CGMS-49 IROWG-WP-01
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Prepared by IROWG
Agenda Item: WGII/3

REPORT OF IROWG ACTIVITIES: OUTCOME AND RECOMMENDATIONSFROM THE IROWG-8 WORKSHOP
Prepared by IROWG1 (www.irowg.org)
Executive Summary
This report summarizes the IROWG-8 meeting held on April 7-13, 2021 as a virtual
conference, hosted by NOAA and UCAR. It provides the main recommendations
from the four IROWG sub-groups: Numerical Weather Prediction; Climate; ReceiverTechnology and Innovative Occultation Techniques; and Space Weather. The key
recommendations for CGMS – endorsed by the IROWG community at the plenary
session – are:•

IROWG reaffirms that all providers of RO observations should classify theseas essential in the sense of WMO Res 40. IROWG stresses the importanceof free, timely and unrestricted access in real time to essential RO data, andfree and unrestricted access to archived raw data (including auxiliary data).

•

IROWG continues to recommend that WMO and CGMS should coordinate
any GNSS-RO data purchases. Specifically, we suggest convening ameeting of all agencies considering procuring these data, in order to discussif, how and when the current 20,000 daily target will be met with global andfull local time coverage.

•

IROWG recommends that CGMS encourages technology and retrieval
developments for improving planetary boundary layer profiling from GNSSRO and their utilization in NWP data assimilation – and the further
exploration of RO-derived water vapor as a climate variable.

•

Per CGMS priority HLPP 1.1.4 (optimized system for atmospheric andionospheric RO observations), IROWG recommends that CGMS
encourages on-going and future GNSS RO and non-RO missions, including
potential commercial providers of RO observations, to incorporate acomplete set of ionospheric measurements.

Full workshop minutes and this CGMS working paper from IROWG-8 will be madeavailable at http://irowg.org/workshops/irowg-8/. All given workshop presentations
can be found at https://cpaess.ucar.edu/meetings/2021/irowg-8.

Action/Recommendation proposed: CGMS is invited to take note and comment.
1

International Radio Occultation Working Group, represented by the rapporteur Anthony Mannucci and theco-chairs Ulrich Foelsche and Sean Healy. Affiliations are listed at the end of this document.

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1

INTRODUCTION

This IROWG working paper reports on the 8th workshop of the International Radio
Occultation Working Group (IROWG-8). The meeting was organized by NOAA andUCAR and was originally planned to take place in the Washington area in the USA. Dueto the COVID situation it had to be re-scheduled as a virtual conference, taking placefrom April 7-13, 2021. IROWG wants to express its gratitude for the excellent organisationof this meeting, which turned out to be the best virtual meeting and one of the mostsuccessful in recent years.
The workshop minutes and full recommendations will be available at:
http://irowg.org/workshops/irowg-8/. All the workshop presentations are at:https://cpaess.ucar.edu/meetings/2021/irowg-8
One and a half years had passed since IROWG-7 (September 2019) and important
progress has been achieved in this period.
372 scientists from 39 countries registered for the virtual conference, and up to about 200
participated on the individual days, including representatives from all the major RO
processing centers, space agencies, the weather prediction centers assimilating RO
data, the research community, and representatives of commercial data providers. 69
talks and 35 posters were presented. Recommendations were developed in dedicatedsub-working groups, and then presented and agreed upon in a plenary discussion on thelast day.
The structure of this report is as follows: Section 2 gives a brief overview of theorganization of the workshop and the sub-groups, Section 3 summarizes sciencehighlights and subgroup-discussions (in order of the presentations on the last day of
IROWG-8), Section 4 summarizes the plenary discussion, Section 5 lists the main
recommendations which were agreed upon by IROWG, and Section 6 concludes themain section of the report. The status of actions and recommendations is covered in the
Appendix.
2

IROWG-8 SETUPIROWG-8 was a full workshop, including presentations, posters and sub-group
discussions. The presentations/posters and the sub-groups were organized according to
the following specific topics, namely:••••

Numerical Weather Prediction (NWP);
Climate;Receiver Technology and Innovative Occultation Techniques;Space Weather.

