SUBSYSTEMS

About the subsystems

Introduction

  • On-board Data Handling Subsystem(OBDH) is the central unit of a satellite
  • It is the subsystem which takes inputs from all other subsystems and decides the functioning of the satellite by coordinating all the other subsystems
  • The subsystem acts as a central processing unit similar to that of a personal computer but in real time and in environmental conditions of space with other subsystems as the input and output devices

Functions of OBDH

  • To coordinate between different subsystems and initiate different modes of operation
  • To store housekeeping data log with respect to time
  • To act as a shared microcontroller with TTC subsystem and act as a bridge between TTC modem and memory
  • To take necessary steps in case of any failure

Components of OBDH

  • Hardware: The hardware consists of a microcontroller, peripherals (i.e. input and output devices), buses and memory
  • Software: The software component consists of a Real time operating system responsible for scheduling the tasks in a correct sequence and also responding to events happening in real time.

Objectives of TTC

  • To design and implement a communication system.
  • Deciding various parameters related to communication like frequency, data rate, modulation techniques, protocols, coding techniques, antenna, transceivers.
  • Link testing for feasibility.

Parts Of Communication System

  • Onboard Commmunication System:
    1. Downlink, Uplink and Beacon unit
    2. The Beacon and Uplink are independent units and Downlink & the Beacon are mutually exclusive, controlled by the OBDH
  • Groundstation:
    1. Serves a link with its end users
    2. Primarily receives the data sent by the satellite and sending commands to the satellite

Operations

  • Downlink: To send status information, payload data and housekeeping data from satellite to space station
  • Uplink: To send control commands from earth to spacecraft

Designing Parameters

  • Data-rate requirement
  • Power Availability
  • Physical Constraints
  • Keplerian elements
  • Pointing Accuracy
  • Ground Station location

The Purpose of the Satellite

Any satellite is launched for a purpose. For Azad-1 the purpose of the mission is to send a telescope in polar orbit (an orbit passing through the north and south pole) to image the outer layers of the sun. The development of the telescope is managed by PAYLOAD subsystem.
The telescope needs to be durable enough to survive in tough space conditions.A telescope is an optical system that converges light to a detector. The detector used is an electronic chip.

Objectives

  • To design the telescope using CAD ( Computer Aided Designing) and to test it using computer simulations
  • To make a ground model of telescope , test it and modify it until it is ready to be launched
  • To design the electronic circuits used in telescope for the electronic chip and to write the code to process image from it
  • Management of telescope once it is active in space

Objectives

  • To design the body of satellite and optimize proper orientation of all the components
  • The subsystem aims to provide a structure so that satellite survives the launch loads
  • Moreover, a suitable thermal environment is required for proper working of all the components throughout all the phases of mission life

Structural

  • The basic structure of our nanosatellite is cubical
  • A nanosatellite’s weight is kept within 10 Kg, thus a proper mass budget was made for each subsystem
  • The basic structure was decided after a number of parameters like placement of components, distance of geometric centre from centre of gravity, stress concentration zones, complexity of mechanisms involved etc. were studied
  • Various static and dynamic analyses are performed on the satellite to determine the load bearing range of the satellite and design it accordingly

Thermal

  • Thermal controls are the arrangements for maintaining the temperature of satellite
  • The temperature limits in the LEO vary between -1000 C to 1200 C, which could damage the components or severely affect their performance
  • Mechanism for thermal control is to be designed, depending on the availability of power and ease of installation

Functions of EPS

  • Power generation
  • Energy storage
  • Power distribution to other subsytems
  • Protection to all electronic components of all subsystems against electrical faults

Energy Storage

Nano Satellites with a mission time of around 1 year use Solar panels for generation of energy and battery for storage of energy. Based on Energy Density, Cycle life and C rate of the battery, Lithium ion which is space tested has been chosen for the mission.

Power Distribution

Power distribution system is centralized and it is done through Voltage buses, generated through converters (DC-DC converters). Decision of switching of the buses is based on the location of the satellite and Energy available in the batteries, which is taken by OBDH subsystem of the satellite.

Protection and Control

Components are protected from cosmic rays through tantalum coating. Constant monitoring of the current drawn and the voltage drop across the subsytems is monitored through current and voltage sensors, accordingly precautions are taken by the microcontroller.

Introduction

  • ADCS stands for ATTITUDE DETERMINATION AND CONTROL SUBSYSTEM
  • Subsystem that is responsible for the determination of satellite's position and orientation in space and Also responsible for their control.
  • It works for most of the other subsystems on the commands given by the OBDH subsystem.

Functions

  • Position determination: To determine the position of the satellite in space, with respect to an inertial reference frame.
  • Attitude Determination: To determine the attitude of the body frame of the satellite with respect to orbit frame.
  • Attitude control: To bring the satellite into earth pointing orientation after ejection and to maintain this attitude within given bounds throughout the period of operation.

Components of ADCS

  • Sensors and Actuators
    1. Attitude Determination – Inertial Measurement Unit (IMU), Sensors, Solar-Cells, etc.
    2. Attitude Control– Reaction Wheels, Magneto-torquer, etc.

  • Modes of Operation
    1. Off mode
    2. Standby mode
    3. Detumbling mode
    4. Nominal control mode
    5. Image capture mode