2.2 SmallSat Communications
Communications are a particular bottleneck for space weather operations,
whether from single SmallSats or from constellations (Hapgood, 2008). As
discussed below, frequency licensing for radio communications is a
complicated and lengthy process even for highly-experienced mission
teams. Frequency licensing and coordination is necessarily
international, since many SmallSats transmit telemetry nearly
continuously while crossing over dozens of countries each orbit.
Additionally, spectrum licensing agencies may impose bandwidth
restrictions for certain radio-frequency (RF) bands depending on the
type of mission (e.g., Earth-imaging versus celestial imaging orin situ measurements) regardless of the actual data volume that
mission may require. In low-Earth orbit (LEO), visibility of ground
stations will limit downlink capacity and hence data “timeliness”
(latency). Adding ground passes to boost downlink capacity or reduce
latency may not be feasible or affordable, and other solutions must be
investigated.
While spectrum-related issues are qualitatively similar for SmallSats as
for larger spacecraft, the speed with which SmallSats, especially
CubeSats, can be developed and launched is outpacing current
coordination processes for spectrum allocation and management.
Procedures for receiving permission for spectrum use are long,
complicated, and in many countries, spread across multiple agencies.
Many researchers deploying science-related CubeSats are unfamiliar with
these rules and regulations, and sometimes discover them late in the
development process, risking denial of a license or, worse, launching
without a license, as was the case with the American startup Swarm
Technologies (Harris, 2018). CubeSat developers have historically
favored lower frequencies (e.g., UHF or S-band), where equipment is less
expensive and more readily available, but these are also the most
congested parts of the radio spectrum. The growing use of CubeSats and
the accompanying explosion in data volume increases the need for higher
bandwidth, which has its own set of costs and challenges (as discussed
earlier). Regulatory authorities also prefer to know details of
satellite orbits when spectrum filings are made, but these parameters
may be uncertain until late in the process, particularly for SmallSats
launched as secondary payloads where the primary may not be known until
only ~12 months before launch. This challenge is
exacerbated for international and joint projects where spectrum
allocations of multiple countries may need to be aligned.
In the next decade, if all proposed constellations are launched (an
unlikely scenario but worth considering), up to 20,000 satellites could
be launched into LEO, most of them under 500 kg (Maclay et al., 2019).
This rapid proliferation of SmallSats places increasing pressure on
coordination in UHF, S, and X bands as well as other space-allocated
bands, since many commercial operators use the same spectral bands as
university or federal government agencies. As more satellites are
launched, the competition for bandwidth will intensify, not just among
satellites in LEO but also in GEO, and in some situations between GEO
and LEO satellites. As RF interference becomes more of a problem,
enforcement of national and international regulations to prevent
interferences will increase, challenging the science community to
continually stay apprised of changes. The International
Telecommunications Union (ITU) has implemented procedures specifically
aimed at regulating bandwidth for SmallSat communication and telemetry
(von der Ohe, 2020).
Optical laser communication is an emerging technology with over 100,000
times more frequency bands than traditional RF, operates at lower power
levels, can achieve much higher data rates, and can potentially be
lighter and smaller (Klumpar et al., 2020). The main problem for laser
communication is cloud cover that can block transmissions, but it may
also be a valuable capability for communications between spacecraft.