# Cross-Pol Transponder with Frequency Shifter for
## 3 Results
### Field Test  
Figure 10 shows the aerial picture of the test site where we tested the radar equipment. The radar was facing a seven-storey building at 130 m distance. The transponder was at the right side of the radar at 22.2 m distance. Figure 11 shows a picture taken by the radar position. Figure 8: Measured phase by rotating the handle of the manual phase shifter. Red represents the phase measured through the up- and downconverters; blue represents the phase measured bypassing the up- and downconverters. Figure 9: Difference between the phase measured when the signal passes through the up- and downconverters and the phase measured directly (bypassing the up- and downconverters). The radar operated in the band 9.915 GHz-10.075 GHz. The number of frequencies was 801. The length of the mechanical scan (orthogonal to the view direction) was 1.82 m. The number of steps along the scan was 180. The transmitted power was 19 dBm. The measured gain of the transponder (shown in picture in Figure 12) was 91 dB. Figure 11: Picture taken from the radar position in front of the building in the field of view of the radar. Figure 10: Aerial view of the test site. Figure 13 shows the obtained monostatic image. The edges of the building marked in the pictures in Figures 9 and 10 are well evident. The amplitude scale of the image is in signal to thermal noise ratio (STNR), which has been calculate as outlined in Reference [9]:  
\[STNR_{ij}=\frac{\left|I_{ij}^{(\textit{fc})}\right|^{2}P_{0}T_{\textit{meas}}A _{\textit{cal}}}{k_{B}T} \tag{6}\]  
where \(I_{ij}^{(\textit{fc})}\) is the radar image focused using the algorithm in [10]; \(P_{0}=19\) dBm is the transmitted power; \(T_{\textit{meas}}\) is the integration time that is given by the time of a single continuous wave (CW) measurement multiplied by the number of frequency (\(N_{f}=801\)) and the number of positions (\(N_{p}=180\)); \(A_{\textit{cal}}=40\) dB is the attenuation of the internal calibration path of the radar head; \(k_{B}\) is the Boltzmann's constant; \(T\) is the absolute temperature (300 K). Figure 12: Picture of the transponder in the field. Figure 13: Monostatic radar image. In order to test the phase stability of the radar system, we acquired 13 images in 7 h, and we calculated the interferograms between any image and its subsequent image. Figure 14 shows one of these interferograms. The pixels relative to the building appear rather stable in-phase. The obtained bistatic image is shown in Figure 15. The image quality is comparable with the monostatic image. The amplitude scale of the image in Figure 14 is STNR calculated using Equation (6), where \(A_{cal}\) is not longer the attenuation of the internal calibration path but is the attenuation of the external bistatic calibration path estimated as follows:  
\[A_{cal}=\frac{G_{TX\_radar}}{4\pi R_{3}^{2}}\frac{G_{RX\_transp}\lambda_{1}^ {2}}{4\pi}A_{att}\cdot A\frac{G_{TX\_transp}}{4\pi R_{3}^{2}}\frac{G_{RX\_radar} \lambda_{2}^{2}}{4\pi}A_{RX\_radar} \tag{7}\]  
With reference to the block scheme in Figure 16, \(G_{TX\_radar}\)=14 dB is the gain of the transmitting antenna of the radar pointed at the transponder; \(R_{3}\) = 22.2 m is the radar-transponder distance; \(G_{RX\_radar}\) = 14 dB is the gain of the receiving antenna of the transponder pointed at the radar; \(\lambda_{1}\) is the  
Figure 14: Interferogram between two monostatic radar images. Figure 9:0 Bistatic radar image. wavelength at 10 GHz; \(A_{tt}=-30\) dB is the attenuation of the attenuator in the calibration channel of the transponder; \(A=91\) dB is the gain of the transponder; \(G_{TX\text{-}transp}=14\) dB is the gain of the transmitting antenna of the transponder pointed at the radar; \(G_{TX\text{-}transp}=14\) dB is the gain of the receiving antenna of the radar pointed at the transponder; \(\lambda_{1}\) is the wavelength at 12 GHz; and \(A_{RX\text{-}radar}=-2\) dB is the gain of frequency shifter of the radar. Even in bistatic configuration, we acquired 13 images.