Validation, or comparison to ground-based reference measurements, is a necessary and ongoing task for any remote sensing instrument in orbit. It is especially important for ageing missions, such as the OSIRIS and ACE satellite instruments that have been measuring atmospheric constituents since 2001 and 2003. Validation, especially in the polar regions, is not an easy task however. The "ground truth" is difficult to come by, since ground-based retrievals face their own challenges at high latitudes. Any intercomparison exercise then has to deal with the problem of inhomogeneous atmospheric tracegas fields. Instruments measure along different lightpaths (direct-sun or scattered-light), and the location and time of the measurements needs to be taken into account. In this talk I will explore various ways of comparing multiple measurements of not quite the same thing. Metrics comparing two or three datasets provide insight into the distribution of the differences, while some effects of the diverse viewing geometries and measurement strategies can be corrected for. Using these tools, I will assess the relationship of ozone and NO2 measurements from OSIRIS and ACE to ground-based data from Eureka in the context of previous validation efforts. The results indicate that the satellite instruments continue to perform well, even after nearly two decades in orbit.