AstroLogic: using XML in a spacecraft-focused client-server system

AstroLogic is an XML-based messaging system which turns around the usual model of spacecraft control. Normally a spacecraft is governed by a top-down, all-seeing command & data handling system, which sends commands and receives responses from subsystems. AstroLogic inverts this by having subsystems send requests for resources to other subsystems, which reply according to their ability to provide those resources. This is the client-server model of information networking and it maximizes flexibility, interchange ability, modularity, and expandability. Each subsystem acts as a client (requesting resources from other subsystems to perform its function) and as a server (providing its functionality as a resource to other subsystems). This model applies within the spacecraft, across a spacecraft constellation, with a ground station, and ultimately including a large set of ground- and space-based resources as one network. Client-server systems must deal with great ambiguity and unpredictability, as a server can never know when a client may arrive or what resources it may ask for. Neither ambiguity nor unpredictability are desirable situations on a spacecraft, so a solid syntactic structure to express both requests and responses is highly beneficial, as a means to make the intent of each side's messages clear. XML provides this structure for AeroAstro's AstroLogic messaging system. This paper will explore some of the more unexpected benefits of XML in this application, particularly: escaping from "bit-width limited" expressions of numerical data; capturing time-based information in a sensible and universal structure; and the ability to integrate tools and third-party standards without added effort