Oceanography : A remote sensing perspective

Reference: https://eastcoast.coastwatch.noaa.gov/cw_olci_chl.php

Introduction

Oceanography, the study of the physical, chemical, biological, and geological aspects of the ocean, plays a crucial role in understanding Earth’s complex marine environment. This field leverages a variety of methodologies and technologies, including remote sensing, to collect, analyze, and interpret oceanic data.

Fundamentals of Oceanography

1. Physical Oceanography: Physical oceanography deals with the study of ocean currents, waves, and tides. It explores the interactions between the ocean and the atmosphere, which significantly influence climate patterns.
2. Chemical Oceanography: Chemical oceanography focuses on the chemical composition of seawater and the biogeochemical cycles. It examines the sources and sinks of various chemical elements and compounds in the ocean.
3. Biological Oceanography: Biological oceanography investigates the marine organisms and their interactions with the ocean environment. It covers the study of marine ecosystems, food webs, and the impact of human activities on marine life.
4. Geological Oceanography: Geological oceanography examines the structure and composition of the ocean floor. It includes the study of plate tectonics, underwater volcanism, and sediment processes.

Key Remote Sensing Instruments

• Radiometers: Measure the intensity of radiation, providing data on sea surface temperature and ocean color.
• Radar Altimeters: Measure sea surface height, which is crucial for understanding ocean circulation and sea level rise.
• LIDAR (Light Detection and Ranging): Uses laser pulses to measure the depth and topography of the ocean floor.

Oceanography is an interdisciplinary field that utilizes advanced technologies, including remote sensing, to study and understand the ocean. By integrating data from various sources and using sophisticated data formats, oceanographers can monitor, analyze, and predict changes in the marine environment. This knowledge is crucial for managing marine resources, mitigating the impacts of climate change, and protecting ocean health.

Sentinel 3 : Satellite

Sentinel-3 is an European Earth Observation satellite mission developed to support Copernicus ocean, land, atmospheric, emergency, security and cryospheric applications. It is jointly operated by ESA and EUMETSAT to deliver operational ocean and land observation services. There are two satellites lanuched: Sentinel-3A on 16 February 2016 Sentinel-3B on 25 April 2018 (a more detail on the satellite and various components are given at official page).

• Main objectives: Systematically measures Earth’s oceans, land, ice, and atmosphere.
• Measure sea surface topography.
• Measure sea and land surface temperature.
• Measure ocean and land surface color.

Additional applications: Sea-level change & sea-surface temperature mapping, water quality management, sea-ice extent and thickness mapping and numerical ocean prediction; land-cover mapping, vegetation health monitoring; glacier monitoring; water resource monitoring; wildfire detection; numerical weather prediction.

Spectral bands: (For more on theoretical basis on the Spectral Calibration details, see page number 13 in the following link)

Sentinel-3 Instruments:

The Sentinel-3 mission includes several key instruments, each serving distinct functions for Earth observation. Here’s a list of the instruments along with a brief explanation of each:

Sea and Land Surface Temperature Radiometer (SLSTR): Measures global sea- and land-surface temperatures daily with an accuracy better than 0.3 K.

• Description: This dual-view (near-nadir and inclined) conical imaging radiometer builds on the heritage of the ENVISAT AATSR instrument. It provides accurate temperature measurements essential for climate monitoring and weather forecasting.
• Includes two thermal infrared channels for active fire detection and fire radiative power measurement.
• covering 9 spectral bands (550–12 000 nm), dual-view scan with swath widths of 1420 km (nadir) and 750 km (backwards), and a spatial resolution of 500 m for visible and near-infrared, and 1 km for thermal infrared channels.

Ocean and Land Colour Instrument (OLCI):

• Function: Captures detailed optical images to monitor ocean and land color, supporting studies of marine biology, water quality, and vegetation.
• Description: This push-broom imaging spectrometer is based on the heritage of the ENVISAT MERIS instrument. It collects data in multiple spectral bands, allowing for comprehensive analysis of oceanic and terrestrial ecosystems.
• Features 21 distinct bands in the spectral region of (400–1040 nm).
• Swath width of 1270 km, overlapping with SLSTR swath.
• covering 21 spectral bands (400–1020 nm) with a swath width of 1270 km and a spatial resolution of 300 m.

Microwave Radiometer (MWR):

• Function: Provides wet atmosphere correction to enhance the accuracy of the altimeter’s measurements.
• Description: This instrument measures the brightness temperature at specific microwave frequencies, helping to correct the signal delays caused by water vapor in the atmosphere. It features independent thermal control and cold redundancy for all subsystems to ensure reliability
• Operates in dual frequencies at 23.8 & 36.5 GHz
• Supports the SAR altimeter.

Synthetic Aperture Radar (SAR) Radar Altimeter (SRAL):

• Function: Measures sea surface height, wave height, and wind speed over the oceans. Provides accurate topography measurements over sea ice, ice sheets, rivers, and lakes.
• Description: SRAL operates in two frequency bands. This dual-frequency SAR altimeter builds on heritage from the ENVISAT RA-2, CryoSat SIRAL, and Jason-2/Poseidon-3 missions. It provides accurate and reliable altimetry measurements essential for ocean topography and sea state monitoring

Precise Orbit Determination (POD) Package:

• Components: Global Navigation Satellite Systems (GNSS) instrument., Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) instrument, Laser retro-reflector (LRR).
• Function: Ensures high-accuracy radial orbit data, which is crucial for precise altimetry measurements.
• Description: This package combines several technologies to provide precise orbital data, which is necessary for the accurate geolocation of the altimetry data.

OLCI module

The Ocean and Land Colour Instrument (OLCI) provides spectral information on the colour of the oceans. It is a radiometer, which measures light reflected by the Earths surfaces. This signal contains a rich wealth of information about the composition of ocean waters, the land surface, and atmosphere. EUMETSAT produces the marine products from the Sentinel-3 OLCI sensors. Ocean colour data provides a window in to the biological activity of the worlds oceans, as well as other oceanographic and coastal processes around sediments and anthropogenic impacts. Sentinel-3 OLCI data can also be used for atmospheric composition applications, including monitoring fires, aerosols, and dust. Current OLCI marine products contain some atmospheric composition related products, and further products are in development. Sentinel-3 OLCI land products are processed by the European Space Agency (ESA).

For example, this data can be used to monitor global ocean primary production by phytoplankton, the basis of nearly all life in our seas. Ocean colour data is also vital to understand climate change — ocean colour is one of the Essential Climate Variables listed by the World Meteorological Organization to detect biological activity in the ocean’s surface layer. Phytoplankton take up carbon dioxide (CO2) during photosynthesis, making them important carbon sinks.

• Ocean colour data can be used to monitor the annual global uptake of CO2 by phytoplankton on a global scale. Using this data we can study the wider Earth system, for instance the El Niño/La Niña phenomena and how these impacts the ocean ecosystem. Beyond climate, ocean colour data is also useful to look at more sporadic events.
• OLCI data can be used track sediment transport, monitor coastal water quality and track and forecast harmful algal blooms that are a danger to humans, marine/freshwater life and aquaculture.

The global picture of ocean ecosystems provided by ocean colour data can guide sustainable marine resource management. Further information on the sensor and its data can be found at http://olci.eumetsat.int and in the Sentinel-3 knowledge base.

Reference:

• https://arunp77.github.io/OLCI.html