CLLARE CM Navigation system
From CSTART Wiki
The navigation system's job is to produce a continuous sequence of accurate estimates of the spacecraft's position and velocity with respect to some coordinate system, and the spacecraft's orientation and angular velocities. The time between consecutive estimates in this sequence may vary depending on the stage of the mission.
[edit] Overview
At its most basic level, the navigation system consists of:
- Input data (measurements from hardware, made by the astronaut, radioed back from Earth)
- Output data (specified by 12 numbers: position and velocity in two 3D coordinate systems, one for position and one for orientation)
- Software to transform input data into output data
Thus the main jobs associated with the navigation system's development are:
- Coming up with possible sources of input data and assessing their usefulness and accuracy
- Coming up with an algorithm to transform input data into output data and implementing this algorithm in software
To some degree these tasks are independent (one can do useful work on one without knowing much about the other)
[edit] Input data
Input data for the navigation system can come from a number of sources:
- GPS (only during launch, possibly during LEO?)
- Accelerometers and gyroscopes (inertial navigation)
- Radio communication analysis
- Visual navigation by the astronaut
[edit] Transformation algorithm
Transforming measurements into position and velocity outputs is not a straightforward matter of integrating acceleration with respect to time to get velocity and integrating measurements of velocity with respect to time to get position. This approach will lead to innacurate estimates as small measurement errors accumulate over time.
In order to make accurate estimates, the navigation system must have some underlying model of the dynamics of the spacecraft (i.e. some idea of how it is supposed to be moving) and some model of the errors in the measurments. The system uses these two things to make intelligent estimates of actual position and velocity.
Kalman filters are well suited to this task, and were used by the Apollo missions.
