Radio Science measurements using phase modulated optical links

Abstract

Radio Science experiments currently rely on unmodulated continuous wave RF signal carrier for spectral purity and maximized signal-to-noise ratio. This requires missions to carefully schedule them away from periods of high rate telemetry. In the era of optical communications, currently designed systems experience the same problem. In this paper, a data processing architecture is derived that will yield high-accuracy link science type of information on the ground from readily transmitted communication signals coming from space assets, through optical links. This technique is intended to save power, bandwidth and scheduling demands on the spacecraft. Our proposed technical approach is applicable to a phase modulated laser thus providing an architectural improvement to present state-of-theart optical communication systems utilized by NASA as well as to future systems. The approach is to, first, obtain the achievable performance of Radio Science measurements from the received optical telemetry signals. This extends our previous results presented on Radio Science measurements for suppressed carrier RF signals. Secondly, a practical system is proposed that approaches the ultimate theoretical performance for estimating the amplitude, phase, and frequency variations due to the changes in the planet atmosphere. For optical links, our previous results to phase modulated CW laser communications are extended. The same information required for radio science data can be extracted by using either differential methods of encoding or phase modulated orthogonal signals and at the optical receiver a non-coherent local laser and an array of photon detectors are used. The performance of these phase modulated schemes is analyzed.