Determining a model error and how to deal with it thus requires a clear understanding of the mathematical model available in the software. Of course, computation or transposition errors can occur during data preprocessing. However, these errors occur less frequently because the process is computerized. A typical example is the critical circle for the plane resection problem. There are many other examples of numerical singularities.
Some satellite constellations can cause singularities in carrier-phase processing.
Continuous kinematic GPS surveying is susceptible to such dangerous conditions, particularly if only four satellites are observed. Another cause for a large a - posteriori variance of unit weight can be premature termination of iterations. These functions are typically linearized around such known quantities as approximate station coordinates. The higher-order term in the linearization can be neglected only if enough iterations have been carried out and the solutions converge rapidly. The software usually provides criteria for determining whether solutions have converged.