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After you have learned about the meaning of spatial resolution and pixel size, this topic is dedicated to the temporal domain of remote sensing data acquisition. The temporal resolution is an instance which can make up for a lower spatial resolution, since the temporal signature a time series can provide has a lot to offer. Let’s find out why.

What is temporal resolution?

The temporal resolution describes the time interval between two overpasses of a given point. Often it is also referred to as the ‘revisit time’ or ‘repitition rate’. The duration of this time span can vary between multiple acquisition per hour (EUMETSAT) up to single acquisition every two (Landsat) or even four weeks (SPOT). A major difference to the spatial domain is that the temporal resolution is not depending and thus not directly related to the sensor but to the satellite platform, the sensor is mounted on.

Temporal dynamics

In the image on the lower left, you can see the significant changes occuring in Ouargla, Algeria. Water levels change drastically as evaporation leads to a water loss at the ground. While rains bring the water back to this oasis during the inter-annual wet seasons, water availability is a crucial parameters for surviving in this area. Satellites with high temporal resolution (as Sentinel-2 in this case) can help to prevent locals from arising water shortages. Near the equator, Sentinel-2 can reach rivisit rates of up to 5 days, allowing the observation of changes happening rapidly. Other satellites with lower temporal resolution (optical or SAR) are therefore e.g. less suited for disaster monitoring.

Another great application of high resolution (spatial and temporal) Sentinel-2 data can be seen in the lower right. Pivot irrigation systems are a very effective way to cultivate areas that are naturally not suited for intensive agriculture. The pivots have a strong intra- and interannual range in vegetation cover. Located in the Saudi Arabian desert these installations can be seen from space!

Weekly Sentinel-2 colour-infrared (RGB) composite for the year 2020 of the city of Ouargla located in Algeria.
Weekly Sentinel-2 colour-infrared (NDVI) composite for the year 2020 of pivot irrigation in Saudi Arabia.
Temporal signature of a pivot irrigation field in Saudi Arabia (graph created in the Sentinel Hub EO Browser.

How do satellite constellations impact the revisit rate?

The temporal resolution of satellites depends on a number of factors. Orbiting satellites (travel time Earth rotation) take more time to revisit the same area than geostationary satellites (travel time = Earth rotation). Most Earth observation satellites are following quasi-polar orbits. Higher temporal resolution can be reached with geostationary satellites (travel time equals the Earth’s rotation). One example for geostationary acquisition are weather satellites such as EUMETSAT and GOES. With higher flight altitudes, satellites can travel more quickly around the Earth. While this is related to less spatial resolution, the temporal resolution is increased. Newer generations of satellites such as the Sentinel fleet have started to operate in chain constellations, increasing the temporal resolution by utilizing more satellites of the identical design. Exemplary, the revisit rate of Sentinel-1A was increased by launching Sentinel-1B in April 2016 from 12 days to 6 days at the equator. This trend is currently drastically increasing the number of satellites in space.

More than 100 of the ‘dove’ satellites operated by Planet Labs Inc. travel through space to complete the dense constallation with very high temporal and spatial resolution (commercial)

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