11.1 How Do Remote Sensing Satellites Collect Data?

Remote sensing satellites are placed in a specific orbital path around Earth. One type of orbit used in remote sensing is geostationary orbit, in which satellites rotate at the same speed as Earth. Since a geostationary satellite takes 24 hours to make one orbit, it’s always in the same place at the same time. For instance, although it’s not a remote sensing system, satellite television utilizes a geostationary orbit—the satellite handling the broadcasting is in orbit over the same area continuously, which is why a satellite reception dish needs to be always pointed at the same part of the sky. Likewise, the WAAS and EGNOS satellites we discussed in Chapter 4 are in geostationary orbit so that they can provide constant coverage to the same area on Earth. There are some remote sensing satellites that use geostationary orbit to always collect information about the same area on Earth’s surface (such as the GOES series of satellites used for obtaining weather imagery).

Many remote sensing satellites operate in near-polar orbit, a north-to-south path wherein the satellite moves close to the North and South poles while it makes several passes a day about Earth. For instance, the Landsat 7 satellite (a U.S. satellite that we’ll discuss in more detail in a few pages) completes a little more than 14 orbits per day (each orbit takes about 99 minutes)—see Figure 11.2 for Landsat 7’s near-polar orbit. Also, check out Hands-on Application 11.1: Examining Satellite Orbits in Real Time (page 358) for two methods of tracking satellite orbits in real time.

While a satellite in continuous orbit passes over Earth, it will be imaging the ground below it. However, during an orbit, a satellite can only image a certain area of ground at one time—it can’t see everything at once. The swath width is the measurement of the width of ground the satellite can image during one pass. A Landsat satellite’s sensor has a swath width of 185 kilometers, meaning that a 185-kilometer-wide area on the ground is imaged as Landsat flies overhead. As satellites orbit, Earth is rotating beneath them, meaning that several days may elapse before an orbital path (and thus, the swath of ground being imaged) takes a satellite over nearby geographic areas. For example, a Landsat satellite may pass over the middle of the United States on Day 1 of its orbit, pass hundreds of miles to the east on Day 2, and not return to the path next to the first one until Day 8 (Figure 11.3).

After a certain amount of time, the satellite will completely pass over all areas on Earth and again resume its initial path. For instance, each one of the Landsat satellites is set up in an orbital path so that it will pass over the same swath on Earth’s surface every 16 days. Thus, when Landsat’s orbit carries it to the swath where it’s collecting imagery of your current location, it will be another 16 days before it’s able to image your location again. A sun-synchronous orbit occurs when a satellite’s orbit always takes it over the same swath of Earth’s surface at the same local time. As a result, images collected from the satellite will have similar sun-lighting conditions and can be used for comparing changes to an area over time. For instance, Landsat 7 has a Sun-synchronous orbit, allowing it to cross the Equator at the same time every day.

The sensors onboard the satellites have different ways of actually scanning the ground and collecting information in the swath being imaged. In along-track scanning, a linear array scans the ground along the satellite’s orbital path. As the satellite moves, the detectors in the array collect the information from the entire swath width below them. This type of sensor is referred to as a “pushbroom.” Think of pushing a broom in a straight line across a floor—in this case, the broom itself is the sensor, and each bristle of the broom represents a part of the array that senses what is on the ground below it. A second method is an across-track scanner; here, a rotating mirror moves back and forth over the swath width of the ground to collect information. This type of sensor is referred to as a “whiskbroom.” Once again, think of sweeping a floor with a broom, swinging the broom back and forth across a path on the floor—the broom is the sensor, and the bristles represent the detectors that collect the data on the ground below them. Satellite systems usually use one of these types of sensors to obtain the energy reflectance measurements from the ground (see Figure 11.4 for examples of the operation of both sensor types).

After a satellite’s sensors have collected data, the data need to be off-loaded from the satellite and sent to Earth. Because data collection is continuous, this information needs to get to people on the surface so that it can be processed and utilized as it becomes available. The Earth Resources Observation Science (EROS) Center located outside Sioux Falls, South Dakota, is one of the downlink stations in the United States that receives data from numerous remote sensing satellites. Satellite data can be directly sent to a receiver at a station or transmitted to other tracking and data relay satellites, which then send the data to Earth. Once received on the ground, the data can be processed into imagery.