  Small corrections in the argument of the latitude can be used to improve the
accuracy of the SGP4 orbit propagator. These corrections have been obtained by
applying the hybrid methodology for orbit propagation to SGP4, therefore
yielding a hybrid version of this propagator. The forecasting part of the
hybrid method is based on a state-space formulation of the exponential
smoothing method. If the error terms that have to be considered during the
model fitting process are taken as Gaussian noise, then the maximum-likelihood
method can be applied so as to estimate the parameters of the
exponential-smoothing model, as well as to compute the forecast together with
its confidence interval. Finally, this hybrid SGP4 orbit propagator has been
applied to data from Galileo-type orbits. This new propagator improves the
accuracy of the classical SGP4, especially for short forecasting horizons.
