On-demand, Dynamic Reconfigurable Broadcast Technology for Space Laser Communication

NASA Glenn's researchers have developed a means of transporting multiple radio frequency carriers through a common optical beam. In contrast to RF infrastructure systems alone, this type of hybrid RF/optical system can provide a very high data-capacity signal communication and significantly reduce power, volume, and complexity. Based on an optical wavelength division multiplexing (WDM) technique, in which optical wavelengths are generated by a tunable diode laser (TDL), the system enables multiple microwave bands to be combined and transmitted all in one unit. The WDM technique uses a different optical wavelength to carry each separate and independent high-frequency microwave band (e.g., L, C, X, Ku, Ka, Q, or higher bands). Since each RF carrier operates at a different optical wavelength, the tunable diode laser can, with the use of an electronic tunable laser controller unit, adjust the spacing wavelength and thereby minimize any crosstalk effect. Glenn's novel design features a tunable laser, configured to generate multiple optical wavelengths, along with an optical transmitter. The optical transmitter modulates each of the optical wavelengths with a corresponding RF band and then encodes each of the modulated optical wavelengths onto a single laser beam. In this way, the system can transmit multiple radio frequency bands using a single laser beam. Glenn's groundbreaking concept can greatly improve the system flexibility and scalability - not to mention the cost of - both ground and space communications.

This invention provides a new method for optical data transmissions from satellites using laser arrays for laser beam pointing. The system is simple, static, compact, and provides accurate pointing, acquisition, and tracking (PAT). It combines a lens system and a vertical-cavity surface-emitting laser VCSEL)/Photodetector Array, both mature technologies, in a novel way for PAT. It can improve the PAT system's size, weight, and power (SWaP) in comparison to current systems. Preliminary analysis indicates that this system is applicable to transmissions between satellites in low-Earth orbit (LEO) and ground terminals. Computer simulations using this design have been made for the application of this innovation to a CubeSat in LEO. The computer simulations included modeling the laser source and diffraction effects due to wave optics. The pointing used a diffraction limited lens system and a VCSEL array. These capabilities make it possible to model laser beam propagation over long space communication distances. Laser beam pointing is very challenging for LEO, including science missions. Current architectures use dynamical systems, (i.e., moving parts, e.g., fast-steering mirrors (FSM), and/or gimbals) to turn the laser to point to the ground terminal, and some use vibration isolation platforms as well. This static system has the potential to replace the current dynamic systems and vibration isolation platforms, dependent on studies for the particular application. For these electro-optical systems, reaction times to pointing changes and vibrations are on the nanosecond time scale, much faster than those for mechanical systems. For LEO terminals, slew rates are not a concern with this new system.

NASA Goddard Space Flight Center has developed a 3D lidar system that consists of microelectromechanical systems (MEMS) beam steering, high performance reconfigurable computing, and an in-depth understanding of systems level integration. Kodiak combines a 3D MEMS scanning lidar with a long range narrow FOV telescope to produce a flexible and capable space flight ranging system. Also included is SpaceCube-level processing power to host a variety of algorithms enabling sensing and 6 degrees of freedom.

The circularly polarized antenna features an integrated prime-fed S-band and Cassegrain-based Ka-band reflector system. The Cassegrain primary and secondary reflectors are specially shaped for optimal Ka-band gain, while a frequency selective surface on the secondary reflector provides reflectivity at Ka-band, and acts as a transparent dielectric radome for the S-band feed antenna. Design innovations include an improved S-band feed antenna and cross-polarization compensation, improved Ka-band horn, and special shaping of the secondary reflector for ease of fabrication. The technology improves upon prior S-band feed antenna designs to provide mechanical robustness and low cross-polarization over a wide field of view. It also improves the front-to-back ratio, providing much higher signal radiated forward, over an increased bandwidth as well. The Ka-band feed horn is based on a prior NASA innovation, the standard Potter horn, but in this innovation has significantly lower sidelobe level performance. This is achieved by using a modified smooth s-curved interior horn profile rather the the typical conical/cylindrical form typical of the standard Potter horn design. The smooth interior wall is also easier to fabricate than alternative corrugated wall designs. Additionally, a cross-polarization cancellation cup is integrated with the Ka-band horn geometry, with the cup being placed around the neck of the horn in the form of a collar, allowing the two to be fabricated together.

For optical modulator drivers, it is important to minimize timing skew in the transmitters to reduce channel distortions. Typical solutions rely on tight tolerances in the design of the path lengths and the use of matched DACs or modulator drivers with built-in channel deskewing provisions. NASA’s novel FPGA design directly drives the optical modulator without DACs or external drivers. This method can reliably align eight 5.76GHz transmitters within 100ps of each other using the built-in transmitter phase interpolator Pulse Position Modulation (PPM) controller and feedback from the optical transceiver. The algorithm is broken down into sections that iteratively align the transmitter channels, relying on the built-in transmitter phase interpolator PPM controller and feedback from the optical transceiver captured on ADCs connected to the FPGA receiver. There are three key differentiating components/methods of the NASA technology. 1. The balun network eliminates the need for high-power expensive DACs or modulator driers to generate the multi-level signal required to drive the optics. 2. The use of the FPGA transmitter phase interpolator PPM controller to deskew the channels. 3. The use of a virtual ADC, standard deviation, thresholding, and moving-average processing techniques on loopback data from a QPSK transceiver for the purposes of transmitter channel deskewing.