Ground-based Fourier Transform Infra-Red (FTIR) measurements are an important component of the global atmospheric monitoring system. Their essential role in validating satellite measurements requires a precise documentation of their quality. Here we present an extensive quality documentation of ground-based FTIR O<sub>3</sub> profiles. This is done in the form of theoretical and empirical error estimations. The latter is achieved by an intercomparison with ECC-sonde O<sub>3</sub> profiles. The FTIR O<sub>3</sub> amounts are obtained by applying the most advanced instrumentation and retrieval strategies and consequently represent the current potential of this remote sensing technique.