Aerospace and Electronic Systems - August 2019 - 26

The Sky is NOT the Limit Anymore: Future Architecture of the Interplanetary Internet

Figure 2.
End-to-end data transfer using DTN. In near-term architecture,
the DSN directly connects to the Mars Orbiter, which relays the
bundle to the rover using BP over LTP.

transmit the data over the long distance - high latency -
between the Earth and Mars. The green continuous line
depicts the data path from the mission center to the Rover.
The purple dashed lines show the hop-to-hop acknowledgments between two neighboring elements gained from
custodian transfer property, a property of DTN functionality supported through utilizing SmartSSR hardware. Currently, Mars and Moon's orbiters use the Proximity-1 data
link protocol to communicate with the surface elements
and the advanced orbiting systems space data link protocol
for communication between orbiters and the DSN antennas on Earth.

IPN MID-TERM COMMUNICATION ARCHITECTURE
For our mid-term architecture, we consider the human colonization of Mars and the further side of the moon, which
will further lead to the long-term goal of colonizing the
whole solar system. As such, we expect an ever-increasing
demand to exchange huge amounts of data in both directions. The lower power consumption, lower mass, higher
range, and higher bandwidth of optical communication
compared with RF make it an auspicious technology to
serve as a communication medium in IPN [20]. Therefore,
we propose using an onboard optical module for spacecrafts and optical communication terminals (OCT) on the
planet's surface to support two-way communication with
high data rates. This design allows us to considerably
reduce the bandwidth asymmetry. These technologies
require less power and considerably reduce the payload.
They are also able to reach longer distances and provide
higher data rates, 10-100× higher than that of the RF.
Figure 3 illustrates the IPN mid-term architecture that
interconnects the Earth with Mars and other planets. In
this architecture, we upgrade the transmission spectrum
from microwave (X, Ka) to an optical communication, or
so-called Free-space optical communication (FSO). The
optical communication is an emerging technology in
which data is modulated onto a laser for transmission. The
laser beam is significantly narrower than a RF beam and,
26

thus, promises to deliver more power and achieve higher
data rates. In outer space, the communication range of
FSO is on the order of thousands of kilometers. Optical
telescopes, therefore, play a pivotal role as beam expanders to bridge interplanetary distances of millions of kilometers. To this purpose, each spacecraft carries a small (a
dozen cm) Cassegrain reflector, a 22 cm aperture, 4 W
laser and contains an isolation and pointing assembly for
operating in the presence of spacecraft vibrational disturbance, and a photon-counting camera to enable the acquisition, tracking, and signal reception.
The planet's ground optical terminals contain photoncounting ground detectors that can be integrated with large
aperture ground collecting apertures (telescopes) for
detecting the faint downlink signal from deep space. The
ground OCT contains six small (a dozen cm) refractive
telescopes for the transmitter and a single bigger reflective
telescope as a receiver. The latter is connected via optical
fibers to the destination. The operating constellations in
this architecture are optical TDRS around Earth, geostationary Mars orbiters (GMOs) and geostationary planet
orbiter. They provide relay services between nodes at the
surface of the outer planet, in-between planets and
between the access network from other planets. The optical deep space network substitutes the DSN ground stations by supporting two communication technologies: RF
microwaves (X and Ka-band) and optical (Lasercom).
This hybrid results in installing optical mirrors in the inner
8 m of a standard DSN 34 m beam waveguide antenna.
The RF communication is kept in order to maintain the
operation in all weather conditions.

IPN LONG-TERM COMMUNICATION ARCHITECTURE
The optical communication in the space is based on line of
sight (LOS), which may experience obstruction or conjunction. For instance, Earth and Mars can be obscured from
each other by the Sun. This obstruction lasts for two weeks
every 26 months. Moreover, LOS communication in space
attenuates because of free-space loss that increases with distances. Therefore, communication between the Earth and
further planets experiences much more attenuation than
communication between the Earth and Mars. If we consider
transmission between the Earth and Pluto, the signal travels
38.44 AU = 5766 million km (0.52 AU for Earth to Mars) in
space and needs 5.4 h to reach its destination.
We propose operating spacecrafts in Sun-Earth's
Lagrangian points to address these problems. Figure 4
shows the positions of the five Lagrangian Points L1, L2,
L3, L4, and L5. At each point, the gravitational forces of
two large bodies (Sun-Earth for instance) cancel the centrifugal force. A spacecraft can, therefore, occupy the
point and move around the Sun without the need for external intervention. These points are commonly used for
observation missions and are envisioned as relays for

IEEE A&E SYSTEMS MAGAZINE

AUGUST 2019

Aerospace and Electronic Systems - August 2019

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