A guiding principle behind the design of the Open System Radar Data Acquisition (ORDA) is to provide an initial build with equivalent functionality to the legacy RDA, while providing the foundation for rapid enhancements of functionality with further builds. The controlled scope of the initial build will enable rapid development and test, and a deployment beginning in late calendar year 2004. Fortunately, implementing legacy RDA functionality in the context of the modern technology of ORDA will enhance mission performance in several areas. This paper will describe the differences between the legacy RDA and the designed functionality of the ORDA initial build.