Detailed Description

This header defines structures to hold the basic data describing a gravitational wave detector.

Author: J. T. Whelan and J. D. E. Creighton

Synopsis

#include <lal/LALDetectors.h>

According to the common frame format specification [16] the geometry of an interferometric detector will be stored in a FrDetector structure, specifying the location of the detector vertex and the orientation of its arms in geodetic coördinates suited to geographical surveying. Resonant bars and other sorts of detectors, if they write their data to frames, are expected to fill this structure with their location and orientation in some way suited to the detector type.

For most data analysis tasks, however, any gravitational wave detector can be described by its location in an Earth-fixed rotating reference frame, as well as a response tensor \(d^{ab}\), constant in the same frame, which defines the "strain" \(h\) measured by the detector in terms of the metric perturbation \(h_{ab}\) as

\begin{equation} h = h_{ab} \, d^{ab} \ . \end{equation}

This header defines a LALFrDetector structure which contains essentially the same information as the FrDetector structure, as well as a LALDetector structure which contains the Cartesian coördinates of the detector along with the components of the response tensor \(d^{ab}\) in the same coördinate system.

The Geodetic Coördinate System

Geodetic coördinates are spheroidal coördinates based on the WGS-84 Earth Model, which is an oblate spheroid with equatorial radius \(a=6.378137\times 10^6\,\textrm{m}\) and polar radius \(b=6.356752314\times 10^6\,\textrm{m}\). Any point in space can be located according to its longitude, latitude, and elevation. The longitude \(\lambda\) is the angle between the half-plane bounded by the symmetry axis of the reference ellipsoid containing the point in question and the half-plane plane containing the Prime Meridian; it is measured in radians, increases to the East, and ranges from \(-\pi\) to \(\pi\). The latitude \(\beta\) is the angle between the ray which is normal to the ellipsoid and passes through the point in question and the equatorial plane; it is measured in radians, increases to the North, and ranges from \(-\pi/2\) to \(\pi/2\). The elevation \(h\) is the signed distance along this ray from the reference ellipsoid to the point in question. This coördinate system is described in more detail in [3] .

Altitude and Azimuth Angles

The LALFrDetector structure stores the directions along the two arms of an interferometer in an altitude/azimuth representation with respect to the local tangent plane to the reference ellipsoid, known as the local horizontal. The altitude \({\mathcal{A}}\) is the angle the direction vector makes with the horizontal, \({\mathcal{A}} > 0\) meaning above horizontal, \({\mathcal{A}} < 0\) below. The azimuth angle \(\zeta\) is found by projecting the direction onto the local horizontal plane, then measuring the angle clockwise from North to this projected direction.

The Cartesian Coördinate System

The position vector and response tensor contained in the LALDetector structure are defined in a simple orthonormal coördinate system with its origin at the center of the earth, an \(x^1\) axis which pierces the Earth's surface at the intersection of the equator and the prime meridian, an \(x^2\) axis which pierces the earth's surface at \(\pi/2\) radians East longitude on the equator, and an \(x^3\) axis which pierces the Earth's surface at the North Pole. The coördinates \(x^1\), \(x^2\), \(x^3\) correspond to the Earth-fixed coördinates \(X_E\), \(Y_E\), \(Z_E\) defined in [3] , respectively.

The relationship between geodetic and Cartesian coördinates is given by

\begin{align} \label{tools_e_cart1} x^1 &=\left( \frac{a^2}{\sqrt{a^2\cos^2\beta+b^2\sin^2\beta}} + h \right) \cos\beta\cos\lambda \\ \label{tools_e_cart2} x^2 &=\left( \frac{a^2}{\sqrt{a^2\cos^2\beta+b^2\sin^2\beta}} + h \right) \cos\beta\sin\lambda \\ \label{tools_e_cart3} x^3 &=\left( \frac{b^2}{\sqrt{a^2\cos^2\beta+b^2\sin^2\beta}} + h \right) \sin\beta \\ \end{align}

Cached Detectors

In practice, we will often be working with fixed unchanging site geometry, e.g., for the LIGO interferometers; to avoid constantly reconstructing the corresponding LALDetectors, we should define some constant LALDetectors describing them. Those are stored in a constant array of LALDetector structures known as lalCachedDetectors, which is declared extern in this header and defined in Module CreateDetector.c.

The LALCreateDetector() routine will first look through the lalCachedDetectors array for a LALDetector structure with matching type and frDetector.name fields; if it finds one, it returns a copy of that; if not, it creates one.

For example, the LALDetector representing LIGO Hanford 4km (H1) in differential mode is lalCachedDetectors[LAL_LHO_4K_DETECTOR].

Modules
	Module CreateDetector.c
	Creates a `LALDetector` structure from a `LALFrDetector` structure and the type of detector.

	Detector Constants
	Constants describing various gravitational wave detectors.

Data Structures
struct	LALFrDetector
	Detector frame data structure Structure to contain the data that appears in a FrDetector structure in frame data. More...

struct	LALDetector
	Detector structure. More...

