Resolvable covering arrays

Two powerful recursive constructions of covering arrays of strengths three and four use difference covering arrays (DCAs). However, what is required in these constructions is not the algebraic structure of differences in a group, but rather that the DCAs produce covering arrays that are resolvable. Both constructions are strengthened by using resolvable covering arrays in place of DCAs. Many new difference covering arrays are found by computational methods, and resolvable covering arrays that do not arise from DCAs are produced. Improvements for bounds on covering array numbers are shown to be substantial.