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Proactive disclosure Print version ![]() ![]() | ![]() | ![]() Satellites and sensors Satellite Characteristics: Orbits and Swaths
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.
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.
"...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?
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?
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.
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