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Satellite comparison update

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When writing about satellite images here i concentrate on open data imagery but i take the launch of a new commercial earth observation satellite by DigitalGlobe earlier today as opportunity to update my previously shown satellite comparison chart.

In addition to WorldView-4 i added the Terra Bella SkySat constellation. These satellites feature an interesting sensor concept and are meant to offer the currently unique ability to record video. There is however very little publicly known on operational plans for this system.

Also added is a column with the daily recording volume in square kilometers for the different systems. The most frequently advertised capability of satellites in addition to the spatial resolution is the revisit frequency which indicates how often a satellite can record images for any single point on the earth surface. A revisit interval of one day however does not mean the satellite system can record daily coverage of the whole earth surface. The recording volume describes what area can actually be covered on a daily basis. There are two variants: the potential recording volume (indicated in red) which is often a more or less theoretical value and the practical recording volume that is recorded in actual operations (in blue). In case of the commercial satellites both is based on claims by the operators obviously.

For the high resolution Open Data systems the numbers are determined based on average actual recordings. For Sentinel-2 this is a bit difficult so i based it on the specification of an average 14 minutes recording per orbit as specified in recent mission status reports. For the low resolution continuously recording systems numbers are – for compatibility – based on half orbit recordings although the longwave bands are of course recorded on the night side as well.

Generally marketing departments of satellite operators often seem to have a lot of fun twisting such numbers to make their systems look better. One thing to keep in mind here as reference: The earth land surface is about 150 million square kilometers. Landsat 8 records this almost fully (except for areas beyond 82.66 degrees latitude) every 16 days – but just barely. To do this it records approximately 25 million square kilometers every day or 400 million square kilometers in the 16 days global coverage interval. Now there are a couple of night side scenes included in that as well as quite a bit of ocean surface due to the width of the recording swath and the constraint of recording fixed WRS2 tiles. But it still illustrates that you need to record much more than just the 150 million square kilometers to actually cover the land surfaces in full, primarily due to the inevitable overlaps due to the orbit and recording footprint geometries.

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