Verification of Guidance, Navigation and Control Systems
Navigation and control systems of space vehicles can be tested on Earth only with restricted degrees of freedom and accuracy of the environmental dynamics before being flown. However, a vehicle’s dynamics can be simulated using numerical computer simulations. It has become general practice to use states from these simulations to stimulate sensors and flight systems for testing. The vehicle’s on-board hardware is used for these tests to verify the combination of sensors, computers and actuators.
The Department of Guidance, Navigation and Control Systems (GNC) runs several laboratories and testing facilities that can be used to test complete navigation and control systems of satellites and space vehicles or parts of GNC systems of space transportation systems.
These hardware-in-the-loop tests can be carried out in two steps:
- In the first step, the mission’s on-board computer can be tested separately. For each interface, a simulation computer provides the on-board computer with simulated sensor data in real time. Additionally, the simulation computer receives output signals from the on-board computer and may incorporate this information into the dynamics simulation.
- In the second step, the simulation computer is connected to sensor stimulators and creates signals for these in real time. The on-board computer then interfaces with the actual sensors instead of the simulator, thus including the sensors in the tests. For this purpose, specialized hardware to test GPS or inertial sensors may be required. Finally, even the actuators may be included. In this case, the actuator actions have to be measured in real time and integrated into the simulation.
Since most interfaces for sensors and actuators are standardized, it is possible to create a system that can test a variety of attitude and orbit control systems for space missions. To adapt the test and verification systems for a particular mission, the interfaces can be reconfigured and communications protocols can be implemented that allow to interact with the sensors and actuators used.