the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 27 May 2024
Assessment of Operational Non-Time Critical Sentinel-6A Michael Freilich Radio Occultation Data: Insights into Tropospheric GNSS Signal Cutoff Strategies and Processor Improvements
Abstract. This study presents an exhaustive assessment of the Sentinel-6A Michael Freilich Radio Occultation (RO) data, focusing on the evaluation of bending angle products derived from the EUMETSAT-provided RO Non-Time Critical (RO-NTC) data collected between September and December 2021.
This analysis confirms the satellite's capability to exceed its target of 770 quality checked bending angle profiles per day, with an availability rate of 99.9 %, demonstrating the robustness of the mission's operational performance. A detailed examination of the Signal-To-Noise Ratio (SNR) and phase noise indicates the high-quality nature of the data. The study also analyses the benefits of employing SNR-based signal cutoff strategies and L2 signal extrapolation in the troposphere, where it is more susceptible to SNR reductions. Furthermore, the paper details some processor enhancements, which led to improved bending angle statistics, particularly below 22 km altitude. Additionally, the analysis revealed terrestrial interference signals on the L2 frequency, confirming that they do not significantly compromise the Sentinel-6A RO data quality.
The validation of the EUMETSAT processed Sentinel-6A RO-NTC data against the European Centre for Medium-Range Weather Forecasts (ECMWF) short-range forecasts and comparisons with Metop-B/C and EUMETSAT-processed SPIRE occultations, highlights the reduction in random error and modifications in the tropospheric bias structure, a result of the enhancements in data processing techniques. This comprehensive analysis confirms the high quality of the EUMETSAT Sentinel-6A bending angle products and underlines the satellite's contribution to the EUMETSAT legacy of precise and reliable radio occultation data for weather forecasting and climate research.
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Status: final response (author comments only)
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RC1: 'Comment on amt-2024-82', Anonymous Referee #1, 29 Aug 2024
The paper presents a nice overview of the radio-occultation mission aboard
the Sentinel-6A satellite and a comprehensive and detailed description of
the developments and improvements of the non-time critical processing of
bending angle data of that mission. It is well readable and can serve as
a reference to those interested in the mission data.The manuscript could nevertheless be slightly improved by addressing some
minor issues and needed clarifications as discussed below. After these
corrections the paper should be acceptable for publication in AMT.
- "Availability rate of 99.9%": clarificationPage 1, line 5 states: "This analysis confirms the satellite’s
capability to exceed its target of 770 quality checked bending angle
profiles per day, with an availability rate of 99.9%, ..."This number is repeated in line 70, and it appears that its meaning
should be clarified. Table 1 gives a list of data gaps, and in view of
figure 3 it should be clear that this cannot mean that 99.9% of the days
the mission requirement of 770 occ/day is reached. If that number
refers to conditions where the instrument is working nominally, the data
downlink works, etc., it should be stated as such.The text also mentions "autonomous or commanded instrument reboots" but
does not give a frequency estimate.In Line 220 a total data loss of around 1.5% is quoted. Assuming a
roughly constant number of measurements per time, I would like to
understand how these numbers relate.
- Line 127: formulation"The Sentinel-6A RO instrument’s performance, is required to provide ..."
I think the following reads better:
"The Sentinel-6A RO instrument requirement is to provide ..."
- Line 168: clarification"For both datasets, bending angle profiles were thinned to 247 vertical
levels"I assume that the bending angle profiles were thinned to the common set
of 247 vertical levels as for near-real time disseminated BUFR products
as used by NWP.
- Line 173: clarification"This forward-modeling process converts ECMWF data on temperature,
humidity, pressure, and geopotential, provided at 137 vertical levels at
the reference occultation position, to 247 bending angle levels."My understanding is that ECMWF provides temperature, humidity, *surface*
pressure, and *surface* geopotential (orography). The forward-modeling
process (e.g. Abel integral) is not simply a conversion; it *derives*
the model-equivalent of the bending angles at the selected 247 levels
from the input variables. Please try to find an adequated formulation.
