Optical terminals for inter-satellite links
The Kepler satellites carry an optical resonator, a frequency comb and terminals for two-way optical inter-satellite links. The latter links are the basis for time and frequency transfer within the constellation, providing the most precise synchronization that has ever been achieved amongst navigation satellites. With Kepler, the synchronization of the satellites does not depend on modeling assumptions as it is with present GNSSs. Furthermore, the two-way optical links also provide inter-satellite ranges with an accuracy of a few tens of micrometers.
The synchronization is performed using a spread spectrum modulated optical carrier. The spread spectrum signal is clocked at a rate of 511×50=25.55 GigaChips per second. This chip rate provides a ranging accuracy of the order of 40 µm and phase measurements with an in the order of 10 nm. The actual accuracy is much lower due to vibrations and other sources of disturbance. The communications signal for exchanging all the intra-system information, is modulated at 50 Megabit per second. At the transmitter, both signals are multiplexed and coherently modulated onto the laser. To this purpose, a BPSK modulation scheme is considered. The electrical and optical signals are synchronized by stabilizing the transmitter laser to the reference cavity and by using the frequency comb to provide the modulation signals. The combinations of both ranging and communication signals, together with the fact that the inter-satellite links are bidirectional, enables global synchronization at system level.
Two types of optical inter-satellite links are being developed: the first one for communications within the same MEO plane, to synchronize all satellites in the same MEO ring, and the second between MEO and LEO satellites, to synchronize across different MEO planes. The former type of links establishes and permanently keeps the connection between neighboring satellites.
Its design needs only to account for the small and slow-varying dynamics between connected platforms. The optical terminals for the latter type of links require a different approach, since each LEO satellite is required to connect with MEO satellites on different orbital planes, and the relative dynamics between LEO and MEO satellites is rather large. Three terminals are carried by each LEO satellite, and are continuously steered towards the connected MEO satellites.
The inter-satellite free-space path does not introduce any distortion of the optical beam wave-front, allowing single-mode fiber coupling at the receiver. Once in the fiber, each optical signal is mixed with a local oscillator and converted to the electrical domain by a balanced receiver. The local oscillator at the receiver is driven by an optical phase-locked-loop (OPLL), to match the frequency and the phase of the incoming light. A second loop (DLL) drives the local spread sequence, to perform the correlation with the received signal, and provides the code measurement. The correlation is done in the optical domain. Once the optical phase-locked-loop is closed, the receiver’s local oscillator is compared to the local cavity stabilized laser to measure the time and frequency offsets. Both transmitter and receiver paths are multiplexed by means of wavelength and polarization, to improve the isolation between both signal directions.
To verify this concept, a demonstrator terminal has been designed and two terminals are being built. These will implement a bidirectional link, with single-mode fiber coupling in the receivers at both sites. The demonstration will take place at the premises of the DLR Institute of Communications and Navigation. A 30 meters long free-space path between two optical laboratories is being set for this purpose. The orbital and satellite parameter such as the required optical power, the pointing accuracy and the divergence for a Kepler MEO satellite are scaled to obtain representative results.
During the system demonstration, the free-space channel between MEO satellites will be emulated. Pointing jitter due to platform vibrations will be induced in laboratory by driving a dedicated mirror with a time varying signal, which statistically describes the expected mispointing due to platform vibrations. The SNR fluctuations and the length jitter will be analyzed together with their impact on the ranging accuracy.