I'm working on a project which uses GPS coordinates. I have a UAV that flies, for example, over a meridian or a parallel. I suppose that the coordinates I'm reading from UAV are always increasing or always decreasing, but it isn't like that. In details, I can have for example 3 decreasing coordinates and 1 increasing coordinate, then (always for example) 10 decreasing coordinate and 1 or 2 increasing coordinates. Is that normal? It could be the precision of the sensor?
There should be no reason for the coordinates to always increase or decrease. It would depend on the direction of the motion, if the UAV is traveling along a straight line you would expect the changes in coordinates to remain similar.
Keep in mind that GPS position estimates have high errors. A simple receiver using a pseudorange observable could have errors even as large as 20-30 meters depending on how it handles ionospheric and tropospheric delays. These errors will result in noisy position estimates, possibly causing the problem you are referring to.