- Line 185: "770 bending angle profile" - use plural: profiles
- Line 200: "Subsequent Sentinel-6A RO processor updates made it more
robust to these kind of issues." - either "this kind" or "these kinds"
- Line 229: "or RO profiles" - "or" should read "of"
- Line 230: clarification"It's worth nothing that lower SNR missions like SPIRE have demonstrated
the ability to systematically detect key atmospheric features."It is certainly a matter of taste what "key atmospheric features" are.
If the authors mean (lower) tropospheric/atmospheric features, they
might say so.
- Figure 6 and text after line 255: clarification neededNot being an expert, I fail to understand yaw flips decrease the SNR
both for rising and for setting occultations even for *GPS* signals.
Figure 5 top row suggests that SNR for GPS is roughly independent of
PRN, so that a naive reader might guess that exchanging the antennae
would decrease the SNR for setting but increase the SNR for rising.
So forgetting GLONASS for the argment, why do we see what figure 6 top
row shows? I cannot understand it from the text in lines 255ff.
- Figure 8: I only see "bias" in the plot, so "standard deviation" should
be removed from the x-axis label.Furthermore, in the caption: "are showed" should read "are shown".
This should also be fixed in more places (fig.9,11,13,16,17,18,19,20).
- Line 369: "the JPL provide navigation bit data file" -> *provided*
- Line 408: "where other error contribution, in particular.." ->
*contributions*.
- Figure 14, caption: "occultation recorder" -> *recorded*
- Line 460: "SRN" -> "SNR"
- Line 469: formulation"An alternative approach consisting on integrating the L2 cutoff ..."
Maybe better:
"An alternative approach of integrating the L2 cutoff ..."
- Line 477: "leading to have a tropospheric bias shift" - remove "have"?
- Line 490: "bending angles profiles" -> "bending angle profiles"
- References, line 563 (and citations in the text):Author "Ho, Shu-Peng" appears as "peng Ho". Not sure if this is a
problem with the bibliography style or the way the reference is handled.
The actually used citation looks strange to me, and a fix should be
attempted.Citation: https://doi.org/10.5194/amt-2024-82-RC1 -
AC1: 'Reply on RC1', Saverio Paolella, 22 Oct 2024
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Line 5: phrases reworked in order to state that 99.9% availability is reached during full operational periods.
Line 70: phrases reworked in order to enphatyze that the analyzed period inlcudes data in commissioning phase and early operational phase, during while several instrument activities were performed. This was done in order to achive the mission target of currently provided 770 quality-checked bending angle profiles per day with a 99.9% availability rate.
Line 157: mentioned that the period under study includes commissioning and early operational phase.
Line 187: mentioned that the reported activities are performed during the commissioning and early operational phase.
Line 215: Better described instrument or processor reboots and indicated the current reboots frequency.
Line 217 to 225: Removed some text lines because they are a leftover from previous investigations now outdated. Indeed the text also refers to June 10th 2021 data, not included in the analyzed period of this paper.Sorry for that.
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Line 127: Fixed, thank you
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Line 168: Yer correct, text updated adding the info you report. Thank you.
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Line 173: Sentences reformulated in order to make the description of what forward modeling is more adequate. Thank you.
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- Lines 185, 200, 229 : Fixed thank you.
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- Line 230: I meant actually "critical atmospheric features, particularly in the lower troposphere". Text updated and, to be more complete, also cited a paper where SPIRE occultations are assessed. Thank you.
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- Line 255: Sorry you are right, the involvement of the beam forming is not that clear. I´ve re-arranged the sentences like this: The time series in this figure illustrates the impact of yaw flips executed in early September and November, particularly when not followed by beam forming adjustments. In such cases, the beam forming that was initially optimized for the rising and setting antennas becomes misaligned once the antennas are switched, leading to a significant reduction in SNR.
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- Figures 8,9,11,13,16,17,18,19,20 all fixed. Thank you.
######################################################
- Lines 369 408, 460, 469, 477, 490: Fixed. Thank you.
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- Figure 14: Fixed. Thank you
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- References, line 563 (and citations in the text): Fixed, Thank you.
