1.1 Una institución en construcción
1.1.6 El modelo de la universidad inglesa
For time dimension, the range is the duration over which observations are made, during a single mission or series of missions and resolution is the temporal sampling rate. For LEO missions, this is defined by the revisit period for a single satellite or constellation of satellites and swath width of an imager. For LEO polar orbiters sun synchronicity is possible: passing over the same area at the same time during the day. Other LEO orbits (such as the International Space Station) pass over the equator
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multiple times day in an inclined orbit that reaches about 53° North and South maximally-this orbit is not sun-synchronous. For GEO orbits with the sensor stationary above fixed location on the equator, the temporal sampling is determined by the image collection rate. In either case, the cloud cover frequency and duration set a lower limit for effective sampling. Temporal sampling duration and resolution are important in determining whether a sensor will observe seasonal, inter-annual, or inter-decadal variation (i.e., tending toward studies of responses to climate change and increasing anthropogenic pressures or providing the ability to distinguish between periodic and secular trends). Episodic changes and diurnal cycles in the environment, including variation in light, temperature, atmospheric aerosol and cloud formation, absorbing trace gas fluxes, surface wind, mixing depth, current, and precipitation and watershed hydrology could influence observations of coral reef, coastal and inland aquatic environments. Coastal tidal waters and wetlands can be subject to a variation with tides, which can periodically change the flux of optically active constituents in the water column. LEO, sun-synchronous polar orbits used for global mapping of environments would sample these processes at the same time of day, only providing one point in diurnal cycles and aliasing the tidal signal. Either long-term time series are needs to wash out these effects or an ~hourly observation strategy must be put into effect. Some regions near coasts will form clouds during the same time of day (e.g., tropics), seasonally, or over most of the year (see Figure 2.10), obscuring observation of all or the same interval of diurnal responses in coastal and inland aquatic environments. The use of models and auxiliary observations from SAR imagers (e.g., Sentinel-1) or surface measurements could help fill in the gaps caused by a persistently or periodically opaque atmospheric in the visible region of the electromagnetic spectrum but only for vegetation or related material that is at the surface or above the surface.
Monitoring phytoplankton blooms or the effects of episodic river discharges requires temporal sampling of a few days or less. Conversely, long-term secular changes in static characteristics of habitats formed by foundational species (e.g., wetland and submerged aquatic vegetation and corals) could be satisfied with less frequent observations, provided tidal, periodic or episodic changes are considered. Temporal sampling of these changes can be as coarse as monthly or seasonal, provide they are observed under consistent conditions. However, changes in phenology would require several observations over short periods of time during the key phases of the growth cycle. Elmendorf et al., (2016) proposed that graminoïd (marshes) and trees (swamps and
mangroves) should be sampled as frequently as once every couple of days during key stages during the growth cycle as part of the design work for observing phenology for the National Ecological Observatory Network (NEON). Thus, observing change in phenological patterns in more stationary communities require higher temporal observations than observing changes in distribution and extent.
Preferences about the temporal frequency of information delivery can change with the aquatic ecosystem parameter considered and its periodic cycle if present (e.g. recurrences of algae blooms with seasonal conditions or growing circles of macrophytes). Conversely, preferences can be imposed by the possible presence of unknown and unpredictable events during periods with particular exploitation pressure of the water resource (e.g. bathing season, water capitation during the drought season, coral bleaching events).
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Figure 2.10 Global annual mean cloud cover derived from three years (2007–09) of Envisat data. The map shows areas with little to no cloud coverage (blue) as well as areas that are almost always cloudy (red). West coasts of most landmasses and open water regions at high latitudes are prone to cloudiness around the year. Data from both the MERIS and AATSR instruments on Envisat were used. Credits: ESA/Cloud–CCIs http://www.esa.int/Our_Activities/Observing_the_Earth/Space_for_our_climate/Highlights/Cloud_cover (on 6 November 2016).
Figure 2.11 Bar charts showing the temporal frequencies preferred by end-users subdivided in groups (left: not working or working on streams and rivers, right: working in Universities/Research Institutions or Local Agencies (Source: C. Giardino, CNR, Italy, from INFORM Project, www.copernicus-inform.eu).
For the temporal frequencies the inland water end-users answers have been analysed dividing them in different representative groups (Figure 2.11). Comparing the two categories of products it can be seen that for the EO derived products there is a general tendency in preferring the monthly
temporal frequency, especially end-users working also on streams and rivers. End-use that requires at least one product per month, will need much more frequent coverage taking into account the risk of sun glint (avoidable by proper sensor and platform geometry design), wave foam, clouds, fog, smoke and haze etc.
0% 20% 40% 60% 80%
Daily Weekly Monthly
EO Products NO STREAMS YES STREAMS 0% 20% 40% 60%
Daily Weekly Monthly
EO Products UNIVERSITIES LOCAL AGENCIES
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With respect to the time of the day a single sun synchronous acquisitions might be insufficient to capture phenomena with a high degree of change, such as re-suspension of suspended matter due to tides or storms or cyanobacterial blooming. Geostationary platforms with suitable earth observing sensors may provide relevant high frequency data, although spatial resolution will be lower
indicating that GEO sensors would be used mainly for medium to large aquatic ecosystem areas. Due to this large variation in end-user requirements and the priority for spatial resolution between ~17 to ~33 m with fine spectral bands with sufficient radiometric resolution the temporal
requirement is as high as is technologically possible and affordable. An absolute minimum would be Landsat frequency as that would deliver seasonal data, whilst the dual sensor Sentinel-2 frequency is preferable (once every five days). But essentially the revisit should be as high as possible (baseline one overpass per day and threshold one overpass once every 3 to 5 days) as especially the water quality characteristics change rapidly. However, larger water bodies (in excess of 1 by 1 km) can be imaged more regularly by the ocean colour sensors such as Sentinel-3. For substratum
measurements and bathymetry these requirements are more relaxed as only one cloud free image per season is required, unless there is an extreme event such as coral bleaching, cyclone/hurricane, tsunami or man-induced damage such as dredging or pollution events that need to be monitored. A mix of LEO sensors in polar orbit, complemented by sensors on board of e.g. the international space station (or LEO sensors) covering the mid and lower latitudes complemented by highest spatial resolution possible geostationary sensors would be the optimum configuration.
Temporal requirements
The temporal requirement is as high as is technologically possible and affordable. An absolute minimum would be Landsat frequency whilst once every one to five days is much better. But essentially the revisit should be as high as possible as especially the water quality characteristics change rapidly. For substratum measurements and bathymetry these requirements are more
relaxed as only one cloud free image per season is required, unless there is an extreme event such as coral bleaching, cyclone/hurricane, tsunami or man-induced damage such as dredging or pollution events that need to be monitored.