## 4 Conclusions  
The Earth's observation from MBS has a long transmission time; its dual-way path in atmosphere and ionosphere, also appears different when viewed from a space-borne platform; its complex environment undoubtedly increases the complexity in frequency domain. In addition, the stop-and-go mode is no longer suitable for MBS; since positions change during a single pulse, the motion status of MBS and corresponding EGT cannot completely be represented by orbit mode used in satellite, but a method including ephemeris that comprehensively considers the revolution or rotation of Earth-Moon motion, has been  
Figure 6: The images generated by using the FBP algorithm when the MBS is at apogee (**a**), perigee (**b**), ascending node (**c**), and descending node (**d**), and their respective time is consistent with the time in Tables 2 and 3.  
used instead. Here, the uplink and downlink components of the pulse transmission path are separately calculated for a better match with reality at four main moments, i.e., the apogee, perigee, ascending and descending nodes, during periodic motion with JPL DE430. Ephemeris has been adopted, and the calculated path method shown in this paper fully represents a real scenario for specific MBS distributed around the near side of Moon. Although the average radius of Moon is much smaller than the average distance between Earth and Moon, this paper no longer considered a point on Moon's surface to be same as the Moon centroid, as adopted by other studies. The results indicate that the longer the synthetic aperture time is, the greater the difference between uplink and downlink will be; therefore, synthetic aperture time is not the longer better for MBS. Meanwhile, the look angle, which determine the position of EGT, also affects the maximum difference, so the dual-path transmission should be calculated separately and then transformed to a high-order polynomial series as a fitting formula. In Earth-Moon motion, the fitting velocity is not close to the real velocity calculated in a specific coordinate system, which is not identical to an air-borne or space-borne platform. For MESR, its value will increase along with the increase in synthetic aperture time and look angle; the smaller its value, the better fitting it will represent. After system parameters and MESR have been set or calculated, the division mode described in Section 2.1 can be used, and the FBP algorithm can be adopted. In contrast to the traditional BP algorithm, the renewed FBP algorithm will no longer project echo to the imaging area, but instead, a centerline will fully represent the imaging area; meanwhile, the synthetic aperture length and image area are divided into sub-parts for fast calculation. Although the FBP algorithm proposed might have a detrimental effect of reduced resolution, its meter-level resolution still meets the demand of MBS with a small width angle. A large amount of data will be generated along the long synthetic aperture time of MBS at the same sampling rate adopted in other platform; lower PRF is not suitable for the MBS to reduce the data received because of the possible azimuth ambiguity. The range interpolation before superposition indicated in this paper will further decrease the error of true position projected on the assumed sub-image area's diagonal, i.e., a segment of the centerline. The compressed results shown in Section 3.2 indicate that imaging processing is efficient in the FBP algorithm, and the results, when MBS is at apogee and perigee, are slightly better than the results at the ascending node and descending node. In summary, the paper explains the necessity of calculating a dual path for MBS's single pulse transmission in Earth-Moon motion, with real ephemeris being adopted; meanwhile, a simplified formula for the dual path of a single pulse is derived. Then, a detailed range difference, mainly at apogee, perigee, ascending, and descending nodes is analyzed, also indicating the necessity of high-order polynomial fitting after the dual path of a single pulse been calculated.