RF Enhanced Laser Pointing

The Project

We propose an RF-enhanced optical acquisition system for use in ground-to-satellite communication systems. This proposed system has potential to reduce the time to acquire a link and increase link availability. Thus far, a literature review, design work, and project planning has been completed, and the next phase will be to put the plan into action. This report serves to communicate the project status, results of planning and initial analysis, our revised approach, and a timeline to complete the study.

As satellites produce and consume more data new communication techniques have been developed to accommodate these growing needs. Optical (laser) communication is the most logical solution to these increased requirements, prompting a number of missions to prove out optical communication capability. Unfortunately, optical communication between spacecraft and ground stations is difficult for a number of reasons. For one, the distances and orbital speeds involved in spacecraft orbits are large; for Low Earth Orbit ≤ 2000 km and around 7 km/s are typical.

For these speeds, distances, and beamwidths the requirement to point a satellite towards a ground station becomes difficult, requiring gimbals with extreme positioning performance, around 0.05◦ and accurate feedback. Although optical communication provides additional benefits, RF communication will remain the standard with optical being used for high-volume and low-latency missions. Current optical link acquisition algorithms scan the pointing uncertainty region until a strong link is found, similar processes are used for RF communications.

This process can be slow due to the narrow beamwidth of the optical beams. We propose enhancement by using the existing RF link to further close the pointing loop. This hybrid system would also increase link availability as light suffers high attenuation due to the atmosphere (clouds, rain), whereas RF can penetrate thick cloud layers. Prior work includes a number of techniques to improve link quality. Typically optical and RF acquisition will occur with some rough estimate of the spacecraft location through use of internal references. Using this internal reference the spacecraft will complete open-loop pointing, typically within 1.5◦ . After this initial pointing has finished the spacecraft will further decrease pointing error (Figure 1) in a closed-loop control system. Feedback for this closed-loop pointing varies, but some examples include received power (optical or RF) and Bit Error Rate (BER).

The goal of this project is to demonstrate a closed-loop system pointing system using both optical and RF feedback. Prior work closed-loop feedback comprises of a photocell array. The photocells produce an electrical current as a response to incoming light. This electrical response is amplified and read by an onboard microcontroller which then steers the spacecraft to reduce pointing error. In addition, we will expand the tested range greatly. In prior work the tested distances are on the order of 1 m whereas our test bed will operate at a distance of 1-2 orders of magnitude greater. Prior work also utilizes fast-steering mirrors whereas our project will use inexpensive consumer grade stepper motors. Finally, our project will utilize existing RF hardware and develop a phased patch antenna array as initial feedback for pointing. We will develop our control system in Simulink and compile for use on a Beaglebone Single Board Computer (SBC).

The Boards

Our project requires two PCBs: a photodiode amplification/sampling board and an RF patch antenna board.

The photodiode amplification circuit comprises of a quad cell photodetector and transimpedance amplifier and filtering. The outputs of these amplifiers are fed into a 2 ksps 24 bit 4 channel ADC and are then read by our microcontroller to provide feedback. The microcontroller commands two stepper motors for motion: a linear stage and a single axis gimbal.

The RF antenna board comprises of a stripline feed and SMT patch antenna. We provide communication over this 2.4 GHz RF link.