Mark S. Asher

Confluence of navigation, communication, and control in distributed spacecraft systems

This research details the development of technologies and methodologies that enable distributed spacecraft systems by supporting integrated navigation, communication, and control. Operating at the confluence of these critical functions produces capabilities needed to realize the promise of distributed spacecraft systems, including improved performance and robustness relative to monolithic space systems. Navigation supports science data association and data alignment for distributed aperture sensing, multipoint observation, and co-observation of target regions. Communication enables autonomous distributed science data processing and information exchange among space assets. Both navigation and communication provide essential input to control methods for coordinating distributed autonomous assets at the interspacecraft system level and the intraspacecraft affector subsystem level. A technology solution to implement these capabilities, the Crosslink Transceiver, is also described. The Crosslink Transceiver provides navigation and communication capability that can be integrated into a developing autonomous command and control methodology for distributed spacecraft systems. A small satellite implementation of the Crosslink Transceiver design is detailed and its ability to support broad distributed spacecraft mission classes is described.

Isolating errors in models of complex systems

One of the steps in creating a mathematical model of a system is to test the model after it has been fully specified, to see whether it is performing adequately. Often, it is found that the model is not performing acceptably (e.g. the model is not giving accurate predictions of the performance of the actual system). The same lack of fidelity can also be observed in established models that had been performing well, indicating a change in the actual system. At this point, it is necessary to diagnose where the problem in the model lies; a process called error isolation. An error isolation technique for detecting the misspecified parameter (or set of parameters) is described. This technique is especially designed for use on state-space models of large-scale systems. The authors report on an example of an application of the methodology to localizing errors in the model of an inertial navigation system. >

Enabling distributed spacecraft system operations with the crosslink transceiver

This research details the development and performance of the crosslink transceiver, an integrated navigation and communication system that enables distributed spacecraft system operations. The crosslink transceiver is a modular, extensible system that supports science operations among multiple, distributed space assets by implementing the essential functions of navigation, communication and control. Distributed spacecraft systems, also called formation flying systems, extend the capabilities of single-spacecraft missions by providing a platform for complex sensing tasks, including multipoint observation, co-observation, and distributed apertures. To accomplish these tasks, such systems rely on the ability to communicate science and coordination information, to determine relative position, velocity and time for command and control operations, and to operate in a coordinated manner to achieve common mission goals. The utility of the crosslink transceiver to support these operations is established by demonstrating its applicability to near-term science and military missions.

Half-duplex relative navigation and flight autonomy for distributed spacecraft systems

Distributed spacecraft systems concepts have been developed to leverage the inherent advantage of multiple, redundant sensing assets, including expanded capability, improved robustness, and graceful functional degradation. Both military and civilian missions have been advanced, with near-term technology demonstration efforts designed to serve as pathfinders to fully capable systems to meet future challenges. Distributing capability among multiple platforms, however, results in a fundamental increase in the complexity of coordinating and operating space systems due to delays in state knowledge and the need to integrate individual spacecraft autonomy within the broader system context. At the same time distributed spacecraft systems typically require added capabilities relative to monolithic spacecraft designs, such as command and control architectures that support diverse communication channels for functions such as crosslink communication and relative navigation measurements. The importance of these additional capabilities is particularly evident in relative navigation functionality, which in many systems may dominate absolute orbit determination requirements. This work describes a technique to address the fundamental need for relative navigation among distributed space assets that focuses on a minimalist hardware implementation that is suited for microsatellites, rovers, and other potential physically limited systems. Test results are provided from experiments implemented on The Johns Hopkins University Applied Physics Laboratorys crosslink transceiver (CLT) operating as a crosslink communication and navigation system in a time-division multiple access modes. To address the coordination and operation of a distributed spacecraft system under conditions that require capabilities such as regular relative navigation and communication, a flight autonomy architecture is defined that specifically addresses the complexities of controlling multiple, distributed assets. This architecture is based on the use of model-based programming and discrete event systems that use explicit logic models of spacecraft hardware and software components coupled with high-level control strategy specifications.