This paper presents a formal method for requirements specification and analysis. Using this method some techniques for step-wise refinement are studied. During the early phases of system development, where the exact requirements are yet unclear, these techniques allow to write incomplete and global specifications, which during successive steps can be refined and completed.At each step the method supports formal analysis of the specification. In particular two abstraction techniques are studied: nondeterminism and uninterpreted symbols. These techniques are explored using a realistic case study that was inspired by the specification of an existing naval command and control system. Specifications are written and analyzed using the language and proof checker of PVS.