Development of a satellite communications software systemand scheduling strategy

[摘要] ENGLISH ABSTRACT: Stellenbosch University and the Katholieke Universiteit Leuven has a joint undertakingto develop a satellite communications payload. The goals of the project are:to undertake research and expand knowledge in the area of dynamically configurableantenna beam forming, to prove the viability of this research for space purposes andto demonstrate the feasibility of the development in a practical application.The practical application is low Earth orbit satellite communication system forapplications in remote monitoring. Sensor data will be uploaded to the satellite,stored and forwarded to a central processing ground station as the satellite passesover these ground stations. The system will utilise many low-cost ground sensorstations to collect data and distribute it to high-end ground stations for processing.Applications of remote monitoring systems are maritime- and climate changemonitoring- and tracking. Climate change monitoring allows inter alia, for the monitoringof the effects and causes of global warming.The Katholieke Universiteit Leuven is developing a steerable antenna to bemounted on the satellite. Stellenbosch University is developing the communicationspayload to steer and use the antenna. The development of the communicationsprotocol stack is part of the project. The focus of this work is to implement theapplication layer protocol, which handles all file level communications and also implementsthe communications strategy.The application layer protocol is called the Satellite Communications SoftwareSystem (SCSS). It handles all high level requests from ground stations, includingrequests to store data, download data, download log files and upload configurationinformation. The design is based on a client-server model, with a Station Serverand Station Handler. The Station Server schedules ground stations for communicationand creates a Station Handler for each ground station to handle all groundstation requests. During the design, all file formats were defined for efficient groundstation-satellite communications and system administration. All valid ground stationrequests and handler responses were also defined.It was also found that the system may be made more efficient by schedulingground stations for communications, rather than polling each ground station untilone responds. To be able to schedule ground station communications, the timeswhen ground stations will come into view of the satellite have to be predicted. Thisis done by calculating the positions of the Satellite and ground stations as functionsof time. A simple orbit propagator was developed to predict the satellite distanceand to ease testing and integration with the communications system. The timeswhen a ground station will be within range of the satellite were then predicted and ascheduling algorithm developed to minimise the number of ground stations not able to communicate.All systems were implemented and tested. The SCSS executing on the Satellitewas developed and tested on the satellite on-board computer. Embedded implementationspossess strict resource limitations, which were taken into account during thedevelopment process. The SCSS is a multi-threaded system that makes use of threadcancellation to improve responsiveness.

[发布机构] Stellenbosch University