# 1.3.2 The scanning law in practice

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

In the time range applicable to Gaia DR2 (25 July 2014 to 23 May 2016), three distinct scanning laws have been employed. All three use a fixed Solar aspect angle of $\xi=45^{\circ}$ (this is the angle between the spacecraft spin axis and the Sun) and a fixed spin rate of $\omega=59.9605$ arcsec s${}^{-1}$. More details on the scanning law can be found in Gaia Collaboration et al. (2016).

EPSL

This is a special, precession-less ecliptic-poles scanning law in which the spin axis of the spacecraft stays in the ecliptic plane. This scanning law was selected to bootstrap calibrations in the science ground segment. In this scanning law, the field-of-view directions scan the northern and southern ecliptic poles on each six-hour spin, giving an extremely dense / frequent sampling in these regions. 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). The EPSL comes in two variants, namely ’leading’, such that the spin axis leads the Sun by 45 ${}^{\circ}$ on the ecliptic, and ’following’, such that the spin axis follows (trails) the Sun by 45 ${}^{\circ}$ on the ecliptic. The EPSL was used in the ’following’ mode (equivalent to a precession phase $\nu=180$${}^{\circ}$; see Gaia Collaboration et al. 2016 for the definition of the precession phase $\nu$ and spin phase $\Omega$).

NSL

This is the nominal Gaia scanning law with an annual precession rate of 5.8 revolutions around the Solar direction. The smooth transition from the EPSL to this scanning law took place on 22 August 2014 (OBMT 1192.1).

NSL-GAREQ1

This scanning law was activated on 25 September 2014 (OBMT 1326.7). It is also nominal in the sense of producing 5.8 revolutions of the spin axis around the Solar direction per year but a discontinuity in the precession phase ($\nu$) and spin phase ($\Omega$) were introduced. This was done to optimize the conditions of quadrupole-light-deflection observations close to Jupiter’s limb as part of the GAREQ experiment (de Bruijne et al. 2010). Results of this experiment are not part of Gaia DR2.