Session
Poster Session 1
Location
Salt Palace Convention Center, Salt Lake City, UT
Abstract
The Polarization-modUlated Laser Satellite Experiment (PULSE-A) is the University of Chicago’s student-led mission to demonstrate an optical downlink at a data rate of 1 to 10 Mbps using circular polarization shift keying (CPolSK). PULSE-A comprises a 3U CubeSat bus carrying a < 1.5U optical transmission terminal and a dual optical-RF ground station. The ground station (GS) system consists of the optical ground station (OGS) and the RF ground station (RFGS). The RFGS is responsible for standard communications and control tasks, while the experimental OGS receives the optical transmission from the satellite’s payload.
To perform satellite-to-ground optical communications, the OGS needs to track, receive, and decode the transmitted signal while providing its own beacon, which allows the satellite to track the OGS. These requirements are accomplished by four assemblies within the OGS: tracking, polarization state preparation, signal decoding, and beacon. The tracking assembly collects, condenses, and collimates incoming laser light from the satellite using an 11” Schmidt-Cassegrain telescope and an optical assembly, which performs fine tracking of the satellite. The polarization state preparation assembly separates light by its left- or right-handed circularly polarized states. The received circularly polarized transmission laser from the payload passes through a quarter-wave plate, converting it into linearly polarized light. This light is then split by a polarizing beam splitter into two separate paths, corresponding to data sent with left- or right-handed circular polarization. Next, the signal-decoding assembly converts the separated polarized light into two voltage channels. These two channels correspond to 1 or 0 bits, which are then digitized by an FPGA. The GS beacon assembly includes the ground beacon laser that enables the satellite to track the ground station.
Since optical communications are directional, the OGS must track the satellite with a pointing accuracy of 1.4 mrad. This requires the GS to conduct a high-precision pointing, acquisition, and tracking (PAT) sequence. First, the RFGS receives telemetry and orbital data, which is passed to the OGS to estimate the satellite’s orbital path. The OGS then coarsely points the telescope towards the satellite’s predicted position. Then, the satellite illuminates the telescope with a beacon and a transmission laser. The payload beacon laser enables a feedback loop which maintains pointing accuracy by moving the telescope and a fast-steering mirror within the OGS in response to deviations in the laser’s position from the center of a tracking camera. This enables the OGS to perform fine corrections to its orbital predictions. As a consequence, the OGS tracks the transmission laser, enabling it to decode the CPolSK transmission.
We present an overview of the design of the GS, focusing on implementing a polarization-based optical communications receiver. This work explores the design of polarization-based optics for space-to-ground optical communications with emphasis on the challenges associated with the directionality of optical communications. We will also cover our independent studies on circular polarization, and how the results affect our design choices.
Document Type
Event
Development of a Polarization-Based Optical Communications Ground Station for the PULSE-A CubeSat
Salt Palace Convention Center, Salt Lake City, UT
The Polarization-modUlated Laser Satellite Experiment (PULSE-A) is the University of Chicago’s student-led mission to demonstrate an optical downlink at a data rate of 1 to 10 Mbps using circular polarization shift keying (CPolSK). PULSE-A comprises a 3U CubeSat bus carrying a < 1.5U optical transmission terminal and a dual optical-RF ground station. The ground station (GS) system consists of the optical ground station (OGS) and the RF ground station (RFGS). The RFGS is responsible for standard communications and control tasks, while the experimental OGS receives the optical transmission from the satellite’s payload.
To perform satellite-to-ground optical communications, the OGS needs to track, receive, and decode the transmitted signal while providing its own beacon, which allows the satellite to track the OGS. These requirements are accomplished by four assemblies within the OGS: tracking, polarization state preparation, signal decoding, and beacon. The tracking assembly collects, condenses, and collimates incoming laser light from the satellite using an 11” Schmidt-Cassegrain telescope and an optical assembly, which performs fine tracking of the satellite. The polarization state preparation assembly separates light by its left- or right-handed circularly polarized states. The received circularly polarized transmission laser from the payload passes through a quarter-wave plate, converting it into linearly polarized light. This light is then split by a polarizing beam splitter into two separate paths, corresponding to data sent with left- or right-handed circular polarization. Next, the signal-decoding assembly converts the separated polarized light into two voltage channels. These two channels correspond to 1 or 0 bits, which are then digitized by an FPGA. The GS beacon assembly includes the ground beacon laser that enables the satellite to track the ground station.
Since optical communications are directional, the OGS must track the satellite with a pointing accuracy of 1.4 mrad. This requires the GS to conduct a high-precision pointing, acquisition, and tracking (PAT) sequence. First, the RFGS receives telemetry and orbital data, which is passed to the OGS to estimate the satellite’s orbital path. The OGS then coarsely points the telescope towards the satellite’s predicted position. Then, the satellite illuminates the telescope with a beacon and a transmission laser. The payload beacon laser enables a feedback loop which maintains pointing accuracy by moving the telescope and a fast-steering mirror within the OGS in response to deviations in the laser’s position from the center of a tracking camera. This enables the OGS to perform fine corrections to its orbital predictions. As a consequence, the OGS tracks the transmission laser, enabling it to decode the CPolSK transmission.
We present an overview of the design of the GS, focusing on implementing a polarization-based optical communications receiver. This work explores the design of polarization-based optics for space-to-ground optical communications with emphasis on the challenges associated with the directionality of optical communications. We will also cover our independent studies on circular polarization, and how the results affect our design choices.
