We derive an expression for the error probability ofM-ary frequency shift keying with differential phase detector and narrowband receiver filter in the satellite mobile (Rician) channel which includes as special cases the Gaussian and land mobile (Rayleigh) channels. We compute the error probability as a function of various system parameters forM= 2, 4 and 8 symbols and the third order Butterworth receiver filter. The error probability increases with Doppler frequency and with the shift of the channel from Gaussian through Rician to Rayleigh. The optimum normalised bandwidth per bit is in the vicinity of one and the optimum modulation index for binary symbols is about 0.6. The threshold for quaternary symbols can be optimised to about 0.9 of the modulation index. For Rician and Rayleigh channels with non zero Doppler frequency there is an error floor, therefore may require diversity or coding in order to achieve a desired error probability.