Ocean remote sensing: Challenges for the future

IEEE Transactions on Geoscience and Remote Sensing, 2000

WindSat, the first polarimetric radiometer on orbit, launched in January 2003, provides the promise of passive ocean wind vector retrievals on a continuous basis, simultaneous with the retrieval of many other geophysical variables such as sea surface temperature, atmospheric water vapor, cloud liquid water, and sea ice extent and concentration. WindSat also serves as risk reduction for the upcoming National Polar-orbiting Operational Environmental Satellite System (NPOESS) Conical Scanning Microwave Imager/Sounder (CMIS). Since the dependence of microwave brightness temperatures on wind direction is small relative to that of other parameters such as wind speed, wind direction retrieval relies on increasingly accurate knowledge of the ocean surface microwave emission, which depends upon surface properties such as roughness and foam due to wave breaking. Coordinated near-surface measurements of ocean surface microwave emission and air-sea interaction parameters are needed to quantify the effects of the processes mentioned above in surface emission models to improve the accuracy of wind vector retrievals. Such coordinated observations were performed during the Fluxes, Air-Sea Interaction, and Remote Sensing (FAIRS) experiment conducted on the R/P Floating Instrument Platform (FLIP) in the northeastern Pacific Ocean during the Fall of 2000. X-and Ka-band partially polarimetric radiometers were mounted at the end of the port boom of R/P FLIP to measure ocean surface emission at incidence angles of 45 , 53 , and 65 . A bore-sighted video camera recorded the fractional area of foam in the field of view of the radiometers. Air-sea interaction parameters that were measured concurrently include wind speed, friction velocity, heat fluxes, and significant wave height. The measured dependence of ocean surface emissivity on wind speed and friction velocity is in good agreement with, and extends, earlier observations and empirical models based on satellite data. Concurrent radiometric measurements and fractional Manuscript area foam coverage data strengthen the possibility of retrieval of sea surface foam coverage using airborne or spaceborne radiometry. The dependence of emissivity on atmospheric stability is shown to be much smaller than the dependence of emissivity on wind speed. Analysis of emissivity dependence on atmospheric stability alone was inconclusive, due to the variation in atmospheric stability with wind speed. The effect of long-wave incidence angle modulation on sea surface emissivity for near-surface measurements was found to be negligible when emissivity measurements were averaged over tens to hundreds of long waves.

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 (T B). At present, two EO space-borne missions are currently planned to be launched in the near future: (1) ESA's 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 " effective " wind speed that minimizes the differences between observations and models. In AQUARIUS, an L-band scatterometer measuring the radar backscatter (σ 0) 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 OPEN ACCESS Remote Sensing 2009, 1 10172 on the other hand the plots (T B-σ 0) 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.

Boundary-Layer Meteorology, 1978

IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing

Reliable, accurate, and timely information about oceans is important for many applications, including water resource management, hydrological cycle monitoring, environmental studies, agricultural and ecosystem health applications, economy, and the overall health of the environment. In this regard, remote sensing (RS) systems offer exceptional advantages for mapping and monitoring various oceanographic parameters with acceptable temporal and spatial resolutions over the oceans and coastal areas. So far, different methods have been developed to study oceans using various RS systems. This urges the necessity of having review studies that comprehensively discuss various RS systems, including passive and active sensors, and their advantages and limitations for ocean applications. In this article, the goal is to review most RS systems and approaches that have been worked on marine applications. This review paper is divided into two parts. Part 1 is dedicated to the passive RS systems for ocean studies. As such, four primary passive systems, including optical, thermal infrared radiometers, microwave radiometers, and Global Navigation Satellite Systems, are comprehensively discussed. Additionally, this article summarizes the main passive RS sensors and satellites, which have been utilized for different oceanographic applications. Finally, various oceanographic parameters, which can be retrieved from the data acquired by passive RS systems, along with the corresponding methods, are discussed.

Dynam Atmos Oceans, 1987

2010

The long-term goal of this project is to fully utilize microwave backscattering data sets that have been collected on the ocean over the last two decades. The prime objective is to better understand the formation of breaking waves on the ocean and their effect on microwave backscatter. SCIENTIFIC OBJECTIVES The scientific objectives of this research are to investigate experimentally wave shadowing and modulation in low-grazing-angle backscatter from the ocean.

1976

The application of two active microwave systems, the synthetic aperture side-looking radar and the dual frequency scatterometer, to the remote sensing of ocean waves is discussed. A brief description of the application characteristics of other microwave systems, such as the nanosecond radar altimeter, the dual-frequency radar interferometer, and the microwave radiometer, is also given. The economic utility of sea state forecasting is included.

Surveys in Geophysics, 2023

This review paper reports on the state-of-the-art concerning observations of surface winds, waves, and currents from space and their use for scientific research and subsequent applications. The development of observations of sea state parameters from space dates back to the 1970s, with a significant increase in the number and diversity of space missions since the 1990s. Sensors used to monitor the sea-state parameters from space are mainly based on microwave techniques. They are either specifically designed to monitor surface parameters or are used for their abilities to provide opportunistic measurements complementary to their primary purpose. The principles on which is based on the estimation of the sea surface parameters are first described, including the performance and limitations of each method. Numerous examples and references on the use of these observations for scientific and operational applications are then given. The richness and diversity of these applications are linked to the importance of knowledge of the sea state in many fields. Firstly, surface wind, waves, and currents are significant factors influencing exchanges at the air/ sea interface, impacting oceanic and atmospheric boundary layers, contributing to sea level rise at the coasts, and interacting with the sea-ice formation or destruction in the polar zones. Secondly, ocean surface currents combined with wind-and wave-induced drift contribute to the transport of heat, salt, and pollutants. Waves and surface currents also impact sediment transport and erosion in coastal areas. For operational applications, observations of surface parameters are necessary on the one hand to constrain the numerical solutions of predictive models (numerical wave, oceanic, or atmospheric models), and on the other hand to validate their results. In turn, these predictive models are used to guarantee safe, efficient, and successful offshore operations, including the commercial shipping and energy sector, as well as tourism and coastal activities. Long-time series of global sea-state observations are also becoming increasingly important to analyze the impact of climate change on our environment. All these aspects are recalled in the article, relating to both historical and contemporary activities in these fields.

Oceanography, 2010

2011

Synchronous and colocated optical and microwave signals from waves in the surf zone are presented and analyzed. The field data were collected using a high-resolution video system and a calibrated horizontally polarized marine radar during the decaying phase of a storm. The resulting changes in the received signals from varying environmental conditions were analyzed. The analysis of the optical signal histograms showed functional shapes that were in accordance with the expected imaging mechanisms from the breaking and nonbreaking waves. For the microwave returns, the histogram shape showed a little dependence on the environmental parameters and exhibited an inflexion point at high returned power that is attributed to a change in the scattering mechanism. The high intensity signals were clearly associated with active wave breaking. However, with either sensor, it can be difficult to effectively isolate the wave breaking signature from other sources, such as a remnant foam or the highl...