Active Pointing and Tracking System in Laser Satellite to Satellite Tracking (SST)

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.

The conventional approaches for measuring focused laser differential interferometry either use a single-point mechanism that cannot calculate velocity or a system that creates non-parallel beams in the testing zone, causing differences in time to travel between beams throughout the testing zone, adding a level of uncertainty to velocity measurements. For this technology, the inventors determined that the best approach is to use a method that ensures all laser beams propagating between the transmitter and receiver sides of the instrument are parallel to one another. This is done by crossing two orthogonally polarized beams at a Wollaston prism located just ahead of the field lens on the transmitter side of the FLDI. The polarization orientation of the two crossing beams must be at ±45 degrees to one another so that the Wollaston prism can further split the beams by a small angle (this gives the instrument its sensitivity to density fluctuations at each measurement point). The use of wedge prisms (that comprise the beam crossing system) to redirect the split beams such that they cross the optical axis minimizes any distortion imparted to the beams. This is in contrast to the use of a spherical focusing lens to redirect the split beams, which can impart undesirable distortions to the beams and affect the focusing properties of the FLDI instrument between its transmitter and receiver sides.

NASA Goddard Space Flight Center has developed a system to more finely point lasers so as to improve the precision of space optical communications and ranging. Through linking a laser beam mirror steering mechanism and associated closed loop control, any residual error in pointing to a desired target is reduced dramatically.

NASA Goddard Space Flight Center has developed a low cost, modular, and flexible space flight laser range finder consisting of optics, electronics, and interfaces for satellite servicing missions (i.e. Restore-L) using customized optics. Built upon previous NASA technologies, the system also consists of a high dynamic range receiver and adjustable laser for a wide range of measurements (i.e. multiples of km to sub-meter).

A new method is described for optical data transmissions from satellites using laser arrays for fine pointing of laser beams that use body pointing. It combines a small lens system and a VCSEL/Photodetector Array in a novel way to provide a fine pointing capability for laser beams that are pointed by body pointing of a CubeSat. As Fig. 1 shows, an incoming laser beam (green or blue, with rightward arrows), transmitted from a ground terminal, enters the lens system, which directs it to an element of the pixel array (gray rectangle). Each element, or pixel, consists of a VCSEL component/photodetector pair. The photodetector detects the incoming beam, and the VCSEL component returns a modulated beam to the lens system (green or blue, with leftward arrows), which sends it to the ground terminal. As the incoming beam changes direction, e.g., from the blue to the green incoming direction, this change is detected by the adjacent photodetector, and the laser paired with that photodetector is turned on to keep the outgoing laser beam on target. The laser beams overlap so that the returning beam continues to point at the ground terminal. The VCSEL component may consist of a single VCSEL or a cluster of VCSELs. Figure 2 shows the propagation of two overlapping laser beams. The system can very accurately point finely focused diffraction-limited laser beams. Also, simultaneous optical multiple access (OMA) is possible from different transceivers within the area covered by the laser array. For this electro-optical system, reaction times to pointing changes and vibrations are on the nanosecond time scale, much faster than mechanical fine pointing systems.