a system with a capability of detecting the platform stationary status is disclosed . the first aspect is to measure three raw acceleration outputs from an apparatus of the three - axis accelerometer unit . the second aspect is to compute the mean of the latest said acceleration outputs obtained from the three - axis accelerometer unit . the third aspect is to subtract the said mean acceleration outputs from the said raw acceleration outputs obtained from the three axis accelerometer unit to find the differential acceleration components between raw and mean values . the fourth aspect is to compute the amplitude of the differential acceleration , i . e ., squared total sum of the three differential acceleration components . the fifth aspect is to count the number of measurements in which the amplitude of the differential acceleration is below a certain threshold , e . g ., 0 . 05 to detect the stationary status if the small amplitude lasts for a certain time length , e . g ., 1 second .