Aerospace and Electronic Systems Magazine April 2018 - 18

Fully Optical Spacecraft Communications
Table 1.

Table 2.

Link Specifications

Transmit power

19.9 dBW

Transmit power

21.76 dBW

Transmitter gain

12 dBi

Transmitter gain

121.0 dBi

522 km

Path length

Path length
Free space path loss

260.5 dB

Free space path loss

Atmospheric loss

2 dB

Atmospheric loss

2 dB

Pointing loss

1 dB

Pointing loss

1 dB

Receiving aperture
Receiver gain
Received power
Bit rate (700 photons/bit)

30 cm

Receiving area

70 cm2

123.6 dBi

Receiver gain

108.9 dBi

−107.3 dBW

Received power

−8.12 dBW

8.85·10 bits/s
4

This is comparable to most high-performance UHF systems
available on the market today and offers a similar bit rate to laser
communication systems on the same scale. The overall system offers an attractive alternative to either solution due to its low cost
and reduced ADCS requirements. With a radiant flux of 8.16 ·
10−9 W/m2, the satellite will have a predicted visual magnitude of
around 1, making it visible under good seeing conditions.
The uplink laser will use a 150-W, YAG (yttrium aluminum
garnet)-pumped, 1,064-nm laser mounted coaxially to the downlink receiver and tracking setup via a 0.3-m aperture transmitter.
The ground station cost is of a lower concern, because such systems do not need to be made available to every spacecraft operator.
The implementation of a pixel array-based tracking system allows
for a tracking error of less than 1.2 arcsecond (3σ) without a fine
steering mirror and for spacecraft identification in emergencies in
which the exact orbit may not be known. A simple radiometric link
budget gives the received power of the link.
At a background irradiance of 1,000 W/m2 (worst-case scenario), with 8.12 dBW, or 0.154 W, of received power, it is predicted
that the peak-to-peak voltage of the signal is 0.45 V. This allows
for relatively easy detection using the electrical power system or
a dedicated channel. The data ceiling of the system is thus bound
mostly by the effective bandwidth of the solar cells.

IMPLEMENTATION
The proposed communication architecture, composed of a VLC
downlink and PV-cell uplink, will be flown on Calypso, a 1U CubeSat developed jointly between Aphelion Orbitals and the Aerospace
Research & Engineering Systems Institute. The two functions are
contained in a dedicated VLC downlink module and a custom EPS
(Electric Power System)-integrated uplink driver. This mission demonstrates the low volume and the ADCS requirements of the system
by implementing it in the given form factor and power budget.

CELL-BASED UPLINK
Reverse biasing of photodiodes is a common technique for improving bandwidth by increasing the number of photocarriers and
18

522 km
255.8 dB

improving drift velocity. The use of self-biasing on solar cells has
been investigated in the VLC industry [2]. This technique has been
demonstrated to improve the −3-dB bandwidth of a PV cell by up
to 60% using a 30-V bias provided by a lightweight upconverter.
Moreover, it was determined that minimal low-energy losses were
incurred, because biasing recovers significant energy expenditure
through increasing PV-cell efficiency.
The use of the uplink as the primary communication system
requires the receiver to be powered on at all times. Thus, we employed a low-power wakeup scheme using an envelope detector
and a 2-MS/s (megasamples per second) COTS ADC (analog-todigital converter), which polls all faces for a period of 100 ms each
at a reduced sampling rate. This scanning process can be implemented to consume minimal standby power as demonstrated by
similar implementations in commercial wakeup receivers [7]. The
ground station broadcasts a link start signal for 5s. When a clock
signal is recovered, the ADC selects the face with the best SNR
and processes telemetry data. The uplink receiver is described in
Figure 3.
One of the primary goals of this payload is to investigate the
feasibility of such a system as an emergency communication and
reset channel. The implementation of the receiver circuitry in the
EPS allows it to power cycle the spacecraft via dedicated control.
At the same time, the received telemetry can be delivered to the
OBC (onboard computer) to override radio link data.

VISIBLE LIGHT DOWNLINK
A survey of COTS LEDs has revealed that, though limited, a number of options are available that provide built-in lensing at a 20°
beam width. However, to open up the possibility for more efficient,
monolithically packaged, high-power diodes, additively manufactured, low-profile lenses were selected. They can be used without
major thermal concern due to the low duty cycle of the transmitter.
The implementation on Calypso uses four high-efficiency, highpower, 620-nm Luminus SBT-90 LEDs capable of 1,600 lumens at
a 13.5-A peak power each (Figure 4).
To maximize transmit antenna gain, we have evaluated a
number of commercial TIR (Total Internal Reflection) lens and

IEEE A&E SYSTEMS MAGAZINE

APRIL 2018

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