Satellite Constellations
A single satellite in a polar sun synchronous orbit could act as a service point for entanglement and key distribution as it covers the entire surface of the Earth in a single day. This works because Quantum Key Distribution is a buffered operation, and not communication. Service and coverage, of course, would increase with more platforms in orbit. We study the different ways the spacecraft could be connected so that it is possible to build better global coverage.
One of the important elements could be inter-satellite links. We have worked with UNSW Canberra to study how inter-satellite quantum links are possible with CubeSats.
Fig. 1 Spacecraft deployment and separation sequence which, after functional checks, separates into a two 6U formation. They are deployed as a single 12U CubeSat spacecraft. These are then maneuvered to fly in formation.
Another type of network would involve having constellations of satellites. In Figure 2, we show the access time available to various types of satellite arrangements.
Fig 2. (a) Total access time of 6 satellites equally distributed in a single-plane over one year for the G20 network. SSO refers to a noon-midnight Sun-SynchronousOrbit and corresponds to an inclination of 96-100 degrees depending on altitude (b) for the Indo-ASEAN network (c) Diminishing return of increasing number of satellites with ISL in constellation leads to optimum size of constellation. Essentially, there is no need for satellite to share keys in excess of the key material they need to re-distribute. (d) Possible types of ISL. Only intra-planar (red) links are considered in this paper as they are less complex and have higher link availability.
With such a constellation, and from available models of cloud cover, it is possible to determine how an Indo-ASEAN network of cities could be served by a constellation of satellites in various orbits. For example in the table below, given the cluster of cities, a 30 degree inclination seems to be good for the Indo-ASEAN region. In our paper we describe also the performance for a global network of cities.
Table 1. Distributed message size after 1 year for an Indo-ASEAN network of ground stations for constellations of 6 and 16 satellites. For example, over one year Singapore could send 10.13 Mbits of secure messages to all cities in this list, provided the cities have enough key of their own. SSO refers to a noon-midnight Sun-Synchronous Orbit. The notation [60 deg 2p] indicates that the constellation is distributed equally over 2 orbital planes with an inclination of 60 degrees. Within one plane the argument of periapsis of the 3 satellites is 360/3 = 120 degrees apart. The longitude of the ascending node of the orbital planes themselves is 360/2=180 degrees apart. The colours are a visual aid for identifying key sizes (from red to green).