Self- and Inter-Crossover Points of Jasons’ Missions as New Essential Add-on of Satellite Altimetry in the Sub-Arctic Seas and the Southern Ocean

Round 1

Reviewer 1 Report

Review of the paper „Self- and Inter-Crossover Points of Jasons’ Missions as New Essential Add-on of Satellite Altimetry in the Sub-Arctic Seas and the Southern Ocean” submitted to Remote Sensing.

The paper demonstrates a method to reduce the differences observed in altimeter records for different satellite crossovers by adjusting the values from consecutive passes in the polar regions and using inter-satellite altimetry data. The paper is well written and interesting. However, some issues must be addressed before the publication.

1. The paper addresses the orbit modeling issues of Jason satellites, however, the topic of orbit modeling issues and the progress of LEO orbit modeling is not sufficiently addressed.
2. Jason satellites (and most altimetry satellites) are equipped with three techniques for precise orbit determination: (1) a GNSS receiver (in the past only GPS was used), (2) a DORIS receiver, and a Satellite Laser Ranging (SLR) retroreflector.
3. The main progress in LEO orbit modeling in terms of GNSS orbits is related to employing proper GNSS antenna calibrations and fixed phase GNSS ambiguities (see https://doi.org/10.1007/s00190-017-1090-2 )
4. The main progress in the SLR data processing, in terms of Jason orbit validation and the calibration of the altimeter instruments is related to proper handling of SLR range biases and timing biases. Large progress in this context was described in https://doi.org/10.1007/s00190-018-1140-4 Moreover, SLR allows for deriving geocenter corrections which are needed to properly address the sea-level rise derived from altimetry
5. The DORIS receivers have now more channels which allow for the improvement of the ground network and many other improvements in DORIS-based Jason orbits (see https://doi.org/10.1016/j.asr.2017.07.044 )

All the technique-specific issues and the progress in GNSS, SLR, and DORIS data processing to improve Jason's orbits should be addressed in the paper, because the precise orbits of Jason are fundamental for altimetry records.  

2. The recent improvement in the precise orbit determination of Jason orbits includes the improvement of the orbit models. Before, the simple cannonball model was employed with the estimation of the constant accelerations in the along-track direction, as well as once-per-revolution accelerations in the along-track and cross-track. Now, the improved macromodels, such as box—wing models are employed to properly handle the non-gravitational forces emerging from the solar radiation pressure, albedo, and atmospheric drag. It seems that the once-per-revolution and twice-per-revolution orbit errors were also found in this study. They are directly related to orbit modeling issues and can be addressed by employing satellite macromodels.
3. The recent progress in orbit modeling allowed to substantially reduce the 118 day issue in the precise orbits, which is directly related to the draconitic year of the satellites (repeatability of the elevation angle of the Sun above the orbital plane). Furthermore, the modeling of the geocenter motion in LEO orbits allowed to improve the altimetry results (see https://doi.org/10.1016/j.asr.2012.06.004 )
4. Other altimetry missions can also be used for the calibration of Jason altimeters, such as Sentinel-3A, Sentinel-3B, Sentinel-6/Jason-CS. Sentinels 3A/B have much larger inclination angles when compared to Jason-1/2/3/CS, therefore they have very dense overlaps around the poles. Sentinel-6/Jason-CS has the same inclination angle as the previous Jason missions. The new altimetry missions and future perspectives should be added to the discussion of the paper.

Author Response

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Author Response File: Author Response.doc

Reviewer 2 Report

The author present a work to explain how the use of self- and inter-crossover can further improve the use of data from satellite altimeter to measure sea conditions, especially at high latitude. After describing several cases where the methodology could be applied the author focused their analysis on the Norwegian Sea area, exploring the characterization of geostrophic wind and wind waves.

This work presents an interesting method to exploit the already known potential of satellite altimeters to quantify sea conditions and therefore deserve to be published.

However, the present manuscript it is difficult to read and require some refinement in the English used. Most of the difficulties in following the analysis is associated to the introduction, which is currently not well structure and does not introduce the objective of the work clearly. I have tried to provide some suggestion for the author that I hope can improve the readability of their text.

