the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 02 May 2024
The GRAS-2 Radio Occultation Mission
Abstract. The second generation of the Global navigation satellite system Receiver for Atmospheric Sounding (GRAS-2) is a Radio Occultation (RO) instrument which is capable of providing 2000 atmospheric profiles per day. The instrument is hosted on all satellites in the Metop Second Generation (Metop-SG) series for polar orbit operation. The GRAS-2 instruments provide occultation measurements from the Galileo, GPS and BeiDou satellites at their common frequencies centred at 1575.42 MHz (L1) and 1176.45 MHz (L5). Using high-gain antennas and an ultra-stable oscillator, neutral bending angles are measured at an unprecedented accuracy of 0.3–0.4 µrad which is better than the requirement of less than 0.5 µrad. The RO signal will be measured deep into the troposphere using a novel open loop tracking scheme utilizing multiple correlator outputs for operation with a tailored ground processing algorithm optimized for extracting signals with low amplitudes approaching the noise floor limitation. Ionosphere measurements to an altitude of 600 km are also acquired.
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Status: closed
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RC1: 'Comment on amt-2024-60', Anonymous Referee #1, 04 Jun 2024
General comments
The authors describe the new GRASS-2 instrument on the Metop Second
Generation (Metop-SG) satellites. GRASS-2 will provide radio
occultation (RO) measurements based on the Galileo, GPS and BeiDou
navigation satellites. GRASS-2 is a further development of the
GRASS receiver on the current Metop satellites. The paper is well
structured and written with clear results and conclusions. My
recommendation is to accept the paper.Specific comments
From the paper ist is not clear when GRASS-2 data will be
available.Lines 57-58: NWP = Numerical Weather Prediction
Line 85, Table 2: What is the advantage of this (very) high sample
rates (250 Hz for Galileo)?Citation: https://doi.org/10.5194/amt-2024-60-RC1 -
AC1: 'Reply on RC1', Joel Rasch, 05 Jul 2024
Thank you for your comments.
1) Data should become available sometime during the first half of 2026. We will add a sentence with this information in the manuscript.
2) NWP = Numerical Weather Prediction, yes you are quite right, we will correct this.
3) Regarding the sampling rate. Judging from COSMIC and GRAS sample rates of 50 and 100 Hz are sufficient to produce high quality RO data for NWP. Higher sample rates are mainly thought to become important in the troposphere where moisture can cause the received signal frequency to change very rapidly. Higher sample rates may contribute to improve the bending angle retrieval in the troposphere. The particular values for sampling rates are chosen so that they align with the navigation data bits of the GNSS signals. The E1b signal has a symbol rate of 250 Hz. We will add some sentences in section 4 with this information.
Citation: https://doi.org/10.5194/amt-2024-60-AC1
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AC1: 'Reply on RC1', Joel Rasch, 05 Jul 2024
-
RC2: 'Comment on amt-2024-60', Anonymous Referee #2, 04 Jul 2024
General comments.
A high level presentation of the GRAS-2 instrument was provided, with some details on the instrument performance, in particular these were measured against its predecessor the GRAS instrument. The information is presented in a clear concise manner and only in a few instances there likely could be some additional qualifying information added to add the context and in some cases the motivation beyond the results presented.
Specific comments.
Around line 55 on page 2, the statement, “a vertical column is sounded over one geographical position” could be misleading to those unfamiliar with details of the technique. There is more detail later in the manuscript describing additional qualifications. It may be helpful to consider how this could be restated, “a vertical column is sounded along the path of the received signal”. Apologies for not having a concrete suggestion for rewording but it may be worth another examination.
The SpaceWire router is mentioned a few times, and its significance may be lost. It may be worth prefacing the first mention of the SpaceWire router than the spacecraft includes a router compliant with the ESA SpireWire standards … .
In the ground processing prototype section there is a bending angle product presented. Is this being requested to be included in the ground processing? Assume the data provided to EUMETSAT likely will be lower level prior to generation of bending angle? Please clearly state that the ground processing prototype also includes processing to bending angle (at the request of EUMETSAT?, and for the estimation of instrument error in bending angle?).
An obvious motivation for signal processing to bending angle, is shown in the next RO instrument performance section. Mentioning this as motivation in the previous section would make sense if you would agree, and tie these sections together more directly.
Lastly, the satellite integration and tests section was very sparse. Assuming Airbus provided the antenna test-caps and led the testing. Are any of the results from these measurements and tests included in this manuscript?
