The next generation GPS standard system is based on operation/operator. One of the most important motivations of the next generation GPS is to reduce specification uncertainty. This paper analyzes the origination of specification uncertainty and the relationship between specification uncertainty and other two important uncertainties in the next generation GPS, correlation uncertainty and measurement. And then the procedure for management of Specification uncertainty is proposed in the paper. Finally, a method on evaluation of specification uncertainty and the general formula for this method is proposed. The embodied formula for estimation of specification uncertainty on the diameter example is deduced in the paper.