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.