As part of the Using Satellite Data for Analysis in Starboard series, this article explains how satellite collections are displayed in the Starboard platform, while introducing the key technical concepts of swaths, footprints, revisit rates, and orbital characteristics where relevant to maritime domain awareness (MDA).
How do I view satellite collection areas on the map?
Satellite data that has been ingested into Starboard can be toggled on or off using your Map layers.
Here, you can choose to view the footprint of any available acquisitions by selecting Collection areas on map in the Satellite data menu. Each individual satellite collection footprint will then appear as geometric shadows on the map, where relevant to your filtered Time and Area.
Select the Collection areas on map layer to view each available acquisition footprint as unique shadows on the map.
You can inspect individual acquisitions of interest by clicking the shadow on the map, or selecting it from the chronological list in the Added satellite data panel.
Learn more: See how this works in practice by visiting our Youtube Masterclass.
The following sections introduce the concept of satellite footprints within wider coverage considerations for each sensor type, and link to deeper technical articles where relevant.
What are satellite footprints?
Most Earth observation satellites circle our planet in Low-Earth Orbits (LEO) at altitudes between 300 to 1,000 km. One revolution of a satellite takes approximately 90 minutes and either follows a sun-synchronous or an inclined orbit. Satellites on sun-synchronous orbits are locked to Earth’s rotation and therefore pass overhead at the same local time of the day. Inclined orbits do not pass over the poles and concentrate coverage on mid to low latitudes, enabling more frequent revisit rates over those regions.
The ‘revisit rate’ is the time taken between satellites in a constellation passing over a single point on the Earth’s surface. In the case of Sentinel-2 (pictured above), the revisit rate is every 5 days.
A satellite’s orbit altitude and sensor aperture both influence the shape and size of its overall swath and constituent footprints.
Sentinel-1 swaths take the shape of long narrow ‘pushbroom’ strips along the Earth’s surface. The swath is processed as smaller image footprints.
A swath refers to the continuous strip of the Earth’s surface captured as a satellite orbits along a set path, while footprints represent the segregated images captured along that swath. The image footprint may be further divided into tiles to facilitate more efficient load times for the end-user. The frequency to which these images can be taken depend on the number of satellites in a provider’s constellation.
Larger footprint sizes are associated with lower-resolution EO and SAR imagery, usually from Government-funded programs which are collected on a pre-determined schedule.
Smaller footprint sizes are associated with higher-resolution imagery, typically requested on-demand through commercial providers.
Indicative footprints for different satellite types when compared to an average patrol aircraft range for the same area. This will vary depending on the individual assets and providers.
Some satellite providers have publicly-accessible acquisition plans which can be used to plot where the satellites will be imaging in the coming days or weeks.
Try it out: Download the latest acquisition plan KML from Sentinel-2, and drop the file onto the Starboard map to view the satellite’s planned footprints in the coming days.
What about Radio Frequency footprints?
In the case of RF, the footprint size is dictated by the different satellite technologies, scan angles, and processing methods used by individual providers, but the frequency of the target signal can also play a role.
RF collection footprints will vary in size and shape based on the provider and target signals.