IEEE Aerospace and Electronic Systems Magazine - July 2020 - 30

Improving Small Satellite Communications and Tracking in Deep Space

Figure 2.
Proximity network topologies (FDMA; time division multiplexing).

4. In addition to saving the development and test cost,
DTN enables a number of benefits to Mission Operations. DTN natively supports reliable deep space
link retransmission services, automatic store and
forwarding routing and data flow management, 256
levels of priority for Quality of Service management
at relay points, and a robust security system.
There is currently an effort underway at Morehead
State University to add DTN capability to their 21-m
ground station3 and to their 6U CubeSat to be launched in
late 2020 [14]. The DSN is currently implementing DTN
as a standard service for space missions [15].

PROXIMITY LINKS
In this article, a proximity link means a link between two
space assets. Typically, a proximity link is used to support
telecom relay when a direct to earth (DTE) link is not
available and/or to support relative navigation between
spacecraft. Another application of proximity links is to
support science. Not all assets can establish direct communications with the ground, so they have to access the DSN
through a relay satellite using a proximity link. The reason
for this is usually the large distance between the two ends
of the deep space link, which may require resources not
feasible for a SmallSat to directly communicate with
earth. Another reason for using a relay is occultation. If a
spacecraft is occulted by a celestial body such as a planet,
direct access to the DSN would not be possible.
Note that a proximity link can be established between a
SmallSat and a mothership or among a mix of space assets.
As will be discussed in the following section, there are a variety of proximity links and associated local area networks.

PROXIMITY NETWORKS

Figure 2 illustrates a number of proximity network topologies. In general, a proximity link is operated either in a
3
This ground station has been affiliated into the DSN with the
intention of using it primarily to service SmallSats.

prescheduled fashion or on demand, where a link is established whenever the need arises. On the left-hand side of the
figure, a federation of relay spacecraft provide relay services
to multiple SmallSats and rovers; in this scenario, it is anticipated that both prescheduled links and on-demand links will
be supported. Prescheduled links are accommodated with
time-division multiple access, whereas on-demand links
normally use frequency-division multiple access.
In the middle, a constellation of assets identified as
"cooperative missions" are connected together via proximity links. Here, the SmallSats are likely to communicate
with each other through a hub, such as a mothership,
which is also capable of relaying information to the
ground. Although less likely, it is possible that the SmallSats may communicate with each other directly.
On the right-hand side of the figure, a fly-by/drop-off
scenario is presented. Due to the short duration of the
encounter, this link has to be prescheduled because the
on-demand option would not be practical.
From Figure 2 it is clear that proximity links come in
many forms with a number of network topologies. The
CCSDS has a suite of protocols called Proximity-1 Space
Link Protocol4 comprised of three Blue Book Publications
for Data Link Layer, Coding and Synchronization Sublayer, and Physical Layer to assist with the design and
operation of proximity networks [16]. Although some of
the features of Proximity-1 have already been implemented and tested in the field, not all have been implemented yet as will be discussed below.
Table 1 summarizes usage scenarios for the three network topologies described in Figure 2 and provides recommendations for improvements needed to support
SmallSat communications. The first column of the

A recently developed protocol called Unified Space Data Link
Protocol (USLP) combines multiple protocols (proximity,
uplink, and downlink) in one with enhanced features. However,
this protocol is not backward compatible. Missions may wish to
employ USLP instead of Proximity-1 to benefit from its
enhanced features: Unified Space Data Link Protocol, CCSDS
732.1-B-1, Blue Book, October 2018. https://public.ccsds.org/
Pubs/732x1b1.pdf#search¼USLP

IEEE A&E SYSTEMS MAGAZINE

JULY 2020

https://public.ccsds.org/pbs/732xlbl.pdf#search=uslp https://public.ccsds.org/pbs/732xlbl.pdf#search=uslp

IEEE Aerospace and Electronic Systems Magazine - July 2020

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