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bilby-dev / bilby

A unified framework for stochastic sampling packages and gravitational-wave inference in Python.
https://bilby-dev.github.io/bilby/
MIT License
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Bias in Triangular Interferometer #806

Open bilby-bot opened 4 months ago

bilby-bot commented 4 months ago

In GitLab by @git.ligo:anna.puecher on Jun 11, 2024, 16:12

In class TriangularInterferometer(InterferometerList): latitude and longitude are computed as

`latitude += ( np.arctan( length

but the longitude should have

/ utils.radius_of_earth * np.cos(latitude*np.pi/180)

instead of just / utils.radius_of_earth, because the "circumference" of the Earth at each latitude is going to be different and not just the equatorial one. If this is not taken into account, the final distance between the triangular detector vertices is going to be biased.

bilby-bot commented 3 months ago

In GitLab by @git.ligo:colm.talbot on Jul 11, 2024, 02:50

Hi @git.ligo:anna.puecher thanks for reporting this, would you be able to write up an example demonstrating this and/or open a merge request with the change?

bilby-bot commented 3 months ago

In GitLab by @git.ligo:anna.puecher on Jul 30, 2024, 14:54

Hi @git.ligo:colm.talbot, sorry for the late reply. I wrote a script that is slightly different from bilby, but it should be the same in the end, you find it attached here triangular_det.py. With this I computed the coordinates of the detector vertices and then checked their distance with different online tools (for example, https://www.nhc.noaa.gov/gccalc.shtml or https://www.meridianoutpost.com/resources/etools/calculators/calculator-latitude-longitude-distance.php).

To give an example it finds (lat,long): A. 50.7231, 6.062458235957906 B. 50.64530365726471 6.017542478135961 C. 50.7231 5.9726267203140155

And the distances: A-B = 9 km A-C = 6 km B-C = 9 km

If I use the 'corrected' longitude, instead, it finds: A. 50.7231, 6.062458235957906 B. 50.64530365726471, 5.991509009187093 C. 50.7231, 5.920677317090751

and the distances are A-B = 10km B-C = 10km A-C = 10km

I will check specifically with the code in bilby and open a merge request.