# Using a Pair of Different-Sized Spheres to Calibrate Radar Data in the Microwave Anechoic Chamber
## 4 Discussions  
The difference between the RCS values of the pair of spheres can be used for verifying whether the scattering region is in the same scattering region because the radar calibration should occur in the same scattering region for the calibration object and the calibrated target. First, we can only use the physical cross-sectional area of the sphere to calibrate the radar data in the optic region, and it requires no special terms for the material of the sphere in the optic region. Second, in the Mie region, we should use an object with a similar material composition to the target to calibrate the radar data. In the Mie region, the material composition of the target contributes to the main scattering field. Therefore, we cannot calibrate a target's radar data using a standard object with a different material composition in the Mie region. For example, if the target is a metallic drone, the calibration object could be a metallic sphere. A metallic sphere would not be appropriate if the target was a bird. Since 65% of a bird's mass is water, a water sphere can be used to describe the scattering field of a bird [12]. In this case, water spheres are more appropriate than metallic spheres when calibrating radar echoes from flying birds using S-band weather surveillance radar. Compared to calibration conditions that consider the shape of the standard object, such as rectangular flat plates and cylinders [4], we should pay more attention to the radar bands and the size of the calibration object. In conclusion, having one object in the optic region and the other object in the Mie region would result in inaccurate radar calibration. The difference between the RCS values of the two spheres also provides a method for ensuring that the test region is in the pure area of the microwave anechoic chamber. Theoretically, the target should be in the far field of the radar scattering field. The distance between the radar antenna and the target must be at least as long as \(R\), which is given by [5; 9]  
\[R>\frac{2d^{2}}{\lambda} \tag{10}\]  
where \(d\) is the target size in the vertical line of incident direction, and \(\lambda\) is the wavelength. In many cases, especially in microwave anechoic chambers, the length of the inside room is shorter than the required range of \(R\). For example, in our case, the length of the anechoic section was approximately 5 m. When measuring a large sphere with a diameter of 30 cm in the Ku-band, a range \(R\) of 9 m is required, which is much longer than the length of the room. Then, how can we ensure that the test area meets the requirements in a compromised way? Or what is the minimum distance between the radar and the target, given the imitated length of the room? The minimum distance is the position where the difference between the two spheres' RCS is equal to the theoretical ones in the optic region. This distance, in our case, is about 2.7 m. Here, we used the Ku-band of the spheres to adjust the antennas and determine the positions of the transmitting antenna and receiving antenna when the  
Figure 4: Radar data of the two spheres within three radar bands: (**a**) HRRPs in the S-band, (**b**) HRRPs in the X-band, (**c**) HRRPs in the Ku-band, (**d**) raw and difference between the two spheres’ data. difference between the radar data of the two spheres was approximately equal to the theoretical difference (i.e., 9.5424 dB). Otherwise, the antenna positions may be incorrect, and the test area around the foam holder may not be in the pure area. It requires many attempts to find the distance successfully. First, two spheres with different diameters, such as the large and small spheres used in our test, must be selected. Second, the locations of the antennas must be adjusted to ensure that the difference between the signal amplitudes of the spheres satisfies the theoretical difference of the optic region. The radar bands in the above tests need to be in the optic region for the large and small spheres. In addition to this verification, the other radar bands should be tested.