GNSS-RO (COSMIC-2·ROM SAF)¶

GNSS Radio Occultation — UCAR CDAAC (COSMIC-2, Metop, commercial RO and others) and EUMETSAT ROM SAF climate data records

Available

At a glance¶

Note on period: ROM SAF CDR v1.0 covers September 2001 to December 2016 (CHAMP, GRACE, COSMIC, Metop) and the Metop ICDR starts in January 2017. COSMIC-2 near-real-time data run from 1 October 2019 to the present, and post-processed Level 2 covers 2019 to May 2022 (day 151 of 2022). UCAR CDAAC also holds earlier missions such as GPS/MET (1995), CHAMP and COSMIC-1 (reprocessings repro2013 and repro2021).

Overview¶

Low-Earth-orbit satellites measure how much GNSS (GPS, GLONASS, Galileo and others) signals bend as they graze the atmosphere (the bending angle), from which refractivity and profiles of temperature, pressure and water vapour are derived. Because the measurement is a phase delay referenced to atomic clocks, it has almost no instrument calibration bias and is unaffected by clouds, making it a climate benchmark for upper-tropospheric and stratospheric temperature and an 'anchor' that reanalyses assimilate without bias correction. In the US, UCAR CDAAC produces Level 0–2 data for COSMIC-2 and many other missions; in Europe, EUMETSAT ROM SAF produces Metop-centred near-real-time and offline data and multi-mission climate records (CDR and ICDR).

What changes with the successor¶

ROM SAF CDR v1.0 (GRM-28/29/30/32/33-R1) + Metop ICDR (GRM-29-I1) → Next ROM SAF reprocessed climate record (CDR v2.0 series) Planned¶

The ROM SAF product status table now includes a 'CDR v2.0 (+ICDR v2)' column, and the operations reports announce the next reprocessing for 2026. No release date or DOI has been confirmed yet.

Sources: ROM SAF Product Status overview (2026-06-22) · ROM SAF News Archive (operations report notices)

Available variables¶

COSMIC-2 (CDAAC) names are from the CDAAC file format documents. ROM SAF Level 3 grids come in one file per variable (six-letter product code in the file name, e.g. RRGMET, RTGMET), and the Level 3 Product User Manual (v3.2, Section 5.3) shows only the refractivity file header (REF), so in-file names of the other variables are left blank. Check ROM SAF Level 1b/2 and other Level 3 variable names in your files.

instantaneous mean accumulated maximum minimum static — instantaneous: value at the time stamp · mean/accumulated/maximum/minimum: over the indicated interval · static: does not change with time

COSMIC-2 Level 2 atmPrf (dry retrieval, full resolution)¶

COSMIC-2 Level 2 wetPf2 (1D-Var moist retrieval, 100 m spacing)¶

ROM SAF Level 3 monthly grids (CDR and ICDR)¶

The full variable list is in the official documentation.

How to get the data¶

• UCAR CDAAC data archive (COSMIC-2, Metop, commercial RO, COSMIC-1 and others) — Direct HTTPS download (no login; tar.gz by date and level)
  • COSMIC-2 near-real-time Level 2 is under cosmic2/nrt/level2/// as daily bundles of atmPrf, wetPf2, bfrPrf (BUFR) and others. The monthly gridded climate record (UCAR RO CDR) is under gridded/cdr/.
• COSMIC-2 data guide (processing streams, latency, citation) — Web
• EUMETSAT ROM SAF product archive (NRT, Offline, CDR, ICDR) — Web download (free registration and login, netCDF)
  • Monthly gridded humidity is also distributed through the C3S Climate Data Store. Products are CC BY 4.0.

Example code¶

Downloads a no-login COSMIC-2 near-real-time wetPf2 daily bundle (about 200 MB), extracts it and opens the first profile. Member names may not end in .nc, so they are not filtered by extension.

# pip install requests xarray netCDF4
import tarfile, requests, xarray as xr

url = ("https://data.cosmic.ucar.edu/gnss-ro/cosmic2/nrt/level2/"
       "2026/284/wetPf2_nrt_2026_284.tar.gz")          # day 284 of 2026 (11 October)
with open("wetPf2.tar.gz", "wb") as f:
    f.write(requests.get(url, timeout=600).content)

with tarfile.open("wetPf2.tar.gz") as tar:
    members = [m for m in tar.getmembers() if m.isfile()]
    tar.extractall("wetPf2", members=members[:1])     # extract a single profile

ds = xr.open_dataset("wetPf2/" + members[0].name, engine="netcdf4")
print(ds[["MSL_alt", "Temp", "Vp", "Pres"]])           # altitude (km), temperature (°C), vapour pressure (hPa), pressure (hPa)

