Climate records are generally assembled from observations made for other purposes, with the result that climate trends are often estimated from unevenly sampled data collected by a variety of measurement systems. The sampling distribution, instrument type, and measurement bias all change over time, making it difficult to distinguish trends in climate from trends in the way the measurements are made and analyzed. Current operational sounders are approaching the level of performance needed for climate studies, but still lack the absolute accuracy required to produce a continuous long term record from multiple sensors. By including internal calibration sources traceable to international standards and an independent verification system, the Climate Absolute Radiance and Refractivity Observatory (CLARREO) mission illustrates one concept for a key component of a true climate observing system.