Infrared horizon sensors are used for determining the orientation of satellites. These sensors usually operate on the principle of detecting the discontinuity in the infrared radiance at the limb. For calibration and testing of the horizon sensors under ground conditions, it is necessary to simulate the infrared horizon of the Earth. The simple, compact simulator described in this paper simulates the radiation of the Earth horizon and the shape of the radiance transition from space to Earth for the spectral range of the horizon sensor. This enables the sensor to be tested as a function of the altitude and orientation of the satellite. The simulator employs a spherical mirror; near its focal surface there are two radiating surfaces simulating space and Earth, respectively. Thermal contrast is obtained by controlling the temperature difference between the two surfaces. Calculated magnitudes of the spherical aberration of the mirror and disposition of the two radiating surfaces with respect to the spherical mirror are used for simulation of the radiance profile shape of the Earth horizon. This paper describes the structure of the simulator, the method of determination of its main parameters and experimental evaluation of the simulator characteristics.