The processing of GPS/GLONASS data requires the specification of an observation model

which consisted of a functional and a stochastic model. The GPS/GLONASS functional model is already well known

but the GPS/GLONASS stochastic model is not. The stochastic model depends on not only the types of the observation and the receiver used

but also the environmental effects. To obtain the precisely combined GPS and GLONASS positioning

the importance of stochastic model of combined GPS and GLONASS observed value is analyzed. The posterior estimation method and iteration of variance and covariance method to estimate stochastic model using residuals of observable errors are proposed. The theoretical and numerical results show that the proposed method which can improve the reliability and precision of ambiguity resolution

is more rigorous in theory and more powerful in practice comparing with the stochastic model

Using iteration of variance and covariance method to estimate stochastic model of combined GPS and GLONASS observables is suggested.