GNSS based SAR system is a passive radar imaging system. Using the reflected GNSS signals, it has been demonstrated that various objects can be identified in GNSS-SAR images. However, compared with conventional SAR systems, the strength of the reflected GNSS signals is significantly lower. How far the objects can be detected by a GNSS-SAR system is a question. In this paper, we derived a theoretical range coverage for GNSS-SAR system for object recognition using object-background scattering contrast, with given false alarm and detection probabilities. Using a GPS software receiver at L1 frequency band, we can detect objects more than 2 km in the land and sea backgrounds, as the field of vision (FOV) for the experiments is approximately 2 km. Theoretically, with such a low cost GNSS software receiver, the operational range can reach 5-8 km.