We learned in the previous section that remote sensing instruments can be placed on a variety of platforms to view and image targets. Although ground-based and aircraft platforms may be used, satellites provide a great deal of the remote sensing imagery commonly used today. Satellites have several unique characteristics which make them particularly useful for remote sensing of the Earth's surface.
The path followed by a satellite is referred to as its orbit. Satellite orbits are matched to the capability and objective of the sensor(s) they carry. Orbit selection can vary in terms of altitude (their height above the Earth's surface) and their orientation and rotation relative to the Earth. Satellites at very high altitudes, which view the same portion of the Earth's surface at all times have geostationary orbits. These geostationary satellites, at altitudes of approximately 36,000 kilometres, revolve at speeds which match the rotation of the Earth so they seem stationary, relative to the Earth's surface. This allows the satellites to observe and collect information continuously over specific areas. Weather and communications satellites commonly have these types of orbits. Due to their high altitude, some geostationary weather satellites can monitor weather and cloud patterns covering an entire hemisphere of the Earth.
Many remote sensing platforms are designed to follow an orbit (basically north-south) which, in conjunction with the Earth's rotation (west-east), allows them to cover most of the Earth's surface over a certain period of time. These are near-polar orbits, so named for the inclination of the orbit relative to a line running between the North and South poles. Many of these satellite orbits are also sun-synchronous such that they cover each area of the world at a constant local time of day called local sun time. At any given latitude, the position of the sun in the sky as the satellite passes overhead will be the same within the same season. This ensures consistent illumination conditions when acquiring images in a specific season over successive years, or over a particular area over a series of days. This is an important factor for monitoring changes between images or for mosaicking adjacent images together, as they do not have to be corrected for different illumination conditions.
Most of the remote sensing satellite platforms today are in near-polar orbits, which means that the satellite travels northwards on one side of the Earth and then toward the southern pole on the second half of its orbit. These are called ascending and descending passes, respectively. If the orbit is also sun-synchronous, the ascending pass is most likely on the shadowed side of the Earth while the descending pass is on the sunlit side. Sensors recording reflected solar energy only image the surface on a descending pass, when solar illumination is available. Active sensors which provide their own illumination or passive sensors that record emitted (e.g. thermal) radiation can also image the surface on ascending passes.
As a satellite revolves around the Earth, the sensor "sees" a certain portion of the Earth's surface. The area imaged on the surface, is referred to as the swath. Imaging swaths for spaceborne sensors generally vary between tens and hundreds of kilometres wide. As the satellite orbits the Earth from pole to pole, its east-west position wouldn't change if the Earth didn't rotate. However, as seen from the Earth, it seems that the satellite is shifting westward because the Earth is rotating (from west to east) beneath it. This apparent movement allows the satellite swath to cover a new area with each consecutive pass. The satellite's orbit and the rotation of the Earth work together to allow complete coverage of the Earth's surface, after it has completed one complete cycle of orbits.
If we start with any randomly selected pass in a satellite's orbit, an orbit cycle will be completed when the satellite retraces its path, passing over the same point on the Earth's surface directly below the satellite (called the nadir point) for a second time. The exact length of time of the orbital cycle will vary with each satellite. The interval of time required for the satellite to complete its orbit cycle is not the same as the "revisit period". Using steerable sensors, an satellite-borne instrument can view an area (off-nadir) before and after the orbit passes over a target, thus making the 'revisit' time less than the orbit cycle time. The revisit period is an important consideration for a number of monitoring applications, especially when frequent imaging is required (for example, to monitor the spread of an oil spill, or the extent of flooding). In near-polar orbits, areas at high latitudes will be imaged more frequently than the equatorial zone due to the increasing overlap in adjacent swaths as the orbit paths come closer together near the poles.
Did you know?
"...the forecast calls for scattered clouds with the possibility of rain..."
...most of the images you see on television weather forecasts are from geostationary satellites. This is because they provide broad coverage of the weather and cloud patterns on continental scales. Meteorologists (weather forecasters) use these images to help them determine in which direction the weather patterns are likely to go. The high repeat coverage capability of satellites with geostationary orbits allows them to collect several images daily to allow these patterns to be closely monitored.
...satellites occasionally require their orbits to be corrected. Because of atmospheric drag and other forces that occur when a satellite is in orbit, they may deviate from their initial orbital path. In order to maintain the planned orbit, a control center on the ground will issue commands to the satellite to place it back in the proper orbit. Most satellites and their sensors have a finite life-span ranging from a few to several years. Either the sensor will cease to function adequately or the satellite will suffer severe orbit decay such that the system is no longer useable.
What advantages do sensors carried on board satellites have over those carried on aircraft? Are there any disadvantages that you can think of?
The answer is ...
As a satellite in a near-polar sun-synchronous orbit revolves around the Earth, the satellite crosses the equator at approximately the same local sun time every day. Because of the orbital velocity, all other points on the globe are passed either slightly before or after this time. For a sensor in the visible portion of the spectrum, what would be the advantages and disadvantages of crossing times (local sun time) a) in the early morning, b) around noon, and c) in the mid afternoon?
The answer is ...
Whiz quiz - answer
Answer 1: Sensors on board satellites generally can "see" a much larger area of the Earth's surface than would be possible from a sensor onboard an aircraft. Also, because they are continually orbiting the Earth, it is relatively easy to collect imagery on a systematic and repetitive basis in order to monitor changes over time. The geometry of orbiting satellites with respect to the Earth can be calculated quite accurately and facilitates correction of remote sensing images to their proper geographic orientation and position. However, aircraft sensors can collect data at any time and over any portion of the Earth's surface (as long as conditions allow it) while satellite sensors are restricted to collecting data over only those areas and during specific times dictated by their particular orbits. It is also much more difficult to fix a sensor in space if a problem or malfunction develops!
Answer 2: An early morning crossing time would have the sun at a very low angle in the sky and would be good for emphasizing topographic effects but would result in a lot of shadow in areas of high relief. A crossing time around noon would have the sun at its highest point in the sky and would provide the maximum and most uniform illumination conditions. This would be useful for surfaces of low reflectance but might cause saturation of the sensor over high reflectance surfaces, such as ice. Also, under such illumination,'specular reflection' from smooth surfaces may be a problem for interpreters. Inthe mid afternoon, the illumination conditions would be more moderate. However, a phenomenon called solar heating (due to the sun heating the surface), which causes difficulties for recording reflected energy, will be near maximum at this time of day. In order to minimize between these effects, most satellites which image in the visible, reflected, and emitted infrared regions use crossing times around mid-morning as a compromise.