Regarding the replacement of CS21/CS22/C30/C40/C50 in the grace_input_months function？

[SingyuTang]

Hello, in the grace_input_months function, why do CS21/CS22/C30/C40/C50 set the grace_month greater than 179 (in the source code, grace_month > 176) to replace the GRACE observations with SLR observations? Where is it? It is mentioned in the literature, can you answer or provide a link to the literature? Thanks!

[tsutterley]

I typically only replace these harmonics for periods with a single (fully) working accelerometer within the satellite pair (GRACE or GRACE-FO). This is when one of the satellites either has either a degraded or non-operational accelerometer. In these cases, non-gravitational (non-grav) effects may not be fully compensated and can contaminate some degrees and orders within the spherical harmonic solutions. I have some notes written up in the project documentation, and within the peer-reviewed literature, I would refer to Bandikova et al. (2019) . Earlier dates for these harmonics could also be replaced with SLR solutions, however, it is best to keep in mind that the ranging-based solutions are improved after the launch of LARES in 2012.
@strawpants or @jgte might be able to provide some more insight.

[tsutterley]

Also, I typically only replace C_2,0 and C_3,0 with SLR solutions in standard practice. When running spherical harmonic solutions from GFZ, I additionally calculate estimates replacing C_2,1, S_2,1, C_2,2 and S_2,2. While I have these as options in the code, doing these replacements should be done with care.

[SingyuTang]

Thank you very much for your help, I think I should improve the learning of background knowledge, which can help me use this toolkit better.

[tsutterley]

Sounds good! Feel free to ask questions here or on the Discussions board.