A Coherent Ka Band Transponder for Doppler Detection of Gravitational Waves

According to the Einstein´s theory, oscillations of large masses generate variations of gravity acceleration. This type of energy may propagate over long distances by a mode called gravitational waves. A possible method for gravitational waves detection is the measure of the relative velocity variations of two masses over a time interval ¿ equal to the expected perturbation period. This measurement becomes signifcant for masses distances greater than c.¿ (c=3E8 m/sec ; ¿=1000 sec), so a pratical testing system may be an Earth Station and an interplanetary spacecraft equipped for high precision relative velocity evaluation performed via Doppler shift detection. This approach needs a very stable microwave signal transmitted from the Earth Station and a phase coherent Transponder on the spacecraft. The gravitational wave characteristics are measured at the ground station comparing the phase of transmitted and receivede signal. The paper describes the design and realization aspects relevant to the implementation of the on-board transponder operating in Ka band. The equipment will be employed for the gravitational waves experiment in the frame of the NASA/ASI Cassini mission and its main characteristic is a low intrinsic phase instability due to the internal phase noise contributors.