In the frame of Silex project, a prototype model of the Acquisition Sensor (AS) was realized by Officine Galileo; scope of this sensor is to detect the optical signal transmitted by another terminal (either on-ground or on-satellite), and to measure the two-axes position of the incident spot beam. The optical system for focusing the beam on the detector is not part of the AS itself. The sensor is split into two units: a Detection Unit (DU), containing the CCD detector and the proximity electronic, and the Electronic Unit (EU) containing the processing electronic that drives the DU and evaluates the coordinates of the spot focused on the detector. The first unit was a flight-oriented realization; an accurate positioning procedure for the detector (few micrometers of error) was developed. The capability of the sensor to operate with minimum arrangements in the wide range of conditions imposed by the different links GEO-GEO and LEO-GEO is investigated. The most critical aspect, the choice of the detection threshold at varying of input signal power, background noise and operating frequency, is well explained. Finally, results of preliminary tests are reported.