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gaia early data release 3 documentation

12.4 Auxiliary tables

12.4.1 commanded_scan_law

This table provides a representation of the Gaia scanning law over the 34 month time period covered by the Gaia Data Release 3 (from 2014-07-25 10:31:26 to 2017-05-28 08:46:29), including the Ecliptic Pole Scanning at the begin of the mission. Note that this is the commanded attitude of the spacecraft, the actual attitude could deviate from it by up to about 30 arcsec. Also, it does not contain any of the data interruptions that occurred during the real mission, of which the main ones are listed in Chapter 1, Section 1.3.
The scanning law has been sampled at a 10 second interval, in which the satellite rotates about 10 arcminutes (the target spin rate is actually 59.9641857803 arcsec/sec). Note that this is several times shorter than a typical field-of-view transit and the scan position angle will be practically constant during this interval.
Notes:

  • The times in columns 1, 2 and 3 are in Julian days in TCB with time origin 2010-01-01T00:00 (JD 2455197.5), following the time coordinate convention used in the Gaia archive. TCB stands for Barycentric Coordinate Time and is the time standard used in Gaia processing, equivalent to the proper time experienced by a clock at rest in a coordinate frame co-moving with the barycentre of the Solar system but outside its gravity well, therefore not influenced by the gravitational time dilation caused by the Sun and the rest of the solar system.

  • Column 1 is the reference time for the spacecraft attitude, while columns 2 and 3 give the times with the (relativistic) corrections applied for the light-travel time to the solar system barycentre, corresponding to an infinitely distant source at the RA, DEC at the centres of FOV 1 and 2, respectively.

  • FOV1 and FOV2 correspond to the preceding (PFOV) and following (FFOV) fields-of-view, respectively.

  • The centres of the field of views are separated by the basic angle of 106.5 deg, see Fig. 2 of Lindegren et al. (2012). Their origin in the focal plane is illustrated in Fig. 3 of the same paper: both originate in the astrometric field (AF) 7, with FOV1 in row 3 and FOV2 in row 5.

  • The scan angle, theta, is the position angle of the direction in which the FOV is moving (also called ‘along-scan’ direction), and is defined in the usual astronomical sense: theta = 0 when the FoV is moving towards local North, and theta = 90 degrees towards local East.

  • All values have been formatted to the default double numerical precision and so this precision should not be interpreted as the accuracy of the data.

Columns description:

jd_time : Time [Julian Date in TCB at Gaia - 2455197.5] (double, Time[Julian Date (day)])

Time at Gaia in units of JD (in TCB) in days -2 455 197.5. The time at which the scan angles and FoV angles are evaluated in TCB (Temps Coordonnée Barycentrique) with an offset of 2 455 197.5 days is applied (corresponding to a reference time T0 at 2010-01-01T00:00:00) to have a conveniently small numerical value.

bjd_fov1 : Time [Julian Date in TCB at barycentre for FOV1 - 2455197.5] (double, Time[Barycentric JD in TCB - 2455197.5 (day)])

Observation time in units of Barycentric JD (in TCB) in days -2 455 197.5, computed as follows. First the observation time is converted from On-board Mission Time (OBMT) into Julian date in TCB (Temps Coordonnée Barycentrique). Next a correction is applied for the light-travel time to the Solar system barycentre corresponding to an infinitely distant source at (ra_fov1, dec_fov1), resulting in Barycentric Julian Date (BJD). Finally, an offset of 2 455 197.5 days is applied (corresponding to a reference time T0 at 2010-01-01T00:00:00) to have a conveniently small numerical value.

bjd_fov2 : Time [Julian Date in TCB at barycentre for FOV2 - 2455197.5] (double, Time[Barycentric JD in TCB - 2455197.5 (day)])

Observation time in units of Barycentric JD (in TCB) in days -2 455 197.5, computed as follows. First the observation time is converted from On-board Mission Time (OBMT) into Julian date in TCB (Temps Coordonnée Barycentrique). Next a correction is applied for the light-travel time to the Solar system barycentre corresponding to an infinitely distant source at (ra_fov2, dec_fov2), resulting in Barycentric Julian Date (BJD). Finally, an offset of 2 455 197.5 days is applied (corresponding to a reference time T0 at 2010-01-01T00:00:00) to have a conveniently small numerical value.

obmt_time : Time at Gaia (OBMT) (long, Time[OBMT])

Observation time at Gaia converted to OBMT using the HATT (High Accuracy Time Transformation).

ra_fov1 : Right Ascension of FOV1 centre (float, Angle[deg])

Barycentric Right Ascension α of Field of View 1 (preceding) in ICRS at given time.

dec_fov1 : Declination of FOV1 centre (float, Angle[deg])

Barycentric Declination δ of Field of View 1 (preceding) in ICRS at given time.

heal_pix_fov1 : FOV1 HEALPix level 12 (int)

Level 12 nested scheme HEALPix containing the Field of View 1 (preceding) right ascension and declination.

This field can be used in conjunction with source_id, whose most significant bits contain HEALPix information.

scan_angle_fov1 : Scan position angle of FOV1 (float, Angle[deg])

Field of View 1 (preceding) scan angle.

ra_fov2 : Right ascension of FOV2 centre (float, Angle[deg])

Barycentric Right Ascension α of Field of View 2 (following) in ICRS at given time.

dec_fov2 : Declination of FOV2 centre (float, Angle[deg])

Barycentric Declination δ of Field of View 2 (preceding) in ICRS at given time.

heal_pix_fov2 : FOV2 HEALPix level 12 (int)

Level 12 nested scheme HEALPix containing the Field of View 2 (following) right ascension and declination.

This field can be used in conjunction with source_id, whose most significant bits contain HEALPix information.

scan_angle_fov2 : Scan position angle of FOV2 (float, Angle[deg])

Field of View 2 (following) scan angle.

solution_id : Solution Identifier (long)

All Gaia data processed by the Data Processing and Analysis Consortium comes tagged with a solution identifier. This is a numeric field attached to each table row that can be used to unequivocally identify the version of all the subsystems that where used in the generation of the data as well as the input data used. It is mainly for internal DPAC use but is included in the published data releases to enable end users to examine the provenance of processed data products. To decode a given solution ID visit https://gaia.esac.esa.int/decoder/solnDecoder.jsp