Watermarking can be modeled as a communication over a steganographic channel. The channel may comprise additive noise which modifies the data value, as well as geometric noise which modifies the data location. We propose here a general model for geometric channels by introducing a random variable, the index variable. We model the jitter attack by a Markov chain. To enhance the watermark retrieval after a jitter, we propose to embed a synchronisation pattern in the cover picture. We use this pattern to estimate the geometric distortions at the receiver side.