Early Radiometric Assessment of NOAA-21 Visible Infrared Imaging Radiometer Suite Reflective Solar Bands Using Vicarious Techniques

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript “Early Assessment of NOAA-21 VIIRS Reflective Solar Bands Using Vicarious Techniques” provided comprehensive comparison of VIIRS Reflective Solar Bands of NOAA-21 with NOAA-20 and SNPP, and it was concluded that the overall radiometric performance of NOAA-21 VIIRS is quantitatively comparable to NOAA-20 for the VIS and NIR bands. The manuscript is well written and interesting to read, and the topic holds significant interest within the VIIRS user community. I have only a minor comment/suggestion to further improve the manuscript.

Section 2 introduced the F-factor (eq. 1), which is very important for the sensor calibration. The results of comparison in this study all depend of the calibration of the three sensors.  However, the result of NOAA-21 sensor calibration information is not included in the manuscript. Was the NOAA-21 sensor calibration published in other paper? (please provide a citation). Otherwise, please add a paragraph or section to provide the calibration results of NOAA-21 with the solar diffuser or lunar calibration.

As mentioned in the Discussion section, disparity with the “truth” radiometric scale could exist for all three VIIRS sensors. Although the relative difference of NOAA-21, NOAA-20 and SNPP is important, as a user of VIIRS instruments, I pay more attention to the sensor degradation. Adding sensor calibration info makes the study more complete and robust.

Author Response

Comments and Suggestions for Authors

The manuscript “Early Assessment of NOAA-21 VIIRS Reflective Solar Bands Using Vicarious Techniques” provided comprehensive comparison of VIIRS Reflective Solar Bands of NOAA-21 with NOAA-20 and SNPP, and it was concluded that the overall radiometric performance of NOAA-21 VIIRS is quantitatively comparable to NOAA-20 for the VIS and NIR bands. The manuscript is well written and interesting to read, and the topic holds significant interest within the VIIRS user community. I have only a minor comment/suggestion to further improve the manuscript.

 

Section 2 introduced the F-factor (eq. 1), which is very important for the sensor calibration. The results of comparison in this study all depend of the calibration of the three sensors.  However, the result of NOAA-21 sensor calibration information is not included in the manuscript. Was the NOAA-21 sensor calibration published in other paper? (please provide a citation). Otherwise, please add a paragraph or section to provide the calibration results of NOAA-21 with the solar diffuser or lunar calibration.

Accepted.  This is a valuable suggestion. We added section 2.1 to provide more details of VIIRS RSB calibration. It also provided on-orbit calibration activities and performance for NOAA21 VIIRS.    

As mentioned in the Discussion section, disparity with the “truth” radiometric scale could exist for all three VIIRS sensors. Although the relative difference of NOAA-21, NOAA-20 and SNPP is important, as a user of VIIRS instruments, I pay more attention to the sensor degradation. Adding sensor calibration info makes the study more complete and robust.

Your comments and suggestion are highly appreciated.

Reviewer 2 Report

Comments and Suggestions for Authors

Nice work which demonstrates the need for vicarious calibration despite the efforts with the on-board systems. This is particularly valuable given the similarity of the instruments and the range of vicarious techniques used.

Some minor comments below:

Page break needed  before section "2.2. Correction for Spectral Differences"

Top of page 11: If reference(s) to the CPF team cross-calibration work with NOAA 20 are available, these should be included.  

Author Response

Comments and Suggestions for Authors

Nice work which demonstrates the need for vicarious calibration despite the efforts with the on-board systems. This is particularly valuable given the similarity of the instruments and the range of vicarious techniques used.

 

Some minor comments below:

 

Page break needed  before section "2.2. Correction for Spectral Differences"

Accepted and changed accordingly

 

Top of page 11: If reference(s) to the CPF team cross-calibration work with NOAA 20 are available, these should be included. 

Accepted. Reference [35] was added based on information provided by the CPF team members.

 

Reviewer 3 Report

Comments and Suggestions for Authors

 

In this paper, the authors have provided an early assessment of the calibration stability and radiometric consistency of the NOAA-21 VIIRS RSB, using vicarious approaches for VIIRS calibration and intercomparison assessment. Before the manuscript can be accepted, several issues need to be addressed.

1.    The Introduction lacks a description of the current research progress related to vicarious approaches, please add.

2.    The bi-directional reflectance function (BRDF) appears several times in the paper, please add a relevant introduction or explain its specific meaning.

3.    The results for the different VIIRS bands are shown in Figures 1-4 (i.e. M1, M3, M7 and M10), why were these particular bands chosen? Please provide a detailed explanation.

4.    Tables 2 and 3 show the results of four vicarious approaches in each band. I suggest adding a comparison and discussion of the results of the different vicarious approaches.

5.    It is glad to see that the results of vicarious approaches to radiometric calibration performance assessment that do not rely on sensor-based components are shown in the paper.

6.    Can you give the results of NOAA-20 and SNPP for Comparison.

Author Response

Comments and Suggestions for Authors

In this paper, the authors have provided an early assessment of the calibration stability and radiometric consistency of the NOAA-21 VIIRS RSB, using vicarious approaches for VIIRS calibration and intercomparison assessment. Before the manuscript can be accepted, several issues need to be addressed.

 

1. The Introduction lacks a description of the current research progress related to vicarious approaches, please add.

Accepted. This is a valuable suggestion and we added more detailed description about vicarious approaches in the introduction section. 

 

2. The bi-directional reflectance function (BRDF) appears several times in the paper, please add a relevant introduction or explain its specific meaning.

Accepted. One statement is added about how BRDF is defined on page 4.  

 

3. The results for the different VIIRS bands are shown in Figures 1-4 (i.e. M1, M3, M7 and M10), why were these particular bands chosen? Please provide a detailed explanation.

The purpose of select these four bands is to represent spectral wavelength ranges in VIS, NIR and SWIR. We added one statement to illustrate this. From calibration point of view, the SD has the largest on-orbit degradation in the VIS spectral range, which brings a significant challenge in maintaining the stability and accuracy of the shortest wavelength, particularly bands M1 to M3. The SWIR bands are located on a separate focal plane from the VIS and NIR bands. For NOAA-21 VIIRS SWIR bands, their gains have been adversely affected by the ice accumulated on the S/MWIR focal plane dewar walls, which caused a rapid degradation in gain and increase in F-factors.

 

4. Tables 2 and 3 show the results of four vicarious approaches in each band. I suggest adding a comparison and discussion of the results of the different vicarious approaches.

Accepted. We added one statement in our discussion of the results from the different vicarious approaches. As we put in the text, the differences among the approaches are up to 2.0%, with the DCC approach having relatively low standard errors for the VIS/NIR bands in comparison with other approaches, while the Dome C results show the largest uncertainties. Regard the existing 2% differences, it is difficult to identify the sources for the root cause and we believe it is the limitation or error from each of the approaches.

5. It is glad to see that the results of vicarious approaches to radiometric calibration performance assessment that do not rely on sensor-based components are shown in the paper.

We really appreciated your valuable comments.

 

6. Can you give the results of NOAA-20 and SNPP for Comparison.

Please refer to Table 2 that shows SNPP and NOAA20 VIIRS RSB comparison.

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

no comment