Spectroscopic measurements of asteroids allow mitigation of differential color refraction effects on ground-based astrometry and orbit prediction accuracy

Data collected with ground-based telescopes accounts for the overwhelming majority of astrometric observations of mainbelt and near-Earth asteroids. Earth's atmosphere subjects these measurements to random error from seeing and to systematic bias from differential color refraction (DCR). The DCR bias when observing solar-illuminated targets with nonuniform spectral reflectances and using non-solar-analog stars as fiducials can be several tens of milliarcseconds, even at low air-mass. The direction of DCR bias is aligned with local vertical at the observing telescope and its varying orientation in inertial space masks its signature in aggregate error analysis performed in inertial coordinates. Until recently, DCR effects of tens of milliarcseconds were dominated by the hundreds of milliarcseconds of systematic bias present in astrometric star catalogs. Improvements in the accuracy of catalogs beginning in 2015 with the 30 milliarcsecond URAT1 catalog, the 2017 publication of the 25 milliarcsecond UCAC5 catalog, and the forthcoming sub-milliarcsecond GAIA catalog have lowered the error floor on achievable accuracy to the point where DCR is now the dominant systematic bias in data taken from the ground. DCR bias depends on the spectral quantum efficiency of the observing instrument, the spectral reflectance of the target, and the spectral types of the fiducial stars. To realize the benefit of star catalogs accurate below the 30 milliarcsecond level, spectroscopic measurements of asteroids and fiducial stars are necessary to correct for DCR bias.