{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T22:41:36Z","timestamp":1760222496668,"version":"build-2065373602"},"reference-count":26,"publisher":"MDPI AG","issue":"12","license":[{"start":{"date-parts":[[2009,12,14]],"date-time":"2009-12-14T00:00:00Z","timestamp":1260748800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/3.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Sensors"],"abstract":"<jats:p>Lack of frequent and global observations from space is currently a limiting factor in many Earth Observation (EO) missions. Two potential techniques that have been proposed nowadays are: (1) the use of satellite constellations, and (2) the use of Global Navigation Satellite Signals (GNSS) as signals of opportunity (no transmitter required). Reflectometry using GNSS opportunity signals (GNSS-R) was originally proposed in 1993 by Martin-Neira (ESA-ESTEC) for altimetry applications, but later its use for wind speed determination has been proposed, and more recently to perform the sea state correction required in sea surface salinity retrievals by means of L-band microwave radiometry (TB). At present, two EO space-borne missions are currently planned to be launched in the near future: (1) ESA\u2019s SMOS mission, using a Y-shaped synthetic aperture radiometer, launch date November 2nd, 2009, and (2) NASA-CONAE AQUARIUS\/SAC-D mission, using a three beam push-broom radiometer. In the SMOS mission, the multi-angle observation capabilities allow to simultaneously retrieve not only the surface salinity, but also the surface temperature and an \u201ceffective\u201d wind speed that minimizes the differences between observations and models. In AQUARIUS, an L-band scatterometer measuring the radar backscatter (\u03c30) will be used to perform the necessary sea state corrections. However, none of these approaches are fully satisfactory, since the effective wind speed captures some sea surface roughness effects, at the expense of introducing another variable to be retrieved, and on the other hand the plots (TB-\u03c30) present a large scattering. In 2003, the Passive Advance Unit for ocean monitoring (PAU) project was proposed to the European Science Foundation in the frame of the EUropean Young Investigator Awards (EURYI) to test the feasibility of GNSS-R over the sea surface to make sea state measurements and perform the correction of the L-band brightness temperature. This paper: (1) provides an overview of the Physics of the L-band radiometric and GNSS reflectometric observations over the ocean, (2) describes the instrumentation that has been (is being) developed in the frame of the EURYI-funded PAU project, (3) the ground-based measurements carried out so far, and their interpretation in view of placing a GNSS-reflectometer as secondary payload in future SMOS follow-on missions.<\/jats:p>","DOI":"10.3390\/s91210171","type":"journal-article","created":{"date-parts":[[2009,12,14]],"date-time":"2009-12-14T10:47:34Z","timestamp":1260787654000},"page":"10171-10189","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":12,"title":["New Passive Instruments Developed for Ocean Monitoring at the Remote Sensing Lab\u2014Universitat Polit\u00e8cnica de Catalunya"],"prefix":"10.3390","volume":"9","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-9514-4992","authenticated-orcid":false,"given":"Adriano","family":"Camps","sequence":"first","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Xavier","family":"Bosch-Lluis","sequence":"additional","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Isaac","family":"Ramos-Perez","sequence":"additional","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Juan F.","family":"March\u00e1n-Hern\u00e1ndez","sequence":"additional","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Nereida","family":"Rodr\u00edguez","sequence":"additional","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Enric","family":"Valencia","sequence":"additional","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Jose M.","family":"Tarongi","sequence":"additional","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Albert","family":"Aguasca","sequence":"additional","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Ren\u00e9","family":"Acevo","sequence":"additional","affiliation":[{"name":"Department of Signal Theory and Communications, Universitat Polit\u00e8cnica de Catalunya & IEEC\/CRAE-UPC Campus Nord D3, E-08034 Barcelona, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2009,12,14]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"480","DOI":"10.1109\/TGRS.1983.350511","article-title":"Considerations for Microwave Remote Sensing of Ocean Surface Salinity","volume":"21","author":"Swift","year":"1983","journal-title":"IEEE Trans. Geosci. Remote. Sens."