Aerospace and Electronic Systems Magazine April 2018 - 19

Huang and Lin

Figure 3.

Topology of the PV-cell uplink with internal reverse biasing. The spacecraft EPS board performs all functions in an integrated module. Only 3 of 10
cells are depicted.

parabolic reflector attachments. Because no suitable lightweight
component was found for the large aperture of the Luminus LEDs,
a set of additively manufactured lenses based on a SLA (stereolithography) process will be produced and subsequently processed
for a suitable surface finish.
The Calypso payload implements a downlink LED driver with
140-W peak power and a 40 Mb/s bandwidth limit. Scaling down
a transmitter of this power represented a sizable technical obstacle
due to thermal dissipation.
This was mitigated through the low specified duty cycle, which
restricts the transmitter to 2 minutes on-time per orbit. An Altera
Cyclone IV handles the stream processing and interfacing with the
OBC. The field-programmable gate array (FPGA), together with

a Raspberry Pi compute module, implements an end-to-end, deep
learning-based signal processing system that drives a 125 MS/s
(Megasamples per second), 14-bit DAC (digital-to-analog converter). This produces a 16-QAM (16-quadrature amplitude modulation) modulated signal that is fed into four high-powered bias tees
after amplification. A dedicated high-discharge battery in the EPS
feeds the synchronized, pulsed metal-oxide semiconductor fieldeffect transistor LED driver.
The module is contained within a single PC104 form-factor
printed circuit board mounted on the Z+ face of the CubeSat,
weighing 150 g. An integrated, 7075 aluminum housing on the
back of the unit is directly coupled to the LEDs and switching circuitry to act as both an electromagnetic interference shield and a
heat sink. Prototypes of the driver circuitry have been constructed,
as shown in Figure 5.
The units have been tested rigorously in Aphelion's facilities
for power characteristics, modulation response, and thermal dissipation under vacuum conditions, with setups used for component
characterization shown in Figures 6 and 7. A second revision will
be made in accordance with the testing, which called for a higherpowered preamplifier.

GROUND STATION AND COMPENSATION

Figure 4.

Luminus SBT-90 LED with an evaluated COTS TIR lens.

APRIL 2018

Free space optical communication systems need to compensate for
channel impairments such as atmospheric distortion and the frequency response of hardware components (i.e., transmitter LEDs).
To account for such problems using an economical and lightweight
method, an innovative method using deep learning was devised.
Using the research of O'Shea and Hoydis [8], we modeled the
downlink as an autoencoder, an unsupervised learning device used
to map high-dimensional data to lower dimensions, acting like a
IEEE A&E SYSTEMS MAGAZINE

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