# Design of Multifunctional Mesosphere-Ionosphere Sounding System and Preliminary Results
## 2 System Description
### Sounding Waveform  
Breaking the limitation of the traditional monopulse mode, the interpulse coding waveform is applied to the MF sounding in MAI system. 16-bit bi-phase complementary sequences are selected as the pseudo-random modulation code for MAI system. The specific waveform of the transmitting signal is shown in Figure 4.  
In Figure 4, _clk_ is the synchronized clock, with the period \(T_{clk}\). \(U(t)\) represents the code sequence. The positive and the complement sequences are shown in Table 2. Further, 0 and 1, respectively, represent the phase 0 and \(\pi\). The duration of each code bit is \(T_{p}=12.8~{}\upmu\)s. \(A(t)\) shows the time-sequence  
Figure 4: The specific waveform of the radar transmitting signal. Driven by the system clock _clk_, the transmitting waveform \(W(t)\) is generated according to the code sequence \(U(t)\) and transmitting pulse \(A(t)\). The positive and complement codes are transmitted alternately. Figure 3: The block diagram of the power amplifier unit. \(4\times 600\) W linear amplifier can provide 2 kW transmission power. Figure 2: The transmission channel of MAI system which is mainly composed of the excitation source and power amplifier. of the transmitting and the receiving. During the sounding period \(T_{r}=320\times 12.8\) us, the sounding signal is transmitted firstly (when \(A(t)\) is in high level), while the remaining time is used to receive the echo signals (when \(A(t)\) is at low). \(W(t)\) is the actual sounding signal waveform. The concept of complement code is first proposed by [PERSON] [30]. By using the feature of the zero autocorrelation sidelobe of a pair of complementary sequences, the complementary code can effectively eliminate the sidelobe interference in the sounding. Figure 5 is from the reference [25], showing the normalized ambiguity function graphs of the 16-bits sequences employed in MAI system. Figure 5a is the three-dimensional ambiguity function graph, Figure 5b shows the sections of zero Doppler and zero shift. Especially, the characteristics of its zero autocorrelation sidelobe intuitively. Obviously, as a kind of PRN sequence, the complementary code has a \"pushpin type\" ambiguity function, which means a high resolution in distance and speed and good sounding accuracy. It is very suitable for soft target sounding applications such as the mesospheric and ionospheric soundings investigated in this paper. According to the waveform system, some specific radar parameters are determined. As the range resolution of bi-phase coded modulation waveform is determined by the duration of each code bit, this parameter of MAI system can be expressed as Equation (1) [31]:  
\[R_{\text{min}}=\frac{cT_{p}}{2}=1.92\text{ km} \tag{1}\]  
where \(c\) is the speed of light. The maximum sounding range \(R_{u}\) and the pulse repetition frequency (PRF) can be determined by Equations (2) and (3) [31]:  
\[R_{u}=\frac{cT_{r}}{2}=614.4\text{ km} \tag{2}\] \begin{table}
\begin{tabular}{c c} \hline
**Sequence** & **Values** \\ \hline The positive sequence A (\(a_{1}\cdots a_{L},L=16\)) & \(1101\_0001\_0111\_1011\) \\ \hline The complement sequence B (\(b_{1}\cdots b_{L},L=16\)) & \(0010\_1110\_0111\_1011\) \\ \hline \end{tabular}
\end{table} 
Table 2: The code sequences of 16-bit bi-phase complementary code. Figure 5: Normalized ambiguity function of complementary code: (**a**) Normalized ambiguity function; (**b**) Normalized ambiguity cut for Delay = 0 μs and Frequency = 0 kHz. It has a “pushpin type”ambiguity. And the section of 0 kHz has only one main lobe with no side lobe. Thus, it is very suitable for soft target sounding. \[PRF=\frac{1}{2T_{r}}=122\ \mathrm{Hz} \tag{3}\]  
Due to the strong near-field coupling of the transmitting signal, there is a certain fade zone \(R_{d}\). When \(L\) is defined as the bit length of the code sequences, \(R_{d}\) can be determined by Equation (4) [31].