Sensors: Image Processing Library
| SeaWiFS (Sea-viewing Wide Field of view Sensor) |
The SeaWiFS sensor.
| SPECTRAL REGION: |
Bands 1-8: | 402-885 nm (range) |
| INSTANTANEOUS FIELD OF VIEW: |
wide | |
| TEMPORAL COVERAGE: |
Ascending Node: | 161 degrees longitude |
| Descending Node: | near-noon | |
| SPATIAL RESOLUTION: | Nadir resolution-LAC | 1.1 km |
| Nadir resolution-GAC | 4.5 km |
The Sea WiFS Concept
Background
The SeaWiFS Mission is a part of NASA's Mission to Planet Earth (MTPE), which is designed to look at our planet from space to better understand it as a system in both behavior and evolution. SeaWiFS is a follow-on project to the Coastal Zone Color Scanner (CZCS), which operated until June 1989. The SeaWiFS sensor was launched aboard the Orbital Science Corporation's (OSC) SeaStar satellite on August 1, 1997. The SeaWiFS sensor is a color-sensitive optical sensor used to observe color variations in the Earth's oceans. The purpose of SeaWiFS is to provide continuous data on the bio-optical properties (those properties of organisms that can be visually recorded) of the oceans. The SeaWiFS project is the first ever NASA/industry collaboration to obtain both scientific and commercial data from the same satellite.Sensor
The sensor has an 8-band observation capability in the visual and near infra-red bands of the spectrum. It is comprised of an optics module for observation, and an electronics module for data processing, storage, and transmission. The sensor's dynamic range has been extended so that it will not be saturated (blinded) over high-albedo concentrations, like clouds or snow banks. SeaWiFS uses a multichannel, time-delay and integration (TDI) processing technique, which allows for a smaller aperture, and thus a lighter instrument. Solar and battery power from the SeaStar satellite drive a brushless DC motor and angular momentum wheel at approximately 2000 rpm to maintain its attitude. The whole sensor can be tilted to -20, 0, or +20 degrees to avoid sun glint reflecting off the oceans.Sensor Applications
SeaWiFS' primary application is to sensing of color variations in the Earth's oceans. Specifically, the color variations are indicative of phytoplankton content, suspended sediments, and the presence of dissolved organic material (DOM or yellow matter). Phytoplankton are single-celled ocean plants that contain chlorophyll for photosynthesis. The green color of chlorophyll stands out from the blue ocean water, thus SeaWiFS can observe the phytoplankton concentrations of the ocean. Because phytoplankton are frequently the first indicator of environmental change, monitoring the concentrations of phytoplankton can have many research and commercial applications.Examples of these applications include:
- providing information on the global cycles of carbon, oxygen, and nitrogen into and out of the oceans
- helping scientists determine the contributions of phytoplankton in reducing global CO2
- showing location of pollutants
- providing information on harmful bacteria (red tides)
- showing scientists where ocean currents provide plant nutrients
- providing information on the location of fish populations for the commercial fishing industry
- knowledge of ocean currents for the shipping industry
- knowledge of ocean eddies and jets for petroleum drilling
 phytoplankton |
Algorithms to identify and categorize the three optically-active elements (phytoplankton, sediments, and DOM) are proving very successful, despite the complexity of the elements' spectral characteristics, which vary with time and location. |
Orbital Characteristics
| Orbits | Circular,sun-synchronous |
| Altitude | 705 km |
| Semi-Major Axis | 7083 km |
| Inclination Angle | 98.21 degrees |
| Orbital period | 1.648 hr |
| Orbits per day | 29 |
| Orbital repeat cycle | every 2 days |
| Swath width - LAC | 2801 km (+/- 58.3 deg.) |
| Swath width - GAC | 1502 km (+/- 45.0 deg.) |
| Weight | 112 lbs. |
Band Characteristics
| Band | Wavelength (nm) | Bandwidth |
| 1 | 402-422 | 20 |
| 2 | 433-453 | 20 |
| 3 | 480-500 | 20 |
| 4 | 500-520 | 20 |
| 5 | 545-565 | 20 |
| 6 | 660-680 | 20 |
| 7 | 745-785 | 40 |
| 8 | 845-885 | 40 |
Data Acquisition
SeaWiFS is a passive optical sensor that detects only the radiation reflected from the Earth's surface back into space. The sensor is able to capture and process images as follows:- First, a rotating telescope on the satellite focuses the image on a rotating half-angle mirror. (The mirror and telescope are phase-synchronized such that polarization of the image is minimized.)
- The reflected image is then sent through a set of beam-splitters, which divide the image into four wavelength intervals. (There are two signals for each of the intervals, encompassing the eight total spectral bands of the sensor.)
- Next the four wavelength intervals are each directed through a set of two spectral bandpass filters to create the eight spectral bands.
- The signals are then pre-amplified and sent to the electronics module, for digitizing and TDI processing.
- Once in the electronics module, the signals are again amplified and filtered, such that noise from the electronics bay itself is minimized. The filtered signal is then digitized by a 12-bit analog-to-digital converter.
- Once digitized, the electronic signal is sent to an on-board procssor for the TDI operation, which is performed in "real-time" as the images are recorded.
- Lastly, the processed signals are added together and divided by four, and rounded to 10-bit numbers for storage. The 10-bit numbers are sent to the on-board data system at an approximate rate of 1.885 Mbps. NASA-approved High Resolution Picture Transmission (HRPT) stations receive the data as the SeaStar satellite passes overhead.
Sensors
Last Modified: Wed Apr 14, 1999
CSR/TSGC Team Web