Delay-Tolerant Satellite Networks addresses a significant and timely issue: will satellite systems allow us to move from a terrestrial world wide web with continuous connectivity to a space wide web with delay tolerant characteristics.
With the advent of mega-constellations, the potential exists for satellite networks that can transfer data in a far timelier fashion, as the book explains in the first few chapters. There is much information here on network theory, some of which seems a little pedantic, but the reader is left with a clear understanding of the challenges, from a communications perspective, of passing data through an intermittently connected network.
The predictability of satellite orbits means that it is possible to calculate a schedule of opportunities for data transfer and the text provides a comprehensive description of how this would be achieved. A limitation of this volume is that the authors seem far more at home with the network-related aspects of their title than the satellite-related issues. As an example, one of the key aspects, from a satellite designer’s point of view, is whether the on-board computer on the satellite will be required to perform these scheduling calculations. In a large, complex network, this could become a significant overhead. The alternative is to perform the scheduling calculations on the ground and then pass them through the network to instruct each satellite. More discussion on the pros and cons of this issue would have been an enhancement, as would a section on the satellite on-board computer and antenna hardware that would be required to implement these networks.
One of the most disappointing aspects of the book was the chapter dealing with example satellite constellations. Some of the information presented (a restatement of Kepler’s Laws, for example) is redundant and too basic for a book of this sort. More worryingly, some of the terminology used to describe the example constellations is in error. In part, this appears to be an editing failure – the text contains frequent examples of linguistic infelicity that are a consequence of the fact that part of the manuscript has been translated. But there is also evidence that the authors are not really satellite orbit devotees. One example that is presented is a Walker constellation, a design method explicitly conceived to distribute the coverage and avoid overlaps between satellites. As an example, it serves to illustrate the tension between coverage designers, who want to minimise the time when the satellites are close together, and the network designers, who want frequent inter-visibility to increase the opportunities to route data. It would have been useful if the authors had spent more time addressing techniques to resolve these important, competing requirements.
