spherical positioning

[sciguy125]

Let's say that I have a device that can suspend itself in the air. It's able to provide thrust in the x, y, and z axes, as well as the appropriate thrust to rotate about each axis. I want to be able to determine what direction is down.

I thought of using 3 accelerometers (one along each axis). You'd be able to calculate the direction of gravity by knowing the acceleration along each axis. The problem comes when you start to move. If you start to move in the +x direction, "gravity" will be biased toward the -x direction.

One thing I thought of was adding 3 gyros to the mix. If you determine gravity at start up, you can use the gyros thereafter. The problem with this is that there's no self-correction. After some time, there WILL be notable error in the measurements, so the system will have to be reoriented. So, how do you know that you're in a stationary state so that you can recalculate gravity?