The communications subsystem is designed to manage incoming and outgoing data transfer with the ground station. It receives commands from the ground station and relays them to the appropriate subsystem. The communications subsystem is also responsible for relaying data from the rest of the satellite to the ground. The comm. subsystem consists of an Atmel and a DATARADIO T-96SR modem and radio. Emerald Radio Protocol is used for ground-satellite communication.
In addition to data routing, the communication system also acts as a scheduler. The scheduler allows EDP commands to be executed automatically at a particular time. Once the EDP command has been executed, its reply is also stored. The scheduler uses two queues for its functionality. The first queue is the Request Queue (RQ). The Request Queue is where the scheduled commands waiting to run are stored. The second queue is the Storage Queue (SQ). The Storage Queue is where the commands that have been executed are stored.
Each command scheduled has the ability to repeat itself, meaning that once it has executed a command and stored its reply in the Storage Queue it can spawn itself back into the Request Queue. Therefore, the user can specify how many times the command is to repeat and at what time interval this should happen. Because a task can repeat, each task has both a Task ID and a Repeat ID. Both of these IDs are specified by the scheduler. These two IDs together form a unique ID for all the tasks in the system.
