R Preethi

Bio: R Preethi is an academic researcher from Indian Space Research Organisation. The author has contributed to research in topics: Acceleration & Terminal (electronics). The author has an hindex of 1, co-authored 1 publications receiving 4 citations.

Analytical optimal guidance algorithm for lunar soft landing with terminal control constraints

Abstract: In this paper it is proposed to augment an existing optimal analytical guidance algorithm for the purpose of achieving terminal angle constraint during the terminal powered descent phase of a lunar soft landing mission. Basic algorithm formulation caters to minimizing the acceleration due to onboard engines to minimize the fuel spend and reach the desired position with desired velocity. However, in order to cope up with terminal angle constraint, an augmentation is proposed in the objective function wherein the aim is not only to minimize the energy but also to minimize the terminal error in acceleration achieved and the desired acceleration specified by the designer. This inadvertently leads to the desired terminal angle. Simulations done for the fine braking phase of lunar soft landing mission prove that terminal attitude constraints are met with the modification in objective function.

Constrained optimal multi-phase lunar landing trajectory with minimum fuel consumption

Abstract: A Legendre pseudo spectral philosophy based multi-phase constrained fuel-optimal trajectory design approach is presented in this paper. The objective here is to find an optimal approach to successfully guide a lunar lander from perilune ( 18 km altitude) of a transfer orbit to a height of 100 m over a specific landing site. After attaining 100 m altitude, there is a mission critical re-targeting phase, which has very different objective (but is not critical for fuel optimization) and hence is not considered in this paper. The proposed approach takes into account various mission constraints in different phases from perilune to the landing site. These constraints include phase-1 (‘braking with rough navigation’) from 18 km altitude to 7 km altitude where navigation accuracy is poor, phase-2 (‘attitude hold’) to hold the lander attitude for 35 sec for vision camera processing for obtaining navigation error, and phase-3 (‘braking with precise navigation’) from end of phase-2 to 100 m altitude over the landing site, where navigation accuracy is good (due to vision camera navigation inputs). At the end of phase-1, there are constraints on position and attitude. In Phase-2, the attitude must be held throughout. At the end of phase-3, the constraints include accuracy in position, velocity as well as attitude orientation. The proposed optimal trajectory technique satisfies the mission constraints in each phase and provides an overall fuel-minimizing guidance command history.

An Optimal Explicit Guidance Algorithm for Terminal Descent Phase of Lunar Soft Landing

Abstract: An explicit guidance algorithm for multi-constrained terminal descent phase of lunar soft landing is presented in this paper. A minimum jerk guidance is designed and extended for this purpose to achieve the terminal control and state constraints. The closed form jerk expression, obtained using the minimum jerk guidance is analyzed to obtain an explicit expression for acceleration command, which is the physical control variable for the guidance loop. The guidance formulation ensures the minimum rate of change of acceleration and vertical touchdown of the spacecraft towards a designated landing site with high terminal accuracy. The design features of the proposed guidance law are demonstrated using simulation results.

Terminal State Constrained Proportional Navigation Law for Lunar Soft Landing Mission

Abstract: With regards to a typical lunar soft landing guidance formulation, it is required to reach the desired position with terminal velocity and orientation constraints. For the terminal phase of lunar powered descent, an existing proportional navigation law developed for missile guidance is modified. Presently in the algorithm, at the beginning of each guidance cycle, a normal acceleration perpendicular to the instantaneous missile-target line-of-sight is computed. The design augmentation proposed in this paper for lunar landing, introduces a polynomial acceleration term along the line-of-sight direction in addition to the existing normal acceleration which would then ensure terminal velocity requirements. It also has the capability to meet zero line-of-sight angles at the end of trajectory maneuver. Simulation results indicate that the developed methodology can be successfully utilized in lunar landing scenarios, especially in the terminal phases where the Lander orientation has to be vertical at the end.