The Advanced Very High Resolution Radiometer
(AVHRR):
A Brief Reference Guide

David A. Hastings
National Oceanic and Atmospheric Administration
National Geophysical Data Center
325 Broadway, Boulder, CO 80303

William J. Emery
Department of Aerospace Engineering Sciences
University of Colorado
Campus Box 431, Boulder, CO 80309

Abstract: The Advanced Very High Resolution Radiometer, carried by NOAA's Polar Orbiting Environmental Satellites, has unique characteristics of spectral response, image geometry, frequency of coverage, and accessibility that make it useful for applications in oceanography, terrestrial sciences, and meteorology. We review the history, engineering characteristics, data availability, applications, and future of this sensor. With cited references, this paper constitutes a starting point for more extensive investigations of the data and their applications.

This is the text of a paper that appeared in the journal Photogrammetric Engineering and Remote Sensing (volume 58, no. 8, August 1992, pp. 1183-1188). Although there is a copyright notice on the published paper, the manuscript was prepared by/for an agency of the Government of the USA. It is therefore not subject to copyright.

INTRODUCTION

THE ADVANCED VERY HIGH RESOLUTION RADIOMETER (AVHRR), operating on the Polar Orbiting Environmental Satellites (POES) of the National Oceanic and Atmospheric Administration, is the direct descendent of the longest-lived and most influential series of Earth observing satellites ever launched. The current generation AVHRR has evolved into a highly accessible data collection system with a wide range of applications in meteorology, climatology, oceanography, and the study of land surfaces. The impressive growth of applications continues with increasing emphasis on regional and global environmental issues, due to AVHRR's unique position as a provider of daily global coverage of digital imagery from two satellites, truly synoptic views, calibrated thermal data, low cost of direct-readout stations and of data from NOAA.

In this paper, we review the development and implementation of the AVHRR sensor hardware, data collection/processing system and data availability. We also provide an overview of applications and summarize needs and plans for successors to the AVHRR.

HISTORY OF WEATHER SATELLITES

NASA's Television Infrared Observation Satellite (TIROS-1) (Allison and Neil, 1962), launched on 1 April 1960, gave our first systematic images of Earth from space. This single television camera was aligned with the axis of this spin-stabilized satellite which meant that it could point at the Earth only for a limited time each orbit (which naturally collected pictures of North America). This experimental satellite series eventually carried a variety of sensors, evolving as technology and experience increased. Working together, NASA and the Environmental Science Services Administration (ESSA, merged into NOAA at the latter's formation in 1970) stimulated improved designs. TIROS-1 through TIROS-X contained simple television cameras, while four of the ten satellites also included infrared sensors.

One interesting development was the change in location of the camera from the spin axis of the satellite was also turned so that now its side, rather than the central axis, pointed towards the Earth. Called the "wheel" satellite, this new arrangement resulted in the camera collecting a series of circular images of the Earth which, when mosaicked, provided the first global view of the Earth's weather systems from space.

Using this wheel concept, cooperation between NASA and ESSA initiated the TIROS Operational System (TOS) with its first launch in 1966. Odd numbered satellites carried improved vidicon cameras and data storage/replay systems that provided global meteorological data, while even numbered satellites provided direct readout Automatic Picture Transmission (APT) video to low cost VHF receiving stations. APT, now derived from AVHRR imagery, is still provided to thousands of simple stations in schools, on ships, and elsewhere worldwide. Nine wheel satellites, called ESSA-1 through ESSA-9, were launched between 1966 and 1969.

The 1970s saw the Improved TOS (ITOS), which combined APT and global data collection/recording in each satellite. The major improvement was the utilization of guidance systems developed for ballistic missiles that made it possible to stabilize the three axes of the spacecraft. Thus, a single camera could be aimed at the Earth, eliminating the need to assemble a series of circular images to map the world's weather. ITOS also introduced day/night acquisitions and a new series of Scanning Radiometers (SRs), which offered vastly improved data. Later, ITOS carried the Very High Resolution Radiometer (VHRR). As part of international weather data exchange, NOAA introduced the direct reception of VHRR data at no charge to ground stations built by an increasing number of users, beginning in 1972. ITOS-1 and NOAA-1, launched in 1070, were transition satellites of the ITOS series, while NOAA-2 through NOAA-5, launched in 1972-1976, carried the VHRR instrument.