Technical Corrections
No major technical corrections were found, there are a few suggestions listed below:
Page 2 around line 39. “Occultation literally means …” is a little colloquial. Maybe: “Occultation in this case refers to the GRAS-2 receiver becoming hidden or disappearing from view of a signal transmitter.”
Page 2 around line 54. “..., whereas the GPS and Galileo satellites … “ consider being a bit more specific. “..., whereas the transmitting GNSS satellite (e.g. GPS or Galileo) complete …”
Page 2, line 55. Regarding the statement that the occulatation takes place in around 5 min. Suggest: “With the current positioning of the LEO receivers and the GNSS transmitters, an occulation takes place over around 5 min, …” .
Page 13, at the end of the conclusions would consider adding another sentence to reprise the previous section (delivery to Airbus). “An initial instrument has been delivered to Airbus in preparation for delivery of the first of the MetOp-SG spacecraft” or something similar.
Citation: https://doi.org/10.5194/amt-2024-60-RC2 -
AC2: 'Reply on RC2', Joel Rasch, 05 Jul 2024
Thank you for your comments.
1) Yes, this sentence could be misunderstood. We will rephrase it. Perhaps something like "For each point in time the signal path between the GNSS and LEO will be a slightly bent line. The majority of the bending is generated when the path makes its closest approach to the Earth and the atmospheric refraction is maximal. The information content of the RO signal for a given sample time is thus generated by a relatively small segment along the lowest part of the signal trajectory. The successive samples of the RO measurement thus forms a vertical column of such segments. During the 5 minutes of the occultation the atmospheric state inside this column will change very little."
2) Yes, perhaps we should say something more about the spacewire. We will add a sentence like you suggested.
3) Indeed, the motivation for the development of the GPP is to be able to assess the instrument error contribution to the bending angle error. We will add some text tying sections 5 and 6 together.
4) No. The test results that we present in the manuscript in section 6 are at instrument level, and as you noted there are no test results in section 7. At instrument level we use a high end GNSS signal generator that is directly fed to the LNA's in the instrument. At satellite integration level signals are generated by a playback unit and fed to the test caps and into the antennas. The playback unit and the test caps adds noise to the bending angle curve and gives a result that is not representative of the instrument performance. So the purpose of the tests at satellite level
are not to judge the performance of the instrument, but rather to ensure that the instrument is undamaged and has been installed correctly. That evaluation is done by comparing with reference measurements made by us using the test caps and the playback unit here in our lab as part of our instrument test campaign.5) We can remove the sentence "Occultation literally means ...". It is not important.
6) Yes, the text you suggest is good. We will use it.
7) Yes, we see your point. We will use a sentence similar to the one you suggest.
8) Yes, we will add a sentence reiterating the current state of the deliveries to Airbus.
Citation: https://doi.org/10.5194/amt-2024-60-AC2
-
AC2: 'Reply on RC2', Joel Rasch, 05 Jul 2024
Status: closed
-
RC1: 'Comment on amt-2024-60', Anonymous Referee #1, 04 Jun 2024
General comments
The authors describe the new GRASS-2 instrument on the Metop Second
Generation (Metop-SG) satellites. GRASS-2 will provide radio
occultation (RO) measurements based on the Galileo, GPS and BeiDou
navigation satellites. GRASS-2 is a further development of the
GRASS receiver on the current Metop satellites. The paper is well
structured and written with clear results and conclusions. My
recommendation is to accept the paper.Specific comments
From the paper ist is not clear when GRASS-2 data will be
available.Lines 57-58: NWP = Numerical Weather Prediction
Line 85, Table 2: What is the advantage of this (very) high sample
rates (250 Hz for Galileo)?Citation: https://doi.org/10.5194/amt-2024-60-RC1 -
AC1: 'Reply on RC1', Joel Rasch, 05 Jul 2024
Thank you for your comments.
1) Data should become available sometime during the first half of 2026. We will add a sentence with this information in the manuscript.
2) NWP = Numerical Weather Prediction, yes you are quite right, we will correct this.
3) Regarding the sampling rate. Judging from COSMIC and GRAS sample rates of 50 and 100 Hz are sufficient to produce high quality RO data for NWP. Higher sample rates are mainly thought to become important in the troposphere where moisture can cause the received signal frequency to change very rapidly. Higher sample rates may contribute to improve the bending angle retrieval in the troposphere. The particular values for sampling rates are chosen so that they align with the navigation data bits of the GNSS signals. The E1b signal has a symbol rate of 250 Hz. We will add some sentences in section 4 with this information.
