have extremely stable timing sources, since error or drift in the time tagging will result in dB losses for a detected signal and inaccuracies in angular resolution.
3. They must have accurate enough timing sources to allow synchronized collections. The collection times need to be offset in time to account for path delays to the various antennas.
4. They must overlap in collection RF. Any differences in center frequency and bandwidth will mean that some of the antennas will be picking up different signals than the other(s). Practically this usually degrades the collection quality.
5. It's desirable to have similar antenna beamwidths or, again, the antennas won't all be picking up the same signals.
6. You need to synchronize the collectors and have accurate estimates for the distances between them. The VLBI systems use methods described here: [How are the atomic clocks synchronised between worldwide VLBI telescopes?](https://astronomy.stackexchange.com/questions/33840/how-are-the-atomic-clocks-synchronised-between-worldwide-vlbi-telescopes/40237#40237),

> **Comments:**
> **username_0**: very informative! i have a better notion of how this works now. thank you  
> **username_2**: You are quite welcome!  Back in the early 2000s we would collect data from disparate antennas on electronic tape and ship it to Green Bank, WV for correlation.  The internet was too slow at that time to accommodate the data volumes.  An astronomer might wait weeks or months to determine if their collection succeeded!  

---

