The compatibility of DORIS with VGOS

The most precise global geodetic reference frame, the International Terrestrial Reference Frame (ITRF), is based on four space geodesy techniques: Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS) and the Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) System. All four techniques complement each other with their observations for the terrestrial reference frames, because of the individual advantage: VLBI is unique for the Earth orientation parameters and the tie of ITRF to the International Celestial Reference Frame (ICRF), SLR is strong in the determination of the centre of mass of the planet Earth and of the scale in the ITRF, GNSS is good for densification of global networks and of orientation and DORIS technique is unique in the most homogeneous global network of reference sites. In order to combine these advantages in a synergetic way, a co-location of these techniques in geodetic observatories is an objective for progress in global geodesy.

The question of how the desired co-location of DORIS at a VLBI site (or vice-versa) can be achieved is under permanent discussion. The VLBI systems are designed to receive extremely faint cosmic signals down to -110 dBm, whereas the DORIS beacon emits signals at a frequency of 2,036 MHz with 40 dBm output power. There is a potential for coupling between DORIS emissions and the VLBI receiving chain generating spurious signals. A risk of overloading or even damaging the VLBI low noise amplifiers (LNA) is possible. Even if in VLBI the same frequency is not being observed, in the worst case the LNA of the VLBI receiver could be saturated by DORIS transmission leading to useless VLBI observations.

Meanwhile, several geodetic observatories collected measurements, made studies, and even co-located active DORIS beacons. The CRAF-VGOS group presents a compatibility study in pycraf with simulations of exclusion zones for DORIS with respect to VGOS radio telescopes. It collates site-specific experiences and may be helpful for future decisions on how to co-locate both techniques at new sites.