Context. Interferometric astrometry at radio and optical wavelengths serves as a fundamental and high-precision method for measuring the positions of celestial objects and for geodetic and Earth orientation measurements. Aims. We determine the importance and form of the degeneracy which arises when the interferometric baselines and the celestial target positions are estimated simultaneously from the same data under circumstances where the declination range of the targets is limited. Methods. Heuristic analyses of an idealised interferometric experiment were used to determine how a degeneracy could arise as a result of limited declination coverage and what form it might take. Singular Value Decomposition of simulated experiments with different declination coverages and different hour-angle coverages were then used to quantify the level of degeneracy present and the form of the degeneracy in example cases. Results. A significant degeneracy was found which depends strongly on the declination coverage and less strongly on the hour-angle coverage of the experiment. The degeneracy is significant even when the declination coverage extends from the pole to the equator. The degeneracy takes the form of a systematic distortion of the measured coordinates in the declination direction which is larger near the equator than at the poles, together with a corresponding baseline distortion. Conclusions. Taking into account this degeneracy in a quantitative way can have a significant effect on the design and interpretation of astrometric and geodetic experiments.
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