The Salinity Sea Ice Working Group

Assessing the scientific importance and technical feasibility of salinity remote sensing

Revised 3 June 1999

SSIWG Information
Salinity Remote Sensing

Meeting Announcements

There is an increased awareness among researchers that surface salinity variability plays an important role in ocean and climate dynamics. This has prompted a general call to increase salinity observations in evolving climate research programs. These proposals are aimed primarily at in situ measurements, which are spatially limited. Due to its global coverage, however, remote sensing may prove to be an invaluable technique in the large scale monitoring of surface salinity.

For two decades, it has been known that sea surface salinity variations can be detected with remote sensing. The signals are small, but experiments with airborne instruments both in the 1970s and in the recent few years have repeatedly demonstrated that scientifically useful measurements can be obtained. With present technology, it may be possible to map the global surface salinity field from satellite on climatological scales, that is ~monthly and ~1-2 degree resolution, with a precision approaching 0.1 psu. Measurements would have to be made with passive microwave radiometryat 1-3 GHz (L & S bands). There may be additional scientific value for sea ice observations in these microwave bands. 

These channels are now being considered for planned future satellite missions. Such observations possibly represent a major advance in global ocean remote sensing capability for climate research. Accordingly, the NASA Physical Oceanography program desires a comprehensive evaluation of the scientific importance and technical feasibility of such a mission.

To this end, a Salinity Sea Ice Working Group (SSIWG) has been formed ad hoc

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Mission of the SSIWG
The current mission of the SSWIG is to:

  1. assess the scientific importance and applications of global surface salinity measurements and sea ice observations at L & S band;
  2. evaluate the potential accuracy and spatio-temporal resolution of satellite data given present and near future technologies;
  3. recommend measurement requirements and identify technology issues and tradeoffs; and
  4. provide guidence for ongoing field measurements and algorithm studies.

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The working groups efforts will provide the background and justification for a mission proposal in response to a future NASA Earth Systems Science Pathfinder (ESSP) or other relevant program announcement. Any proposed mission using these microwave frequencies is also likely to be in collaboration with a program to measure terrestrial soil moisture (which uses similar channels). However, it remains necessary to fully explore and articulate the scientific benefit and technical feasibility of salinity and sea ice measurements to provide the strongest possible justification for a satellite project. The SSIWG's findings will be made freely available to all groups preparing mission proposals and SSIWG members will be free to participate on any mission teams.

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The initial SSIWG meeting was held February 7-8, 1998 in La Jolla, CA. (see SSIWG-1 report for more information).

The second meeting was held 19-21 April 1999 at Goddard Space Flight Center, Greenbelt, MD (see SSIWG-2 report).

The third meeting was held Jan 22-23, 2000 in San Antonio, TX. (see SSIWG-3 report).

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Participation in the SSIWG is voluntary and informal.
Please Notify the Chairmam, Gary Lagerloef if you are interested in joining the effort.

Participants at each meeting are listed in the meeting reports

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Satellite Options and Opportunities

Principal engineering challenge: Very large antenna structures are required for L-band systems to achieve a footprint size ~tens of kilometers.

Terrestrial surface soil moisture is measured with similar microwave frequencies. The signals are more robust than salinity and some satellite designs have greater emphasis on soil moisture science requirements.

Hydrostar: This mission was proposed to the NASA ESSP-2 announcement in 1998, but did not get selected. HYDROSTAR was to be a thinned array aperture synthesis design proposed by University of Michigan and GSFC to launch in early 2002. It entailed a cross-track scan, single channel (L- band, H-pol) passive radiometer primarily for soil moisture science and was considered to be experimental for SSS. For more information, contact the Principal Investigator, Dr. Tony England <>. The non-selection is an unfortunate set back for the soil moisture and oceanographic communities. For the latter, HYDROSTAR represented our earliest opportunity to obtain L-band data from space and make initial progress at mapping global SSS and improving algorithms.

MIRAS/RAMSES/SMOS: A thinned array, 2-dimensional synthesis design being developed in Europe, that has been called MIRAS and RAMSES during various development programs in ESA and CNES. It will address soil moisture and surface salinity with an H&V-polarized L-band radiometer system. The mission was proposed to ESA in November 1998 as SMOS (Soil Moisute Ocean Salinity) and approved for Phase A study in June 1999. For more information, contact the Principal Investigator Dr. Yann Kerr <>, or visit the website at The SMOS proposal, available on this website, gives a good review of the scientific and technical merits for salinity remote sensing and the contribution to CLIVAR objectives.

OSIRIS: A large mesh antenna design under design at NASA/JPL. OSIRIS is aimed at attaining the highest possible SSS measurement accuracy with current technology. It includes a conical scan, multi channel (L&S-band, H&V- pol) radiometer and optional L-band radar for wind and sea state correction. Pre-Phase A studies are proceeding with the NASA Instrument Incubator Program. This system was also described for NASA's post-2002 Request for Information in August 1998. For more information, contact the Principal Investigator Dr. Eni Njoku <>.

Other 2-Dimensional aperture systesis designs are being supported by the NASA Instrument Incubator Program at NASA/GSFC for future soil moisture and salinity observations. For more information, contact the Principal Investigator Dr. David LeVine <>.

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