# Gaia scanning law - Gaia DR2 time period

Here is a representation of the Gaia scanning law over the 22 month time period covered by the Gaia Data Release 2 (from 2014-07-25 10:31:26 to 2016-05-23 11:36:26), 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 Lindegren et al. (2018) (astrometry) and Riello et al. (2018) (photometry).

#### Scanning law DATA

Please connect with an FTP client of your choice to ftp.cosmos.esa.int and navigate to the folder GAIA_PUBLIC_DATA/GaiaScanningLaw. The data can be found in this folder along with a readme.txt file. Please have a look at this readme file as well.

#### Sampling

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.

#### Columns

The columns in the output file are:

1. JulianDayNumberRefEpoch2010TCB@Gaia = Time [JD in TCB at Gaia - 2455197.5]
2. JulianDayNumberRefEpoch2010TCB@Barycentre_1 = Time [JD in TCB at barycentre for FOV1 - 2455197.5]
3. JulianDayNumberRefEpoch2010TCB@Barycentre_2 = Time [JD in TCB at barycentre for FOV2 - 2455197.5]
4. ra_FOV_1(deg) = Right ascension of FOV1 centre [deg]
5. dec_FOV_1(deg) = Declination of FOV1 centre [deg]
6. scanPositionAngle_FOV_1(deg) = Scan position angle of FOV1 [deg]
7. ra_FOV_2(deg) = Right ascension of FOV2 centre [deg]
8. dec_FOV_2(deg) = Declination of FOV2 centre [deg]
9. scanPositionAngle_FOV_2(deg) = Scan position angle of FOV2 [deg]

#### 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 deg 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.