satellite image

SATELLITE HURRICANE OBSERVATIONS

Satellite Sensor and Processing



Satellite image


Satellite Platform and Instrumentation

The satellite instrument used to remotely assess hurricane impacts on coastal areas is the Advanced Very High Resolution Radiometer (AVHRR). The AVHRR is a broad-band, four or five channel (depending on the model) radiometer that senses in the visible, near-infrared, and thermal infrared portions of the electromagnetic spectrum. This sensor has been carried aboard NOAA's Polar Orbiting Environmental Satellites (POES), beginning with TIROS-N in 1978.

The NOAA POES satellites are sun-synchronous and view approximately the same location on earth twice a day. Since the late 1980s, there have generally been two NOAA POES satellites active at any one time, yielding a view of the same earth location every six hours, or four times a day. Each satellite pass provides a 2339 kilometer-swath-width (1491 mile-swath-width) at a nominal resolution of 1.1 kilometers at nadir. The satellites orbit at an average elevation of 833 kilometers (517 miles).

The five-channel AVHRR instrument senses radiation in the visible (0.58 to 0.68 micrometers), near-infrared (0.73 to 1.1 micrometers), and thermal infrared bands (3.55 to 3.93 micrometers, 10.3 to 11.3 micrometers, 11.5 to 12.5 micrometers). The four-channel AVHRR contains only the first four channels; although it has five channels, the fifth channel is a repeat of channel four. Using an internal blackbody as a calibration source, the thermal infrared bands can be used to detect the brightness temperature of the earth's oceans; the brightness temperature from two or three channels can be used to accurately derive a sea surface temperature (SST) value. Satellite-derived SST imagery is the product most scientists and coastal managers are familiar with due its availability on the Internet and through NOAA. The SST imagery on this CD-ROM was generated using NOAA's National Environmental Satellite, Data and Information Service (NESDIS) "split window" algorithms that have a root mean square (RMS) accuracy of 0.7 degrees Celsius. Generally, the imagery is accurate to within +/- 0.5 degrees Celsius.

The ocean turbidity images are products derived from the channel 1 and 2 visible and near-infrared sensors. The product is based on a positive correlation between ocean reflected (scattered) light and the concentration of sediments in the water column. As sediment concentrations increase, as may be expected after the passing of a severe storm such as a hurricane, reflected light increases commensurately and the brightness of visible light in channel 1 can be used as a proxy measure for the change. In order to make a product that is comparable over space and time, the atmospheric contribution to the channel 1 signal must be accounted for. To correct for these atmospheric effects, channel 1 is corrected for both Rayleigh scattering and aerosol scattering, the latter correction made using signal information from channel 2 (near-infrared).


Data Sources and Processing

The satellite data telemetry for this project came from a number of sources. Some of the data was directly received from a High Resolution Picture Transmission (HRPT) downlink station at the Coastal Services Center (CSC) in Charleston, South Carolina that has been operational since 1996. A large portion of the data were acquired through Dr. Rick Stumpf from the U.S. Geological Survey (USGS). Other data were ordered from the National Climatic Data Center (NCDC) satellite archives. The data were processed from level 1b format to products of water turbidity and SST. The imagery was rectified to an Albers Equal Area projection.


Image Navigation

The initial image rectification was only as accurate as the orbital elements available for a particular pass. All passes exhibited additional geo-locational errors due to satellite attitude and timing errors as well as off-nadir image distortion. To correct for this and co-register the imagery, the individual passes were interactively "navigated" using an image processing program. The program allowed the user to reference the satellite image to a series of known ground control points (GCP) or landmarks. Each image had approximately 12 GCPs. The navigation was completed by using a second order function derived from the GCPs to "warp" the satellite image into the referenced continental outline. This co-registered all passes to within approximately one pixel or +/- 1 kilometer.


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