On Ground Simulations
The control law needs to be verified to check whether it meets the requirements imposed on it. It must be checked whether the attitude reaches withing a permissible band around desired attitude and whether the time required by the attitude to reach into this band in according to the payload requirements. It must be checked whether any physical parameters are not taking unbounded values. It must be checked whether actuators have the capacity to give the control input throughout (in other words, confirm that the actuators are not getting saturated at any point in time). The control law must be tested for robustness in case of errors in (like mounting errors, damage during launch on in flight which may increase noise or add bias) or complete failure of actuators and sensors. Battery simulations are done to check whether adequate power is being generated to keep the entire satellite alive throughout its mission life. Verification is generally done by numerical methods, by running extensive simulations for different initial conditions. A model for numerical simulations is made in which the environment, satellite dynamics, estimation algorithm, sensors, controller and actuators are simulated according to the best known models. Initial conditions for simulations are the initial states of all actuators and sensors; launch date, time and location; initial angular rates and attitude at the point of launch.
On-board In-Loop Simulation (OILS)
OILS is On-Board In-Loop Simulation. In OILS, essentially, the controller in the numerical model of the control law is replaced by the On Board Computer(OBC). Sensor data as simulated in the numerical model is fed to the OBC. The OBC performs the necessary calculations on the input data and returns the desired control input to the the actuators. This desired control input is fed back to the numerical model and satellite dynamics are simulated. This process goes on in a loop. This helps us qualify the performance of the OBC code.
Hardware In-Loop Simulation (HILS)
HILS is Hardware-In-Loop-Simulation. The entire satellite hardware is tested to check the performance of the actuators and control law. Sensor inputs are not simulated like in OILS but sensors are given inputs of actual physical quantities (for example a special light simulates the sun). Satellite is generally suspended on an air-bearing table to simulate it’s dynamics in space.
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