Open source RTOS implementation for on-board computer (OBC) in STUDSAT-2

Abstract

STUDSAT-1 is the first pico-satellite developed in India by the undergraduate students from seven engineering colleges across South India. After the successful launch of STUDSAT-1 on 12th July, 2010, the team is developing STUDSAT-2 which is India's first twin satellite mission. STUDSAT-2 is undertaken by the undergraduate students from seven engineering colleges from the State of Karnataka, India. STUDSAT-2 consists of two Nano Satellites each having the dimension of 30 × 30 × 20 cm cube and weighing less than 10 kg. Each satellite has two payloads, one is the CMOS Camera with a resolution of 80 m and other is the ISL facility. The satellites are in along-the-track constellation architecture with the Master Satellite sending position data of the first image location on orbit and velocity of master satellite from GPS module to the Slave satellite via ISL for improving temporal resolution. On-board computer is the brain of a satellite whose operating system is designed to operate in resource constrained environments and is often tailored to the specific needs of its host system, while a real time operating system ensures that interrupts and other time critical tasks are processed when required. In order to maximize the accessibility the operating system must be inexpensive, and readily available, which demands an open-source OS. Implementation of a Real-Time Operating System (RTOS) provides several priority levels for tasks execution. FreeRTOS fulfills the requirements by having real-time capabilities, full availability of the source code, functional development environment, cross-platform support and community support. After an extensive literature survey, the ARM microcontroller with low power and high performance capability, STM32 ARM Cortex-M4 with 168 MHz CPU having 210 DMIPS is chosen as the main controller that supports FreeRTOS that can run on ARM Cortex-M cores. The On-Board Computer controls and coordinates the tasks of all other subsystems of the satellite. The different tasks executed by these subsystems have multi-threading and preemptive multitasking characteristics. A real-time operating system, FreeRTOS, uses advanced task scheduling techniques and a preemptive kernel, which allows multi-threading of processes to occur. This paper presents the technical justication for using FreeRTOS in STUDSAT-2. It presents implementation of FreeRTOS on STM32Cortex M4 controller which will increase the functional integration and performance of small satellite that simplifies software development, enables code modularity, and results in maintainable and expandable high-performance that will reduce the effort required to develop software for STUDSAT-2A/2B Mission.