Aerospace and Electronic Systems - April 2019 - 48

Deep Space Network in the CubeSat Era
loop receivers, which can record the IF signal without
tracking the carrier and do other signal processing on it
(e.g., Radio Science open loop processing).

CUBESAT DESCRIPTION
CubeSats are small spacecraft, with limited capability.
CubeSats are specified in units of U, where a U is a cube
with 10 cm sides. A 3U CubeSat is three cubes connected in one dimension; a 6U CubeSat has the same
volume as two 3U CubeSats (e.g., two rows of three
cubes). So, a 6U CubeSat's dimensions are approximately 30 cm Â 20 cm Â 10 cm (the actual dimensions
may be slightly different) [4].
Given their limited volume, CubeSats are generally single instrument spacecraft and have limitations in their operations, due to thermal and power constraints [5]. For
example, the Mars CubeSat One (MarCO) CubeSats that
were launched with the InSight mission to Mars will only
act as a UHF relay for the InSight Entry, Descent, and Landing phase [6]. These limitations mean that communication
periods between the spacecraft and Earth will be limited.
CubeSats are normally launched as secondary payloads on a launch vehicle. For the EM-1 mission, the
primary spacecraft is the Orion vehicle and there will be
13 secondary 6U CubeSat payloads [7].

CHALLENGES WITH CUBESATS
CubeSats are expected to provide several challenges to the
DSN, especially during the initial acquisition and early
operations phases that occur immediately post launch.
As discussed previously, CubeSats will be secondary
payloads on a spacecraft launch; they will not have a
launch vehicle of their own. So, at the start, they will be
competing for tracking assets with the prime mission of
the launch. And, it is very likely that there will be multiple
CubeSats as part of the launch, all desiring tracking support as soon as possible (for example, for the EM-1
launch, there may be up to 13 CubeSats of which at least 9
require DSN tracking services during the launch [7], [8]);
this will be an increase of nearly one-third the number of
spacecraft the DSN currently tracks. Given that a launch
for the prime mission will be considered a Level 1 event
(a high-priority event that requires elevated DSN support),
two antennas will be dedicated to it, potentially leaving
just one antenna available to support the CubeSat initial
acquisition.
Another concern is the independence of the CubeSat
missions. Unlike a formation flying mission (such as Magnetospheric Multiscale Mission (MMS) [9], with its 4
spacecraft), all the CubeSats will be independent missions,
which increases the difficulty of allocating and coordinating support to each mission. Even at Mars, where there
48

are multiple independent orbiters and landers, there is a
system called the Mars relay operations service [10] that
coordinates the activities of the different landers and relay
spacecraft.
Then there are the limitations of the CubeSat itself.
Due to limitations in power and thermal control, the
spacecraft may only be able to have its telecommunications active for a limited time every 24 hours. This will
put additional pressure on the ground operations (both the
spacecraft team and the DSN) to get these limited times
scheduled (when competing with the more standard missions that the DSN tracks, which often require hours of
time per antenna). The size of the CubeSat limits the size
of its antenna for communications, which will inhibit the
transmission link to/from Earth (for example, the MarCO
antenna was only allocated 4 percent of the available volume and had to weigh less than a kilogram [11]). Additionally, the CubeSat transponder may have limited data
rates it can generate (unlike standard spacecraft, which
often have multiple data rates), which, when coupled with
the limited time it can transmit, may constrain the amount
of data the CubeSat can return in a pass. A further limitation on achievable data rate is the ranging signal; if the
CubeSat requires ranging for its navigation, modulating
the turned around ranging signal reduces the power available for the telemetry, which reduces the data rate that can
be supported.
Finally, due to the limited testing, a CubeSat will
undergo prior to launch (one factor in keeping its cost low),
it is anticipated that there will be a higher potential for
spacecraft anomalies during the CubeSat mission, which
will require real-time scheduling changes to resolve.

CURRENT IMPLEMENTATIONS
The prime issue with the near-term CubeSats, such as
those that will be ejected from the EM-1 launch vehicle,
will be the need to track a large number of them after
ejection from the launch vehicle. The EM-1 launch vehicle will have up to four different points in time when
CubeSats will be ejected sequentially. These points,
referred to as "bus stops" may eject four or more CubeSats
over a short time window (at the time of this paper's writing, the actual time between CubeSat ejections is still
being discussed) [12].
To support this, the DSN will be using capabilities
that have been supporting other missions that have similar
scenarios as do the CubeSats. Specifically, the issue of
handling multiple spacecraft in the antenna beamwidth
(MSPA, discussed earlier) is a scenario that the DSN deals
with on a daily basis with the missions that are on or orbiting Mars. In this case, each spacecraft has a different uplink
frequency and downlink frequency, so the spacecraft do not
interfere with each other. The current capability of the DSN

IEEE A&E SYSTEMS MAGAZINE

APRIL 2019

Aerospace and Electronic Systems - April 2019

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