1. Long-duration navigation method by means of a first strapdown navigation unit generating a first state vector whose components give position, attitude and speed values of a carrier with a small instantaneous error, comprising actions consisting of: generating by means of a second navigation unit, a second state vector whose components give position, attitude and speed values of the said carrier with a small long-term error; combining the first and second state vector to obtain an error observation vector; generating an error estimate vector from the error observation vector, by means of a filter; combining the error estimate vector thus obtained with the first state vector so as to obtain an estimated vector of the carrier position, attitude and speed values with a small instantaneous error and a small long-term error.