Pictures of the earth’s surface are captured from satellites orbiting the earth and are a vital tool for mapping and monitoring fires. The image below is from the MODIS satellite showing smoke coming from many fires in the Top End of the NT in July 2015.
Earth Resource satellites like MODIS are specifically designed to measure aspects of the earth’s surface using on-board ‘sensors’ (cameras). These satellites orbit/circle the Earth in a so they pass over the same spot on the earth’s surface at regular intervals.
There are many Earth resource satellites, each with different sensor and orbit characteristics. It is important when using information derived from these satellites to understand the type of information each provides and how it can be used. The satellites cannot always ‘see’ fires, so understanding their limitations is key to their effective use.
Earth Observing Fleet (December 2021): This animation shows the orbits of NASA’s fleet of Earth observing spacecraft that are considered operational as of December 2021. Visualization Credits: Greg Shirah (NASA/GSFC): Lead Visualizer, Ernie Wright (USRA): Visualizer and Kel Elkins (USRA): Visualizer. Source: NASA’s Scientific Visualization Studio.
The following explanation will focus on the three satellites most commonly used in fire mapping and monitoring across northern Australia. They are the Landsat, Sentinel and MODIS earth resource satellites.
Key terms in satellite data.
Key to understanding satellite data are their spatial and temporal characteristics.
Spatial (space) in the context of satellite data refers to the size of the image.
This is described in terms of resolution and extent.
Spatial resolution refers to the on-ground size of a pixel in a satellite image.
Spatial extent refers to the on-ground width of a satellite image overpass.
Temporal (time) resolution refers to the time it takes for a satellite to revisit and capture imagery at the same location. Sometimes referred to as temporal frequency or revisit frequency.
The graphic to the right shows the on-ground resolution or pixel size of three earth resource satellites (1) MODIS with each pixel 250×250 metres, Landsat 28×28 metres and the Sentinel 2 satellite with a 10 x 10 metre resolution.
The on-ground resolution has significant implications for a satellite’s ability to accurately map on-ground fire activity.
The graphic below shows an aerial incendiary run near Nitmiluk National Park from the 10th May 2015 using higher resolution (28 m) resolution Landsat imagery (left) and lower resolution (250 m) MODIS imagery (right).
The incendiary line appears very clearly in red in the Landsat image. Importantly, in the Landsat image it is clear where holes/breaks in the burn line occur that fires could creep through later in the year. None of this is clear on the 250m resolution MODIS imagery.
While it is clear from the example above that Landsat imagery provides the most detail for mapping burnt areas from satellite; there are two primary limiting factors for using that imagery for daily fire monitoring. First is the relatively small spatial extent of each Landsat image/scene. Second is it’s low temporal resolution (16 days).
Every Landsat orbit (overpass) captures imagery along a 185km-wide path. This means that to capture satellite pictures for all of north Australia, many orbits and hundreds of images would be required.
Each square in the image to the right is a single Landsat image/scene.
In contrast; a MODIS satellite overpass captures imagery over a 2,330 km-wide path. This means that MODIS can map all of northern Australia in three overpasses or images.
Temporal Resolution (Revisit frequency)
The other key factor is revisit frequency, or the amount of time between when a satellite captures an image. With MODIS, because the spatial extent is greater, it requires fewer orbits of the earth, thus it’s overpass return frequency (temporal resolution) is higher. This higher temporal resolution means that the MODIS satellite will pass over the same place every day whilst Landsat takes 16 days before revisiting the same location.
Furthermore, the MODIS sensor (camera) is actually on board two satellite platforms, one named Aqua and one named Terra. This results in a MODIS sensor imaging the same place on the earth’s surface up to twice a day. This very high temporal resolution is critical for mapping, monitoring active fires and responding quickly to emergency situations.
Aqua Satellite and MODIS Swath: The orbit of of the Aqua Satellite from August 27-31, 2005, showing the orbit track relative to the ground observation track in the Earth’s frame of reference. Visualisation Credits: Horace Mitchell (NASA/GSFC): Lead Animator, Norman Kuring (NASA/GSFC): Scientist Gene Feldman (NASA/GSFC): Scientist. Source: NASA/Goddard Space Flight Center Scientific Visualization Studio
Due to these attributes MODIS satellites are currently the basis for most global and continental scale fire mapping. As well as providing the data for mapping burnt areas. They also provide hotspot data depicting active fires.
Monthly Active Fires: Monthly active fires detected by MODIS, March 2000 to 2013. Based on maps were created by Reto Stockli, NASA’s Earth Observatory Team, using data courtesy the MODIS Land Science Team at NASA Goddard Space Flight Center. Credits: Marit Jentoft-Nilsen: Visualizer and Reto Stockli (NASA/GSFC): Visualizer. Source: NASA Goddard Space Flight Center.
In addition to MODIS, there are a number of other satellite sensors that contribute to collecting hotspot data. This reduces the amount of time for the collection of hotspot data down to a few hours. However, the accuracy of hotspot locations varies between satellites.
The table below summaries the major characteristics of the main earth resource satellites used in fire management.
|Resolution: Burn area mapping
|2.5 – 15m
The National Aeronautics and Space Administration’s (NASA) video below explains how Landsat orbits the earth.
Source: NASA | Landsat’s Orbit