We propose a novel physical layer secret key generation method for the
inter-spacecraft communication links. By exploiting the Doppler frequency
shifts of the reciprocal spacecraft links as a unique secrecy source,
spacecrafts aim to obtain identical secret keys from their individual
observations. We obtain theoretical expressions for the key disagreement rate
(KDR). Using generalized Gauss-Laguerre quadrature, we derive closed form
expressions for the KDR. Through numerical studies, the tightness of the
provided approximations are shown. Both the theoretical and numerical results
demonstrate the validity and the practicality of the presented physical layer
key generation procedure considering the security of the communication links of
spacecrafts