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"2196","DOI":"10.1109\/TGRS.2004.834649","article-title":"The Determination of Surface Salinity with the European SMOS Space Mission","volume":"42","author":"Font","year":"2004","journal-title":"IEEE Trans. Geosci. Remote. Sens."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"804","DOI":"10.1109\/TGRS.2003.819444","article-title":"The WISE 2000 and 2001 Field Experiments in Support of the SMOS Mission: Sea Surface L-Band Brightness Temperature Observations and their Application to Multi-Angular Salinity Retrieval","volume":"42","author":"Camps","year":"2004","journal-title":"IEEE Trans. Geosci. Remote. Sens."},{"key":"ref_4","doi-asserted-by":"crossref","first-page":"3180","DOI":"10.1109\/TGRS.2007.894925","article-title":"New Instrument Concepts for Ocean Sensing:Analysis of the PAU-Radiometer","volume":"45","author":"Camps","year":"2007","journal-title":"IEEE Trans. Geosci. Remote. Sens."},{"key":"ref_5","first-page":"331","article-title":"A Passive Reflectometry and Interferometry System (PARIS): Application to Ocean Altimetry","volume":"17","year":"1993","journal-title":"ESA J."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"951","DOI":"10.1109\/36.841977","article-title":"Scattering of GPS Signals from the Ocean with Wind Remote Sensing Application","volume":"38","author":"Zavorotny","year":"2000","journal-title":"IEEE Trans. Geosci. Remote. Sens."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"2914","DOI":"10.1109\/TGRS.2008.922144","article-title":"Correction of the Sea State Impact in the L-Band Brightness Temperature by Means of Delay-Doppler Maps of Global Navigation Satellite Signals Reflected Over the Sea Surface","volume":"46","author":"Camps","year":"2008","journal-title":"IEEE Trans. Geosci. Remote. Sens."},{"key":"ref_8","unstructured":"Marchan-Hernandez, J.F., Valencia, E., Rodriguez-Alvarez, N., Ramos-Perez, I., Bosch-Lluis, X., Camps, A., Eugenio, F., and Marcello, J. (2009). IEEE Geosci. Remote. Sens. Lett., submitted for publication."},{"key":"ref_9","unstructured":"Valencia, E., Marchan-Hernandez, J.F., Camps, A., Rodriguez-Alvarez, N., Tarongi, J.M., Piles, M., Ramos-Perez, I., Bosch-Lluis, X., Vall-llossera, M., and Ferr\u00e9, P. Experimental Relationship between the Sea Brightness Temperature and the GNSS-R Delay-Doppler Maps: Preliminary Results of the ALBATROSS Field Experiments. Cape Town, South Africa."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"4392","DOI":"10.3390\/s8074392","article-title":"PAU\/RAD: Design and Preliminary Calibration Results of a New L-Band Pseudo-Correlation Radiometer Concept","volume":"8","author":"Camps","year":"2008","journal-title":"Sensors"},{"key":"ref_11","unstructured":"Bosch-Lluis, X., Ramos-Perez, I., Camps, A., Marchan-Hernandez, J.F., Rodr\u00edguez-\u00c1lvarez, N., Valencia, E., and Guerrero, M.A. Initial Results of a Digital Radiometer With Digital Beamforming. Boston, MA, USA."},{"key":"ref_12","unstructured":"Bosch-Lluis, X., Ramos-Perez, I., Camps, A., Rodriguez-Alvarez, N., Marchan-Hernandez, J.F., Valencia, E., and Nieto, J.M. Digital Beamforming Analysis and Performance of a Digital L-Band Pseudo-Correlation Radiometer. Cape Town, South Africa."},{"key":"ref_13","unstructured":"ESA's SMOS Available online: http:\/\/www.esa.int\/esaLP\/LPsmos.html (accessed on November 12, 2009)."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1755","DOI":"10.1109\/TGRS.2002.802489","article-title":"Polarimetric mode of MIRAS","volume":"40","year":"2002","journal-title":"IEEE Trans. Geosci. Remote. Sens."