Peer review of: Design and flight results of the VHF/UHF communication system of Longjiang lunar microsatellites (2020)

 

Reviewer #2 (Remarks to the Author): 

Paper Summary 

The author of this paper describes and elaborate about the design and implementation of the 
VHF/UHF radio and the waveforms used by China's Chang'e-4 lunar far side mission. Moreover the 
paper reports about the mission which was composed by two lunar microsatellites for low frequency 
radio astronomy, amateur radio and education, together with the Queqiao L2 relay satellite. The 
communication system operated in lunar orbit by means of a VHF/UHF SDR for small ground 
stations. The authors also provide downlink performance analysis and interferometry experiment. 

Comments 

The paper is very interesting, solid and sound and provides exhaustive results. 

Nonetheless, the reviewer deems that the SotA can be more effectively referred by adding more 
references to Small sats. To this aim the reviewer suggest to cite references such [1-2] in the 
introduction part. 

Paper Recommendation 

On the reviewer’s opinion, the paper can be accepted even if an answer to the minor point that is 
defined in the review would be beneficial. 

References 

[1] Burleigh S. C. et alii: “From Connectivity to Advanced Internet Services: A Comprehensive Review 
of Small Satellites Communications and Networks”, Wireless Communications & Mobile Computing 
(WCMC) Journal, Special Issue on Emerging Multiple Access Technologies, Vol 2019. 

[2] F. Davoli et alii: “Small satellites and CubeSats: Survey of structures, architectures, and 
protocols,” International Journal of Satellite Communications and Networking, 2018. 




Reviewer #3 (Remarks to the Author): 


Well crafted paper that described in detail the design and results of the communication system on 
the Longjiang-1 and 2 spacecraft. Some comments: 

- Line 5, item b) on page 3: Did you exercise this option to validate it could work in the near Earth 
orbital region? 

- Line 41 on page 3: Did you identify the IF in the paper? I thought I read something about 70khz, but 
I couldn't find it again. 

- Line 10, right hand paragraph, believe the word "satellite" should be capitalized. 

- Line 15 on page 4, right hand paragraph: It was not apparent why the FER results are different for 
Longjiang-2 vs 1. What was the physical difference between the two satellites? Or is this referring to 
Longjiang-2 achieving lunar orbit thus higher path loss? 

- Line 42 on page 6, the reference to anti-jamming improvement did not identify a reference point, 
from where it was improved. The assumption here is that its better, but not quantified, and the 
value of improvement supports hacking to the signals while in Lunar orbit. 

- Line 5 page 6, did you validate that the status data of the radio that was sent down made sense? 
That is, if it was temperature or power useage, you could validate the numbers were reasonable, 
and thus all working correctly? A typical use of a beacon is also to get some health and status of the 
satellite itself in the case of the larger downlink not operating. Was this not included? 

- Line22, page 6 right hand paragraph: You reference overheating of batteries, can you provide any 
other context? Was this due to the UHF system operating or to multiple other systems on board 
operating? Just curious if this points to higher than average power draw from the RF system. 

- Figure 4c is a nice output from the camera onboard looking back. Given the very low data rate of 
the UHF, can you identify how long it took to send all of this images data, or how many passes it took 
to take it out of memory and then send along? 

- Line 22 on page 8, right hand paragraph: Can you explain why the 20Hz jump occurred to the 
TCXO? Do you think this was a component issue onboard? 

- Line 39 on page 8, right hand paragraph; It is uncertain if you can claim that the receiving system 
had good performance at very low SNR; given that the signal was very low at Harbin you could not 
pull out distinguishable I/Q points. Could you quantify "very low" referring to a PSD that conforms to 
what was receieved at Shahe? That was coherent and recognizable, albiet a bit noisy. How far down 
in PSD is that from Dwingeloo that had the best? No arguement the system has good performance, 
just what is meant by "good" is relative to what is received from the best performing ground 
antenna. 

- Line 8 on page 10: Can you provide some quantitative output of the VLBI results? The oscillators 
has good signal output accuracy, thus were you able to correlate the resultant signals to a data 
parameter sent from the Moon? For future study this would be interesting to understand what is 
possible on the surface for science sensing.