<p>If the signal is detected at the same time at two sites, then the source must be in the plane bisecting the sites.  This constrains the source to a great circle in the sky.</p>
<p>Generally with two detectors, the direction can be limited to a circle in the sky. With &quot;clever techniques&quot; (perhaps comparing the quality of the signal in the two detectors) Some more direction information can be squeezed out of the data, but probably not enough to determine the direction.</p>
<p>So to get an accurate location, a third detector is needed.  This allows you to triangulate to find the location on the sky. More detectors can mean greater accuracy. This is not unlike how GPS works on Earth: by measuring the difference is time from several know locations you can pinpoint your location.  Likewise by combining the timing data from three or more GW detectors, the source can be located to a point in the sky.</p>
<p>Occasionally the GW will also have an associated signal in light (or radio etc) In this case, the source can be located with great accuracy. Merging black holes have no light signal, but merging neutron stars do, in a &quot;kilonova&quot;.</p>

CC BY-SA 4.0
2021-08-01T22:21:34.693Z
Is there a third detector in LIGO? I always thought there are two.So after making three GW measurements (shortly lasting) another EM based can be made. Except for BHs.
Deschele Schilder
CC BY-SA 4.0
2021-08-01T22:31:20.517Z
There are two Advanced LIGO observatories (in New York and Louisiana), Advanced Virgo (in Italy) and KAGRA (in Japan), with LIGO-India expected to be online in a couple of years.  So four operational detectors and another on the way.
James K
CC BY-SA 4.0
2021-08-01T22:32:59.823Z
A hopefull future! Thanks for the answer!
Deschele Schilder
CC BY-SA 4.0
2021-08-02T02:12:48.003Z
Is this method how the speed of gravity was measured? Three measurement are not enough either if you dont know the speed of gravity. But if you *assume* it to be c and see light coming from the assumed spot it seems the assumption is right.
Deschele Schilder
CC BY-SA 4.0
2021-08-02T08:34:40.027Z
Well the "multi-messenger" observations  of gravitational waves and, simultaneously, optical transients does put very sharp limits on any deviation from the speed of light of gravitational waves. The graviton is very widely expected to be massless.
James K
CC BY-SA 4.0
2021-08-02T08:43:36.730Z
If the speed of gravity was different from c then there wouldnt be a visible process going on at the place where it would if the speed was equal to c. Is a process at the right place for the speed to be c observed? That is, after the triangulated spot, with gravtyspeed=c, was a transient process seen at that spot (confirming that sg=c)?
Deschele Schilder
CC BY-SA 4.0
2021-08-02T08:46:46.727Z
Don't think on the speed of gravity not being "c".  For information about the kilonova multimessenger observations see https://en.wikipedia.org/wiki/GW170817
James K
CC BY-SA 4.0
2021-08-02T08:48:10.807Z
Nono. I dont think its not c. I just imagine how it was confirmed. Im not *that* crackpotty...
Deschele Schilder
CC BY-SA 4.0
2021-08-01T22:14:41.160Z
2021-08-01T22:14:41.160Z
James K
<p>When a gravitational wave arrives on Earth, a detector can observe tiny fluctuations of space metric. But there is no direction associated with this. Only when the second observation is made, far away from the first measurement, you have some more information. Assuming the speed of gravity to be the speed of light there is still insecurity about the the source of the wave. If the two signals are measured at the same time, there is still a whole plane of possibilities.</p>
<p>Say the tell-tale signatures are indeed simultaneously found. How can you find out from where the signal came? How can you say where on the plane of possible sources the real source is located? What other observation can pinpoint it?</p>

CC BY-SA 4.0
2021-08-01T22:27:23.423Z
Duplicate of: [Techniques for locating origin of gravitational waves](https://astronomy.stackexchange.com/questions/16255/techniques-for-locating-origin-of-gravitational-waves)
Daddy Kropotkin
<observational-astronomy><gravitational-waves>
How can we know where a gravitational wave comes from?
Deschele Schilder
# How can we know where a gravitational wave comes from?

(Asked by: username_0 on 2021-08-01)

When a gravitational wave arrives on Earth, a detector can observe tiny fluctuations of space metric. But there is no direction associated with this. Only when the second observation is made, far away from the first measurement, you have some more information. Assuming the speed of gravity to be the speed of light there is still insecurity about the the source of the wave. If the two signals are measured at the same time, there is still a whole plane of possibilities.

Say the tell-tale signatures are indeed simultaneously found. How can you find out from where the signal came? How can you say where on the plane of possible sources the real source is located? What other observation can pinpoint it?

**Tags:** observational-astronomy, gravitational-waves
> **Comments:**
> **username_1**: Duplicate of: [Techniques for locating origin of gravitational waves](https://astronomy.stackexchange.com/questions/16255/techniques-for-locating-origin-of-gravitational-waves)  

---

## 1 answers

### Answer by username_2 on 2021-08-01. Score: 3 | Accepted answer

If the signal is detected at the same time at two sites, then the source must be in the plane bisecting the sites. This constrains the source to a great circle in the sky.

Generally with two detectors, the direction can be limited to a circle in the sky. With "clever techniques" (perhaps comparing the quality of the signal in the two detectors) Some more direction information can be squeezed out of the data, but probably not enough to determine the direction.

So to get an accurate location, a third detector is needed. This allows you to triangulate to find the location on the sky. More detectors can mean greater accuracy. This is not unlike how GPS works on Earth: by measuring the difference is time from several know locations you can pinpoint your location. Likewise by combining the timing data from three or more GW detectors, the source can be located to a point in the sky.

Occasionally the GW will also have an associated signal in light (or radio etc) In this case, the source can be located with great accuracy. Merging black holes have no light signal, but merging neutron stars do, in a "kilonova".

> **Comments:**
> **username_0**: Is there a third detector in LIGO? I always thought there are two.So after making three GW measurements (shortly lasting) another EM based can be made. Except for BHs.  
> **username_2**: There are two Advanced LIGO observatories (in New York and Louisiana), Advanced Virgo (in Italy) and KAGRA (in Japan), with LIGO-India expected to be online in a couple of years.  So four operational detectors and another on the way.  
> **username_0**: A hopefull future! Thanks for the answer!  
> **username_0**: Is this method how the speed of gravity was measured? Three measurement are not enough either if you dont know the speed of gravity. But if you *assume* it to be c and see light coming from the assumed spot it seems the assumption is right.  
> **username_2**: Well the "multi-messenger" observations  of gravitational waves and, simultaneously, optical transients does put very sharp limits on any deviation from the speed of light of gravitational waves. The graviton is very widely expected to be massless.  
> **username_0**: If the speed of gravity was different from c then there wouldnt be a visible process going on at the place where it would if the speed was equal to c. Is a process at the right place for the speed to be c observed? That is, after the triangulated spot, with gravtyspeed=c, was a transient process seen at that spot (confirming that sg=c)?  
> **username_2**: Don't think on the speed of gravity not being "c".  For information about the kilonova multimessenger observations see https://en.wikipedia.org/wiki/GW170817  
> **username_0**: Nono. I dont think its not c. I just imagine how it was confirmed. Im not *that* crackpotty...  

---

