Categories: GIS Data

Satellite Mapping of Oceans and Coral Reefs

In 2018, NASA launched the ICESat-2 satellite (which stands for Ice, Cloud, and land Elevation Satellite) to monitor levels of ice. However, soon after the satellite’s launch, it became evident this satellite can also measure coral reef systems, and specifically oceanic depths that were deeper than expected, providing an important monitor for these fragile ecosystems.

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The ICESat-2 has a laser altimeter which is used as a signal to measure the height of ice, which scientists are using to monitor climate change of polar regions and glaciers. However, while testing and monitoring data from this satellite as it collected data near Bikini Atol, site of earlier nuclear tests, scientists realized the lasers on board also generate underwater reflections that were captured by the satellite system. These underlying reflections showed coral regions along coastal zones. While the deep ocean is still largely unmapped, many regions within 5 meters depth are also not well known, as these depths are too shallow for vessels to penetrate. Having this laser system now allows oceanic zones in these depths to be bathymetricly mapped, giving a clear picture of shallow ocean (epipelagic zone) ecosystems.

ICESat-2 uses six beams on three tracks to measure the height of ice. Image: NASA.

Using ICESat-2 to Monitor Ocean Health

The laser systems scans on three tracks and each part of the scan covers every spot on Earth four times a year, giving seasonal coverage and monitoring. The laster fires 10,000 times per second and has 60 reflected photons that can be detected by the satellite’s telescope. Initially, the plan was for ICESat-2 to have greater laser ocean penetration than ICESat, which was the earlier satellite it replaces, although the depths were intended to be very shallow (within 2 meters depth). However, it was realized the depths are closer to 5 meters or more, making it now applicable for oceanographers and other scientists monitoring near-shore ocean health including coral life. Data such as a coral reef’s depth and slope, along with texture data that reveals the quality and health of corals, can be used for monitoring and evaluating oceanic change. One recent benefit, due to the coronavirus outbreak causing major shutdowns in Venice and other well-known harbors, is measuring the depths of these well-known areas that would typically be too busy and full of unsettled sediment and debris to allow easy measurements. In addition to benefits to scientists monitoring the coral health of near-shore environments, others are beginning to see the benefits of the satellite’s use. For instance, both the US Coast Guard and National Geospatial-Intelligence Agency are looking to use the data from ocean depth measurements to begin to create better maps to use for their personnel.[1]

To validate what scientists had noticed with the potential for ICESat-2’s measurements for oceanic depth, its data have been compared to other data collected from different sensors and sources. Overall, it is evident that error measurements for depth have a range between 0.43—0.60 m root mean square error (RMSE) over 1 m grid resolution. This levels of accuracy is more than sufficient for most applications. Additionally, in recent research, it was also shown that the depth in which ICESat-2’s Advanced Topographic Laser Altimeter System (ATLASA) can reach is potentially up to 38 m, making it even more useful to measure not only coral systems but other oceanic life and habitats. This could now mean deeper parts of the ocean may also be mapped soon.[2]

Geolocated bottom return photons for the Great Bahama Bank site. This site contains 316 km of continuous bathymetry from ICEsat-2 ATLAS. Source: Parrish et al., 2019.

Using ICESat-2 to Measure Forest Canopy Height

There are other benefits to ICESat-2, including its ability to measure the heights of forest canopies and be used to estimate biomass in regions, that scientists are also applying the literally trillions of data points the satellite sends over any given region. In fact, monitoring changing forest structure is another benefit, particularly in regions vulnerable to land use change or deforestation. However, for such areas, numerous monitors are already available, although this satellite does provide more precise and frequent data gathering than many older systems. The ability for the lasers to penetrate deeper into the ocean makes ICESat-2 unique among all satellite systems, giving it a unique utility for oceanic mapping that was typically done by costly expeditions and addressing at least some long-standing goals for scientists to better map and monitor near-shore systems.[3]

Overall, ICESat-2, as the name implies, was intended to give scientists another tool to monitor critical changes to ice globally. While this is still the case, accidental discoveries now mean the application of ICESat-2 helps to address other critical knowledge gaps in our global monitoring efforts, mainly in the oceans where data are still relatively limited. Most of the oceans still cannot be mapped but now we can at least begin to map areas near shores and monitor the health of corals. This is also critical for climate change monitoring since corals are sensitive to ocean temperature change.

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References

[1]    For more on the benefits of ICESat-2 and how these previously unknown benefits were determined, see: https://www.sciencemag.org/news/2020/04/ice-tracking-space-laser-could-also-map-sea-floor-and-monitor-health-coral-reefs


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[2]    For more on test on the quality of data captured by ICESat-2, see:  Parrish, C.E., Magruder, L.A., Neuenschwander, A.L., Forfinski-Sarkozi, N., Alonzo, M., Jasinski, M., 2019. Validation of ICESat-2 ATLAS Bathymetry and Analysis of ATLAS’s Bathymetric Mapping Performance. Remote Sensing 11, 1634. https://doi.org/10.3390/rs11141634

[3]    For more on the uses of ICESat-2 to monitor forest growth, see:  Narine, L.L., Popescu, S., Neuenschwander, A., Zhou, T., Srinivasan, S., Harbeck, K., 2019. Estimating aboveground biomass and forest canopy cover with simulated ICESat-2 data. Remote Sensing of Environment 224, 1–11. https://doi.org/10.1016/j.rse.2019.01.037

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