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BUG: fix triangular interferometer coordinates #1016

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Merged
ColmTalbot merged 1 commit into from
Jan 28, 2026
Merged

BUG: fix triangular interferometer coordinates #1016

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38 changes: 17 additions & 21 deletions bilby/gw/detector/networks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -358,6 +358,8 @@ def __init__(
if isinstance(maximum_frequency, float) or isinstance(maximum_frequency, int):
maximum_frequency = [maximum_frequency] * 3

brng = 90 - xarm_azimuth

for ii in range(3):
self.append(
Interferometer(
Expand All @@ -376,29 +378,23 @@ def __init__(
)
)

xarm_azimuth += 240
yarm_azimuth += 240

latitude += (
np.arctan(
length
* np.sin(xarm_azimuth * np.pi / 180)
* 1e3
/ utils.radius_of_earth
)
* 180
/ np.pi
phi1 = np.radians(latitude)
phi2 = np.arcsin(
np.sin(phi1) * np.cos(length * 1e3 / utils.radius_of_earth) +
np.cos(phi1) * np.sin(length * 1e3 / utils.radius_of_earth) * np.cos(np.radians(brng))
)
longitude += (
np.arctan(
length
* np.cos(xarm_azimuth * np.pi / 180)
* 1e3
/ utils.radius_of_earth
)
* 180
/ np.pi
latitude = np.degrees(phi2)

lam1 = np.radians(longitude)
lam2 = lam1 + np.arctan2(
np.sin(np.radians(brng)) * np.sin(length * 1e3 / utils.radius_of_earth) * np.cos(phi1),
np.cos(length * 1e3 / utils.radius_of_earth) - np.sin(phi1) * np.sin(phi2)
)
longitude = np.degrees(lam2)

brng += 240
xarm_azimuth += 240
yarm_azimuth += 240


def get_empty_interferometer(name):
Expand Down
2 changes: 1 addition & 1 deletion test/gw/detector/networks_test.py
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Original file line number Diff line number Diff line change
Expand Up @@ -387,7 +387,7 @@ def c(a):
for pair in list(combinations(self.triangular_ifo, 2)):
delta_lat = np.radians(pair[1].latitude - pair[0].latitude)
delta_long = np.radians(pair[1].longitude - pair[0].longitude)
pair_a = a(delta_lat, delta_long, pair[0].latitude, pair[1].latitude)
pair_a = a(delta_lat, delta_long, np.radians(pair[0].latitude), np.radians(pair[1].latitude))
pair_c = c(pair_a)
distance = bilby.core.utils.radius_of_earth * pair_c
self.assertAlmostEqual(distance / 1000, pair[0].length, delta=1)
Expand Down
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