IEEE Aerospace and Electronic Systems Magazine - July 2020 - 15

Davarian et al.
experience with wireless terrestrial networks. At the Kaband region of the spectrum, 500 MHz of uplink and
downlink bandwidths are allotted to deep space communications by regulatory bodies (see Table 2). Radios that can
transmit at rates of 100 Mb/s or more have been, or are
being, developed [2]-[4]. Some future applications, however, may require even higher rates. A good alternative to
RF communications is optical signaling that can deliver
higher data rates than the RF approach while reducing
mass, power, and volume for a given data rate. Near-infrared wavelengths can provide bandwidths supported by
current laser technology that are orders of
magnitude greater than RF signals. At lunar distances,
optical links may provide data rates approaching 1 Gb/s
[5]. The primary reason for this astonishing performance
is that optical transmitters focus the light energy in a
narrow beam reducing the amount of light energy spread
in space and wasted. Moreover, unlike the RF links, bandwidth becomes mostly a nonissue for optical links.
Additionally, space-based optical transmitters tend to
have lower size, weight, and power (SWaP) than RF
transmitters.
Because of the very narrow beamwidth of a typical
optical link, spacecraft must be able to keep the beam
pointed within a small tolerance in order that the beam
can be intercepted by the receiving telescope on the
ground. For this reason, optical space communication
requires an elaborate pointing control system that may
include star trackers, optical sensors, imaging cameras,
and ground beacons. Optical control systems usually
operate within the typical pointing control and knowledge uncertainty of conventional spacecraft attitude
control systems, though improved spacecraft stability
may result in further performance improvement in the
future.
Optical systems rely on line-of-sight, and Earth atmospheric conditions can interfere with optical signals. Optical ground stations are located in environments that
minimize this concern, but for high reliability missions, an
RF communication system is typically also needed on the
spacecraft to be used for emergencies and as a backup link.
Optical communications from Earth orbiters have
been successfully demonstrated by multiple organizations,
particularly in the U.S., Europe, and Japan. The most
recent experiment of interest is the one that demonstrated
lasercom downlinks from a low Earth orbit (LEO), 1.5U,
2.3 kg CubeSat to an optical ground station [6]. Two
vehicles, built by the Aerospace Corporation, were
launched in November 2017 and placed in a 450-km circular orbit. Subsequent to on-orbit checkouts and preliminary pointing calibration and utilizing on-board star
trackers, communications links up to 100 Mb/s with bit
error rates near 10À6, without any forward error correction, were demonstrated. Furthermore, the two CubeSats
were also used to demonstrate laser ranging.
JULY 2020

In another example, The German Aerospace Center,
DLR, has demonstrated optical communication from a
LEO orbit in a series of experiments called OSIRIS for
Optical Space Infrared Downlink System [7]. Of particular
interest to the CubeSat community is DLR's planned 2019
demonstration for a 0.3U optical terminal with data rates
up to 100 Mb/s and 8 W power consumption. Active beam
steering and spacecraft body pointing are also part of this
demonstration.
In addition to the above examples, there have been
other campaigns that have successfully concluded (or are
continuing) by NASA, ESA, JAXA, industry, and others to
demonstrate high-rate optical communication from space.
It should be noted that the developments discussed
above have been based on Earth-orbiting satellites with
typical distances of a few hundreds to thousands of kilometers, with one exception from the Moon distance [5].
This situation will soon change due to a JPL program
called Deep Space Optical Communications (DSOC),
which is developing an optical terminal and ground support for a deep space demonstration. The plan for DSOC
calls for demonstrations from deep space onboard NASA's
Psyche mission, scheduled for launch in 2022 [8]. DSOC
utilizes single photon detection to combat signal attenuation caused by extreme planetary distances. The DSOC
goal is to increase spacecraft communications performance
and efficiency by 10 times over conventional means, e.g.,
Ka-band communications, all without increasing the mission burden in mass, volume, power, and/or spectrum.
DSOC architecture is based on transmitting a laser
beacon from Earth to assist line-of-sight stabilization to
make possible the pointing of a downlink laser beam, see
Figure 4. The laser beacon to DSOC will be transmitted
from JPL's Table Mountain Facility, in the Angeles
National Forest (Southern California). DSOC's beaming
of data from space will be received at a large-aperture
ground telescope at Palomar Mountain Observatory in
California, near San Diego, California. DSOC will be
demonstrated at ranges up to 2.5 AU.
To summarize, there is interest in developing optical
space communications technology by space agencies,
national laboratories, and industry to achieve very high
data transmission rates while reducing equipment size.
Whereas several demonstrations have taken place using
Earth-orbiting satellites, no deep-space optical communication has been demonstrated yet. The first deep space
flight (DSOC) is planned for 2022. As discussed above,
DSOC uses an uplink optical beacon to stabilize the optical terminal platform and requires a large telescope as the
receive end on the ground. Therefore, it is mainly suitable
for big investments such as a large mission that will generate vast amounts of science data not suitable for transmission via RF links. Likewise, human voyage to Mars is
expected to produce large volumes of data needing highrate optical links for transmission. The use of an uplink

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IEEE Aerospace and Electronic Systems Magazine - July 2020

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