IEEE Aerospace and Electronic Systems Magazine - July 2020 - 31

Davarian et al.
Table 1.

Protocol Trade Space and Recommended Upgrades5
#

Applicable Scenario(s)

Traffic
Uncertainty

System
Throughput

No. of
Relay Users

Orbital relay network and
fly-by/drop-off encounter

Low

Low, High

Few

None

Orbital relay network

Low

Low, High

Many

None

Cooperative mission

Low

Few, Many

None

Cooperative mission

Low

Low, High

Few

None

Cooperative mission

Low

High

Many

Upgrades to support
multiple instantiations of
Proximity-1 protocol and
multiple frequency channels

Cooperative mission and
orbital relay network

High

Few, Many

Upgrade to support demand
assignment multiple access
using FDMA

table indicates various mission scenarios (Orbital Relay
Network, Fly-by/Drop-off Encounter, Cooperative Mission, and the combination). The second column indicates the assumed level of traffic uncertainty. For
example, if a priori knowledge of data traffic exists, the
uncertainty is low. The third column provides a measure of required throughput. The fourth column indicates the size of the user group. The last column
provides a recommendation on whether an upgrade is
needed for the given scenario as defined by topology,
traffic knowledge, throughput, and network size, and
the nature of the upgrade is specified for the cases
where upgrades are recommended.
In Table 1, the first four scenarios (rows in the table)
do not need any enhancement to the Proximity-1 protocol
implementation currently in the use for Mars relay. The
fifth mission type requires a multichannel radio on the
mothership (the hub or relay satellite of a cooperative mission) capable of supporting multiple, simultaneous instantiations of the CCSDS Proximity-1 protocol, each
operating on a separate frequency channel. It can operate
with a fixed channel assignment and schedule for each
SmallSat, due to good knowledge of traffic load.
The last scenario requires an upgrade to the current
implementation of Proximity-1 to enable on-demand
access. This upgrade will provide a proximity relay radio
capable of bidirectional hailing (both relay and user initiated), operating on separate hailing/data channels, and collision detection/back-off. When high traffic with demand
uncertainty exist, frequency division multiple access
(FDMA) will be used.
5

The upgrades in the last column are with respect to the existing
Proximity-1 implementation used in Mars proximity links.

JULY 2020

Recommended Upgrades

PROXIMITY LINK-DTN LAYER INTERACTION
Thus far, our focus has been on analyzing the physical and
link layer attributes and performance of various proximity
link configurations. In the following, we will discuss the
interaction between proximity links with DTN.
In general, DTN operates according to a contact
graph-a database containing the current best knowledge
of the radio link communication schedule between a
spacecraft and other assets and the expected, achievable
data rates. This contact graph lays out a time horizon over
which relay and routing decisions are made. When multiple data products with varying data volume and time-criticality are held in storage, it is the function of the DTN
network layer to determine the best out-going links, the
appropriate relay assets (a mothership, other peer SmallSats, etc.), and the order in which multiple data products
should be transmitted to meet both latency and data volume targets. The more accurate the DTN contact graph
reflects the true schedule of the radio system and its
achieved throughput, the higher is the overall user qualityof-service. The task of maintaining consistency between
the proximity link and the DTN contact graph is of primary importance.
Depending on the degree of traffic uncertainty in each
underlying use case, different interactions between the
proximity link and the DTN layer will be necessary. For
mission scenarios with highly deterministic science operations, the communications schedule and optimal data rate
settings can be precomputed accurately and uplinked to
the spacecraft well in advance of the actual time of data
transmission. Once loaded, the radio system and the DTN
network layer can operate independently but consistently
to the same underlying plan.

IEEE A&E SYSTEMS MAGAZINE

IEEE Aerospace and Electronic Systems Magazine - July 2020

Table of Contents for the Digital Edition of IEEE Aerospace and Electronic Systems Magazine - July 2020