IROWG-8 participants were asked to summarize relevant activities within the scope ofthe sub-group in dedicated sub-group meetings and express recommendations which
could either be relevant to CGMS, to the GNSS (Global Navigation Satellite System, e.g.
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GPS) RO community, to providers of RO data, or within the IROWG. These werediscussed in the open plenary. Furthermore, the subgroups assessed the status of therelevant CGMS actions.
Sean Healy will be stepping back as IROWG co-chair; an election process for a new cochair was therefore initiated.
The next IROWG-workshop (IROWG-9) will be held in September 2022 in Seggauberg,
Austria, in conjunction with the OPAC-7 meeting (Occultations for Probing Atmosphere
and Climate).
3

SCIENCE HIGHLIGHTS AND WORKING-GROUP-DISCUSSIONS

Note that this is a summary; the entire list of recommendations will be made available in
the full meeting minutes. Here we focus on the main recommendations per subgroup,
which we regard as highest priority, even though not all of them can be promoted as
IROWG key recommendations.
Receiver Technology and Innovative RO Techniques
Chair: Rob Kursinski (PlanetiQ), Rapporteur: John Braun (UCAR)
GNSS-RO can provide very high vertical resolution, all-weather, thermodynamic profilingof the planetary boundary layer (PBL) that is difficult to achieve from any other remotesensing technique. The unique contribution from GNSS-RO for spaceborne PBL profilingof temperature and water vapor as well as PBL height has been recognized in the U.S.
2017–2027 Decadal Survey for Earth Science and Applications from Space.
Nonetheless, important issues do exist with RO profiling of the PBL which include:• a persistent negative refractivity bias within the moist PBL• increased noise in the lowermost troposphere data from COSMIC-2 despite its
unprecedented high signal to noise ratios (SNR),• limited utilization of lower troposphere RO data by NWP systems, as evidenced
by high data rejection rates at low altitudes.
• depth of penetration
Ducting or super-refraction can cause RO-derived refractivity to be negatively biased in
the PBL. Unbiased refractivity retrievals can be achieved when ducting is present, but
doing so requires knowing whether or not ducting is present on a profile-by-profile basis–
which is supported by observing deep occultation events, which require very high SNR
observations.
Innovative techniques like polarimetric RO, LEO-LEO, grazing angle GNSS-R, andairborne observations deliver very promising results, which could be operationallyexploited.
Radio Frequency Interference (RFI), e.g. through GPS jammers, has been identified asa major and increasing problem.