Practical tips¶

• COSMIC-2 atmPrf Temp is in Celsius and Pres in hPa (written mb in the files). Convert units before comparing with reanalyses (K, Pa).
• atmPrf carries a quality flag bad (1 means the profile failed QC). Remove those before computing statistics.
• atmPrf attributes include the lowest height of the straight-line GPS–LEO path (hslbot) and signal-quality indicators (snr1avg, s4 and others). Use them to select profiles for lower-tropospheric analysis.
• The COSMIC-2 initial-operations notice (2020-03) reported larger bias and standard deviation at about 19–29 km for GPS L2P occultations than for others. Split by signal type for precise lower-stratosphere work.
• Dry temperature equals the real temperature only where water vapour is negligible, in the upper troposphere and stratosphere. ROM SAF and UCAR climate records mainly provide dry-temperature grids in that range (UCAR CDR temperature 10–40 km).
• ROM SAF CDR (2001–2016) and ICDR (2017 onward) come from different processing systems, so there may be a seam. Use a matching CDR–ICDR pair (e.g. GRM-29-R1 + GRM-29-I1) and mark the join.
• Metop-A ended in November 2021, so later Metop data come from Metop-B and -C (and from November 2025 also Metop-SG A1). Check for sampling changes when the satellite mix changes.
• The UCAR RO CDR monthly grids use COSMIC-2 only between 30°N and 30°S and drop Spire data south of 60°S, so the mission mix varies with latitude.

Notices and change history¶

References for your paper¶

Dataset citation¶

COSMIC-2 Data Products (UCAR) — DOI 10.5065/T353-C093

@misc{ucar2019cosmic2,
  author    = {{UCAR COSMIC Program}},
  title     = {{COSMIC-2} Data Products},
  year      = {2019},
  publisher = {UCAR/NCAR - COSMIC},
  doi       = {10.5065/T353-C093},
  note      = {Accessed DD MMM YYYY}
}

ROM SAF multi-mission reprocessed climate record GRM-28-R1 (monthly grids) — DOI 10.15770/EUM_SAF_GRM_0001

@misc{romsaf2021grm28,
  author    = {{EUMETSAT ROM SAF}},
  title     = {Radio Occultation Reprocessed Climate Data Record - Multimission ({GRM-28-R1})},
  year      = {2021},
  publisher = {EUMETSAT},
  doi       = {10.15770/EUM_SAF_GRM_0001}
}

ROM SAF Metop climate record GRM-29-R1 (ROM SAF citation example) — DOI 10.15770/EUM_SAF_GRM_0002

@misc{romsaf2019grm29r1,
  author    = {{EUMETSAT ROM SAF}},
  title     = {{ROM SAF} Radio Occultation Climate Data Record - Metop ({GRM-29-R1}), v1.0},
  year      = {2019},
  publisher = {EUMETSAT},
  doi       = {10.15770/EUM_SAF_GRM_0002}
}

ROM SAF Metop interim climate record GRM-29-I1 (2017 onward) — DOI 10.15770/EUM_SAF_GRM_0006

@misc{romsaf2019grm29i1,
  author    = {{EUMETSAT ROM SAF}},
  title     = {{ROM SAF} Radio Occultation Interim Climate Data Record - Metop ({GRM-29-I1})},
  year      = {2019},
  publisher = {EUMETSAT},
  doi       = {10.15770/EUM_SAF_GRM_0006}
}

Reference papers¶

COSMIC-2 first results — DOI 10.1029/2019GL086841

@article{schreiner2020cosmic2,
  author  = {Schreiner, W. S. and Weiss, J. P. and Anthes, R. A. and Braun, J. and Chu, V. and Fong, J. and Hunt, D. and Kuo, Y.-H. and Meehan, T. and Serafino, W. and Sjoberg, J. and Sokolovskiy, S. and Talaat, E. and Wee, T. K. and Zeng, Z.},
  title   = {{COSMIC-2} Radio Occultation Constellation: First Results},
  journal = {Geophysical Research Letters},
  volume  = {47},
  number  = {4},
  pages   = {e2019GL086841},
  year    = {2020},
  doi     = {10.1029/2019GL086841}
}

Acknowledgements¶

UCAR recommends citing the DOI rather than a URL because DOIs persist. ROM SAF products are CC BY 4.0: figures should carry 'EUMETSAT ROM SAF [product ID]' (e.g. EUMETSAT ROM SAF GRM-29-R1), text should give the product name, version, product ID and DOI, and authors are asked to send the title and DOI of peer-reviewed papers to romsafom@dmi.dk.

Information on the datasets used in this study was compiled with the help of the Korean Climate Data Navigator (KCDN) (Sim, 2026; https://doi.org/10.5281/zenodo.23300923).

Links¶

• UCAR COSMIC — COSMIC-2 mission overview
• UCAR COSMIC — data table for other RO missions
• UCAR RO Climate Data Record (monthly grids)
• CDAAC file format document (atmPrf)
• ROM SAF product documents (user manuals, validation reports)
• ROM SAF usage policy and citation guidance