Radio frequency compatibility of VGOS and DORIS stations

Within the framework of the Global Geodetic Observing System (GGOS), the co–location of space and satellite geodetic techniques is strongly promoted to achieve the GGOS goal of establishing a network of stations with 1 mm position accuracy and 0.1 mm/yr in stability. The lastest realization of ITRF, ITRF2020, includes only a number of 10 local ties between VLBI and DORIS stations, so there is a high demand to increase the number of VLBI and VGOS stations co-located with DORIS at the same site. The installation of a DORIS beacon at a VGOS site seems difficult without affecting radio astronomical measurements. In fact, VLBI systems are designed to receive extremely faint cosmic signals down to –110 dBm, whereas the DORIS beacon emits signals at a frequencies of 401.25 MHz and 2.036 GHz with 40 dBm output power. Although the lower frequency is outside the frequency range of a VGOS system (2 GHz–14 GHz), the higher frequency falls within the lower S-band. It is therefore highly recommended to evaluate each radio telescope site carefully before installing a DORIS system in its vicinity. We present compatibility studies, carried out with Pycraf (open source software) under different scenarios, to investigate different types of exclusion zones with respect to the new Matera VGOS radio telescope. Path propagation loss, effective antenna gains, and available free digital terrain models have been considered. The results can be used to search for potential sites for a new DORIS station in the area. The analysis also considers real cases from individual European VGOS observatories with experience of installing DORIS stations.