# Design of Multifunctional Mesosphere-Ionosphere Sounding System and Preliminary Results
## 5 Conclusions  
In this paper, we propose a new type of sounding system which integrates the MF radar's sounding function of the mesosphere (which also belongs to the lower ionosphere) and the ionosonde's sounding function of the ionosphere above E layer into one hardware platform which is denoted as the middle atmosphere-ionosphere (MAI) system. It breaks through the monopulse mode adopted by traditional MF radar. MAI system innovatively adopts the pseudo-random phase code modulation on the waveform design. Relying on the pulse compression, an extra correlation gain would be obtained, and thus the transmitting power can be reduced to simplify the equipment and consequently the cost. The employment of the complementary code sequences also enables MAI system to have a strong anti-jamming capability. For the antennas employed in MAI system, unlike the traditional MF radars, a mode of one transmitting and six receiving is adopted for the mesospheric (lower ionospheric)  
Figure 17: Electron density profile at the local time of 17:50, 5 January 2018.  
sounding. The progressive-wave antennas are employed for receiving, which miniaturizes the array structure, reduces the difficulty of erection, and satisfies the large bandwidth for the ionospheric sounding. Although the efficiency may be lost, the waveform gain would compensate for this. Using the developed prototype device, we conducted a verification experiment. Based on the experimental data, using FCA and DAE algorithms, the drift velocities of the wind field and the electron density profiles of the lower ionosphere are estimated or calculated. By sweeping in high frequency band, MAI system conducted the ionospheric vertical sounding. After inversing with the QPS model based on the ionogram, the ionospheric electron density profile could be acquired. The successful development of MAI system makes it possible to sound the two regions on the same platform at the same location. This is of significance to the study of the coupling characteristics and physical processes of them. With the further improvement of experiments, the scientific research based on MAI system will be more desirable.  
For the current MAI system, in order to better carry out future research tasks, we think that we need to improve some of its shortcomings. Firstly, the current signal quality needs to be strengthened. Both the wind field estimation and the low ionospheric electron density inversion have some data points missing. While this will not affect the macroscopic spatial and temporal analysis, the performance details need improvements. Secondly, at present, the electron density profiles of the lower ionosphere and the upper ionosphere have not been connected in series. Further, if we combine the sounding results together, the continuous electron density profiles from the D layer to the F layer is very promising. The cause of the abnormal phenomenon of the electron density at lower ionosphere is also worthy of in-depth study. Finally, the current two sounding functions are time-independent. If the time-sharing operation is modified to offer a continuous mode, the sounding results would be more scientifically meaningful.  
Data curation: T.L.; Investigation: G.Y.; Methodology: T.L.; Project administration: Z.Z.; Software: Y.L. and C.J.; Validation: C.Z., B.N. and Y.H.; Writing--original draft: T.L.; Writing--review & editing: C.Z., B.N. and P.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (42327002/41304127/41727804/41604133), Hunan Provincial Natural Science Foundation of China (2018)J3210/2018J2154), the Research Foundation of Education Bureau of Hunan Province, China (17C0716).  
The authors declare no conflict of interest.