Laboratory Characterization of Optical Inter-satellite Links for Future GNSS

Optical inter-satellite links are proposed in the Kepler constellation to connect satellites in a Global Navigation Satellite System (GNNS) constellation for optical ranging, time transfer and data transmission [1]. A laboratory demonstrator is being developed to verify all three aspects. The demonstrator is constituted by two terminals, performing a bidirectional free-space optical link in the laboratory, with single-mode fiber coupling in the receivers at both sites. The optomechanics is based on commercial off-the-shelf (COTS) components. The optical terminal includes a point-ahead assembly, which compensates for the point-ahead angle (PAA) between the two linked satellites. The absence of the PAA under laboratory conditions allows this mirror to be used for pointing jitter emulation instead, i.e. to emulate the expected satellite platform angular vibrations. The ranging is performed by using a 25.55 Gc/s binary phase shift keying (BPSK) phase modulation of the optical carrier. This high modulationrate allows ranging accuracy in the order of 100 µm. The data-communication channel is multiplexed to the ranging signal at a rate of 50 Mb/s and allows exchanging satellite and timing information. Both the optical carrier and the spreading sequence are synchronized to the on-board reference and a reference clock input for the sequence generator Field Programmable Gate Array (FPGA) is generated. Thus, the information exchanged though the data communication channel and precise ranging are used to support highly accurate two-way time transfer between the linked satellites. The objective of this paper is to present the hardware developments of optical terminals, which will demonstrate range measurements and communications. The first characterizations of the main terminal components are shown, namely the real-time Digital Signal Processing (DSP) system based on FPGA for ranging and data transfer and the optical phase-locked-loop (OPLL) for locking and tracking the incoming optical carrier. Further, static homodyne ranging estimation is evaluated and the impact on the frequency stability of the optical assembly and other main components is shown.