Remote sensing (oceanography)

Remote sensing in oceanography mostly refers to measuring properties of the ocean surface with sensors on satellites or planes, which compose an image of captured electromagnetic radiation.

All remote sensing instruments carry a sensor to capture the intensity of the radiation at specific wavelength windows, to retrieve a spectral signature for every location.

Capturing the spatial variation of the ocean with remote sensing is considered extremely valuable and is on the frontier of oceanographic research.

Remote sensing enables temporal analysis over vast spatial scale, since satellites have a constant revisit time, provide a wide image and are often operational for multiple consecutive years.

This concept of constant data in time and space was a breakthrough in oceanography, which previously relied on measurements from drifters, coastal locations like tide gauges, ships and buoys.

All in-situ measurements either have a small spatial footprint or are varying in location and time, so do not deliver constant and comparable data.

[10] Landsat 1 delivered the first multi-spectral images of features on land and coastal zones all over the world and already showed effectiveness in oceanography,[11] although not specifically designed for it.

In 1978 NASA makes the next step in remote sensing for oceanography with the launch of the first orbiting satellite dedicated to ocean research,[12] Seasat.

Seasat was only operational for a few months but, together with the Coastal Zone Color Scanner (CZCS) on Nimbus-7, proved the feasibility of many techniques and instruments in ocean remote sensing.

The Advanced Very-High-Resolution Radiometer (AVHRR) Is the sensor carried on al NOAA missions and made SST retrieval accessible with a continuous time-series since 1979.

Sentinel-3 is now one of the best equipped missions to map the ocean hosting a SAR altimiter, multispectral spectrometer a radiometer and several other instruments on multiple satellites with alternating orbits providing exceptional temporal and spatial resolution.

Sensors on remote sensing instruments capture radiation, which can be translated back to deduce the physio-chemical properties of the surface.

Complete derived products from remote sensing often use simple calculations or algorithms to transform the spectral signature from a cell to a physical value.

In land remote sensing change detection is used for example: to assess the impact of a volcano eruption,[13] check the growth of plants through time,[14] map deforestation,[15] and measure ice sheet melt.

Another way to infer change from only 1 acquisition is by computing the dynamical component and direction from a static image which is leveraged in RADAR altimetry to deduce surface current velocity.

The radiation coming from the earth's surface with a wavelength within the atmospheric window can be captured by a passive radiometry sensor at satellite height.

Radiometry captures the surface skin temperature (~10 micron depth) of the ocean, which significantly differs from bulk SST in-situ measurements.

The SST is a clear climatological indicator linking to the ENSO cycles, weather and climate change but can also highlight movement of ocean water.

SST anomalies can highlight mesoscale eddies, ocean fronts and regions of upwelling, vertical mixing or river outflow as the water is locally more cold or warm due to transport.

Algal blooms are often caused by a local enrichment of the water system with nutrients, which temporarily remove the limiting growth factor of photosynthetic organisms like cyanobacteria.

Due to oxygen depletion, blocking sunlight and the release of possible toxins algal blooms can be harmful to their environment.

Algal blooms are used to study internal wave structures, up-welling and river outflows,[21] which all bring nutrients to surface waters, since they are correlated with algae concentration .

Pollution often coincides with high nutrient waters, making algal blooms good indicators for the severity and impact of water pollution[22] RADAR altimeters send microwave pulses to the surface and catch the reflection intensity over a short time period measuring the two-way travel time of the signal.

If assuming geostrophic balance, the velocity anomaly and direction of surface currents perpendicular to the satellite overpass can be computed using the formula:

Often the spatial resolution of RADAR altimeters is not too high but their temporal coverage is tremendous, allowing constant monitoring of the ocean surface.

The color of open ocean basins is mostly controlled by phytoplankton and travel predictably or covary with other constituents in the water column like chlorophyll a.

Finally, removing the effects of the atmosphere is difficult to achieve because of the complex and dynamic mix of coastal aerosols and sea spray.

While methods of interpolation and modelling can be developed to a high degree of statistical accuracy, they are in their essence a educated guess based on surrounding conditions.

[25] Modern technology has provided UAV users with numerous platforms able to be outfitted with commercial or custom made sensor packages.

Satellites and crewed aircraft require shore-based facilities or ships capable of supporting take-off and landing operations.