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High-priority subgroup recommendations:1) Encourage technology and retrieval developments for improving planetary boundary
layer profiling from GNSS-RO and their utilization for NWP data assimilation - including
formation of a working group focused on PBL related issues.
2) Advance LEO-LEO occultation development towards a demonstration mission. The
subgroup recommends that CGMS adopt an action asking international space agencies(in particular CAS (Chinese Academy of Sciences) and ESA, where development work
based on proposals towards initial LEO-LEO demonstration missions is ongoing) to
support mission preparation projects that implement the next steps towards a LEO-LEO
microwave and infrared-laser occultation (LMIO) demonstration mission.
3) Develop NWP forward operators for polarimetric RO, LEO-LEO, grazing angle
GNSS-R, and airborne observations and promote the use of these new data for NWP
and science applications
Numerical Weather Prediction
Co-Chairs: Hui Shao (JCSDA), Neill Bowler (Met Office), Ben Ruston (NRL)
Recent results confirm that the 20,000 target is well founded, and very likely to further
increase the impact of GNSS-RO in NWP systems.
A number of operational NWP centers have now demonstrated the positive impact ofincreasing the number of observations, with the assimilation of FORMOSAT-7/COSMIC2 and, in some cases, commercial data in 2020-21. For example, NASA GMAO haveshown that the assimilation of both COSMIC-2 and Spire data leads to GNSS-RO being
the third most important observing system, based on the Forecast Sensitivity to
Observation Impact (FSOI) metric. Some impact on tropospheric humidity is also being
reported – thanks to the high quality of COSMIC-2 data. In addition, new OSSEs by NASA
GMAO suggest that there are still benefits moving from assimilating 50,000 to 100,000
profiles per day. Overall, the results presented at IROWG-8 reinforce the case for thecurrent 20,000 daily target, which we note is achievable if the agency-led missions areaugmented with commercial data.
We encourage CGMS members to consider any forthcoming missions of opportunity to
fly additional GNSS-RO receivers, noting the recent progress with the Sentinel-6 Michael
Freilich mission.
The NWP group notes that measurements made with GPS, GLONASS, Galileo and
QZSS signals have now been successfully used in operational NWP (via commercial
data provision); and that rapid operational implementation of new data is more likely if
results and experience are shared between NWP centres.
High-priority subgroup recommendations:1) The NWP group reaffirms providers of RO observations should classify these as
essential in the sense of WMO Res 40. IROWG stresses the importance of free, timely
and unrestricted access to essential RO data including archived raw and auxiliary data.
2) The NWP group continues to recommend that WMO and CGMS should coordinate
any GNSS-RO data purchases, by convening a meeting of agencies actively considering
procuring data. Sufficient commercial data should be purchased, with a world license fornear-real time data, to meet the target of 20,000 globally available occultations per day.
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3) There is a need to include additional information in a BUFR template. The NWP
subgroup emphasizes the need for receiver IDs and a measure of SNR. This new
template should accommodate new observation types, e.g., airborne GNSS-RO, andpolarized RO (GNSS-PRO). Definition of a new GNSS-RO template should proceed evenif specifics on how to calculate elements is ongoing.
Space Weather
Chair: Bill Schreiner (UCAR), Riccardo Notarpietro (EUMETSAT)The high spatial and temporal resolution of COSMIC-2/FORMOSAT-7 data allows forstudying the ionosphere (at low to mid latitudes) in much more detail than ever before,
thereby breaking new ground for ionospheric research.
The space weather sub-group (SWSG) increased the priority of the recommendation toincorporate ionospheric measurements into satellites carrying RO instruments. In
particular, the sub-group specifically endorses the merit of ionospheric upgrades to EPS
being implemented for ionospheric data, as done in the Metop-A end-of-life test. In
addition, the sub-group acknowledges that the Sentinel-6 hardware is capable ofacquiring ionospheric observations in addition to the atmospheric RO, and endorses
plans for implementing ionospheric capabilities via software upgrade to the Sentinel-6
receiver.
High-priority subgroup recommendations:
1) Per CGMS priority HLPP 1.1.4 (optimized system for atmospheric and ionospheric RO
observations), the Space Weather sub-group of IROWG recommends CGMS encourage
on-going and future GNSS RO and non-RO missions, including potential commercial
providers of RO observations, to incorporate a complete set of ionosphericmeasurements (including negative and positive elevation angles – i.e. also data fromzenith viewing precise orbit determination (POD) antennas …).
2) The SWSG recommends that CGMS expend all reasonable effort to acquire highquality near real-time GNSS RO space weather measurements, including all raw data,
meta data, and documentation needed for future reprocessing, at all latitudes with full
local time coverage.
3) The SWSG acknowledges the value of direct broadcast for low-latency ionosphericdata, and endorses agency efforts to make use of direct broadcast channels forionospheric data, wherever the missions are designed to provide such capability (e.g.:
EPS-SG), to further decrease the latency of the ionospheric observations.
ClimateChair: Andrea Steiner (WEGC), Rapporteur: Panagiotis Vergados (JPL)The value of GNSS-RO data for climate monitoring is increasingly acknowledged by thebroader climate science community. IROWG has been invited by the IPCC to contribute,and this chapter will appear in the upcoming assessment report.
The high spatio-temporal resolution of COSMIC-2/FORMOSAT-7 data allows forstudying the tropical atmosphere in unprecedented detail, thereby breaking new groundfor atmospheric research, e.g. related to different types of atmospheric waves and thediurnal cycle in the stratosphere.
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High-priority subgroup recommendations:1) Ensure continuity and long-term availability of climate quality RO measurements with
global coverage over all local times. Operational GNSS RO missions for continuous
global climate observations need to be established and maintained as backbone toensure continuity with at least 20,000 occultations per day.
2) Acknowledging CGMS recommendation R46.01 on long-term data storage, we
recommend that data providers ensure that all information necessary for independent
processing towards climate data products is freely available (following WMO resolution40), including long-term archiving and public data access, thus ensuring full climate
traceability.
3) We recommend assessment of the uncertainty in the refractivity coefficients that
impacts the accuracy and traceability of RO climate time series and trends. Significant
progress was made at JPL in implementing an experiment to measure the refractivity of
air, but currently is lacking the accuracy needed by the climate group. Required steps toimprove accuracy have been identified by NASA/JPL, however further financial support
is needed to implement these steps. IROWG is pleased to see these initial refractivity
experiments and encourages CGMS agencies to support this activity.
4

