A prototype sensor designed to use of Gamma Ray Bursts (GRBs) as signals of opportunity for relative positioning, navigation and timing (PNT)in deep-space applications is described. GRBs are intense and aperiodic electromagnetic emissions which can last from a few fractions of a second to hours in length. What makes them suitable for navigation is that they are ubiquitous and can be sensed easily. The design of and results from laboratory testing of a prototype GRB sensor known as the Gamma Ray Burst Incidence Detector (GRID) is presented. Concept of operation for relative PNT using GRBs in CubeSat missions is also described.