CHALLENGES IN THE MISSION MANAGEMENT OF POLAR SATELLITE LAUNCH VEHICLE OF ISRO DURING DEVELOPMENT - SOME REFLECTIONS

摘要

Polar Satellite Launch Vehicle (PSLV) is the work-horse of the Indian Space Research Organisation (ISRO) for launching satellites for various missions, having logged 13 consecutive launch successes in the last 15 years. Development of PSLV started in early 1980s. PSLV was originally designed to inject a 1000 kg remote sensing satellite in a 900 km sun-synchronous orbit when launched from Sriharikota (India's) launch station. Quantum technological jumps were required in the areas of solid motors, liquid propulsion system, avionics, navigation guidance and control (NGC) systems, stage auxiliary systems, vehicle engineering and most importantly, the mission management. Mission management required an integrated approach to the tasks of mission analysis, aerodynamic design, on-board NGC system, mission simulations and mission synthesis. This paper presents the challenges faced by the team during the development of PSLV at Vikram Sarabhai Space Centre (VSSC) in the areas of navigation, guidance and autopilot system, mission analysis and mission simulation. It was decided to provide guided injection for PSLV with small dispersions in injection conditions. This called for an inertial navigation system, closed loop guidance and digital autopilot system on board. 16 bit Motorola 68000 processor based onboard computers, one for the navigation system and another for guidance and autopilot system were designed. An 8 -bit processor was designed for data handling in the first stage control system. A hot-standby was provided for each of these on-board computers, which called for a robust failure, detection and isolation (FDI) logic. Implementation of application software in OBCs with the required priority, protection and accuracy along with system software were the real challenges. Simulations with on-board computers- in- the loop (OILS) and hardware-in-loop (HLS) were carried out to validate and end-to-end check of NGC system. A large number of wind tunnel tests were carried out in India and abroad to derive aerodynamics coefficients with acceptable error bands. The trajectory requiring pitch and yaw manoeuvring was optimised to derive maximum payload meeting the constraints of land impacts, injection conditions and visibility from ground stations.Author who was personally involved in these developments recalls in the paper a few cases / problems highlighting the efforts made by the team to overcome the problems, faced during the development. These relate to navigation system, closed loop guidance system, the sequencing system - real time decision (RTD) for stage sequencing and gain switchings in the autopilot system, and in the area of aerodynamics. Simulation problems are also highlighted. The intention of the author is to highlight the spirited approaches undertaken by the development team at VSSC to design a reliable NGC system and mission software for PSLV and solve problems faced in the shortest possible time.