The latest generation of this series has been operational since 1978. TIROS-N (for TIROS- NOAA) and NOAA-7 through the latest NOAA-12 include the Advanced Very High Resolution Radiometer (AVHRR), discussed in the following section. The major advance introduced with this satellite series was the shift from an analog data relay to a fully digital system. Now the data are digitized onboard the spacecraft before being transmitted to the Earth. Also the size and weight of the satellite has changed from under 300 kg with the ESSA series of satellites to over 1200 kg with the TIROS-N satellites. There has been one change in the TIROS-N series and we now have the advanced A-TIROS-N along with the advanced A-AVHRR-2. The primary difference in the AVHRR is the addition of a second thermal infrared band to help in the correction for water vapor attenuation when computing sea surface temperature.

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.

SENSOR HARDWARE

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: satorder@ncdc.noaa.gov

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.

APPLICATIONS OF AVHRR DATA

Initially, the NOAA satellites were designed to observe the Earth's weather in the form of cloud patterns. Infrared sensing extended this view from daytime-only imagery to systems that sampled both day and night. Experience with the SR and the VHRR clearly demonstrated that these scanners could be used for more than monitoring the weather. Subsequent sensors were, therefore, specifically designed to measure other phenomena such as sea surface temperature, terrestrial vegetation, etc.

NOAA designed AVHRR for the following tasks: Channels 1 and 2 were to be used to discern clouds, land-water boundaries, extend of snow and ice, and the inception of snow/ice melting, and to monitor terrestrial vegetation employing the computation of the NDVI; Channels 3, 4, and 5 were to be used to measure the temperature of clouds and the sea surface, and for nighttime cloud mapping. Applications have extended far beyond these original objectives. Discussion of current applications is too extensive to be adequately covered in this brief review. Good compendia can be found in the proceedings of regional symposia on AVHRR imagery. Such compendia include Prata (1986), Hastings et al. (1987), Thomas et al. (1989), and Gagliardini (1989). In addition, the International Journal of Remote Sensing, Remote Sensing of Environment, and professional meetings of conferences in meteorology, (especially in Europe) and oceanography are excellent sources of research papers on current applications.

METEOROLOGY

Simple mapping of cloud patterns is still an important application of AVHRR data, especially at high latitudes where data from geostationary satellites are severely distorted (due to Earth curvature). Meteorologists in northern Europe, for example, use AVHRR much more extensively than do those in the southern United States. Such meteorological applications include interpreting cloud top temperatures and heights for predicting and monitoring storms (Saunders, 1988); differentiating ice, water, shadow, and other aspects of clouds (Scorer, 1989; Karlsson, 1989); deriving information about winds at high latitudes from monitoring cloud motions (Turner and Warren, 1989); boundary layer airflow patterns (Winiger et al., 1989); water vapor content of the lower atmosphere (Schluessel, 1989); and the study of tropical cyclones (Hille et al., 1986).

It should be acknowledged that the AVHRR does not fly alone on the TIROS-N satellites but rather flies jointly with the TIROS-N Operational Vertical Sounder (TOVS). The TOVS is really not a single instrument but rather a combination of the High-resolution Infrared Radiation Sounder (HIRS, now the HIRS-2), the Microwave Sounding Unit (MSU), and the Stratospheric Sounding Unit (SSU). Together, these systems provide a suite of infrared and microwave channels that can be used to profile atmospheric temperature and, to a lesser degree, atmospheric moisture. The TOVS is presently the mission-critical sensor for meteorological applications. Climatology is also studied. Typical applications include the extend of snow cover, vegetation conditions, the advance/recession of shorelines, and relations between sea surface temperature and Sahelian drought (represented by the Normalized Difference Vegetation Index). Many of these applications remain in the research domain, and have not yet matured into products that can be routinely marketed.

