The Advanced Very High Resolution Radiometer (AVHRR) sensor is carried on NOAA's Polar-orbiting Operational Environmental Satellites (POES) starting with TIROS-N in 1978 and on the MetOP series satellites owned and operated by EUMETSAT. Onboard the TIROS-N, NOAA-6, 8 and 10 POES Satellites, the AVHRR Sensor measures in four spectral bands, while on the NOAA-7, 9, 11, 12 and 14 POES Satellites, the sensor measures in five bands. The AVHRR/3 sensor on NOAA-15, 16, 17, 18 and 19 measures in six bands though only five are transmitted to the ground at any time.
The visible data values may be converted into albedos and the IR data into radiances or temperatures using the calibration information which is appended but not applied. Latitudes and longitudes of 51 benchmark data points along each scan are included. Other parameters appended are: time codes, quality indicators, solar zenith angles, and telemetry.
The objective of the AVHRR instrument is to provide radiance data for investigation of clouds, land-water boundaries, snow and ice extent, ice or snow melt inception, day and night cloud distribution, temperatures of radiating surfaces, and sea surface temperature, through passively measured visible, near infrared and thermal infrared spectral radiation bands.
The AVHRR for TIROS-N and the follow-on satellites is a scanning radiometer with either four or five channels, which is sensitive to visible/near IR and infrared radiation. The instrument channelization has been chosen to permit multispectral analyses which provide improved determination of hydrologic, oceanographic, and meteorological parameters. The visible (0.5 micron) and visible/near IR (0.9 micron) channels are used to discern clouds, land-water boundaries, snow and ice extent, and, when the data from the two channels are compared, an indication of ice/snow melt inception. The IR window channels are used to measure cloud distribution and to determine the temperature of the radiating surface (cloud or surface). Data from the two IR channels is incorporated into the computation of sea surface temperature. By using these two channels, it is possible to remove an ambiguity introduced when clouds fill a portion of the field-of-view.
On later instruments in the series, a third IR channel was added for the capability of removing radiant contributions from water vapor when determining surface temperatures. Prior to inclusion of this third channel, corrections for water vapor contributions were based on statistical means using climatological estimates of water vapor content.
AVHRR data have been used for many diverse applications. In general, AVHRR applications encompass meteorological, climatological and land use. Obvious meteorological and climatological applications include detection and analysis of: cold fronts; plumes; weather systems; cloud movement; squall lines; boundary clouds; jet stream; cloud climatology; floods and hurricanes. In addition, land use applications of the AVHRR include monitoring of: food crops; volcanic activity; forest fires; deforestation; vegetation; snow cover; sea ice location; desert encroachment; icebergs; oil prospecting and geology applications. Other miscellaneous AVHRR applications include the monitoring of: migratory patterns of various animals; animal habitats; environmental effects of the Gulf War; oil spills; locust infestations; and nuclear accidents such as Chernobyl.
NOAA POES and EUMETSAT's Metop satellites obtain global imagery daily and are sent to designated receiving stations worldwide. These data are relayed to the NOAA Satellite Operations Facility located in Suitland, Maryland, for processing and distribution.
As a result of the design of the AVHRR scanning system, the normal operating mode of the satellite calls for direct transmission to Earth (continuously in real-time) of AVHRR data. This direct transmission is called HRPT (High Resolution Picture Transmission). In addition to the HRPT mode, about 11 minutes of data may be selectively recorded on board the satellite for later playback. These recorded data are referred to as LAC (Local Area Coverage) data. LAC data may be recorded over any portion of the world, as selected by NOAA/NESDIS, and played back on the same orbit as recorded or during a subsequent orbit. LAC and HRPT have identical Level 1b formats.
The full resolution data are also processed on board the satellite into GAC (Global Area Coverage) data which are recorded only for readout by NOAA's CDA stations. GAC data are reduced resolution data. The data volume and resolution are further reduced by averaging every four adjacent samples and skipping the fifth sample along the scan line.
POES satellites operate in relatively low orbits, ranging from 830 to 870 km above the earth. They circle the earth approximately 14 times per day (with orbital periods of about 102 minutes). The orbits are timed to allow complete global coverage twice per day, per satellite (normally a daytime and a nighttime view of the earth) in swaths of about 2,600 km in width. High resolution (1 kilometer) data are transmitted from the satellite continuously, and can be collected when the satellite is within range of a receiving station. Recorders on board the satellite are used to store data at a 4 kilometer resolution (processed by the on-board computers) continuously, and a limited amount of data at a 1 kilometer resolution on demand. The recorders are dumped when the satellite is within range of a NOAA receiving station.
