RGS data are individual events. The wavelength, , assigned to a photon of spectral order is determined by the diffraction equation
where is the grating spacing. The dispersion angle is fixed by the photon's position on the CCD detector and the constant geometry of the detector and grating assemblies. The only remaining variable, therefore, is , the angle of incidence of light on the gratings so that the X-ray photon wavelength reduces to a function of the source celestial coordinates only:
and it is the analyst's responsibility to make sure that they are as accurate as possible. Differentiation of the diffraction equation shows that a systematic error of up to 2.3 mÅ is introduced for every arcsecond error in the source coordinates. The PPS procedure makes an automatic choice between the proposal's pointing coordinates and X-ray coordinates derived from the simultaneous EPIC images, both of which have been known to be unreliable for a number of reasons. It is therefore worthwhile to take as much care as possible over the coordinates used in the analysis including proper motion as necessary. The SIMBAD  and NED  reference databases are excellent in this regard. The source coordinates required by rgsproc are thus best explicitly supplied by the analyst in decimal degrees to six decimal places, wherever possible. In the absence of any better information, the proposal pointing coordinates are used by default.