Advanced Very High Resolution Radiometer (AVHRR) Sensor/Instrument Document

Advanced Very High Resolution Radiometer, AVHRR

The AVHRR represents and improvement over the Very High Resolution Radiometer (VHRR) sensor flown aboard the Improved TIROS Operational System (ITOS) series of operational satellites (the last of which was NOAA-5). The AVHRR is a cross-track scanning system similar to the VHRR, but features four or five spectral channels, compared to just two channels for the VHRR sensor. The AVHRR flown aboard TIROS-N, NOAA-6, NOAA-8, and NOAA-10 has four channels, and the AVHRR aboard NOAA-7, NOAA-9, NOAA-11, NOAA-12, NOAA-13, and NOAA-14 has five channels. Subsequent satellites in the series will have either four or five channels. Provision has been made for five channels in the data format for all satellites. Channel-5 contains a repeat of channel-4 data, when only four different channels are available.

The AVHRR provides data for real-time transmission to the NOAA local read-out services, termed High Resolution Picture Transmission (HRPT). The sensor also provides data for storage on the two spacecraft digital tape recorders for later playback. The objective of the instrument is to provide radiance data for investigation of clouds, land-water boundaries, snow and ice extent, ice or snow melt inception, data and night cloud distribution, temperatures of radiating surfaces, and sea surface temperature. It is an integral member of the payload on the advanced TIROS-N spacecraft and its successors in the NOAA series and, as such, contributes data required to meet a number of operational and research-oriented meteorological objectives.

Radiance and Imagery > Infrared

Radiance and Imagery > Visible

The NOAA AVHRR processing flow begins with sensor data receipt by the Command and Data Acquisition (CDA) stations (Wallops Island, Virginia and Gilmore Creek, Alaska) where the data are re-broadcast via communications satellites to NOAA/National Environmental Satellite, Data, and Information Service (NESDIS) in Suitland, Maryland. The ephemeris data (orbital reference information) are funneled through the Advanced Earth Location Data System (AELDS) software on an International Business Machines (IBM) mainframe which feeds an IBM 921 for Level-1b production. Earth location and calibration data are appended (but not applied) to the data as part of the Level-1b processing. The AELDS is a completely on line, data-driven system within the preprocessing system. The Earth locations are computed for specific pixels using the data time corrected for clock drift, if any. The physical algorithm used to compute Earth locations is described in the memorandum titled, "NESS Tech. Memo 107: Appendix C". The Earth location values are computed by using the exact equations of the physical algorithm. The use of raw data time as corrected for satellite clock drifts, combined with the use of exact equations, eliminates a major source of error in the Earth locations. Further, AELDS uses the most up-to-date User Ephemeris File (UEF) available within NOAA in contrast to the one-day-old UEF data used by the previous system. This, in itself, represents a significant improvement in the accuracy of Earth location data.

The constant instrument mounting and / or time dependent attitude errors for NOAA satellites are minimized by the Attitude Determination and Control System (ADACS) carried aboard the spacecraft. This system orientates the spacecraft in such a way that the nadir position of the scanning instruments always points towards the sub-satellite point. The residual errors of this system are reported in the data stream. Eventually, AELDS will be using these residual errors to correct for any attitude errors, significantly increasing the accuracy of the resulting Earth location data.

The optical system consists of an afocal 20.3-cm (8-inch) aperture telescope combined with secondary optics, separating the radiant energy into discrete spectral bands. The optics are then focused onto their respective field stops.

The IFOV for all channels is specified to be 1.3 +/-0.1 milliradians, leading to a resolution at the satellite subpoint of 1.1 km for a nominal altitude of 833 km. The scanning rate of the AVHRR is 360 scans per minute. The time within each scan line of AVHRR data represents IFOV #1.

Polarization effects were minimized to the extent practical by proper orientation of internal optical components. The instrument has been designed such that the IFOV of the four channels can be made coincident within +/-0.1 milliradians (8 percent).

International Telephone and Telegraph Corporation

Not applicable.

The AVHRR solar channels are initially calibrated (i.e., pre-launch calibrated) against an NBS-traceable integrating sphere, while the thermal channels are calibrated against a laboratory blackbody source during thermal/vacuum testing. The infrared detectors are cooled to cryogenic temperatures to improve their sensitivity. A passive blackbody mounted on the baseplate assists in-flight calibration of the thermal bands. No inflight calibration of the visible and near-infrared bands is available.

The infrared channels are calibrated during flight using a view of a stable blackbody and space as a reference. No in-flight, visible-channel calibration is performed, although the spaceview is available as one reference point. Although the calibration will vary from instrument to instrument, the design goals for the infrared channels were an Nedt of 0.12 deg.K (@ 300 deg.K) and a S/N (signal to noise ratio) of 3:1 @ 0.5% albedo. The calibration of the visible channels 1 and 2 can be performed using pre-launch calibration coefficients for AVHRR on NOAA-10, NOAA-12, and NOAA-14, and time-adjusted values for AVHRR on NOAA-7, NOAA-9, and NOAA-11.

Kidwell, K.B., 1995, NOAA polar orbiter data user's guide: Washington, D.C., National Oceanic and Atmospheric Administration [variously paged].

Paris, C.A., 1994, NOAA polar satellite calibration-- A system description, NOAA Technical Report NESDIS 77: Washington, D.C., National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service [variously paged].

Planet, W.G., ed., 1988, Data extraction and calibration of TIROS-N/NOAA radiometers, NOAA Technical Memorandum NESS 107--Rev. 1: Washington, D.C., National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service [variously paged].

Schwalb, Arthur., 1978, The TIROS-N/NOAA A-G satellite series, NOAA Technical Memorandum NESS 95: Washington, D.C., National Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service, p. 18-51.

Schwalb, Arthur., 1982, Modified version of the TIROS-N/NOAA A-G satellite series (NOAA E-J)--advanced TIROS-N (ATN): NOAA Technical Memorandum NESS 116, Washington, D.C., National Oceanic and Atmospheric Administration, National Earth Satellite Service [variously paged].

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EOSDIS
IMS
User
EOSDIS Glossary

ADACS -- Attitude Determination and Control System
AELDS -- Advanced Earth Location Data System
AVHRR -- Advanced Very High Resolution Radiometer
CDA -- Command and Data Acquisition
GAC -- Global Area Coverage
HRPT -- High Resolution Picture Transmission
IBM -- International Business Machines
IFOV -- Instantaneous Field of View
ITOS -- Improved TIROS Operational System
ITT -- International Telephone and Telegraph Corporation
LAC -- Local Area Coverage
LP DAAC -- Land Processes Distributed Active Archive Center
NESDIS -- National Environmental Satellite, Data and Information Service
NOAA -- National Oceanic and Atmospheric Administration
POES -- Polar Operational Environmental Satellites
TIROS -- Television and Infrared Observation Satellite
UEF -- User Ephemeris File
URL -- Uniform Resource Locator
VHRR -- Very High Resolution Radiometer
EOSDIS Acronym List