AVHRR Level 1b data are grouped into four data types:: HRPT, LAC, GAC and FRAC (Full Resolution Area Coverage) FRAC applies only to Metop satellites. GAC data are available globally since about 1978. The LAC and HRPT data types are most complete from 1985 forward and are limited to specific areas of the world. As a general rule, HRPT data are available for the U.S. and coastal areas and LAC data are available over foreign land masses. FRAC data is available for the entire globe since late October 2006.
The AVHRR data series is archived at Level 1b (following FGGE terminology) is raw data in 10 bit precision that have been quality controlled, assembled into discrete data sets, and to which Earth location and calibration information has been appended, but not applied. Other parameters appended are: time codes, quality indicators, solar zenith angles, and telemetry.
The AVHRR provides a global (pole-to-pole) on-board collection of data from all spectral channels. At an 833 km altitude, the 110.8 degree scan equates to a swath 27.2 degrees in width (at the Equator), or 2,600 km, centered on the subsatellite track. This swath width is greater than the 25.3 degree separation between successive orbital tracks, providing overlapping coverage (side-lap).
For LAC, HRPT, and FRAC the instantaneous field-of-view (IFOV) of each channel is approximately 1.4 milliradians (mr) leading to a resolution at the satellite subpoint of 1.1 km for a nominal altitude of 833 km. Since GAC data contain only one out of three original AVHRR lines and the data volume and resolution are further reduced by averaging every four adjacent samples and skipping the fifth sample along the scan line, the effective resolution is 1.1 x 4 km with a 3 km gap between pixels across the scan line. This is generally referred to as 4 km resolution.
Each scan of the AVHRR views the Earth for a period of 51.282 milliseconds (msec). The analog data output from the sensors is digitized on-board the satellite at a rate of 39,936 samples per second per channel. Each sample step corresponds to an angle of scanner rotation of 0.95 milliradian (mr). At this sampling rate, there are 1.362 samples per IFOV. A total of 2,048 samples for the LAC/HRPT/FRAC data are obtained per channel per Earth scan, which spans an angle of +/- 55.4 degrees from the nadir (subpoint view). Successive scans occur at the rate of 6 per second, or at intervals of 167 msec.
For GAC data, successive sets of 4 out of every 5 samples in every third scan line are averaged to obtain an array of data spaced at intervals of 125 msec along the scan and at 500 msec along the satellite track. This leads to a data rate of 49,080 samples-per-minute and 2 scans-per-second. There are a total of 409 samples for the GAC data per channel per Earth scan.
Because the satellite is sun-synchronous, visible data revisit time is daily. Infrared imaging is accomplished twice daily with the second visit occurring during the pass over the dark side of the Earth. Instrument operation is continuous.
The overall coverage of the archived AVHRR data base is shown in the following tables. However, associated with equipment malfunctions, there may be short gaps in the time ranges.
CLASS offers an on-line digital image browse feature for selected satellite image data sets. This image browse feature is primarily intended to support data set selection for order, by allowing users to visually judge overall image quality, determine the extent of cloud cover, and/or verify geographic coverage. The first satellite image data set to be supported with the browse feature is the AVHRR Level 1B data set. The sample browse images below show Hurricane Andrew and Norway.
Additional information on the AVHRR sensor and data for satellites TIROS-N through NOAA-14 can be found at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/podug/html/c3/sec3-0.htm
Details of the AVHRR/3 sensor can be found at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c3/sec3-1.htm while calibration information for AVHRR/3 is at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c7/sec7-1.htm and data format details for AVHRR/3 LAC and HRPT are at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c8/sec831-3.htm and data format details for AVHRR/3 GAC are at: http://www.ncdc.noaa.gov/oa/pod-guide/ncdc/docs/klm/html/c8/sec831-4.htm)
Satellite Data Readers
There are several sources of AVHRR data readers and converters available to the CLASS user.
For a listing of commercial readers please go to the FAQ section in CLASS - second from last bullet item. A freeware reader we have heard about is available at SatSignal for HRPT data type. It even mentions using data from CLASS.
For AVHRR GAC data NOAA's National Climatic Data Center offers a level1b to netcdf Java based converter. Please click here to access.