An Analytical technique is developed to evaluate the error rate performances of quadrature phase shift keying (QPSK) and 16-ary phase shift keying (16-ary PSK) in a land mobile satellite communication channel. A regenerative repeater is considered. The effects of satellite nonlinearity and mobile channel on the performances of both systems have been considered. A frequency non-selective fading has been assumed. The combined effect of the traveling wave tube (TWT) power amplifier which exhibits AMfAM and AMfPM cnwersions and the multipath fading and shadowing of the mobile channel in addition to lhe additive white Gaussian noise (AWGN) are taken into consideration in the analysis. The effect of TWT back-off from saturation which determines the degree of nonlinearity has also been taken into account. An expression for the error probabilities for both systems are derived and computed numerically.