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gaia early data release 3 documentation

1.3 Spacecraft and ground-segment status applicable to Gaia Early Data Release 3

1.3.2 Implementation of the scanning law

Author(s): Jos de Bruijne, Gonzalo Gracia-Abril, Neil Cheek, Cian Crowley, Claus Fabricius, Juanma Martín-Fleitas, Alex Hutton, Alcione Mora

In the time range applicable to Gaia EDR3 (25 July 2014 to 28 May 2017), two distinct scanning laws have been employed (Table 1.8). Both use a fixed Solar-aspect angle of ξ=45(this is the angle between the spacecraft spin axis and the Sun; Figure 1.8) and a fixed spin rate of ω=59.9605 arcsec s-1. The scanning law that has been commanded to the spacecraft during the time interval corresponding to Gaia EDR3 is part of the release and is described in Section 12.4.1. The difference between the commanded attitude and the actual attitude is less than 30 arcsec, which is sufficient for the scientific objectives of the mission (what matters is strict spin-rate control and knowledge of the pointing). More details on the scanning law can be found in Section 1.1.4 and Gaia Collaboration et al. (2016b).

Table 1.8: Scanning laws used in Gaia EDR3. Both laws share the same solar-aspect angle (ξ=45) and spin rate (ω=59.9605 arcsec s-1). The actual transition from the EPSL to the NSL in August 2014 was continuous and spanned revolutions 1189.3–1192.1. OBMT stands for on-board mission time(line) in units of six-hour revolutions since launch.
Start date Start OBMT Scanning law
25-07-2014 1078.4 EPSL following
22-08-2014 1192.1 NSL
25-09-2014 1326.7 NSL spin- and precession-phase change
08-02-2017 4795.4 NSL spin-phase change
EPSL

This special ecliptic-poles scanning law, during which the spin axis of the spacecraft stays in the ecliptic plane (Section 1.1.4), has been designed to bootstrap calibrations in the science ground segment. This scanning law has been active during the initial 28 days of the nominal mission, between 25 July 2014 (OBMT 1078.4) and 22 August 2014 (OBMT 1192.1; Table 1.8). The EPSL was used in the ‘following’ variant such that the spin axis follows (trails) the Sun by ξ=45on the ecliptic; this is equivalent to a precession phase ν=180(see Gaia Collaboration et al. 2016b for the definition of the precession phase ν and spin phase Ω).

NSL

This is the nominal scanning law of Gaia with an annual precession rate of 5.8 revolutions around the Solar direction at a fixed Solar-aspect angle of ξ=45(Section 1.1.4). The smooth transition from the EPSL to this scanning law took place on 22 August 2014 (Table 1.8). On 25 September 2014 (OBMT 1326.7), a discontinuity in the precession phase (ν) and spin phase (Ω) were introduced and a further spin-phase change was activated on 8 February 2017 (OBMT 4795.4). These operations were performed to optimise the conditions of quadrupole-light-deflection observations close to Jupiter’s limb as part of the GAREQ experiment (e.g., de Bruijne et al. 2010). Results of this experiment are not part of Gaia EDR3.