A radio telescope's antenna pattern (or radiation pattern) is the pattern of its sensitivity to radio arriving from different directions, i.e., a sensitivity function on the sphere of all directions. Even if it is highly directional, such as a dish, refraction causes it to be somewhat sensitive to "rings" of directions around its primary direction of sensitivity (the same principles as the Airy disk). Thus, when the antenna is pointed and receiving, the presence of a signal is ambiguous: it may be from that direction, or alternately from some other direction in its antenna pattern. A means of working around the ambiguity is to change the antenna's direction slightly and make use of the observed changes in signal strength to determine the actual direction of the incoming signal(s). (This is somewhat equivalent to angular differential imaging in optical telescopes.)
Antenna patterns are a fact of life for all antennas, e.g., in communication, and radar, and affect transmission as well as reception.