 

General comments

Abstract

Perhaps ‘Satellite altimetry has been successfully used during the past three decades to measure sea level, …’

 

1. Introduction

The introductory section appears fragmentary and topics are discussed before having been introduced. There should be here a clear tread that guide the reader trough the literature, highlights existing gaps such that to introduce the purpose of the work. I therefore suggest to restructure this section as follows.

-1 General description of crossover and definition of SC and IC (from lines 40-43) and requirements to obtain quality measurements, i.e. lines 44 -60.

e.g. Satellite altimetry has been successfully used during the past three decades for studies within the ocean environment. Applications include sea level, ocean dynamics, coastal oceanography, planetary  waves, ocean tides, wind and wave, ice cover, Earth’s gravity field, and climatology research [1-9].

The crossovers, that is, points of intersection of ground tracks of altimeters play an exclusive role in maintenance of functionality and performance of satellite altimetry. …. Unitl line 60

Topex/Poseidon–Jasons’ missions provide special crossover datasets that are free of the above mentioned difficulties.

-2 Introduce and describe Topex/Poseidon-Jason’s missions and their characteristics., lines 27-34 There is no need to use bullet point here. 

Line 62 Substitute ‘For example’ with ‘ Because of these’ , individual (for SC) and synchronized (for IC) orbits of Jason’s altimeters generate regular in space and periodic in time measurements in a number of special fixed  … until line 68

-3 Introduce paper objectives. I think this sentence somehow contains part of the work objectives. Special subsets of altimetry data with improved sampling can provide additional information on ocean dynamics and new opportunities for sea monitoring.

-4 To achieve these aims we collected data of High Latitude Crossovers (HLC) with minimal time intervals between consecutive measurements and explore both self- and inter-crossover types for theTopex/Poseidon–Jason’s missions (this will introduce the reader to section 2).

-5 Present the paper structure lines 69-73

 

2. The self- and inter-crossovers

Rewrite as. 2.1 Self-crossovers.Standard jason’s orbit. Since self-crossover is the key word here

Same for 2.2 with Inter-crossovers.

The following sentences (lines 93-95) would probably fit better in the introduction and possibly help to introduce the aim of the work.

Here one should emphasize that SCs occur regularly at the same points and time of the satellite cycle (i.e. for fixed pairs of track numbers), thus, comprising an abundant data subset. Surprisingly, this specific data set is not widely discussed and used to our knowledge.

 

4. High latitude crossovers in the Norwegian sea

Line 388 Figure 5c,d are missing, although I think what it is here described is already the background of Figure 5a,b. Please check and correct accordingly.  

4.1 better writing wind waves rather then wind sea state since later in 4.3 the term wind wave is used.

Line 426 Present this result first ‘result is good agreement of two consecutive measurements in the crossovers as illustrated by Figure 9a.’ and then the one not shown.

 

5. Discussion and Conclusions

At the end of this section (after line 493) it would be nice to have some comments from the authors on the results that can be expected if the methodology would be apply to the other cases described in section 2. This will give more sense to the description of different cases that are currently described but not mentioned through the manuscript. At the present the paper will not change if only the Norwegian sea case is presented as case study. However I retain nice to have several cases where the method could improve measurements. Because of this I think it would be good to mention the other cases in the discussion.

 

Figures

Figure 2 In the legend add IC and SC tot the number for consistency with table and text. Also use different colours for clusters and crossovers. Now this is generating confusion in interpreting the values in panel b. I suggest to use black symbols for crossovers since are different.

 

Figure 3 Please, slightly increase the legend font, particularly in panel g).

 

Figure 5 Substitute Current velocity modul with Absolute value of current velocity. Also indicate in the caption the symbols associated to the clusters.

Figure 9 Indicate in the caption that B and G stand, respectively, for Baludin and Gommenginger model. C) wave period from single-track vs…, d) wave periods as predicted by the two models.

 

Specific comments

Line 5 Substitute publications with literature.

Line 5-6 Remove “” from self- and inter- crossover.

Line 47 rephrase as. For measurements of the ocean surface, ….. For wind-wave studies

Line 148 Don’t use “number” just the number, and add SC or ICS to numbers to increase readability.

Line 150 substitute an with the.

Line 153 nautical miles(nmi) because then nmi is used.

