# A Novel High-Precision Imaging Radar for Quality Inspection of Building Insulation Layers
## 1 Introduction  
The building insulation layer can effectively reduce the loss or infiltration of heat and keep the indoor temperature stable, which not only saves energy but also provides a more comfortable living environment for humans [1, 2, 3, 4, 5]. In addition, the insulation layer can protect the building structure and improve environmental quality [6]. The origins of building insulation layers can be traced back to ancient humans' preliminary exploration of building comfort and energy conservation. With the development of building technology and materials, insulation technology has gradually evolved and improved [7]. Since the 1960s, with the improvement in energy-saving consciousness, building insulation technology has developed rapidly. Relevant building energy conservation standards have been introduced internationally, promoting the popularization of external building insulation systems. Highly efficient insulation materials are commonly used in the exterior walls, roofs, and windows of modern buildings, and external building insulation systems have become part of the standard design. The number of accidents in which the insulation system falls off and injures pedestrians and damages property has been increasing year by year, which mainly stems from cracks, hollowing, water seepage, and insufficient adhesion in the external insulation system for exterior building walls. If the quality of the insulation layer is not properly handled, it may lead to poor building energy efficiency, compromise building comfort, and even cause safety hazards. Therefore, the quality inspection of building insulation layers is an important part of ensuring insulation effectiveness and building safety. The amount of bonding mortar is closely related to the quality of the insulation layer. If the amount of mortar is too low, it will result in an insufficient bonding area between the insulation board and the substrate, which will reduce tensile strength and wind pressure resistance, making it prone to hollowing and detachment. If the mortar layer is too thick, cracks may occur due to increased shrinkage stress, which weakens the bonding effect. The standard commonly used to detect the quality of the insulation layer is that the bonding area measured by the point-frame method should not be less than 40%, and in high wind pressure areas, a full bonding method (100% coverage) should be used. Therefore, how to conduct non-destructive testing and accurately calculate the bonding area of the bonding mortar within the insulation layer is an important research topic in the field of the quantitative detection of insulation layer quality [8]. Existing means of the non-destructive testing of building insulation systems include laser scanning, infrared thermal imaging, ultrasonic testing, acoustic emission, traditional ground-penetrating radar, etc. [9, 10, 11, 12]. Laser scanning technology can acquire 3D point cloud data of the surface of insulation layers, which can then be used to analyze the flatness, thickness variations, and potential delamination risks of the surface. However, it cannot penetrate the insulation layer to detect internal structures [13]. Infrared thermography detects defects such as voids, cracks, or delamination inside the insulation layer by monitoring the surface temperature distribution. However, this method requires appropriate temperature differences, is greatly influenced by environmental factors, and cannot quantitatively assess the internal conditions of the layer [14, 15, 16, 17]. Ultrasonic technology can penetrate the insulation layer to determine if there are voids, cracks, delamination, or other defects inside. However, the hammering used to excite signals during testing can damage the insulation layer, and the identification of defects relies on human experience, which can be affected by subjective factors [18]. Acoustic wave detection technology monitors the acoustic waves generated by the insulation layer during stress or temperature changes by placing sensors on the surface.