  We present an analysis of coronal mass ejections (CMEs) observed by the
Heliospheric Imagers (HIs) on board NASA's Solar Terrestrial Relations
Observatory (STEREO) spacecraft. Between August 2008 and April 2014 we identify
273 CMEs that are observed simultaneously, by the HIs on both spacecraft. For
each CME, we track the observed leading edge, as a function of time, from both
vantage points, and apply the Stereoscopic Self-Similar Expansion (SSSE)
technique to infer their propagation throughout the inner heliosphere. The
technique is unable to accurately locate CMEs when their observed leading edge
passes between the spacecraft, however, we are able to successfully apply the
technique to 151, most of which occur once the spacecraft separation angle
exceeds 180 degrees, during solar maximum. We find that using a small
half-width to fit the CME can result in observed acceleration to unphysically
high velocities and that using a larger half-width can fail to accurately
locate the CMEs close to the Sun because the method does not account for CME
over-expansion in this region. Observed velocities from SSSE are found to agree
well with single-spacecraft (SSEF) analysis techniques applied to the same
events. CME propagation directions derived from SSSE and SSEF analysis agree
poorly because of known limitations present in the latter. This work was
carried out as part of the EU FP7 HELCATS (Heliospheric Cataloguing, Analysis
and Techniques Service) project (http://www.helcats-fp7.eu/).
