IEEE Aerospace and Electronic Systems Magazine - July 2020 - 35

Davarian et al.

Figure 7.
SmallSat proximity communications (UHF, LGA-LGA; S, HGA-LGA; and X, HGA-LGA lines fall on top of each other).

PROXIMITY COMMUNICATION LINKS
In this section, we examine the performance of proximity
links. As discussed in the previous section, the most used
proximity radio is Electra, which operates at UHF frequencies.
Although UHF continues to be a good choice for links where
low-gain antennas are used, for more demanding proximity
links that use medium- or high-gain antennas, a higher frequency regime, such as S, X, or Ka, may be more desirable.
Figure 7 illustrates the data rate as a function of distance for a
number of antenna and frequency options. The assumptions
for this analysis are listed in Table 2. A few existing or past
missions are overlaid on the chart for reference. These examples include the MarCO/InSight UHF relay link, the Huygens/
Cassini8 S-band link, and the MRO/MSL UHF relay link. The
findings of this investigation are summarized in Table 3 for
binary phase shift keying modulation. The MRO/MSL curve
shows the measured performance of the Electra radio and its
ability to adjust bit rate to match the distance between the two
ends of the channel. The sharper drop at larger distances is
due to the loss of antenna gain at the beam's edge.
Table 3 provides useful information for a proximity
link designer. Depending on link distance and data rate
requirements, recommendations on frequency preference
and on antenna directivity are provided. Ofcourse, the

Cassini is the name of a mission that explored the Saturnian system
and landed the Huygens probe on Saturn's biggest moon Titan,
decommissioned in 2017: https://www.jpl.nasa.gov/missions/cassinihuygens/.

JULY 2020

findings depend on the assumptions that were made for
calculating link performance in Figure 7. For example,
transmit powers for both X and Ka bands are assumed to
be 4 W. As technological improvements are made to
SmallSat power systems in general, and to amplifiers in
particular, Figure 7 conclusions may change. Therefore,
the findings in Table 3 should be viewed as guidelines,
noting that each proximity link design will be influenced
by the available technology at the time of usage.
Another consideration is the availability of hardware.
Whereas UHF radios are readily available commercially,
X- or Ka-band proximity radios may not be available commercially and will have to be developed. Therefore, either
an existing commercially available proximity radio, such
as Electra, must be upgraded to the desired band, or a
DTE radio with X (or Ka) capability, such as Iris, should
be redesigned to also offer proximity services. As mentioned earlier, an X-band relay radio was developed for
the use in the Europa Clipper mission [19]. Hence, this
may provide a path to having an X-band proximity radio
commercially available for the future use.
The recent interest in robotic and human exploration
of the moon calls for radios that support proximity links
as well as DTE communications from Lunar orbits. In
response to this realization, considerations are given to
upgrading the Iris radio to support proximity links in addition to direct-to-earth communications. Table 3 provides a
guideline for channel frequency selection and antenna
type for proximity communications. For example, a recent
Lunar-orbiter proposal uses a mission design that employs

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https://www.jpl.nasa.gov/missions/cassini-huygens/. https://www.jpl.nasa.gov/missions/cassini-huygens/.

IEEE Aerospace and Electronic Systems Magazine - July 2020

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