OCEANOGRAPHY

It was recognized early-on in the NOAA satellite program that one of the environmental variables easiest to compute from the operational infrared (IR) imagers was sea surface temperature (SST). The VHRR and its immediate successor, the AVHRR-1, used a single thermal infrared channel in the 10- micrometre window, to measure (SST). It was realized that by adding an 11-micrometre channel, the difference in the values of the two thermal IR channels would help to correct for the atmospheric attenuation of the SST signal due to the presence of water vapor in the atmosphere. This then gave rise to the Multi-Channel SST (MCSST) which is computed from channels 4 and 5 of the AVHRR.

One of the problems of computing SST from the infrared AVHRR channels is that, even though the IR channels are calibrated onboard the satellite (with both internal hot and external cold loads), the SST must be compensated for intervening atmospheric effects and the drifts of the internal calibration sources. At present, the MCSST is calibrated against SST values measured in situ by satellite tracked, drifting buoys. These buoys are constrained to measure SST at anywhere from 0.5 to 1.0 m below the surface while the IR satellite sensor receives energy emitted from the mm thin skin layer of the ocean (Schluessel et al., 1987; Emery and Schluessel, 1989; Schluessel et al., 1990). An alternative is to calibrate the AVHRR derived SST with skin SST reference measurements made by a radiometer mounted on a ship.

Infrared AVHRR imagery has also proven very useful in mapping mesoscale ocean features in terms of their SST signatures (Evans et al., 1984; Cornillon et al., 1987). Major ocean currents, such as the Gulf Stream, are readily visible by their marked SST gradients. Techniques have been developed for mapping ocean current variability from their signatures in the AVHRR SST imagery (Cornillon et al., 1987, Emery et al., 1986).

Another oceanographic application of AVHRR data is in the study of sea ice. Properly filtered for clouds over ice, AVHRR imagery can be used to compute sea ice concentration and ice edge location (Emery et al., 1991b). Also, a sequence of AVHRR images, either visible or thermal infrared for polar winters, can be used to compute ice motion (Ninnis et al., 1986; Emery et al., 1991a).

TERRESTRIAL SCIENCES

The AVHRR has evolved into an invaluable resource for studying the land surface. AVHRR's frequent day/night synoptic coverage (morning and afternoon views give it twice the chance to have cloud-free coverage), its ability to see huge areas in stereo, its IR calibration, and the ability to be rigorously reprojected, are features unavailable in any other system.

While many geologists are concentrating on very small areas, with systems carrying anywhere from 7 to 256 spectral channels, AVHRR offers almost unused stereoscopic views of entire structural provinces and their surroundings - offering the opportunity to guide previously unattainable regional geological syntheses (Hastings, 1988), e.g., Monitoring of volcanic eruptions (and their effects on climate and safety of aircraft movements) (Matson, 1984; Prata, 1989).

In the area of monitoring terrestrial vegetation, the AVHRR-derived NDVI has proven to be a very robust and useful quantity. There is considerable discussion on how to interpret the NDVI, and how to relate the satellite-derived model to climate and terrestrial phenomena (Tucker et al., 1981; Tucker and Sellers, 1986). However, there is little doubt that the NDVI provides a useful space perspective monitor of vegetation, land-cover, and climate, if used carefully. The index has been produced and utilized globally (Kidwell, 1990) and regionally (Tucker et al, 1985,86; Goward et al., 1987; 1991; Sakamoto et al., 1987).

