The design of a robust downlink communication system for nanosatellites in UHF amateur radio band involves many challenges like radio spectrum constraints, time-varying Doppler, clock drifts and synchronization in a LEO satellite channel. This paper presents the system designed and implemented for IITMSAT, a student satellite project at IIT Madras, which successfully overcomes these challenges using a robust physical layer design implemented on a generic Software Defined Radio platform. The performance of the proposed design is verified through simulations as well as hardware. The communication system gives a good performance which is only about 1 dB away from theory. © 2015 IEEE.