An Analytical Search for Efficient Microcontroller-On Board Computer for Veltech-Nanosat

International Journal of Advanced Science and Technology Vol. 29, No. 6s, (2020), pp. 232-236

An Analytical Search for Efficient Microcontroller-On Board Computer for Veltech-NanoSat

1Prabhu Kumar Surarapu, 2Tumuluri Sree Sravya, 3D.Ruthra Prabha 1Assistant professor, 2,3UG student Department of ECE Veltech Rangarajan Dr.Sagunthala R&D Institute of Science and Technology, Chennai

Abstract On Board Computer (OBC) isthe heart of the satellites and particularly, It is the main subsystem in the Nano satellite.OBC is responsible for communication, data storage and functional control between all subsystems in the satellite. Right Choice of micro controller is very important task since complete monitoring of the subsystems depends on the OBC. The probability of the success of the satellite mission depends on the reliability of microcontroller and components. In this paper we have listed down, compared and analysed the microcontrollers that were used earlier in nanosatellites and pico satellites like PRATHAM, JUGNU, SATYABAMASAT, ICUBESAT,STUDSAT and SWAYAM. Key words--- On Board Computer (OBC), Microcontroller, VeltechNanoSat, Nano satellite

I. INTRODUCTION

Veltech-NanoSat, the first satellite initiative of Veltech group of institutions, Chennai, Tamilnadu,India, is in the process of development at Veltech Rangarajan Dr.Sagunthala R&D Institute of Science and Technology under guidance of Indian Space Research Organisation (ISRO) . This project was initiated with the purpose enabling the students and faculty who are involved with design, analysis and testing. The important mission of this nano satellite is to monitor the agricultural fields, land sliding, forest fire, flow and quantity of water. Like other satellites Veltech-NanoSat has also various subsystems which include Payload, Power, Communication, Attitude control, Structure, Mechanisms, Thermal and On Board Computer (OBC). Among all subsystems, On Board Computer (OBC) plays a vital role in the satellite. OBC major function includes the monitoring of satellite maintenance and control, payload operations and diagnostics. This paper discusses the importance of OBC and makes an analytical study of several microcontrollers used earlier in national and international organisations for nano satellites.

II. OVERVIEW OF ONBOARD COMPUTER

The on board computer system development is primarily divided in to hardware design and software design [4].OBC is responsible for the begging of the satellite to until it ejected in to orbit [5].The basic requirements of hardware design are mission, system requirements, system configuration, subsystem requirements, subsystem configuration and equipment requirement [5]. The overall architecture of OBC shown in Figure 1 and represented the important function OBC in a satellite. OBC has to collect the data from various subsystems and need to monitor the complete heath of satellite [1].The complete operation of nano satellite depends on the effective function of OBC. Figure number 1 mentioned down indicates the communication of OBC with various subsystems. Different interfaces like Universal Asynchronous receiver-transmitter (UART), Serial Peripheral Interface (SPI), Inter-Integrated Circuit(I2C) can be used for communication between various systems.

Figure 1. OBC Architecture

Fig 2. Main functions of OBC

The main functions mentioned in Fig 2 displays the functions OBC for maintenance and control, Payload operation and diagnostics. These functions of OBC monitor the health of each subsystem and proper communication between subsystems [2].These are the fallowing functions of OBC: a) Satellite maintenance and control  Power on initialisation  Telemetry and tele command process  Attitude control  Orientation Modes  Power management  Operations mechanism  Thermal control mechanism  Emergency mode and safe mode recovery b) Payload Operation  Wireless sensor data storage and transmission  Payload on/off and automatic operation  Payload programming c) Diagnostics  Memory check  Routine power, voltage and current checks

Design procedure for OBC derived from European Cooperation for Space Standardization (ECSS)[4].Figure 3 demonstrates the design hierarchical for OBC.

Fig 3. OBC Design Hierarchical

III. SELCTION OF MICROCONTROLLER

Selection of Micro Controller strictly depends on the requirements of OBC.

A. Approach for selection The selection of microcontroller depends on the design constrains. Two approaches were used to opt best microcontroller. The first approach was to select the microcontroller with design specifications. The next approach is based on comparisonsoto select the best microcontroller used earlier in space applications. [3]. B. Design Constraints  Entire system should consume power less than or equal to 1 W.  Mass about 300 grams.  It can capable of operating between -100 C to +500 C temperature.  It must occupy less space.  It should have all compatible interfaces. C. Essential specifications and Peripherals required Data interface with other subsystems is one of the vital functions of OBC. To interface with various subsystems requires different peripherals.

TABLE I OBC SPECIFICATIONS AND PERIPHERALS Data bus SPI Bus, UART, I2C Telemetry Bit Rate: 1200 bps Data Coding: NRZS/PCM Analog Channels: Bi-Level Channels: DS Channels: BER:10-6 Telecommand Bit Rate: 100 bps Data Coding: NRZS/PCM On/Off Commands: Serial Load Commands: BER:10-6 Data Interface with all other SPI Bus-All sensors subsystem GPIO-Communication UART-Magnetometer, GPS etc I2C-EPROM (Most of the EEPROMs are I2C Complaint)

A. Memory unit All subsystems in the satellite are interfaced with OBC since it is the main monitoring system in the satellite. The interfaced systems share the data with OBC. The shared information storage is necessary for monitoring heathof subsystems and for the decision making. So the selected microcontroller should be capable of having access to huge external memory [3]. B. Comparison of Microcontrollers The microcontrollers used in earlier nano satellite is the focus of discussion in this chapter. Microcontroller ATmega128 used in IIT Bombay’s satellite PRADHAM.ATmega128 was selected on basis its low power consumption and peripherals support [1].The microcontroller AT91SAM7X series were used in IIT Kanpur’s satellite JUGNU and Sathyabhama University’s satellite SATYABAMASAT because of the following features: protective mode access, thumb instruction, pipeline instruction and advanced memory controller [2].

TABLE II MICROCONTROLLERS USED FOR NANO SATELLITES

Name of Nano Satellite Organisation Microcontroller PRADHAM IIT Bombay, India ATmega128 JUGNU IIT Kanpur, India ARM7-AT91SAM7X SATYABAMASAT SatyabamaUniversity,Chennai,India AT91SAM7A1 ICUBE II Institute of Space Technology, C161 PI Pakistan

ARM CORTEX M0 is most suitable microcontroller for Veltech-NanoSat’s OBC because of the salient features listed in Table III. The comparative analytical study of both approaches testifies the fact.

TABLE III KEY FEATURES OF ARM CORTEX M0

Parameters Description Architecture 32 bit RISC On Chip RAM 8 Kbytes on Chip SRAM and 32 K FLASH Programming memory

Address Space Up to 64Kbytes Optional External Memory Space

GPIO 11 Watchdog Timer YES

Operating Range 3.3.-3.6V

Temperature Range -400 C to +1500 C

Advanced Power Saving Features YES

ADC 10 bit ADC (5 Channel)

UART/SPI 2 Speed 12 MHz

IV. CONCLUSION

The functions, design constrains, approach towards the section of microcontroller of OBC and the key features of ARM CORTEX M0 used in Veltech-NanoSat are discussed in this paper.This microcontroller,ARM CORTEX M0 with these sailent features, watchdog Timer, temperature range and address space used in Veltech-NanoSat makes it unique.

REFERENCES

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