OK, so suppose I am not interested in measuring small fluctuations in the force of gravity, because I am not looking for buried iron ore deposits or big meteors.Suppose I want to measure the direction of the graviational force, and I want to do this from a moving platform like a car or an aircraft.If I get a 3D MEMS accelerometer, that will do a good job if the vehicle is stationary or moving at a constant velocity, but otherwise, it won't.Is there another way to measure the direction of the gravitational force which will work in an erratically moving vehicle ?
How about a magnetometer if you only want to measure the direction of the acceleration due to gravity. I am pretty sure that a magnetometer will not be affected by linear or rotational acceleration.
This reminds me of how we measured it back in high school...It involved a battery operated bell and a huge long strip (as long as the drop from the window to the ground) of carbon paper with a weight on the end. Then dropped the weight while the bell was ringing, with the strip between the bell and the ringer lever thingy. The impact of the ringer made marks on the carbon paper, and of course the marks got further apart as the weight accelerated. Measured the time it took to drop from the science lab window to the ground. Counted the total number of marks. That gave the number of marks per second or seconds per mark. Measured the (varying) distance between marks. That, with the now known time between marks, gave the (varying) velocity at any instant.And hence the acceleration....But using a pendulum is by far the easiest way to do it... (As long as the bob is very heavy compared to the string, the whole mass may be deemed to be at the bob's CoG.) Beauty of the pendulum method is that since the period T is constant regardless of how wide the swing is, it can as suggested above be measured over 100s of swings and that reduces the impact of the reaction time when the stop watch is started and stopped.
Well the idea is, if you know the accelerations and gyro readings in 3D, then you can calculate the change in orientation. But you get variations and drift in the gyro readings which make this not actually work in practise. You need to keep correcting the gyro-derived orientation using some other means, and two of the available means are the direction of "down" and the direction of the earth's magnetic field.The problem is, a lot of the feasible schemes for doing this ( for example, read Magdwick's papers ) kind of assume that the acceleration of the body is small and that therefore the accelerometer reading indicates the direction of gravity ( either "up" or "down" ). I've developed another scheme which also works and has the advantage ( for me ) that I can understand it.It would be useful to have some means of identifying the direction of gravity independently from acceleration but I guess if Einstein says you can't, we it is probably right. Now a very small higgs boson detector might be able to detect the direction where they are coming from, independent of the actual motion of the device.