},{"key":"ref_15","doi-asserted-by":"crossref","first-page":"6131","DOI":"10.3390\/s90806131","article-title":"Calibration of Microwave Radiometers Using Pseudo-Random Noise Signals","volume":"9","author":"Camps","year":"2009","journal-title":"Sensors"},{"key":"ref_16","unstructured":"Ramos-Perez, I., Camps, A., Bosch-Lluis, X., Marchan-Hernandez, J.F., and Rodr\u00edguez-\u00c1lvarez, N. Synthetic Aperture PAU: a new instrument to test potential improvements for future SMOSops. Barcelona, Spain."},{"key":"ref_17","unstructured":"Ramos-Perez, I., Bosch-Lluis, X., Camps, A., Valencia, E., Marchan-Hernandez, J.F., Rodriguez-Alvarez, N., and Canales-Contador, F. Preliminary Results of the Passive Advanced Unit Synthetic Aperture (PAU-SA). Cape Town, South Africa."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"3005","DOI":"10.3390\/s8053005","article-title":"PAU\/GNSS-R: Implementation, Performance and First Results of a Real-Time Delay-Doppler Map Reflectometer Using Global Navigation Satellite System Signals","volume":"8","author":"Camps","year":"2008","journal-title":"Sensors"},{"key":"ref_19","unstructured":"Valencia, E., Camps, A., Marchan-Hernandez, J.F., Bosch-Lluis, X., Rodr\u00edguez-\u00c1lvarez, N., and Ramos-Perez, I. GPS Reflectometer Instrument for PAU (griPAU): Advanced Peformance Real Time Delay Doppler Map Receiver. Noordwijk, The Netherlands."},{"key":"ref_20","unstructured":"Camps, A., Aguasca, A., Bosch-Lluis, X., Marchan-Hernandez, J.F., Ramos-Perez, I., Rodr\u00edguez-\u00c1lvarez, N., Bou, F., Ib\u00e1nez, C., Banqu\u00e9, X., and Prehn, R. PAU One-Receiver Ground-based and Airborne Instruments. Barcelona, Spain."},{"key":"ref_21","unstructured":"Valencia, E., Acevo, R., Bosch-Lluis, X., Aguasca, A., Rodr\u00edguez-\u00c1lvarez, N., Ramos-Perez, I., Marchan-Hernandez, J.F., Glenat, M., Bou, F., and Camps, A. Initial Results of an Airborne Light-Weight L-Band Radiometer. Boston, MA, USA."},{"key":"ref_22","doi-asserted-by":"crossref","unstructured":"Acevo-Herrera, R., Aguasca, A., Bosch-Lluis, X., and Camps, A. On the Use of Compact L-Band Dicke Radiometer (ARIEL) and UAV for Soil Moisture and Salinity Map Retrieval: 2008\/2009 Field Experiments. Cape Town, South Africa.","DOI":"10.1109\/IGARSS.2009.5417480"},{"key":"ref_23","doi-asserted-by":"crossref","unstructured":"Gleason, S. (2006). Remote Sensing of Ocean, Ice and Land Surfaces Using Bistatically Scattered GNSS Signals From Low Earth Orbit. [Ph.D. Thesis, University of Surrey].","DOI":"10.1109\/IGARSS.2006.792"},{"key":"ref_24","unstructured":"Camps, A., Rodriguez-Alvarez, N., Bosch-Lluis, X., Marchan, J.F., Ramos-Perez, I., Segarra, M., Sagues, L., Tarrago, D., Cunado, O., Vilaseca, R., Tomas, A., Mas, J., and Guillamon, J. (, January March,). PAU in SeoSAT: A Proposed Hybrid L-band Microwave Radiometer\/GPS Reflectometer to Improve Sea Surface Salinity Estimates from Space. Florence, Italy."},{"key":"ref_25","unstructured":"Rodr\u00edguez-\u00c1lvarez, N., March\u00e1n, J.F., Camps, A., Valencia, E., Bosch-Lluis, X., Ramos-P\u00e9rez, I., and Nieto, J.M. Soil Moisture Retrieval Using GNSS-R Techniques: Measurement Campaign In A Wheat Field. Boston, MA, USA."},{"key":"ref_26","unstructured":"Rodriguez-Alvarez, N., Monerris, A., Bosch-Lluis, X., Camps, A., Vall-llossera, M., Marchan-Hernandez, J.F., Ramos-Perez, I., Valencia, E., Mart\u00ednez-Fern\u00e1ndez, J., Sanchez-Martin, N., Baroncini-Turricchia, G., and Perez-Gutierrez, C. Soil Moisture and Vegetation Height Retrieval Using GNSS-R Techniques. Cape Town, South Africa."}],"container-title":["Sensors"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/1424-8220\/9\/12\/10171\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T22:11:59Z","timestamp":1760220719000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/1424-8220\/9\/12\/10171"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2009,12,14]]},"references-count":26,"journal-issue":{"issue":"12","published-online":{"date-parts":[[2009,12]]}},"alternative-id":["s91210171"],"URL":"https:\/\/doi.org\/10.3390\/s91210171","relation":{},"ISSN":["1424-8220"],"issn-type":[{"type":"electronic","value":"1424-8220"}],"subject":[],"published":{"date-parts":[[2009,12,14]]}}}