IRS-1C User's Handbook

Space Segment

The space segment carries out the following functions:

• Images the earth in all the required spectral bands
• Formats the payload sensor data and transmits to ground stations in X-band and also records the video data for later transmission
• Provides necessary power (with a margin) for mainframe and payload subsystems in all operating conditions
• Provides attitude stability required for imaging
• Provides housekeeping information for monitoring the satellite health and accepts telecommands to control the spacecraft

The structure of spacecraft consists of :

• Main platform
• Payload platform

The main platform consists of four vertical panels and two horizontal decks supported on a central load bearing cylindrical shell of 930 mm diameter and 1123 mm height. The bottom of the cylinder is attached to an interface ring which interfaces with the launch vehicle. The vertical panels and horizontal decks carry the major mainframe subsystem packages. The Sunside and Anti-Sunside panels additionally support solar arrays and the power transfer assemblies. The earth viewing panel carries the payload data transmission antenna, the TTC antenna and Sun sensors.

The payload platform accommodates the PAN, LISS-III and WiFS cameras. In addition, it accommodates Earth sensors and Star sensors. A Carbon Fibre Reinforced Plastic (CFRP) monocoque cylinder of 930 mm diameter and 370 mm height separates these two platforms and provides thermal isolation to minimize thermal distortion effects on imaging.

The PAN payload has a capability to tilt upto an angle of ±26 deg in the direction of pitch. A Payload Steering Mechanism (PSM) supporting the PAN camera enables this rotation. The PSM is initially held by a hold down mechanism during launch. Later, it is released by activating a pyrocutter by a command from the ground. Four Reaction Control System (RCS) propellant tanks made of titanium of 390 mm diameter are mounted on either side of a 30.7 mm thick stiffened honeycomb deck of 875 mm diameter which is fixed inside the main cylinder. The thermal control system maintains the temperature of different subsystems within specified limits. It employs semi-active and active elements like heaters and temperature controllers in addition to passive elements like paints, multi layer insulation blankets and optical solar reflectors. The power requirements of IRS-1C are met by six deployable solar panels (size 1.1 m x 1.46 m each). Three panels are mounted on the Sun side and three are mounted on the anti Sun side. The panels have a capacity to generate 813 W of power at EOL (End Of Life) at normal incidence. Besides the Sun tracking panels, two batteries of twenty eight cells with a capacity of 21 Ah (Ampere hours) are provided to support peak power requirements and power during eclipse.

IRS-1C satellite

The TTC system is configured to work in S-band. It comprises of telemetry and telecommand subsystems and a transponder.

The telemetry system collects the house keeping data from each subsystem and then formats and modulates it on the subcarrier. There are two formats, viz., dwell and normal. Dwell mode and normal mode formats can be simultaneously received. An onboard storage facility of 2.75 million bits exists for recording the house keeping data of one orbit period or sampled data in 1:5 ratio in sampled mode for four orbits period. The normal telemetry rate is 512 bits per second (bps) while the playback data from storage is at 6.4 Kbps. Telemetry system except for storage has full redundancy. The telemetry system also houses another storage facility for raw Star sensor data for a period of 25.6 minutes.

The telemetry data is transmitted on two Phase Shift Key (PSK) subcarriers of 25.6 KHz and 128 KHz. The normal telemetry is modulated on 25.6 KHz subcarrier, while the 128 KHz subcarrier is used for playback data or dwell data or Star sensor data.

The telecommand system employs a shortened B-C-H code for command reception. It provides time tag command execution facility with edit, block execution and memory error detection features. The time tag facility permits execution of 255 commands per decoder. TC supports auto commanding for autodeployment and safemode operations. It also houses programmable and fixed duration timers to control the operation of payload and data handling system. It provides special logics to configure the payload and data handling system for various operational modes.

The transponder serves to transmit housekeeping data, receive telecommand signals, demodulate ranging tones and retransmit it to ground with a fixed turn around ratio of 240/221.

The Attitude and Orbit Control System (AOCS) for IRS-1C is configured to achieve three axis body stabilisation of the spacecraft in Sun synchronous orbit. The AOCS system is basically configured around two systems, one redundant to the other. Each system in turn consists of a processor based system and a hardware based system as a back up. The AOCS system is associated with necessary sensors and actuators to carry out the control functions. All the three axes are controlled using actuators, reaction wheels, magnetic torquers and thrusters. The attitude control electronics package generates control signals for these actuators depending upon the attitude errors sensed by Earth sensors, Gyros and Sun sensors. The system provides for initial three axis acquisition, in-orbit three axis control and orbit control.

The isometric view of IRS-1C spacecraft (Stowed mode) is as shown in the following figure.

Isometric view of IRS-1C spacecraft