The Short-term Stability of a Simulated Differential Astrometric Reference Frame in theGaiaEra

Abstract

We use methods of differential astrometry to construct a small field inertial reference frame stable at the micro-arcsecond level. Such a high level of astrometric precision can be expected with the end-of-mission standard errors to be achieved with the Gaia space satellite using global astrometry. We harness Gaia measurements of field angles and look at the influence of the number of reference stars and the star's magnitude as well as astrometric systematics on the total error budget with the help of Gaia-like simulations around the Ecliptic Pole in a differential astrometric scenario. We find that the systematic errors are modeled and reliably estimated to the µas level even in fields with a modest number of 37 stars with G <13 mag over a 0.24 sq.degs. field of view for short time scales of the order of a day with high-cadence observations such as those around the North Ecliptic Pole during the EPSL scanning mode of Gaia for a perfect instrument. The inclusion of the geometric instrument model over such short time scales accounting for large-scale calibrations requires fainter stars down to G = 14 mag without diminishing the accuracy of the reference frame. We discuss several future perspectives of utilizing this methodology over different and longer timescales.

Differential astrometric framework for the Jupiter relativistic experiment with Gaia

“…In this paper we have further developed a differential astrometric framework for the analysis of Gaia observations, initially introduced in Abbas et al (2017). We have specifically targeted high-cadence, short timescale observations in one of the fields of the GAREQ experiment, which is set to provide an unprecedented measurement of the relativistic deflection of light due to Jupiter's quadrupole and a firsttime measurement in the optical of the light deflection due to Jupiter's monopole (expected to be ∼16mas for a grazing light, Crosta & Mignard 2006) .…”

“…10 in de Bruijne 2012). However, the advantage of using sources brighter than G ∼ 13 mag is that they will always be observed as two-dimensional images, and therefore have high-precision measurements in both coordinates, though the precision of the AC coordinate is a factor of five worse than the AL one due to the rectangular shape of the CCD pixels (de Bruijne 2005, also see Table 1 in Abbas et al 2017).…”

“…the fiducial position free from any calibrations. This operation has the added advantage of reducing the number of calibration unknowns by a factor of three with respect to the astrometric model used in Abbas et al 2017 as will be shown below.…”

“…In this paper we perform a differential astrometric analysis of Gaia simulated observations of a GAREQ event that occurred in February 2017, to gauge the best approach towards obtaining a reference frame stable at the µas level. The analysis described here improves on Abbas et al 2017 in several respects, including a) a novel implementation of the methodology to significantly reduce the number of unknown parameters, b) the inclusion of stellar parallaxes, and c) the investigation of the effects on the stability of the reference frame in the presence of unmodeled proper motions and parallaxes as well as when the known astrometric parameters are removed a priori.…”

“…In Abbas et al 2017 we provided an initial assessment of this approach using Gaia simulated high-cadence observations around the ecliptic poles as a backbone, finding that the so-defined local reference frame would be stable at the µas level with a modest number (30-40) of stars over a 0.24 × 0.24 deg field. In this paper we further develop our methodology, focusing on the observational scenario setup for the purpose of the Gaia relativistic light deflection experiment around Jupiter, particularly due to its flattened mass distribution that induces a quadrupole term (hence the name GAREQ -Gaia Relativistic Experiment on Jupiter's Quadrupole, Crosta & Mignard 2006;Crosta et al 2008a,b).…”