Citation: https://doi.org/10.5194/amt-2024-82-AC1
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AC1: 'Reply on RC1', Saverio Paolella, 22 Oct 2024
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RC2: 'Comment on amt-2024-82', Anonymous Referee #2, 08 Oct 2024
Review of: “Assessment of Operational Non-Time Critical Sentinel-6A Michael
Freilich Radio Occultation Data: Insights into Tropospheric GNSSSignal Cutoff Strategies and Processor Improvements” by Paolella et al 2024.
This is a nicely written manuscript about the updates done in the data processing algorithm chain for Sentinel-6A NTC product. For example, it includes a detailed analysis on daily number of occultations including quality of checked bending angles profiles of S6A. Loss of data was attributed to unstable number of GLONASS satellites, yaw flips or other instrument manoeuvres. Also, a good discussion about the importance of L2 cutoff point in this manuscript which potentially is important to other GNSS-RO data processing algorithms, too. Furthermore, the role of navigation was illustrated nicely with much detail.
I highly recommend its publication with some very minor edits listed below:
-page 5, l. 126: delete “then”
-page 5, l. 132: use “emphasize” instead of “underscores”
-page 7, In caption Table 1remove “)”
-page 8, l. 173: Which option in the forward operator is used? 1D or 2D?
-page 8, l. 177 and “which” after “phase, “
-page 8, l. 184 add “number” after “combined total”
-page 10 l.237: “PRN” Please write out acronym when used for the first time.
-page11, in caption of Figure 4: “SLTA” This is explained later on page 14 but maybe best to do it here, when used for the first time.
-page 11, l.258: “nothing” should be “noting”
-page 12, Fig.5: Add an x – axis, e.g “Various satellites”. Also, legend is difficult to read (too small).
-page 12, Fig.6: Please add more details in the caption what can be seen in the different panels. Especially for top right and bottom left it is difficult to see the difference between them (at least from the title).
-page 17, Fig.9: Define the different lines. Std dev in bold and bias are thin lines?
-page 18, l. 370: “these” should be “this”
-page 21, Fig.13. “bite” should be “bit”
Citation: https://doi.org/10.5194/amt-2024-82-RC2 -
AC2: 'Reply on RC2', Saverio Paolella, 23 Oct 2024
- page 5, l. 126: Fixed, thank you
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- page 5, l. 132: Fixed, thank you.
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- page 7, In caption Table 1: Fixed, thank you.
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- page 8, l. 173: Added 1D-Var in the text. Thank you
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- page 8, l. 177: Fixed, thank you
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- page 8, l. 184: Fixed thank you
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- page 10 l.237: Yes you are right, but I´ve decided to removed the usage of PRN because the PRN more identifies GPS satellites than GLONASS ones. So I moved to using the more general "satellite number" instead of PRN. Do you agree? Thank you.
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- page11, in caption of Figure 4: SLTA is now explained the first time it is used. Thank you.
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- page 11, l.258: Fixed, thank you.
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- page 12, Fig.5: Plot updated. Legend removed and satellites names moved directly to the X-Axis. Does it look better now? Thank you.
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- page 12, Fig.6: Do you mean Figure 6 in page 13? in case so I have updated the caption this way: "Daily average mean SNRs measured between 60 and 80 km SLTA, categorized by GNSS signal code and split between rising and setting occultations. The top left panel shows values for GPS and GLONASS L1C codes, while the top right, bottom left, and bottom right panels report SNR values for GPS L2C, GPS L2P, and GLONASS L2CA codes, respectively". Titles are removed but legends now better indicate the content of the plots. Thank you
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- page 17, Fig.9: You are right. Fixed for the plot you indicate but also for the other plots where the biases and standard deviations are shown. Thank you.
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- page 18, l. 370: Fixed, thank you.
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-page 21, Fig.13: Fixed, thank you.
Citation: https://doi.org/10.5194/amt-2024-82-AC2 -
AC3: 'Reply on RC2', Saverio Paolella, 24 Oct 2024
Sorry concerning page 8, l. 173: Added 1D and not 1D-Var in the text. Sorry for the mistake in the answer.
Citation: https://doi.org/10.5194/amt-2024-82-AC3
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AC2: 'Reply on RC2', Saverio Paolella, 23 Oct 2024
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