GNSS/INS-Equipped Buoys for Altimetry Validation: Lessons Learnt and New Directions from the Bass Strait Validation Facility

Round 1

Reviewer 1 Report

Dear Editor,

This study focuses on the evaluation of the uncertainty characterizing Sea Surface Height (SSH) measurements by GNSS equipped buoys. The survey takes place over the Bass Strait validation facility, where stable sea surface conditions favor this kind of evaluation. The authors provide a rigorous description and quantification of the primary sources of uncertainty (both random and systematic) that affect elevation data from these sensors.

The authors analyze two key systematic error sources that have not been considered in previous studies: the variable buoyancy position of the buoy as a function of external forcing (current and waves), and the effect of dynamic changes in antenna position/orientation as a function of the sea state. The authors analyze the uncertainties associated with the two error sources and propose two approaches aimed at modeling their amplitude and at reducing their effect on the final SSH measurements.

The results presented in the study provide critical insight on how to improving the precision of measurements from GNSS equipped buoys, which data are essential for the validation of current and future altimetry missions.

In my opinion, the topic considered here is of scientific interest and fits the journal scope. I recommend the manuscript publication on Remote Sensing after some minor revisions.

 

Other Comments:

Lines 141-142: it is probably not necessary to cite reference 15 two times.

Lines 153 -163: I think that the manuscript would benefit from a more detailed description of the "Existing GNSS Processing". Several more details could be added either in this section or reported in the supplementary material. 

Line 329: Please, report the value of the "reasonable correlation" between model output and residual in amplitude and phase for each buoy deployed.

Figure 6: Top section - The black and the red line are not really distinguishable. It would be probably better to represent the top one of the two curves with a dashed line.

Figure 8: the definition of Dark- and Light-colored lines is kind of vague. Please consider revising this figure.

Figure S1: The figure seems to be upside down. Please correct either the orientation of the figure or the direction of the labels.

Author Response

We thank the reviewer for their constructive comments.

Please see the attachment for the detailed response.

Author Response File: Author Response.docx

Reviewer 2 Report

In this paper, the authors focused on two previously ignored problems: changes to buoyancy as a function of external forcing, and biases induced by platform dynamics. Inertial navigation system was deployed in GNSS buoy in order to improve the accuracy of the results. In my option, this work is interesting and meaningful for exploring the validation methods of future wide-swath altimeter such as SWOT.However, there are some questions and comments. Thus, my recommendation is minor revision.

Overall issue:

1. Comparing SSH from GNSS buoy with the mooring data is an effect way to evaluate the result. However, it could be better to describe in detail how the  SSH is derived from mooring.
2. The introduction section and background section could be combined.

Minor issue:

3. Line 81：full name of  ‘CNES’ should be added.

4. Line 124: the location of the buoy 3, buoy 4 and the mooring could be shown in Figure 1.

5. Line 173: The introduction of the European Centre for Medium-Range Weather Forecasts (ECMWF) model should be added.

6. Figure 7-8: The color bar should be added.

 

Author Response

We thank reviewer 2 for their constructive comments.

Please see the attachment for detailed response.

Author Response File: Author Response.docx

Reviewer 3 Report

RemoteSensing-901553, Review

GNSS/INS equipped buoys for altimetry validation – lessons learnt and new directions from the Bass Strait validation facility

Boye Zhou, Chris. Watson et al.

Context & General comments
Satellite altimetry, by its principle, needs equipped ground sites at least to calibrate new instrument-missions (radar, etc. it’s mandatory)
and if possible to monitor their behavior along time (several years).
In addition to maintaining a couple of instruments with high geodetic requirements on a given site, satellite altimetry needs, in fact,
a network of cal/val sites (e.g., Harvest, Corsica, Kavaratti, Bass Strait, etc.). The main efforts conducted on each site are relevant in
terms of accuracy, long-term stability and durability (decades).

Buoys equipped with a GPS receiver have been developed and used since the ENvisat mission roughly (in the 90’s); most of the tide gauges used during the mission calibration activities were located very near to the coast where backwater effects occur. In contrast, high-sea buoys can be placed at dedicated locations (crossover point, etc.).

However, dedicated sites have many advantages on the long term including the ability to treat new altimeter missions and eventually several missions together.
The improvement of all instruments needed to ensure an accurate calibration (sea state bias) at a level < 1 cm is thus an important goal especially on the long term (decades); because it is necessary to ensure enough stability (infrastructures, oceanographic & geophysics effects, etc.) between successive missions.

It is the case of the Bass Strait altimeter validation facility which has contributed to multiple altimeter missions.

The comparison of SSH measured both from a floating platform equipped with GNSS and a tide gauge ‘’obviously’’ depends on the baseline (but not only) which ranges from 100 m, 1 km to tens of km. It depends also on the sea state; thus, the addition of an autonomous inertial measurement system (of n degree-of-freedom) to the floating platform can thus be applied to enhance the
accuracy of positioning during the tilt of the platform.

Abstract
Nothing to say.

Introduction
The Introduction followed by the Background (Sect. 2) is clear and concise. The authors make large references to related works.
However, it should be added some comments (and may be references) about the studies which were using an inertial measurement system, even for
monitoring high-frequency ocean signals (e.g. Remote Sensing and Spatial Information Sciences, 2016).

3. Data
3.1 & 3.2: Nothing to say.

4. Method
4.1: OK

4.2:
-> 4.2.2: I would suggest authors to add a figure with a scheme to better explain (in its principle) how the SSH measurement is compensated by the INS measurement and processing.

4.3
I should say that the inclusion of 4.3.3 (Tether) into that Section which seems to be dedicated to the precision assessment (buoy against mooring and between buoys) is not ‘’natural’’.
->
Because authors wrote Section 5 with 4 sub-sections, I would suggest to follow the same order for Section 4.
As an example, I would suggest to move 4.3.3 to 4.3 (Tether Tension, as it is the case in 5.3) and then moving 4.3 to 4.4
(Buoy Precision) that would conserve both sub-sections ‘’Overarching’’ and ‘’Relative’’ precision.

5. Results

It is very difficult to treat data acquired from roughly 2012 to 2018, with no possibility of new/recent measurements; for example,
the role of the external forcing for deployments having stdDev > 20 mm is not clear. In addition, the contribution of the in situ ‘’ground truth’’ (potential errors and/or remaining biases) is also not very clear, even if authors are very careful about that.

Case Study: Tether and Orientation
The reader is a bit disappointed due to the difficulty to clearly show the improvement(s) due to both systems (not applicable for all deployments). However, authors are always very clear about that, notably with figures 6. and 7. (for this last, caption is too long: -> please make it shorter).

6. Discussion
I should say that the Discussion is a bit too long for a regular reader. I think that even if there is a real difficulty to enhance performances thank to Tether and INS/Orientation measurements, it is not necessary to develop a long discussion.

-> The only thing which should be saved, I think, is the part 6.4 dedicated to SWOT. The rest (6.1 to 6.3) should be strongly shortened and then included in the previous section 5. (Results).

I think that it will help the reading in term of clarity: arguments,
advantages and so on.

Author Response

We thank reviewer 3 for their constructive comments.

Please see the attachment for detailed response.

Author Response File: Author Response.docx

Round 2

Reviewer 3 Report

dear authors,

thank you for considering remarks and suggestions.

I do reget the so long Section 6 (Discussion) which for me do not help the reader to have a concise and precise ''feeling'' about development & test of equipped floating platform.

Regards.