## 5 Conclusions  
Although the air-launched 3D GPR is nondestructive, time-saving, and less labor-intensive to assess density and thickness, it can only provide indirect measurement data under testing area conditions and strongly depends on the explanations by prediction models. This study focused on the adaptability and accuracy of different prediction models to calculate the dielectric constant, thickness, and density of an asphalt concrete base. By choosing proper calculation methods, the thickness and density of this layer were predicted, and homogeneity was assessed. The main findings were given below:  
1. The Common Mid-Point Method, using the air-launched bowknot monopole antennae matrix, had a satisfactory performance in determination of the dielectric constant of the asphalt concrete base and presented an error of 1%, when compared with the calculated dielectric constant from cores. The results indicated potential of the Common Mid-Point Method to be adopted for providing reliable estimation results on the dielectric constant of an asphalt concrete layer and obtaining accurate calculated thickness.
2. In terms of GPR and the Common Mid-Point Method, thickness homogeneity assessment on the tested filed found that the thickness of an asphalt concrete base showed the thin area to be close to the edge of the pavement and the middle section of the asphalt concrete base.
3. Among the Birefringence, Boettcher, Linearity indicator, and Rayleigh models, the Rayleigh model was suggested to predict density, and the predicted density exhibited a good correlation coefficient with the measured density. The results of the Rayleigh model presented an average correction factor of 1.15, when compared to the density from the non-nuclear density gauge.  
However, the Common Mid-Point Method was only able to detect a line area and the detection dielectric constants were a representative value of small areas. The variation of dielectric constants in the small area should be considered. In this study, only asphalt pavement layers were researched. The applications of GPR also included cement concrete pavements.  
[PERSON], [PERSON], [PERSON] and [PERSON] conceived and designed the experiments. [PERSON], [PERSON], [PERSON] and [PERSON] performed the experiments. [PERSON], [PERSON] and [PERSON] analyzed the data. [PERSON], [PERSON] and [PERSON] contributed reagents/materials/analysis tools. [PERSON] and [PERSON] wrote the paper. [PERSON] and [PERSON] revised the paper. All authors have read and agreed to the published version of the manuscript.  
Figure 10: Density distribution of the tested area.  
**Funding:** This research was funded by the National Key R&D Program of China, grant number 2018 YFB1600103, project ZR2020 QE271 was supported by Shandong Provincial Natural Science and Foundation and Shandong province key research and development program, grant number 2019 GSF109020, No. 2019 GGX101042.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** Data sharing is not applicable to this article.  
**Conflicts of Interest:** The authors declare no conflict of interest.