## 5 Experiments and Results
### Adaptive Beamforming of Oblique Backscatter Ionogram  
Figure 14 show the oblique backscatter ionograms processed by the SMI and ESB adaptive beamforming algorithms. To truly reflect the suppression effect of vertical echoes and interference signals, Channel 1 is still taken as the reference in this section. Obviously, the adaptive beamformers greatly weaken the vertical echo signals much better than conventional DBF as Figure 10c. It is also reasonable to believe that the 2-hop echoes will not interfere with the oblique backscatter signals. Figure 11: Normalized signal-to-noise ratio (SNR) of \(5.2\,\mathrm{MHz}\); (**a**) Channel 1; (**b**) Directly sum; (**c**) digital beamforming (DBF). Figure 12: Normalized SNR of 7 MHz; (**a**) Channel 1; (**b**) Directly sum; (**c**) DBF. Figure 13: Normalized SNR of 6 MHz; (**a**) Channel 1; (**b**) Directly summing; (**c**) DBF. Therefore, in the whole frequency band of 5-9.2 MHz, the signals over 520 km can be traced and extracted as the oblique backscatter echoes. It is equivalent to extending the sounding range towards the transmitting station. For typical RF interference signals, such as 6 MHz, 9.4 MHz and 9.8 MHz, compared with the conventional DBF, the adaptive beamformers have much better suppression effects as shown in Figure 14b,c. Especially for ESB algorithm, some strong interference signals such as 9.4 MHz and 12.2 MHz are almost completely filtered out. It is undeniable that both SMI and ESB seem to reduce the SNRs. When the adaptive beamformers suppress the vertical signals in the low frequency band greatly, the SNRs of the higher frequencies are not improved. This is because that whether it is SMI or ESB, the basic idea is to keep the desired signals while minimizing the output power of the whole array. This makes the total output power of the array decrease when the desired signal or interference exist. But when there is no desired or interference signals, the noise power cannot be reduced equally. And due to the effect of the spatial filtering, echo signals from other azimuths caused by the side lobe of the transmitting antenna are also weakened substantially together. In contrast, the effect of this signal processing method for obtaining the leading edges of the desired pointing direction should be more meaningful. In order to better show the application effect of adaptive beamforming in the oblique backscatter sounding, some of the specific typical frequencies, such as 5.2 MHz with strong vertical signal, 6 MHz with great interference, 7 MHz with a strong desired signal and 8.8 MHz with a weak one, have been extracted as Figures 15-18. For 5.2 MHz, in Figure 15a, it is obvious that the 2-hop of the vertical echo signal of channel 1 is a pseudo leading and can easily be mistaken for the desired echo signal. But through the adaptive beamforming, the vertical incident signals are well suppressed. If the same attenuation coefficient (24 dB) of the 2-hop echo signal in Figure 15a is referred, the 2-hop signals in Figure 15b,c are limited near the background noise level and will not affect the discrimination of the leading edge. Therefore, while it seems that the SNR in Figure 15a looks better, in fact, Figure 15b,c are more accurate. Figure 14: **(a)** The oblique backscatter ionogram of Channel 1; **(b)** The oblique backscatter ionogram of SMI; **(c)** the oblique backscatter ionogram of ESB. Figure 15: Normalized SNR of 5.2 MHz; **(a)** Channel 1; **(b)** SMI; **(c)** ESB. As for the strong interference signal at 6 MHz. From Figure 16a, it can be seen that the strength of the interference signal almost equal to the 2-hop of the vertical echo signal. At the group distance of 583.7 km, the strength of the signal is only 23.57 dB higher than the average background level. While in Figure 16b,c, this value can reach 45.69 dB and 44.85 dB. For the area from 300 km to 580 km, the suppression ability of interference signal is even better than DBF as Figure 13c. Especially for the SMI algorithm, Figure 16b shows a very clear and steep leading edge.