We study the problem of amplify-and-forward relaying in a hybrid satellite-terrestrial communication system with imperfect channel gains. Signals are received by masked destination earth station (ES) with the help of a relaying ES, situated at the ground with good line-of-sight. No direct transmission is possible from the satellite to the destination ES. The satellite-relay ES link is assumed to be Shadowed-Rician fading, and relay ES-destination ES (terrestrial link) is assumed to follow Nakagami-m fading. We propose a scheme for channel estimation and detection of channel gains and transmitted signal from the satellite. The estimation of terrestrial-link is performed separately, whereas, the channel estimation of cooperative hybrid link (satellite-relay ES-destination ES) and symbol detection is performed jointly. The average symbol error rate of the considered system for M-ary phase shift keying constellation is derived by using moment generating function (M.G.F.) approach. Moreover, the analytical diversity order of the considered system with estimated channel gains is also derived. It is shown by simulation and analysis that the diversity order of the system is not affected by the estimation errors. By using the expression of M.G.F., the average capacity of the system is also derived.