SUMMARY OF PLENARY-DISUCSSIONSAll of the sub-groups recognized the importance of Level 0 (raw) data. It was stressedthat raw data should be included in data purchase plans from commercial providers.
Action within IROWG: A working group will be formed to develop an exchange format
for raw data that is common to the different receiver types and that can be used for dataexchange among users.
Radio Frequency Interference (RFI), e.g. through GPS jammers, has been identified asa major problem – receiver developers need to address this issue.
Technology and retrievals for the PBL, and their utilization for NWP have been discussed:
Action within IROWG: Form a working group on PBL – how best to extract profile
information.
Significant discussion occurred on the provision of commercial data. Dallas Masters ofSpire recommended that NOAA purchase Level-2 data to increase access to data frommultiple constellations. Several members of the community, however, reiterated that, forclimate monitoring purposes, Level-2 data are not sufficient and raw data must be
available.
The recommendations of the IROWG regarding coordination of data purchases acquired
more urgency due to a large increase in the production of commercial RO data,
particularly from Spire. GeoOptics is the sole provider of operational radio occultation
data products under NOAA’s Delivery Order-2 contract (1300 profiles/day). The near realtime data are only available to US Government users.
The Numerical Weather Prediction (NWP) sub-group recommended that CGMS
agencies convene a meeting to discuss coordinated data purchases. This suggestion
was well received.
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The IROWG is addressing action item A47.31 (WGII) by developing material that shows
gaps in RO data coverage from current and planned agency-led missions. These gaps
occur particularly in local time above 40° latitude. Ben Ho of NOAA agreed to provide
maps showing geographic and local time distributions for agency-led radio occultation
profiles.
Regarding action A48.02 (WGII), the NWP sub-group suggested that agencies send
representatives to an IROWG meeting so that detailed information on quality control
methods and statistics used by the different agencies can be discussed, leading to theirimproved documentation.
IROWG should verify that the WMO OSCAR database properly documents the abilitiesof current and future missions to obtain ionospheric data.
NWP and climate requirements are not exactly the same – care is needed whenpurchasing commercial data and climate requirements need to be considered.
New RO data probe the lower troposphere better than before.
Action within IROWG: Establish a task force for the lower troposphere. Explore ROderived water vapor further as a climate variable.
A dedicated launch of satellites carrying RO receivers was suggested by the climate subgroup to address the gap in local time coverage that has emerged since the demise of
COSMIC-1 and the limited latitudinal coverage of COSMIC-2. The existing RO
constellation does not serve the needs of climate monitoring. Dedicated launches, either
by agency-led missions or by commercial data providers, are an effective approach to fill
the gap.
5

MAIN RECOMMENDATIONS FROM IROWG-8

5.1 IROWG reaffirms that all providers of RO observations should classify
these as essential in the sense of WMO Res 40. IROWG stresses the importanceof free, timely and unrestricted access in real time to essential RO data, and free
and unrestricted access to archived raw data (including auxiliary data).
This is important for NWP, Climate and Space Weather. For applications in climate
science it is particularly important that all information necessary for independent
processing towards climate data products is freely available. Data latency is particularly
important for Space Weather applications (requirement of 30 minutes (median), with agoal of 15 minutes).
5.2 IROWG continues to recommend that WMO and CGMS should coordinate
any GNSS-RO data purchases. Specifically, we suggest convening a meeting of
all agencies considering procuring these data in order to discuss if, how andPage 7 of 11

