Communication As a Space Microdatacenter Bottleneck¶
The above analysis did not consider communication of EO data from the satellites to an SµDC. Fig. 10 shows a small constellation supported by two SµDCs in ‘ring’ topology [87]. Data from distant EO satellites is relayed to the SµDC by more proximal EO satellites. Thus, the number of connected EO satellites is potentially limited by the capacity of the ISL between the SµDC and the closest EO satellites.
A ring topology in which the SµDCs are part of the same orbit as the EO satellites has clear benefits. By flying the SµDCs in formation with the EO satellites and using a fixed ring topology, ISLs are also fixed. This is important when ISLs are optical, since optical ISLs can take seconds or even minutes to orient [32, 75]. Small satellites which contain optical ISLs often orient the ISL by rotating the entire satellite [75]. This means that the satellite cannot perform simultaneous communication and imaging. However, by using a ring topology with fixed distance and angle between satellites, satellite designers can design the ISL and camera such that they are usable simultaneously.
If one SµDC can support the computation of \(n\) satellites, but the capacity limitation of the ISL between the SµDC and the closest EO satellites dictates that the SµDC can only receive data from \(m < n\) satellites, then the number of clusters (and thus SµDCs) needed is \(\\frac{64}{m} > \\frac{64}{n}\). In this case, the constellation is ISL-bottlenecked. If \(m \\ge n\), the constellation is ISL-unconstrained.
For lightweight applications, the minimal number of SµDCs that are needed in a ring topology may not be set by the total amount of computation required, but rather by the number required to mitigate the ISL bottleneck. Table 8 shows how many EO satellites an SµDC can support before becoming ISL-bottlenecked at various
data rates and for several ISL capacities based on RF [101, 137] and optical [41, 89] LEO to LEO ISLs. This table assumes that a base (at 3 m) 4K RGB image is generated every 1.5 s on each EO, and transmitted via ISL to the SµDC. As resolution improves, so does the number of pixels in the image (i.e., the imaged area remains constant). In a ring topology, the limiting links are the ones between the SµDC and its adjacent EO satellites. Thus, for example, at 3 m resolution and \(1 \\text{ Gbit } s^{-1}\) ISL capacity, each ISL can support transmitting over four images every 1.5 s. Since the SµDC has two ISL receivers, it can support up to nine EO satellites.
The results show that \(< 100 \\text{ Gbit } s^{-1}\) ISLs are often insufficient to support even a single EO satellite for high data rates. Even \(100 \\text{ Gbit } s^{-1}\) ISLs fail at 10 cm resolutions. On the other hand, a single SµDC can support a large number of EO satellites at low data generation rates (i.e., coarse resolution and high early discard rates) — more than what would realistically be placed into a single orbital plane. This table data, combined with Fig. 9 indicates when ISL-bottlenecks or computational requirements dictate the number of SµDCs needed. Fig. 11 shows that the number of clusters, and thus SµDCs, is set by the ISL-bottlenecks for many applications — especially for high-power SµDCs. As ISL capacity increases, the bottleneck goes away, and the number of clusters required matches the number of SµDCs needed to support the computation, as in Fig. 9.
In general, it is preferable for a constellation to be ISL-unconstrained as an ISL-bottlenecked constellation means more SµDCs are launched than are strictly required based on computational power requirements. This increases constellation equipment, launch, and management costs.
Our results also show that ISLs considerations can have important influence on SµDC design for lightweight applications — high power SµDCs are more likely to be ISL-bottlenecked than a low power SµDCs. They also suggest that ISL network topology may play an important role in enabling high SµDC utilization. Thus ISL considerations will impact EO/SµDC satellite constellation design.