Enumerations
enum	LALDetectorEnum { LAL_TAMA_300_DETECTOR = 0 , LAL_VIRGO_CITF_DETECTOR = 1 , LAL_VIRGO_DETECTOR = 2 , LAL_GEO_600_DETECTOR = 3 , LAL_LHO_2K_DETECTOR = 4 , LAL_LHO_4K_DETECTOR = 5 , LAL_LLO_4K_DETECTOR = 6 , LAL_CIT_40_DETECTOR = 7 , LAL_ALLEGRO_DETECTOR = 8 , LAL_AURIGA_DETECTOR = 9 , LAL_EXPLORER_DETECTOR = 10 , LAL_NIOBE_DETECTOR = 11 , LAL_NAUTILUS_DETECTOR = 12 , LAL_ACIGA_DETECTOR = 13 , LAL_KAGRA_DETECTOR = 14 , LAL_LIO_4K_DETECTOR = 15 , LAL_ET1_DETECTOR = 16 , LAL_ET2_DETECTOR = 17 , LAL_ET3_DETECTOR = 18 , LAL_ET0_DETECTOR = 19 , LAL_NUM_DETECTORS = 20 }
	Enumeration of Detectors: follows order of DQ bit assignments. More...

enum	LALDetectorType { LALDETECTORTYPE_ABSENT , LALDETECTORTYPE_IFODIFF , LALDETECTORTYPE_IFOXARM , LALDETECTORTYPE_IFOYARM , LALDETECTORTYPE_IFOCOMM , LALDETECTORTYPE_CYLBAR }
	Detector type, which determines how the detector response is determined. More...

Files
file	DetectorSiteTest.c
	Tests the detector response and site parameter structures and the routine to create one from the other.

Variables
const LALDetector	lalCachedDetectors [LAL_NUM_DETECTORS]
	Pre-existing detectors. More...

Error Codes
#define	LALDETECTORSH_ENULLP 1
	Null pointer. More...

#define	LALDETECTORSH_ETYPE 2
	Unsupported detector type. More...

Detector DQ bit assignments (2 bits per detector)
#define	LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(d) (LAL_INT8_C(1) << 2 * (d))

#define	LAL_TAMA_300_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_TAMA_300_DETECTOR)

#define	LAL_VIRGO_CITF_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_VIRGO_CITF_DETECTOR)

#define	LAL_VIRGO_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_VIRGO_DETECTOR)

#define	LAL_GEO_600_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_GEO_600_DETECTOR)

#define	LAL_LHO_2K_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_LHO_2K_DETECTOR)

#define	LAL_LHO_4K_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_LHO_4K_DETECTOR)

#define	LAL_LLO_4K_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_LLO_4K_DETECTOR)

#define	LAL_CIT_40_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_CIT_40_DETECTOR)

#define	LAL_ALLEGRO_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_ALLEGRO_DETECTOR)

#define	LAL_AURIGA_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_AURIGA_DETECTOR)

#define	LAL_NIOBE_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_NIOBE_DETECTOR)

#define	LAL_NAUTILUS_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_NAUTILUS_DETECTOR)

#define	LAL_ACIGA_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_ACIGA_DETECTOR)

#define	LAL_KAGRA_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_KAGRA_DETECTOR)

#define	LAL_LIO_4K_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_LIO_4K_DETECTOR)

#define	LAL_ET1_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_ET1_DETECTOR)

#define	LAL_ET2_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_ET2_DETECTOR)

#define	LAL_ET3_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_ET3_DETECTOR)

#define	LAL_ET0_DETECTOR_BIT LAL_DETECTOR_DQ_BIT_FROM_DETECTOR_ENUM(LAL_ET0_DETECTOR)

Enumeration Type Documentation

◆ LALDetectorEnum

enum LALDetectorEnum

Enumeration of Detectors: follows order of DQ bit assignments.

Enumerator
LAL_TAMA_300_DETECTOR
LAL_VIRGO_CITF_DETECTOR
LAL_VIRGO_DETECTOR
LAL_GEO_600_DETECTOR
LAL_LHO_2K_DETECTOR
LAL_LHO_4K_DETECTOR
LAL_LLO_4K_DETECTOR
LAL_CIT_40_DETECTOR
LAL_ALLEGRO_DETECTOR
LAL_AURIGA_DETECTOR
LAL_EXPLORER_DETECTOR
LAL_NIOBE_DETECTOR
LAL_NAUTILUS_DETECTOR
LAL_ACIGA_DETECTOR
LAL_KAGRA_DETECTOR
LAL_LIO_4K_DETECTOR
LAL_ET1_DETECTOR
LAL_ET2_DETECTOR
LAL_ET3_DETECTOR
LAL_ET0_DETECTOR
LAL_NUM_DETECTORS

Definition at line 168 of file LALDetectors.h.

◆ LALDetectorType

enum LALDetectorType

Detector type, which determines how the detector response is determined.

Since data from bars as well as interferometers can be written to frames, we need an additional piece of information to interpret the site geometry data specified in the LALFrDetector structure; for instance, is the x arm really the x arm or is it the long axis of a bar? The LALDetectorType enumeration provides a way to keep track of that.

Enumerator
LALDETECTORTYPE_ABSENT	No FrDetector associated with this detector.
LALDETECTORTYPE_IFODIFF	IFO in differential mode.
LALDETECTORTYPE_IFOXARM	IFO in one-armed mode (X arm)
LALDETECTORTYPE_IFOYARM	IFO in one-armed mode (Y arm)
LALDETECTORTYPE_IFOCOMM	IFO in common mode.
LALDETECTORTYPE_CYLBAR	Cylindrical bar.