IEEE Aerospace and Electronic Systems Magazine - July 2020 - 22

Improving Small Satellite Communications in Deep Space
launch volumes such as an ESPA ring [43]. It is likely other
antennas, such as large folded panel reflectarray antennas
[44], will find application in a MicroSat.

ACTIVE ARRAYS FOR SMALLSATS
An active array is a type of phased array antenna in which
each array element is fed by an active electronics module
known as a Transmit-Receive Module, or TRM [45]. Each
TRM includes a PA for transmit mode, a low noise amplifier for receive mode, and a phase shifter that is used to
control the transmitted and received phase of the element,
thereby creating an electronically steerable beam. Electronic beam steering offers unique advantages. For
instance, it is useful for missions in which spacecraft
instrument operations preclude using the spacecraft Attitude Determination and Control System to point the telecom antenna beam toward a ground station. Active array
beam agility offers advantages for 5G mobile communications [46] that may be also prove to be useful for inter-satellite communications in distributed swarm antenna arrays
[47], CubeSat clusters or constellations.
An important advantage of an active array for telecom
systems is spatial power combining. As discussed in the
"Power Amplifiers for SmallSats" section, it may be difficult to develop very-high-power SSPAs with good power
efficiency. Typically, SSPAs achieve high power by combining several (typically four or more) PAs in parallel using
microwave circuits, and then transfer that power to the
antenna through a coaxial transmission line. The loss in
this combining process reduces overall efficiency. Active
arrays, on the other hand, do this by combining the PA output signals in space, thereby eliminating the transmission
line loss and achieving higher efficiency. Since active
arrays have a larger number of PAs, the power required by
each one is generally lower. For example, a 16-element
active array with 2 W amplifiers feeding the elements will
generate the same Equivalent Isotropic Radiated Power as
a single antenna of similar gain fed by a 32 W amplifier
(neglecting transmission line losses). An active array also
has the potential to simplify the thermal design.
Cost is a key drawback of active arrays. Two new
technologies show promise to reduce cost for small satellites. Recently, very low-cost Silicon Germanium (SiGe)
active arrays have been developed [48]. The low cost and
rapid development time enabled by large-scale wafer-level
integration [49] and other architectures [50] may make
this a useful technology for SmallSat applications. SiGe
phased arrays have proven to give excellent beam scan
performance and offers reduced susceptibility to space
radiation. A second technology of interest is the electronic
scanned reflectarray [51], [52], which has the promise of
providing a low-cost electronic beam steering capability.
The concept is to have a phase shifter integrated into each
reflectarray element in order to enable electronic scanning.
22

In principle, each element could have a Transmit/Receive
module, making it a fully active array. However, it is
likely that such an array will not be cost competitive with
SiGe phased array technology. Several university-level
demonstrations of scanning reflectarrays have appeared in
the literature (e.g., [53]), but this design concept has a relatively low TRL for space applications.

FUTURE DIRECTIONS FOR SMALLSAT ANTENNAS
The vision of deploying large numbers of low cost SmallSats to support expanded deep space exploration, constellations, swarms, etc., requires low cost, high reliability
antennas that can be designed quickly to meet unique mission requirements. The key technical requirements for
these HGAs are low mass, compact stowage, and high efficiency. Deployable antennas to address these needs have
come a long way and enjoy a degree of maturity both at
X- and Ka-bands. However, additional work is needed in
deployment concepts and mechanisms that will improve
stowage efficiency. Stowage efficiency varies considerably for various antenna technologies. As noted earlier,
the ISARA and MarCO antennas consume almost no payload volume, but it is not clear that this technology is scalable to create large aperture (> 1m2) SmallSat antennas
with low aerial mass density (e.g., < 2kg/m2). On the
other hand, mesh reflectors can create large apertures with
low aerial mass density, but at the cost of stowage efficiency. For example, the RainCube 50 cm diameter mesh
reflector takes nearly 1.5U of volume and the KaTENna 1
m diameter antenna requires about 3U of stowage volume,
or 25% of a 12U bus.
Flight experience with deployable SmallSat antennas
is very limited. As of 2018, only three reflectarrays
(ISARA and two MarCO spacecraft) and a small number
of deployable CubeSat mesh reflectors have flown in
space. These antennas are essentially technology demonstration units that were developed on a very limited budget
and were not subjected to the level of rigorous flight project testing typically expected of a deep space mission. A
technology maturation effort is needed to address issues
that were discovered in developing these antennas. Moreover, there is a need to productize promising antenna
designs to realize low production cost. For example, the
intricate mechanical deployment mechanism used in a
mesh reflector includes a large number of unique parts
that must be assembled by highly trained technical personnel. It may be possible to redesign these antennas to
enable robotic assembly with fewer parts.
Finally, it appears likely that active array electronically
steered antennas will ultimately find use in SmallSats, at
least for medium gain antenna applications. Active array
antennas are inherently compact and offer the ability to
steer the beam without using the spacecraft attitude determination and control system, which enables the telecom

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IEEE Aerospace and Electronic Systems Magazine - July 2020

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