In this paper, we proposed a new cross -correlation sum making use of the complex analytic signal to calculate the coherency of the signals for velocity spectrum analysis. It can estimate dielectric constant from multi-channel dataset robustly even the signal with low SNR. On the other hand, a vector network analyzer based GPR system with an antenna array was developed. The antenna array consisted of one transmit antenna and five receive antennas and it can detect the refection signals at different antenna offset with a high signal-to-clutter ratio. Air layer, gypsum wall and concrete wall of different thickness from 10 cm to 30 cm were tested and four field datasets on asphalt pavement of highway road were collected. The estimation results of dielectric constant and layer thickness validated the accuracy. The maximum errors of the thickness estimation results were less than 1.3cm and the relative errors were smaller than 11%.