variety of studies including interstellar molecular spectroscopy (e.g. [@Blaszkiewicz2004]) and VLBI (e.g. [@Bartkiewicz2005]).

OCRA is a planned 30GHz 100–element continuum receiver (see [@Browne2000]). A prototype receiver, OCRA-p, funded by a grant from the Royal Society Paul Instrument fund, has been on the telescope since 2004. The instrument is described in detail in [@LowePhD] and [@Lowe2007]; here we present a short summary. The basic radiometer design is based on the prototype for the Planck Low Frequency Instrument (LFI, [@Mandolesi2000]), and is similar to the WMAP K-band receivers [@Jarosik2003]. OCRA-p provides only two horn–feeds, the beams of which are separated by $3'.1$ and have FWHM $1'.2$. As the beam separation is small, it is possible to reduce the effects of atmospheric and gain fluctuations by switching between the beams and taking the difference. This can be improved upon by further levels of switching. The full switching strategy for SZ observations is described in Section \[sec:obs\]. In addition to the SZ program, OCRA–p has been used to study planetary nebulae ([@Pazderska2009]) and for various radio source studies including flux density measurements to support the Very Small Array CMB program (see [@Gawronski2010] for details of the OCRA measurements, and Genova–Santos et al. (in preparation) 