Citation: https://doi.org/10.5194/amt-2024-60-AC1
-
AC1: 'Reply on RC1', Joel Rasch, 05 Jul 2024
-
RC2: 'Comment on amt-2024-60', Anonymous Referee #2, 04 Jul 2024
General comments.
A high level presentation of the GRAS-2 instrument was provided, with some details on the instrument performance, in particular these were measured against its predecessor the GRAS instrument. The information is presented in a clear concise manner and only in a few instances there likely could be some additional qualifying information added to add the context and in some cases the motivation beyond the results presented.
Specific comments.
Around line 55 on page 2, the statement, “a vertical column is sounded over one geographical position” could be misleading to those unfamiliar with details of the technique. There is more detail later in the manuscript describing additional qualifications. It may be helpful to consider how this could be restated, “a vertical column is sounded along the path of the received signal”. Apologies for not having a concrete suggestion for rewording but it may be worth another examination.
The SpaceWire router is mentioned a few times, and its significance may be lost. It may be worth prefacing the first mention of the SpaceWire router than the spacecraft includes a router compliant with the ESA SpireWire standards … .
In the ground processing prototype section there is a bending angle product presented. Is this being requested to be included in the ground processing? Assume the data provided to EUMETSAT likely will be lower level prior to generation of bending angle? Please clearly state that the ground processing prototype also includes processing to bending angle (at the request of EUMETSAT?, and for the estimation of instrument error in bending angle?).
An obvious motivation for signal processing to bending angle, is shown in the next RO instrument performance section. Mentioning this as motivation in the previous section would make sense if you would agree, and tie these sections together more directly.
Lastly, the satellite integration and tests section was very sparse. Assuming Airbus provided the antenna test-caps and led the testing. Are any of the results from these measurements and tests included in this manuscript?
Technical Corrections
No major technical corrections were found, there are a few suggestions listed below:
Page 2 around line 39. “Occultation literally means …” is a little colloquial. Maybe: “Occultation in this case refers to the GRAS-2 receiver becoming hidden or disappearing from view of a signal transmitter.”
Page 2 around line 54. “..., whereas the GPS and Galileo satellites … “ consider being a bit more specific. “..., whereas the transmitting GNSS satellite (e.g. GPS or Galileo) complete …”
Page 2, line 55. Regarding the statement that the occulatation takes place in around 5 min. Suggest: “With the current positioning of the LEO receivers and the GNSS transmitters, an occulation takes place over around 5 min, …” .
Page 13, at the end of the conclusions would consider adding another sentence to reprise the previous section (delivery to Airbus). “An initial instrument has been delivered to Airbus in preparation for delivery of the first of the MetOp-SG spacecraft” or something similar.
Citation: https://doi.org/10.5194/amt-2024-60-RC2 -
AC2: 'Reply on RC2', Joel Rasch, 05 Jul 2024
Thank you for your comments.
1) Yes, this sentence could be misunderstood. We will rephrase it. Perhaps something like "For each point in time the signal path between the GNSS and LEO will be a slightly bent line. The majority of the bending is generated when the path makes its closest approach to the Earth and the atmospheric refraction is maximal. The information content of the RO signal for a given sample time is thus generated by a relatively small segment along the lowest part of the signal trajectory. The successive samples of the RO measurement thus forms a vertical column of such segments. During the 5 minutes of the occultation the atmospheric state inside this column will change very little."
2) Yes, perhaps we should say something more about the spacewire. We will add a sentence like you suggested.
3) Indeed, the motivation for the development of the GPP is to be able to assess the instrument error contribution to the bending angle error. We will add some text tying sections 5 and 6 together.
4) No. The test results that we present in the manuscript in section 6 are at instrument level, and as you noted there are no test results in section 7. At instrument level we use a high end GNSS signal generator that is directly fed to the LNA's in the instrument. At satellite integration level signals are generated by a playback unit and fed to the test caps and into the antennas. The playback unit and the test caps adds noise to the bending angle curve and gives a result that is not representative of the instrument performance. So the purpose of the tests at satellite level
are not to judge the performance of the instrument, but rather to ensure that the instrument is undamaged and has been installed correctly. That evaluation is done by comparing with reference measurements made by us using the test caps and the playback unit here in our lab as part of our instrument test campaign.5) We can remove the sentence "Occultation literally means ...". It is not important.
6) Yes, the text you suggest is good. We will use it.
7) Yes, we see your point. We will use a sentence similar to the one you suggest.
8) Yes, we will add a sentence reiterating the current state of the deliveries to Airbus.
Citation: https://doi.org/10.5194/amt-2024-60-AC2
-
AC2: 'Reply on RC2', Joel Rasch, 05 Jul 2024
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