New FIRMS Data Product: Satellite Swath Outlines

In remote sensing, a “swath” is the horizontal width of the Earth’s surface (perpendicular to the satellite’s orbit track) imaged by the sensor on a single overpass. In a previous EarthData blog, we reviewed the size of the respective swaths for each polar-orbiting satellite sensor used in NASA’s Fire Information for Resource Management System (FIRMS). The concept of a swath is crucial for users to understand the coverage, or “what is seen,” by a given satellite sensor during an overpass and to understand its relationship to temporal and spatial resolution.

FIRMS has long provided the daily ascending and descending orbit tracks of all polar-orbiting satellites used in the application. These data allow users to visualize the orbital path of a satellite over the ground during an overpass, as well as the time of observation.

FIRMS now provides the swath outlines, which show the extent of Earth’s surface within view of a sensor for each orbit. These data complement the satellite orbit track data, which is the centerline within each swath. The swath outlines are derived from the bounding coordinates in the metadata for Level-2 swath satellite products that are stored in the NASA Common Metadata Repository (CMR). These bounding coordinates are calculated and may not perfectly align in places. Consequently, users should not use them to precisely assess areas at the edge of swaths. 

How to Access Swath Outline Data in FIRMS

To access the swath outlines for polar-orbiting satellite sensors in the advanced mode of the FIRMS map viewer, find the Satellite Swath Outlines & Orbit Tracks group in the legend.

When you expand this group by clicking the “+” sign, you will see a separate layer for each satellite on its descending and ascending node, plus an option to display the orbit track and swath outline. If you toggle one of the layers to display it, by default it will display both the orbit tracks and the swath outlines. 

Figure 1 provides an example of the swath outlines for the ascending node for Aqua MODIS on May 31, 2026. The outlines represent the extent of the imagery collected by Aqua MODIS during daytime observations. As noted above, the orbit tracks form the centerline of the swath, while the time of overpass is depicted in one-minute intervals along the track.

The swath outlines are divided into segments or granules based on temporal extent. The swath granules for MODIS are 5 minutes in duration, while VIIRS granules are 6 minutes in duration. The coordinates for the swath outlines are extracted from the metadata for the near real-time (NRT) MODIS and VIIRS active fire detection data products, MODIS Thermal Anomalies/Fire 5-Min Level 2 Swath data, and the VIIRS Active Fires 6-Min Level 2 swath data. 

The user interface for the Satellite Swath Outlines & Orbit Tracks group can be toggled to allow the orbit tracks and swath outlines to be simultaneously displayed for multiple satellite sensors. It can also be narrowed down to display the orbit tracks or swath outlines of one satellite sensor (Figure 2).

Use Cases for Swath Outline Data

The inclusion of swath outline data in FIRMS supports multiple information needs and use cases for active fire detection, imagery, and related FIRMS data products. Selected examples include:

• Understanding relative swath widths for different polar-orbiting sensors used in FIRMS: Swath outlines provide an appreciation of the differences between sensors. For example, MODIS has a swath width of 2,300 km (Figure 1), while VIIRS has a swath width of 3,040 km and its swaths overlap (Figure 2). The swath outlines make it possible to visualize the relative coverage of these two instruments. Use the Measuring Tool in the FIRMS map viewer to verify the swath width for each.
• Identifying gaps and overlaps in daily observations by satellite sensors: Figure 3 displays the Aqua MODIS swath outlines for May 31, 2026, along with the collected True Color Composite corrected reflectance imagery. Note that a single MODIS sensor cannot quite image the entire globe on a given day. There are coverage gaps between approximately 30 degrees north and 30 degrees south in between consecutive Aqua MODIS orbits. However, VIIRS has a swath width of 3,040 km, so swaths from consecutive orbits overlap, as shown in the NOAA-20 VIIRS example in Figure 3. Consequently, any VIIRS sensor easily images all of Earth on a given day. 
• A given location towards the edge of a VIIRS swath can potentially be imaged twice within 100 minutes on consecutive orbits. And due to the size of their swath widths and convergence, the overlap between swaths by either MODIS or VIIRS increases with latitude, resulting in the opportunity to retrieve images for locations at high latitudes as many as six to eight times per sensor.

• Understanding when and if a particular area/location was or will be imaged: Swath outlines are coded with several key attributes to enable users to discern the date, start/end time, and the orbital node (day or night) of each swath granule. With the “allow multi layer selection” enabled on the Satellite Swath Outlines & Orbit Tracks legend group, users can display the swath outlines for multiple satellite sensors. Users can click on a location of interest to view when this area will be observed (or was observed, if conducting a retrospective examination) by each of these satellite sensors. (See Figure 4).

Coming soon: The user interface for the Satellite Swath Outlines & Orbit Tracks group will also include a satellite overpass prediction tool to enable users to see the anticipated overpass times for the next 31 days.