Sensors
Remote sensors can be grouped according to the number of bands and the frequency range of those bands that the sensor can detect. Common categories of remote sensors include panchromatic, multispectral, hyperspectral, and ultraspectral sensors.
Panchromatic sensors cover a wide band of wavelengths in the visible light or near infrared light spectrum. An example of a single band sensor of this type would be a black and white photographic film camera.
Multispectral sensors cover two or more spectral bands simultaneously typically from 0.3 m to 14 m wide.
Hyperspectral sensors cover spectral bands narrower than multispectral sensors. Image data from several hundred bands are recorded at the same time offering much greater spectral resolution than a sensor covering wider bands.
Figure 2.1 Diagram of a Hyperspectral image.
Ultrasprectral sensors are still under development and not yet in use. These sensors will cover thousands of bands with an even narrower bandwidth than hyperspectral sensors.
Digital Image Data Delivery Systems
Scanner Sensor Systems
Electro-optical and spectral imaging scanners produce digital images with the use of detectors that measure the brightness of reflected electromagnetic energy. Scanners consist of one or more sensor detectors depending on type of sensor system used.
One type of scanner is called a whiskbroom scanner also referred to as across-track scanners. It uses rotating mirrors to scan the landscape below from side to side perpendicular to the direction of the sensor platform, like a whiskbroom. The width of the sweep is referred to as the sensor swath. The rotating mirrors redirect the reflected light to a point where a single or just a few sensor detectors are grouped together. Whiskbroom scanners with their moving mirrors tend to be large and complex to build. The moving mirrors create spatial distortions that must be corrected with preprocessing by the data provider before image data is delivered to the user. An advantage of whiskbroom scanners is that they have fewer sensor detectors to keep calibrated as compared to other types of sensors.
Figure 2.2 AVIRIS whiskbroom scanner
Another type of scanner, which does not use rotating mirrors, is the pushbroom scanner also referred to as an along-track scanner. The sensor detectors in a pushbroom scanner are lined up in a row called a linear array. Instead of sweeping from side to side as the sensor system moves forward, the one dimensional sensor array captures the entire scan line at once like a pushbroom would. Some recent scanners referred to as step stare scanners contain two-dimensional arrays in rows and columns for each band. Pushbroom scanners are lighter, smaller and less complex because of fewer moving parts than whiskbroom scanners. Also they have better radiometric and spatial resolution. A major disadvantage of pushbroom scanners is the calibration required for a large number of detectors that make up the sensor system.
Scanner Platform Systems
Aircraft Systems
Airplanes have served as remote sensing platforms starting with Wilber Wright carrying the first camera into the air. Aircraft have several useful advantages as platforms for remote sensing systems. Aircraft can fly at relatively low altitudes thus allowing for sub-meter sensor spatial resolution. Aircraft can easily change their schedule to avoid weather problems such as clouds, which may block a passive sensor's view of the ground. Last minute timing changes can be made to adjust for illumination from the sun, the location of the area to be visited and additional revisits to that location. Sensor maintenance, repair and configuration changes are easily made to aircraft platforms. Aircraft flight paths know no boundaries except political boundaries. Getting permission to intrude into foreign airspace can be a lengthy and frustrating process. The low altitude flown by aircraft narrows the field of view to the sensor requiring many passes to cover a large area on the ground. The turnaround time it takes to get the data to the user is delayed due to the necessity of returning the aircraft to the airport before transferring the raw image data to the data provider's facility for preprocessing.
Figure 2.4 40 band sensor installed inside an airplane.
Satellite Systems
Satellite platforms flown from space provide a very wide field of view for the sensor and regular systematic repetitive revisits. Resolution is limited due to the satellite's fixed altitude and orbital path flown. Satellites know no political boundaries allowing them to cover any corner of the globe unheeded by foreign government interference. Expensive ground support facilities are required to operate satellites. The satellites systems are capital intensive costing hundreds of millions of dollars and have relatively short operating life spans of usually five years or less.
Major Satellite Programs
Some major satellite programs delivering images used in agriculture today include the following:
Landsat 5 uses a thematic sensor ("TM") which operates in 7 bands with a resolution of 30 meters except thermal infrared with has a resolution of 120 meters. Space Imaging EOSAT of Thornton, Colorado is the exclusive distributor of Landsat images.
Spot 1,2,3, and 4 use high-resolution visible ("HRV") sensors that operate in 4 bands with a resolution of 10 m panchromatic and 20 m multispectral. Spot images are distributed by Spot Image headquartered in Toulouse, France.
IRS-1C uses three sensors: the LISS-III, with 23 meter resolution in four spectral bands, a panchromatic sensor, with 5.8 m resolution, and a Wide Field Sensor ("WiFS"), with 188 m resolution. IRS images are distributed by Space Imaging EOSAT of Thornton, Colorado under an exclusive license from ANTRIX Corp. Ltd. of India, the commercial marketing company of the Indian Space Research Organization.
Investing in commercial satellites can be a risky business. TRW's Lewis satellite with hyperspectral sensors was lost shortly after launch in August of 1997. EarthWatch also lost its EarlyBird satellite four days after launch in December of 1997.
Terrestrial Systems
Terrestrial remote sensing systems are ground-based sensor systems. Some research has been done using remote sensors attached to long hydraulic booms hoisted above the crop canopy from the ground. Images collected from such a close distance have resolutions that are much greater than images from aircraft or satellites. Other ground-based systems use vehicle-mounted sensors that control variable rate applicators in real time. For example, remote sensors that can distinguish weeds from the crop are mounted on sprayers that change the application rate of herbicides applied on the go (Figures 2.5 and 2.6). A form of remote sensing technology called machine vision is used to sense weeds in the crop and control the sprayer. (Steward and Tian, 1998).
Figure 2.6 Sprayer prototype developed at the University of Illinois at Urbana-Champaign