PDS 456 is the most luminous object in the local universe and has one of the most extreme X-ray and UV spectra of any active galactic nucleus. The X-ray data reveal a massive, ionised outflow, which carries as much kinetic energy as the bolometric output of this quasar. Such outflows are thought to be disk driven, should be common when AGN are feeding/merging and may explain the M - sigma relation through a feedback mechanism. Situated at low redshift (z=0.184), PDS 456 provides an excellent opportunity to study such an outflow in detail. A short HST/STIS observation taken in 2000 also revealed an extraordinary UV spectrum, including the most blueshifted CIV emission line know (at -5000 km s-1) and a broad, blueshifted absorption feature probably due either to Ly-alpha or NV 1240 angstroms. More recently, PDS 456 has been shown to have a highly variable X-ray spectrum, and during 2013/14 we will monitor the outflow in this quasar using large programs allocated on Suzaku and XMM-Newton. Here we request a HST/COS observation towards the end of the X-ray monitoring campaign to provide contemporaneous UV data in order to constrain the physical properties of the outflow. In particular, we wish to search for associated UV/X-ray absorption features and relate the UV emission and absorption characteristics of PDS 456. We propose a HST/COS observation of PDS~456 using the G140L grating which give a suitable velocity resolution over the required wavelength range from blueward of Ly$\alpha$ to redward of HeII $\lambda 1640$. We adopt the previous HST/STIS spectrum as a guide to the target flux, and assume no major UV continuum variability. Comparing the HST/STIS observed flux with the XMM-Newton OM UV magnitudes provides fluxes consistent to well within a factor of two. Simulations show that we can measure the required spectral features with sufficient accuracy to achieve the science goals of our proposal by attaining a signal-to-noise ratio of 20-30 per HST/COS spectral resolution element. We require this S/N particularly at the location of the broad absorption feature (observed wavelength $\approx 1350$\AA) and the NV and CIV emission lines (observed wavelengths $\approx 1450$ \AA\ and 1800\AA). We have used the on-line HST exposure-time calculators to determine and compare the required exposure times. From these we have concluded that HST/COS is more efficient than HST/STIS and hence is our chosen instrument. To cover the spectral range required, we request observations using the G140L grating for 95 minutes. The previous HST/STIS observation shows that the host-galaxy of PDS~456 does not contaminate the UV spectrum significantly. Given the declination of PDS~456, we have 54 minutes per orbit of visibility. AGN/QUASARS Default false true false false false SUPPORTED GALAXY QSO QUASAR ULTRALUMINOUS IR GAL This object was generated by the targetselector and retrieved from the SIMBAD database. J2000 J2000 The observed continuum flux at 1350 angstroms is aproximately 4e-15 erg cm^-2 s^-1 COS/FUV Spectroscopic No No G140L 1280 The scheduling data was generated by CASM 21.2. 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