The accurate monitoring of climate change im- poses strict requirements upon observing systems, in partic- ular regarding measurement accuracy and long-term stability. Currently available data records of the essential climate vari- ables (temperature- T , geopotential- p , humidity-RH, wind, and cloud properties) in the upper-air generally fail to fulfil such requirements. This raises serious issues about the abil- ity to detect, quantify and understand recent climate changes and their causes. GCOS is currently implementing a Ref- erence Upper-Air Network (GRUAN) in order to fill this major void within the global observing system. As part of the GRUAN implementation plan we provide herein funda- mental guidelines for establishing and maintaining reference quality atmospheric observations which are based on prin- cipal concepts of metrology, in particular traceability. It is argued that the detailed analysis of the uncertainty budget of a measurement technique is the critical step for achieving this goal. As we will demonstrate with an example, detailed knowledge of the calibration procedures and data process- ing algorithms are required for determining the uncertainty of each individual data point. Of particular importance is the careful assessment of the uncertainties introduced by correc- tion schemes adjusting for systematic effects.