During an observation, a radio telescope beam points towards a specific point in the sky, identified with its coordinates right ascension and declination. The declination is by definition 0o for the equator, +90o for the celestial north pole and -90o for the celestial south pole. The right ascension runs along the celestial equator.
The radio sources of interest are distributed over the whole celestial sphere. The part of the sky in which a radio telescope can observe these sources depends on the construction of the instrument. Most radio telescopes have been built to observe sources over the whole sky visible from the telescope, i.e. all elevations. Noting that many important radio sources, such as the Galactic Centre, have negative (thus: southernly) declinations, studies of these sources from Europe require that elevations down to 0o must be accessible. However, for specific reasons some radio telescopes have been built to operate above a minimum elevation a few degrees above the horizon. The minimum elevation that a radio telescope can track a celestial source is obviously a telescope specific parameter and, therefore, values above 0o cannot be used in compatibility studies when generic scenario are addressed.