GNSS-instruments (Global Navigation Satellite System) are the standard field surveying equipment (in addition to tachymeter and levels) for geodetic network establishment and detail surveying. As in the case of other geodetic instruments, it is essential to pre-analyse GNSS-receiver quality parameters, obtained from laboratory calibration and/or field testing of the specific instrument and/or measuring method. Thus, the relevance of the results, as indicated by manufacturer, is obtained that may explain the suitability of a specific GNSS-instrument for field measurements. In 2007, the International Organization of Standardization (ISO), Technical Committee 172, Subcommittee 6 (ISO/TC 172/SC6), presented a comprehensive GNSS field testing procedures for real time measurements, based on statistical evaluation and verification of the manufacturer's hardware and firmware. The test can be performed anywhere on the field assuming that the test area includes minimal potential influences to GNSS measurements. At the same time, a test does not require any additional processing software, because the test data evaluation is based on elementary statistics. This paper presents the theoretical basis of GNSS instrument testing in accordance with the ISO 17123-8 guidelines and further examination of specific measurements on the selected site.