In this paper, we report the development of the design verification model (DVM) of Rb atomic frequency standard for the Indian Regional Navigational Satellite System (IRNSS) programme. Rb atomic clock is preferred for the space applications as it is light-weight and small in size with excellent frequency stability for the short and medium term. It has been used in all other similar navigation satellite systems including GPS, GLONASS Galileo etc. The Rb atomic frequency standard or clock has two distinct parts. One is the physics package where the hyperfine transitions produce the clock signal in the integrated filter cell or separate filter cell configuration and the other is the electronic circuits which include frequency synthesizer for generating the resonant microwave hyperfine frequency, phase modulator and phase sensitive detector. In this paper, the details of the Rb physics package and the electronic circuits are given. The reasons for the mode change in Rb lamp have been revisited. The effect of putting the photo detector inside the microwave cavity is studied and reported with its effect on the resonance signal profile. The Rb clock frequency stability measurements have also been discussed.