THE AVHRR SENSOR
Throughout this developmental process, NOAA has followed a philosophy of meeting operational requirements with instruments whose potential has been proved in space. Predecessor instruments were flown experimentally on experimental satellites before they were accepted and implemented on operational monitoring satellites. These instruments were redesigned to meet both the scientific and technical requirements of the mission; the goal of the redesign was to improve the reliability of the instrument and the quality of the data without changing the previously proven measurement concepts (Barnes and Smallwood, 1982). This philosophy bring both benefits and challenges to the user. Benefits are centered around relative reliability, conservative technology, continuity of access, and application of the data compared to other satellite systems. Challenges include desires to use the system beyond its original design that could have been advanced more rapidly (but at considerably more cost and/or the loss of continuity of data characteristics). Challenges also include conflicting desires by users for greater support for their own particular scientific disciplines with more advanced sensors and more sophisticated customer support (while often also desiring even lower-cost imagery) from NOAA. NOAA's interpretation of its mission has resulted in the following characteristics of AVHRR.
AVHRR's ancestors were the Scanning Radiometers (SRs), first orbited on ITOS-1 in 1970. These early SRs had a relatively low spatial resolution (8 km) and fairly low radiometric fidelity. The VHRR was the first improvement over the SR and for a while flew simultaneously with the SR. Later, the VHRR was replaced by the AVHRR which combined the high resolution and monitoring functions. There are two series of AVHRR instruments. Built by ITT Aerospace/Optical Division in the mid 1970s, the AVHRR/1 is a four-channel, filter-wheel spectrometer/radiometer (Appendix) while the AVHRR/2, built in the early 1980s, is identical except for the addition of Channel 5.
The AVHRR instrument comprises five modules: the scanner module, the electronics module, the radiant cooler, the optical system, and the baseplate. Schwalb (1978, 1982) and ITT (1982) provide detailed descriptions of AVHRR and POES hardware; the Appendix summarizes these characteristics.
AVHRR channels 1 and 2 are designed, and calibrated before launch, to provide direct, quasi- linear conversion between the 10-bit digital numbers and albedo. In addition, the thermal channels are designed and calibrated launch as well as in space to provide direct, quasi-linear conversion between digital numbers and temperature in degrees Celsius. As the thermal infrared channels were optimized for measuring the skin temperature of the sea surface, their range is approximately -25 to +49 degrees Celsius for channel 3, -100 to +57 degrees for channel 4, and - 105 to +50 degrees for channel 5 for a typical NOAA 11 scene.
DATA INGEST, PROCESSING, AND DISTRIBUTION
There are four classes of AVHRR data: (1) High Resolution Picture Transmission (HRPT) data are full-resolution (1-km) data received directly in real-time by ground stations; (2) Global Area Coverage (GAC) are sampled on-board to represent a 4.4-km pixel, allowing daily global coverage to be systematically stored and played back to NOAA ground stations at Wallops Island, Virginia, and Fairbanks, Alaska, and a station operated at Lanion, France, by the Centre National d'Edudes Spatiales (CNES); (3) Local Area Coverage (LAC) are 1-km data recorded on- board for later replay to the NOAA ground stations; and (4) Automatic Picture Transmission (APT) is an analog derivative of HRPT data transmitted at a lower resolution and high power for low-cost VHF ground stations. Kidwell (1991) provides a handbook for users of AVHRR data.
Special acquisitions of LAC data may be requested by anyone (Weaks, 1987). HRPT, LAC, and GAC data are received by the three stations just mentioned, and are processed at NOAA facilities in Suitland, Maryland. In addition, relatively low-cost, direct-readout stations can be set up to read the continuously broadcast HRPT data. Further information about these data, and about NOAA's on-line cataloging and ordering system, can be obtained from the
National Oceanic and Atmospheric Administration National Environmental Satellite Data and Information Service (NESDIS) National Climatic Data Center (NCDC) Federal Building Ashefille, NC 28801-5001 Telephone: (704) 271-4800 FAX: (704) 271-4876 email: firstname.lastname@example.org
Users of large quantities of data may be able to obtain them more rapidly from NOAA by making the appropriate individual arrangements with the
Chief, NOAA/NESDIS Interactive Processing Division (E/SP22) Room 510, World Weather Building Washington, DC 20233 Telephone: (301) 763-8142
Over 200 HRPT ground stations operate worldwide. Most of these stations collect imagery primarily for meteorological forecasting. Most stations do not archive data; others save a few select scenes based on institutional interests. However, as interest in regional and global environmental studies increase, efforts are being made to develop internationally cooperative ventures to save data from several HRPT stations, supplementing these data with LAC coverage to obtain periodic global full resolution AVHRR data. For example, the European Space Agency has anticipated increased interest in AVHRR data for Europe and neighboring Africa by developing a coordinated archive and dissemination system with on-line catalog (Fusco et al., 1989). Currently, a consortium of laboratories is developing a program to collect and combine global 1-km HRPT data to map the global land surface at least twice a month.
Digital NOAA satellite data are ready for correcting from digital counts to albedo and radi- ance/temperature, and are amenable to rigorous geometric reprojection (using an orbital model) from the original space oblique mercator projection to the user's desired projection. Non- rigorous contrast enhancement and geometric transformation by rubber-sheeting can also be utilized. The data can also be converted form the original 10 bits to 8 bits for display and interactive analysis. These procedures reduce the fidelity of AVHRR data. On the other hand, accepting the data as accurate measurements of albedo and temperature places too much faith in the physics of the measurements, the optical path of the electromagnetic radiation that reaches AVHRR, and in the subsequent processing of the data. Rigorous correction methods are the subject of present research which may eventually lead to new correction methods making the quantitative analysis of AVHRR more realistic.
OPERATIONAL AVHRR DATA PRODUCTS
NOAA presently produces a number of operational products from AVHRR imagery. Calculated Normalized Difference Vegetation Index (NDVI), Sea Surface Temperature, atmospheric aerosols, and sea ice cover data are available globally, while snow cover is mapped for the northern hemisphere. These derived parameters are discussed in more detail below, in Ohring et al. (1989) and in Kidwell (1991).
DIRECT READOUT OF AVHRR DATA
NOAA extracts no fees for establishing and operating an HRPT direct-readout ground station. Indeed, it does not even require station operators to make themselves known to NOAA. The agency recommends, however, that operators be on NOAA's mailing list and make use of its on- line bulletin board, so that they can keep current with news of current and planned satellite operations. In addition, there is no charge for, or control of, APT ground stations that can be conveniently operated from moving platforms such as ships. NOAA has several references (most notably Barnes and Smallwood (1982) available to potential operators of HRPT or APT ground stations. It maintains an office to support such stations:
Coordinator, Direct Readout Services NOAA/NESDIS Washington, DC 20233
Besides the more than 200 HRPT ground stations, which can now be constructed for under
$100,000 using commercial equipment, some enterprising radio amateurs have constructed systems for
several hundred dollars, with a personal computer and surplus or homemade antennas and circuit
boards. Also, an APT-only receiving station can be set up for under $2,000.
HISTORY OF WEATHER SATELLITES
APPLICATIONS OF AVHRR DATA
SOME EFFECTS OF THE SATELLITE SYSTEM ON APPLICATIONS
THE FUTURE OF THE AVHRR
Entire paper (a bit long at 45 kilobytes)
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