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when the current 20,000 target will be met with global and full local timecoverage.
It is likely that commercial data will need to be purchased, with a world license fornear-real time data, to meet the current target of 20,000 globally available
occultations per day.
IROWG notes that commercial data are of operational quality and have been usedsuccessfully by several agencies, as demonstrated by the successful operational
assimilation by the ECMWF, UK Met Office, and USAF of over 7000 daily, near realtime Spire Level 2 RO BUFR profiles over more than four months in 2020. The daily
target of 20,000 – although possible within 2021 with the inclusion of commercial
data – will be difficult to achieve without sharing the data. These data should beevenly distributed in local time and latitude.
Climate requirements should be taken into consideration when purchasing
commercial data, and the potential climate quality of commercial data needs to beassessed.
A world license has numerous benefits, including avoiding data duplication, sharingof experience with data sets among different centres, fairness among countries, andreducing the timeline toward implementation.
IROWG will provide links to current real time statistics on the number of assimilated
GNSS-RO data in various systems, and their performance.
5.3 IROWG recommends that CGMS encourages technology and retrieval
developments for improving planetary boundary layer profiling from GNSS-RO
and their utilization in NWP data assimilation – and the further exploration of
RO-derived water vapor as a climate variable.
GNSS-RO can provide very high vertical resolution, all-weather, thermodynamic
profiling of the planetary boundary layer (PBL) that is difficult to achieve from anyother remote sensing technique. The unique contribution from GNSS-RO for
spaceborne PBL profiling of temperature and water vapor as well as PBL height has
been recognized in the U.S. 2017–2027 Decadal Survey for Earth Science andApplications from Space.
Atmospheric radio-refractivity is heavily dependent on water vapor. GNSS-RO data in
the lower troposphere therefore contain a lot of water vapor information, which has
so far only been partly exploited.
5.4 Per CGMS priority HLPP 1.1.4 (optimized system for atmospheric andionospheric RO observations), IROWG recommends that CGMS encourages
on-going and future GNSS RO and non-RO missions, including potential

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commercial providers of RO observations, to incorporate a complete set ofionospheric measurements.
•
across all negative elevation angles as defined in the LEO local level framefrom orbit altitude down to near the Earth’s limb to allow for ionospheric profiling (forRO missions)•
across all positive elevation angles in the zenith direction forionospheric/plasmaspheric sensing•
dual frequency signal amplitudes or SNRs, pseudoranges, phases, slant TECs,
amplitude and phase scintillation indices•with the maximum sampling rate allowed by the receiver (at least for all the timeintervals for which the level of ionospheric scintillation is significant).
IROWG endorses the merit of ionospheric upgrades to EPS being implemented forionospheric data, as done in the Metop-A End-of-Life test. In addition, the sub-groupacknowledges that the Sentinel-6 hardware is capable of acquiring ionosphericobservations in addition to the atmospheric RO, and endorses plans for implementing
ionospheric capabilities via software upgrade to the Sentinel-6 receiver.

6

CONCLUSIONSThe “GPS” RO Technique is now a true “GNSS” RO Technique, where signals from all
GNSS constellations are being exploited.
GNSS-RO data demonstrate a high impact in NWP. This impact clearly increases with
the number of high-quality profiles – without any sign of “saturation”.
GNSS-RO climate data advance climate change monitoring and contribute to theupcoming IPCC assessment report.
GNSS-RO data with high spatial and temporal resolution allow for unprecedented studies
of atmospheric and ionospheric phenomena.
A better penetration into the lowest kilometres allows for studying the planetary boundary
layer – including tropospheric water vapor.
Overall, the aim of the community is to ensure the long-term continuity of the GNSSRO measurements, and to maximize the number of high-quality GNSS-RO
observations, providing good spatial and local time coverage, which can be freely
exchanged. Specifically, the researchers need access to the raw data, not justretrieved products. The provision and funding of long-term archiving of both the raw
GNSS-RO data and all the meta-data is essential for climate studies, for reanalyses,
and for any reprocessing activities. The researchers also need access to information
about the instrument performance. Overall, it is important that multiple centers have all
the information required for them to process and re-process GNSS-RO from bothgovernment sponsored and commercial missions. Therefore, there is strong support
for a “backbone” of agency-funded GNSS-RO missions with long-term commitment.
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Commercial GNSS-RO missions have reached operational quality (at least in the UTLS),
and could help to close the identified gaps in geographic and local-time coverage –provided that they are made available for the scientific community.