Multi-channel imagery from the AVHRR has also proven to be useful in snow cover mapping. The frequent coverage of the AVHRR is again the prime advantage in being able to distinguish clouds from snow cover with their similar albedo signature. Methods have been developed for routinely computing snow cover from AVHRR imagery (Dozier et al., 1981; Carroll et al., 1989). Combined with topographic relief information, snow cover from AVHRR can be converted to snow-water equivalent to give an estimate of the amount of water reserve represented by the winter snowpack. Again the primary limitation is cloud cover blocking the signature of the snow cover, but the frequent coverage of the AVHRR helps to lessen the impact of cloudiness.

Another use of AVHRR data lies in monitoring forest fires in terms of their IR signature. Because forest fires are intensely hot, the thermal IR signature can be spotted in the AVHRR as soon as the fire size becomes large enough to dominate the 1-km pixel of the AVHRR. In addition, the visible and IR channels will clearly reflect the smoke plume associated with the forest fire, which assists in mapping the fire location and estimating its progress in time. NDVI is also a good estimator of the damage done to vegetation after the fire, and can be used to map the burnout made before the fire.

Finally, the AVHRR can be used to assist in mapping forest fire fuel potential. Again, a vegetative index, such as the NDVI, is the primary indicator of where dead trees are abundant. Because the NDVI only responds to healthy, growing vegetation (it really indicates the health of the growth), those areas densely populated with dead or dying trees will show a decrease in NDVI. While it may take a few in situ samples to verify the patterns suggested by the NDVI mapping, the satellite perspective provides a unique way to survey the distribution of dead trees that make up the forest fire fuel potential.

SOME EFFECTS OF THE SATELLITE SYSTEM ON APPLICATIONS

Note that the statement, "sun synchronous", used to describe the TIROS-N orbit is an approximation. In reality, the orbit precesses in an effort to maintain the same local time. Thus, over a 14-day cycle the actual local time varies slightly. Besides the variation in nominal equatorial crossing times and the annual variation in sunrise and sunset, there is a tendency for actual equatorial crossings to occur somewhat later during the lifetime of any given satellite. This change is several minutes monthly, amounting to as much as two hours over the lifetime of a satellite.

Also, is should be observed that the polar orbit of the satellite, combined with the 3000-km swath of the AVHRR, leads to considerable overlap of the satellite imagery at polar latitudes. At equatorial latitudes the orbit is designed so that only a few kms overlap on adjacent orbits, but at 80 degrees N/S the sequential AVHRR swaths overlap by about one-third. This greatly increases the coverage at polar latitudes which is important for the removal of clouds in this persistently cloudy region of the world.

Several additional "departures from perfection" of the AVHRR system are discussed in the references. One should not think, however, that the AVHRR sensor is inferior or superior to other systems currently in orbit. Scientists use AVHRR for such a wide variety of applications that one naturally expects this system to be short of perfection for any single application.

THE FUTURE OF THE AVHRR

AVHRR already has two successors in various stages of design and development. The names for these systems change fairly frequently, so are inappropriate for mention here. A transitional sensor plans to split Channel 3 into daytime and nighttime sensors, but otherwise remain largely similar to the current AVHRR. Refined channel allocations and design are planned for the two full generations of successors to AVHRR.

The MODerate-resolution Imaging Spectrometer (MODIS), planned for the Earth Observing System later this decade, is intended to have slightly higher spatial resolution, and 64 spectral channels. It is intended to be a narrower image area, with a nadir-and tilt-pointing capability, to enable it to view in along-track stereo.

Several other experimental or commercial systems of moderate resolution have reached the drawing-board stage. It is safe to assume that desires will continue for adapting some of the more popular applications of AVHRR to more specialized systems.

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APPENDIX

Spacecraft Characteristics, advanced TIROS-N/NOAA E-J series:

3.71 m long, 1.88 m diameter maximum. 1009 kg spacecraft mass (excluding expendables); 386 kg satellite payload (all instruments and support). 2.37 by 4.91-m solar array. 515 Watts minimum output; 475 Watts power requirement at full operation. Attitude Determination and Control System incorporates (1) Earth Sensor Assembly (for pitch and roll); (2) Sun Sensor Assembly (yaw); (3) three orthogonal gas-bearing gyros to measure change in attitude in three axes (a fourth canted gyro is available as a spare); (4) Four Reaction Wheel Assemblies which provide corrective torques; (5) two Roll/Yaw Torquing Coils and two Pitch Torquing Coils electrically dissipate roll/yaw and pitch angular momentum. On board computers and continuously updated ephemeris models transmitted from Earth assist this process.

