IEEE Aerospace and Electronic Systems Magazine - July 2020 - 29

Davarian et al.
distances, for example, beyond Neptune's orbit. In practice,
however, radio navigation does provide very accurate results
for cruise, orbit insertion, and EDL when the orbit of the target is well known, e.g., Mars. However, for targets with large
orbit and shape uncertainty, such as asteroids and comets,
optical navigation tends to outperform radio navigation for
applications such as orbit insertion and rendezvous.
Another noteworthy difference between the two options
is that earth-based radio navigation has nanoradian accuracy
that allows the spacecraft to stay on the planned orbit during
both cruise and approach. Although this level of accuracy is
not usually possible with space-based optical navigation far
from the target; when spacecraft-target distance becomes
small, (e.g., less than 0.1% of earth-target distance) optical
navigation may become more accurate than the RF option.
But some missions may not want to wait that far into the
flight to obtain high accuracy navigation-they may view it
as risky. Therefore, optical navigation may be ideal for some
missions but not suitable for others.
From the above-mentioned discussion, it is evident
that the navigation-type preference (radio or optical) is
mission dependent. For many small body missions, the
combination of both radio and optical navigation systems
would yield the best navigation result; radio during cruise
and optical during approach. Therefore, radio and optical
navigation each have their particular merits and should be
chosen by a project per the mission need.

DISRUPTION TOLERANT NETWORKING (DTN)
Customarily, communications with the spacecraft during
mission operations is managed by the spacecraft team.
Transmission and reception episodes are individually configured, started, and ended by command in a prescheduled
format. If data are lost, retransmission is manually
requested by the operations team in separate uplink command sessions that are often days later. Even the relaying
of data from Mars rovers through Mars orbiters is managed by the operations team via sending transmission
commands to the rover and the orbiter.
An alternative approach would be to implement an
automatic space data communications network, similar in
capability to the Internet. The Internet protocols themselves, however, are generally unsuitable for this purpose
because communication links to and from the spacecraft
are often subject to interruption and, for deep-space missions, signal propagation delays may be very large. Disruption-Tolerant Networking (DTN) is an alternative network
architecture that is designed to address the unsuitability of
terrestrial internet protocols for the use in most space applications. DTN is the keystone of the Solar System Internetworking concept and is now nearly mature enough to put
into the widespread operational use [9], [10]. Our study
examined the use of DTN for SmallSats. The conclusions
of this examination are summarized as follows.
1. DTN is recommended by the Interagency Operations Advisory Group (IOAG)1 for the use in Solar
System Internetworking (SSI) and has been standardized by an international standards body, the
Consultative Committee for Space Data Systems
(CCSDS) [11], [12].

ONE-WAY RADIO NAVIGATION
One-way navigation has recently attracted the interest of the
deep space navigation community. The idea is to use, for
example, only the DSN uplink (or only downlink) rather
than coherent two-way signaling for navigation. This
approach requires a highly stable frequency and timing
source onboard (clock stability depends on navigation accuracy requirement). Although this level of stability was not
available in the past, recent work on space-worthy atomic
clocks [7] has shown promise. While, the technology in [7]
may be appropriate for large satellites, its mass and power
are not desirable for SmallSats. A fitting option for SmallSats may be a technology known as the chip scale atomic
clock (CSAC), which is available with the power consumption of about 120 mW, weight about 35 g, and short-term stability of 10À11. The radiation hardened version of this
technology is commercially available [8], but the clock has
not flown on any deep space mission yet. Due to its limited
accuracy, CSAC is suitable for missions that do not demand
a very high level of navigation accuracy; hence, it seems
more suitable for SmallSats than larger missions.
Facilitated by stable space clock availability, one-way
navigation may be used to enable radio-based semiautonomous navigation and form part of a robust onboard system
that combines both radio and optical data processing for
enhanced navigation.
JULY 2020

2. DTN has been flight proven on the International
Space Station (ISS).
3. SmallSat programs will be able to save time and
money by using DTN and leveraging the current
developments in NASA and ESA programs that will
use DTN. These programs include DTN-enabled
space radios. Standardized ION-DTN2 flight-qualified software is available on Sourceforge [13]

The IOAG was established by the Interoperability Plenary (IOP),
which started in June 1999. The first IOP meeting was convened as
a result of a European Space Agency (ESA) and NASA Interagency,
Tracking, Communications, and Operations Panel (ITCOP) Meeting.
The IOAG provides a forum for identifying common needs across
multiple international agencies for coordinating space communications policy, high-level procedures, technical interfaces, and other
matters related to interoperability and space communications.

The Interplanetary Overlay Network (ION) software distribution is
an implementation of DTN architecture, as described in Internet
RFC 4838, which is intended to be usable in embedded environments including spacecraft flight computers. https://sourceforge.net/
projects/ion-dtn/

IEEE A&E SYSTEMS MAGAZINE

https://www.sourceforge.net/projects/ion/dtn https://www.sourceforge.net/projects/ion/dtn

IEEE Aerospace and Electronic Systems Magazine - July 2020

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