Updated Wednesday, 21-Jan-2004 09:52:06 EST
Sensing physical properties from a distance
Used when direct measurement is impractical (because of great distance) or when a greater breadth of data can be obtained (e.g., wider view from high above Earth)
Electromagnetic Radiation from Warm Objects
- Any object with temperature above absolute zero (-273C or -459F) is a warm object
- Any warm object emits electromagnetic (e/m) radiation -- some of the energy that keeps the object warm is being radiated away.
- This radiation has all possible wavelengths, but some are stronger (more intense) than others
- Spectrograph measures intensity of radiation for each wavelength and generates a plot as shown below. This data may represent radiation from an entire object or just from a specific area on the object's surface
(1 micron = 1 micrometer = 10-6 m.)
- Wien's Law relates the temperature (in degrees Kelvin) of the observed area to the wavelength L at which the intensity is greatest.
- In the plot above, the intensity (I) is maximum at a wavelength L = 1.25 microns. So the temperature of the observed object is T = (2900 microns K)/1.25 microns = 2320K.
- Conversion from Kelvin to Fahrenheit is
T(oF) = (9/5)*T(oK) - 459.67o
Characteristic Spectrum of a Substance
- Viewed in detail, the spectrogram of an object reveals many peaks.
- Every substance has a unique pattern of these peaks (called its spectrum).
Variation of Intensity with Distance
- Major problem for remote sensing is reduction of intensity with distance
- Intensity falls off in proportion to the square of the distance
Instrument A is 10000 km from a warm object; instrument B is 30000 km away from the same object. Instrument A will receive 9 times greater intensity of radiated energy than instrument B.
- Satellites at higher altitudes (1000 -- 30,000 miles) can observe large portions of Earth's surface all at once -- useful for meteorological observations.
- For detailed observations, satellite observes only a small area at a time. As satellite moves along its orbit, it views a swath that follows the satellite's ground track.
- Pushbroom scanning -- electromagnetic (e/m) radiation from a rectangular area on the ground is focused onto an array of sensors (sometimes called a mult-cell sensor). All the cells are identical, and are designed to measure some part of the spectrum. Each cell is responsible for measuring properties of the e/m radiation from a small section of the rectangular area. In the figure below, e/m radiation from the section marked in red is focused onto the corresponding cell (sensor) in the satellite, etc. The sensor then sends the spectral information to the onboard computer. The same process occurs for all of the small sections across the swath simultaneously. A fraction of a second later, the satellite has moved along its orbit, and is then viewing the next rectangular area along the swath (which can be less than a mile across or up to several hundred miles in width).
This is commonly called pushbroom scanning because the entire width of the swath is swept by the sensors at the same time. It requires somewhat simpler optics (lenses, etc.) but also requires a more complex and expensive multi-celled sensor.
- Side scanning -- e/m radiation from the rectangular area on the ground is focused onto a single sensor, but the swath width must be scanned by a mirror in the satellite that pivots back and forth. The pivoting is controlled by the computer so that at any instant, the computer can associate information received by the sensor with the correct small section on the ground. In the figure below, the mirror is first tilted so that e/m radiation from the section marked in red is directed to the sensor, but a fraction of a second later, the mirror is repositioned so that e/m radiation from the section marked in yellow in then directed to the sensor.
This method is called side scanning since the pivoting mirror scans sideways across the swath. As with pushbroom scanning, the small sections on the ground can vary from less than a mile to several hundred miles. This method requires more complex optics (the pivoting mirror must be controlled by the computer very rapidly and precisely), but has the advantage of needing only one sensor.
- Ground track -- refers to the path traced out by a point on the ground directly underneath the satellite. If the Earth did not rotate, the ground track would be a circle, but because of Earth's rotation, the ground track crosses a different part of the Earth each time the satellite goes around its orbit. For remote sensing, this allows the swath of the sensors to cover all of Earth's surface eventually.
- For example, the ground track covering an 8-hour interval for the TOPEX satellite (which measures ocean height and temperature) is shown below (image courtesy of the Jet Propulsion Laboratory). Here the colors indicate ocean height. If this ground track were shown on a three-dimensional map (globe) of the Earth, it would appear as a set of circles, with each one shifted to the left (west) as Earth rotated under the satellite. The flat map (Mercator projection) gives the ground track a distorted appearance here.
Copyright © 1998, Robert G. Melton
Updated Wednesday, 21-Jan-2004 09:52:06 EST