Attitude normally maintained within 0.12 degrees, 0.2 degrees maximum. Attitude measurement accuracy approximately 0.1 degrees. Five digital tape recorders, each with 168 m of tape, for 45 MB of data, enough for a full orbit (110 minutes) GAC, 10 minutes LAC. Tapes designed for 25,000 passes on each transport unit. Data processing all digital (APT translated digital to analog). Two-Year design lifetime.

Nominal orbit:

Height        Inclination         Orbital Period       Orbital Increment        Orbits Reception
(km)*            (deg.)                  (min.)           (deg./deg. W)     Per Day         Time (min.)**

833 + 18.5    98.739+0.5              101.38             25.40          14.18                13.0

*    Difference between apogee and perigee less than 56 km.         
**  Time visible to an antenna directly under the orbital track, assuming interference when the  satellite 
is less than 5 degrees above horizon.
Sensor characteristics:

Optics: 20.3-cm diameter focal Cassegrain telescope.
Scan mirror: Ribbed beryllium; 29.46-cm major axis, 20.96-cm minor axis.
Scanner: 360 rpm 80-pole hysteresis synchronous motor with ribbed beryllium scan mirror.
Power consumption 4.5 Watts maximum.
Cooler: Two-stage radiant cooler, Infrared detectors operate at 105k.
Data Output: 10-bit binary, channels simultaneously sampled at a 40 kHz rate.
Power: 28 Watts Maxim (total instrument).
Size: 79 cm by 28 cm by 41 cm maximum.
Mass: 30 kg maximum.
Detector Type: Silicon (Ch 1 & 2); InSB (Ch 3); HgCdTe (Ch 4 & 5).
Scan Angle: 55.4 degrees on either side of nadir.
IFOV: 1.3 milliradians.
RFOV: 1.1 km at nadir; 6 + km at edge of scan.
Image Width: 2048 pixels, about 2800 km.
Calibration: Blackbody surface inside spacecraft (290 K) and space (0 K) and space (no albedo) for reflected channels.

Channel #    Description                                Band Width                    IFOV****

     1*      visible (green) channel                    0.58  to 0.68 micrometers     1.39
     2       reflected infrared channel                 0.725 to 1.05 micrometers     1.41
     3       hybrid reflected/thermal infrared channel  3.55  to 3.92 micrometers     1.51
     4**     thermal infrared channel                   10.3  to 11.3 micrometers     1.41
     5***    thermal infrared channel                   11.5  to 12.5 micrometers     1.30
*     0..55 to  0.90 micrometer on TIROS-N.
**    10.5 to 11.5 micrometer on TIROS-N and NOAA 40-channel AVHRR/1 series.
***   No channel 5 on  TIROS-N and NOAA 4-channel series. Channel 4 data are repeated.
****  Instantaneous Field of View (in milliradians)

Satellite   Dates of Operation  Descending  Daylight    # of 
                                  Node     Acquisition  channels
 
TIROS-N     10/19/78-01/30/80   0300         AM          4
NOAA-6      06/27/79-03/05/83   0730         PM          4 
            07/03/84-11/16/86              
NOAA-7      08/24/81-02/01/85   0230         AM          5
NOAA-8      05/03/83-06/21/84   0730         PM          4 
            07/01/85-10/31/85              
NOAA-9            02/25/85      0220         AM          5
NOAA-10           11/17/86      0730         PM          4
NOAA-11     11/08/88-09/13/94   0140         AM          5
NOAA-12           09/01/91      0730         PM          5      
NOAA-14           12/30/94      0140         AM          5

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