Satellite Imagery: Passive Sensors (MODIS, Landsat, Sentinel-2)

Several public agencies collect daily/weekly passive sensor (sunlight, including visible-spectrum) imagery that can be very helpful for trip planning. These resources are particularly valuable in the fall and spring, when trying to get a sense for snow cover, frozen lakes, etc.

I use three satellite sources: MODIS, Landsat, and Sentinel-2. Imagery is limited by daylight (a problem during Alaska’s winters) and cloud cover. For those conditions, I’ve been exploring the application of Sentinel-1, which is radar rather than spectral imagery. See Satellite Imagery: Synthetic Aperture Radar.

MODIS

MODIS Aqua and Terra satellites collect imagery daily, but the resolution (250m) is not great. I use this imagery to get a quick sense for conditions.

NASA Worldview is my go-to. The site is well-designed with a powerful time-slider to compare seasonal images. For Alaska, you will need to change to polar projection and rotate to north.

Landsat 8

Landsat has less frequent imagery (8 day revisit interval), but with higher resolution (30m) than MODIS.

Sentinel Hub Playground provides an elegant interface to view Landsat imagery, as well as MODIS and Sentinel. Unfortunately, the MODIS resolution is not as high as provided by NASA Worldview, and Sentinel not as high as via EO Browser (see below).

Select Dataset

Click the calendar icon to view available acquisition dates (takes some time to render)

Choose cloud cover

If you choose a low cloud cover percentage, an older image will load, regardless of the calendar date selected. For this reason I prefer to view the acquisition date on the image. Select Show acquisition dates from the Effects tab

Sentinel-2

Sentinel-2 imagery is available several times per week (5 day revisit time), and provides the highest (free) resolution (10m). There are two versions of Sentinel-2 imagery, L2A and L1C. L2A is a bottom of atmosphere reflectance, larger filesize, and generally (always?) crisper.

The easiest source for Sentinel-2 imagery is Sentinel Hub Playground, see the Landsat section above. You can also explore Sentinel-2 via the Sentinel Hub EO Browser. EO Browser is a more complex site than the Playground, but the payoff can be worth it depending how deep you want to dig.

From the Search panel, select Sentinel-2, L2A

From the Results panel, either select Visualize from the tabular results, or click on a blue ‘coverage’ region.

Note the cloud cover percentage in the results… you might want to scroll back to a date with less cloud cover.

Proof of Concept

These tools, particularly MODIS and Sentinel-2, were critical in pulling off a remote ice skating trip to Katmai National Park and Preserve.