Issues related to the non-co-location of different observing systems are currently being studied and depend critically on the horizontal distance as well as the local geography. Another important aspect is whether a non co-located routine sounding site is able to modify or amend current routine observations. Additional ground checks or more extensive meta data collection may be required to satisfy the needs of GRUAN. Thus it is important to understand the level of co-operation or control that a GRUAN site has over a non co-located radiosonde site. The critical question will be, whether it is possible to establish and verify measurement uncertainties for the routine sonde operations and whether it will be possible to compare these results in a quantitative fashion with remote sensing or special sonde operations at the core GRUAN site. If a quantitative connection between two non co-located sites cannot be established, their combined use may be limited.