<p><a href="https://en.wikipedia.org/wiki/Very-long-baseline_interferometry" rel="nofollow noreferrer">Very Long Baseline Interferometry</a> or VLBI such as the <a href="https://en.wikipedia.org/wiki/Event_Horizon_Telescope" rel="nofollow noreferrer">Event Horizon Telescope</a> or JPL's delta-DOR (see below) uses baselines of the order of the Earth's diameter.</p>
<ul>
<li><a href="https://space.stackexchange.com/q/15347/12102">Spacecraft position determination</a></li>
<li><a href="https://space.stackexchange.com/q/21256/12102">How do we track the exact location of the spacecraft which is millions or billions of miles away from us?</a></li>
<li><a href="https://space.stackexchange.com/q/31467/12102">Help understanding BepiColombo's weak capture at Mercury's L1 and need for delta-differential one-way range measurements</a></li>
<li><a href="https://space.stackexchange.com/q/6528/12102">Tracking Arc and Time for Precise Orbit Determination</a></li>
</ul>
<p>The Earth's diameter is about 12,000 kilometers.</p>
<p>The Russian radio telescope satellite <a href="https://en.wikipedia.org/wiki/Spektr-R" rel="nofollow noreferrer">Spektr-R</a> allows for an even larger baseline, with an apoapsis that can reach to 300,000 kilometers!</p>
<ul>
<li><a href="https://space.stackexchange.com/q/27183/12102">What artificial satellite has the farthest orbit around the Earth?</a></li>
<li><a href="https://space.stackexchange.com/q/33564/12102">What are Spectr-R's major contributions to radio astronomy that could not have been done from Earth?</a> (currently unanswered, <em>should I move it here?</em>)</li>
</ul>
<p>These result in maximum arrival times of about 42 milliseconds and 1 second, respectively.</p>
<p>In order to combine these signals, mathematical correlation is used, and the &quot;beam is steered&quot; by varying the time delay in the correlation. This can be done in real time, or during &quot;playback&quot;. In the case of the <a href="https://en.wikipedia.org/wiki/Event_Horizon_Telescope" rel="nofollow noreferrer">Event Horizon Telescope</a> (from which we are waiting for a press conference next Wednesday) suitcases full of hard drives are flown on airplanes from the various sites and collected before the correlation can even begin.</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/20082/7982">How does the Event Horizon Telescope implement the interferometry?</a></li>
</ul>
<p><strong>Question:</strong> Have there been observations with much longer delays used in correlation of radio signals? Minutes? Months?</p>

CC BY-SA 4.0
2019-04-06T03:30:57.977Z
When gravitational lensing produces multiple images there can be time delays   ([days](http://spiff.rit.edu/classes/phys240/lectures/lens_results/lens_results.html)) between the arrival of light, so presumably also radio waves. That is the only way I can think of that such correlations could arise.
Keith McClary
CC BY-SA 4.0
2019-04-06T03:38:29.687Z
Oh, I see that's where you're coming from.
Keith McClary
CC BY-SA 4.0
2019-04-06T03:47:10.977Z
@KeithMcClary [That is](https://astronomy.stackexchange.com/q/30243/7982) certainly an answer, please consider posting it. There may be other, very different situations as well, perhaps multiple paths through a large cloud of (ionized?) gas acting as a refractive lens, or [some kind of (as-yet unspecified) mirror in space](https://astronomy.stackexchange.com/q/22681/7982). I can't think of anything specific right off the bat, but that's precisely why I'm asking the question.
uhoh
CC BY-SA 4.0
2019-06-01T16:48:42.740Z
@KeithMcClary there's a bounty that's got 21 hours + 24 hours grace period left. Consider writing at least a short answer?
uhoh
CC BY-SA 4.0
2020-07-10T02:17:28.627Z
@KeithMcClary I've added another bounty...
uhoh
<observational-astronomy><radio-astronomy>
Examples of radio correlations over times much longer than interferometric baselines?
uhoh
# Examples of radio correlations over times much longer than interferometric baselines?

(Asked by: username_0 on 2019-04-06)

[Very Long Baseline Interferometry](https://en.wikipedia.org/wiki/Very-long-baseline_interferometry) or VLBI such as the [Event Horizon Telescope](https://en.wikipedia.org/wiki/Event_Horizon_Telescope) or JPL's delta-DOR (see below) uses baselines of the order of the Earth's diameter.

* [Spacecraft position determination](https://space.stackexchange.com/q/15347/12102)
* [How do we track the exact location of the spacecraft which is millions or billions of miles away from us?](https://space.stackexchange.com/q/21256/12102)
* [Help understanding BepiColombo's weak capture at Mercury's L1 and need for delta-differential one-way range measurements](https://space.stackexchange.com/q/31467/12102)
* [Tracking Arc and Time for Precise Orbit Determination](https://space.stackexchange.com/q/6528/12102)

The Earth's diameter is about 12,000 kilometers.

The Russian radio telescope satellite [Spektr-R](https://en.wikipedia.org/wiki/Spektr-R) allows for an even larger baseline, with an apoapsis that can reach to 300,000 kilometers!

* [What artificial satellite has the farthest orbit around the Earth?](https://space.stackexchange.com/q/27183/12102)
* [What are Spectr-R's major contributions to radio astronomy that could not have been done from Earth?](https://space.stackexchange.com/q/33564/12102) (currently unanswered, *should I move it here?*)

These result in maximum arrival times of about 42 milliseconds and 1 second, respectively.

In order to combine these signals, mathematical correlation is used, and the "beam is steered" by varying the time delay in the correlation. This can be done in real time, or during "playback". In the case of the [Event Horizon Telescope](https://en.wikipedia.org/wiki/Event_Horizon_Telescope) (from which we are waiting for a press conference next Wednesday) suitcases full of hard drives are flown on airplanes from the various sites and collected before the correlation can even begin.

* [How does the Event Horizon Telescope implement the interferometry?](https://astronomy.stackexchange.com/q/20082/7982)

**Question:** Have there been observations with much longer delays used in correlation of radio signals? Minutes? Months?

**Tags:** observational-astronomy, radio-astronomy

> **Comments:**
> **username_1**: When gravitational lensing produces multiple images there can be time delays   ([days](http://spiff.rit.edu/classes/phys240/lectures/lens_results/lens_results.html)) between the arrival of light, so presumably also radio waves. That is the only way I can think of that such correlations could arise.  
> **username_1**: Oh, I see that's where you're coming from.  
> **username_0**: @username_1 [That is](https://astronomy.stackexchange.com/q/30243/7982) certainly an answer, please consider posting it. There may be other, very different situations as well, perhaps multiple paths through a large cloud of (ionized?) gas acting as a refractive lens, or [some kind of (as-yet unspecified) mirror in space](https://astronomy.stackexchange.com/q/22681/7982). I can't think of anything specific right off the bat, but that's precisely why I'm asking the question.  
> **username_0**: @username_1 there's a bounty that's got 21 hours + 24 hours grace period left. Consider writing at least a short answer?  
> **username_0**: @username_1 I've added another bounty...  

---

## 0 answers

