It is pointed out by Rothcaher et al., 1996, that the apriori model of the tropospheric delay is not sufficient if highest accuracy is required. Thus, it is necessary to estimate a correction for the apriori tropospheric model. Since it is still difficult to separate the real term orbit-biases from the tropospheric biases, the combined effect of orbit-biases and the tropospheric biases is considered in thie current paper as a single error, namely the geometric error. The statistical behavior of the geometric error in the double difference sense is studied. A developed real-time model for reducing the effect of the geometric biases over long baselines is introduced and verified