Line 170: delete for 

Line 348 add the after gradient due to

Line 349 substitute and with ,

Line 350 Write orthogonal instead of orth and rephrase the second part as  ..’vector, n is the vector normal to s, and U is the velocity vector which is orthogonal to the pressure gradient.’

Line 353 allow the components of the geostrophic current, which are perpendicular to track directions (dashed lines in Figure 4), to be estimated.

Line 354 add equation before (1) and also eases the estimate of both wind values and associated errors.

Line 356 Please rephrase this sentence.

Line 357 Perhaps better Local differences in current velocity can be used to estimate changes in water transport, …

Line 360 Analogously to the above scheme, the physical model of wave periods from altimetry data is also based on the estimation of wave height spatial gradient. Model result can be expressed …

where \alpha_ss = 0.67 is the dimensionless parameter of the theory for wind-driven waves under the hypothesis of week turbulence. Remove the equation (2) is based on.

Line 363 Substitute It allows to … with This allow the variations of wave eight to be related with spectral flux of energy ….

Line 365 Substitute Secondly, with Second,

Line 377 In the following section we will discuss…

Line 380. Here, HLCs form  ….clusters, providing at least ….

Line 387 Remove , between 17 and October

Line 400 Rephrased as Fourier decomposition of a current velocity component G as function of longitudine \lambda

Line 402 Add description of the terms within equation 4, i.e. In equation (4) n is the mode of the Fourier expansion, A its amplitude, i the imaginary unit, k represents the boundary direction (I guess)

Also, shouldn’t in the following lines the spatial frequency keeps k subscript for consistency?

Lines 409-410 Please maintain the use of zonal and meridional to avoid confusion

Lines 444 results from the comparison between the physical model of Badulin and its empirical counterpart of Gommenginger et al. are reported in Figure (d and show a remarkable similarity …

Line 446 rephrase as The excess of the period between these two models is essentially …  

Line 449 rephrase as Such difference can be related to altimeters characteristics [ ] or, as an alternative explanation, to the effect of high latitudes …

Line 464 rephrase as Features of self- and inter-crossovers relevant for improving the measurements of sea conditions are listed below.

Line 468 Substitute altimeters’ tracks with altimeter tracks

 

 

 

Author Response

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Author Response File: Author Response.doc

Reviewer 3 Report

The manuscript presents an add-on of altimetry for oceanography. It is interesting and helpful for the community.

My concerns:

(1) A section of DISCUSSION should be split from “Discussion and Conclusions” in the present form. And the results from analysis current, wind/wave using the proposed “self-crossover” and “inter-crossover” should be compared with those using traditional altimetry and discuss a little bit here. The “discussion” stated in the last section is too concise.

(2) I recommend a section named dataset (or others), where Jason-2/3 satellites could be introduced. The paper mainly focused on altimeters of Jason, so somewhere should be apparent to specify this (not other altimeters).

(3) Table I is not completed.

(4) It is not very clear for me to understand how to determine the wave group direction from the “self-crossover” illustrated in Fig 4 b and equations (2),(3). Even the parameter of wave group direction  did not appear in equations (2)&(3).

Author Response

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Author Response File: Author Response.doc

Round 2

Reviewer 1 Report

Second review of the manuscript „Self- and Inter-Crossover Points of Jasons’ Missions as New Essential Add-on of Satellite Altimetry in the Sub-Arctic Seas and the Southern Ocean”.

 

Unfortunately, the Authors did not address the issues raised in the first review report. Therefore, I cannot recommend this paper for publication. The paper discusses the issues of the crossovers for Jason satellites. However, the reason for systematic effects observed in the altimetry data emerges from the systematic effects in satellite orbits. Therefore, a detailed discussion of all limitations and systematic orbital errors, as well as techniques for precise orbit determination of Jason satellites, is needed. Otherwise, the paper will be incomplete and will lack a thorough and exhausting analysis. I strongly encourage the Authors to address all the issues from the original review report and address them in a proper way.

Author Response

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Author Response File: Author Response.pdf

Reviewer 3 Report

The authors respond all my concerns. No further questions.

Author Response

Dear Referee,

Thank you a lot for your efforts and fruitful suggestions

On behalf of the authors

Sergei Badulin