7

APPENDIX

Status of CGMS Actions/Recommendations relevant to IROWG

A47.31: CGMS baseline and RO: IROWG and 7th WMO Impact Workshop needs tovalidate the current baseline in terms of the coverage, number, quality and samplingof RO○ IROWG notes that the current CGMS baseline (6000 occultations from
low inclination orbits distributed geographically and temporally; 1000
occultations from drifting high inclination orbits; and 7600 occultations
from sun‐synchronous orbits) does not necessarily reflect the IROWG
recommendations regarding geographic and local time coverage.
○ E.g., the requirement “7600 occultations from sun‐synchronous orbits”(without further specification) can result in an adequate sampling of thediurnal cycle – or not (see below).
○ It is mandatory, that RO data are globally distributed and provide agood sampling of the diurnal cycle. This can be either achieved with adedicated constellation with orbits that drift sufficiently fast in local time
(typically in six orbit planes, including high-inclination orbits), or withsatellites in six or more sun-synchronous orbit planes that provide anadequate sampling of the diurnal cycle (or by a combination of theseapproaches).
○ The current (and near future) situation is unsatisfactory: Due to thecancellation of its polar component, COSMIC-2 data are confined tolatitudes below ~40° Metop as well as Metop-SG satellites are/will belimited to one orbit plane, and there is currently only one sunsynchronous satellite (FY-3D), which covers other local times thanMetop(-SG). This leaves a serious gap in local time coverage at mid
and high latitudes – even more pronounced for ionospheric data, which
are not provided by the first generation of Metop satellites.
○ This situation can/will be slightly improved (but certainly not mitigated)by including RO data from the upcoming FY-3E satellite with different
equator-crossing time and data from Sentinel-6 in a drifting orbit. Initial
Sentinal-6 data look very promising, and will – ultimately – cover all
local times, however, it takes almost two months to achieve this.
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○ Actual profile numbers tend to be smaller than the nominal ones. Basedon the nominal performance documented in the WMO OSCAR
database, one would assume that the current constellation ofoperational RO missions would deliver at least 10 000 profiles per day(April 2021); however, the actual number of post-QC profiles (averageof the past month) that can be assimilated at ECMWF is less than8000.
○ IROWG shares the concern, expressed in the recent WGIII risk
assessment, that there is “Continuity risk for the number andgeographic distribution of radio occultations; especially in the low- to
mid- latitudes”, since there is currently no plan for a successor of the
COSMIC-2 mission.
○ This baseline is currently not sufficient to meet the HLPP objective (1.2)
to advance the atmospheric Radio Occultation constellation, with thelong-term goal of providing 20,000 occultations per day on a sustained
basis.
A48.02: Data providers to document data processing QC processes (including a monthof QC statistics, e.g. rejection percentage at each QC step) and space sampling
information and provide to IROWG.
○ IROWG notes that this is a CGMS recommendation to data providers.
○ IROWG suggests that the relevant agencies send representatives tothe next IROWG meeting so that detailed information on quality control
methods and statistics used by the different agencies can be
discussed, leading to their improved documentation.
○ The need for this has been recognised, and members of the community
will take steps to document their procedures.
○ Certain WG members (from EUMETSAT, Spire and NOAA-NESDIS)
agreed to provide information on QC numbers in the next IROWG
meeting.

Affiliations
Anthony Mannucci is with the Jet Propulsion Laboratory, California Institute ofTechnology. Sean Healy is with the European Center for Medium Range Weather
Forecasts. Ulrich Foelsche is with the University of Graz, Austria.
© 2021. All rights reserved.

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