TITLE: POES NOAA AVHRR HRPT Data [NCAR/EOL] CONTACTS: Steve Williams NCAR/EOL P.O. Box 3000 Boulder, CO 80307-3000 phone: 303-497-8164 email: sfw@ucar.edu 1.0 DATA SET OVERVIEW This data set contains 1-km resolution 5-channel AVHRR (Advanced Very High Resolution Radiometer) HRPT (High Resolution Picture Transmission) data from the NOAA POES satellites (K/L/M/N). These data were retrieved from the NOAA CLASS archive. The data are in Level 1-b format and cover the T-REX period (1 March to 30 April 2006). They cover the region from 34-40 N and 115-126 W. 2.0 INSTRUMENT DESCRIPTION Complete information on the NOAA POES AVHRR instumentation is available from the NOAA KLM User's Guide: http://www2.ncdc.noaa.gov/docs/klm/ 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. 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 and 16 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 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 and HRPT, 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. 3.0 DATA COLLECTION AND PROCESSING NCAR/EOL conducted no processing or quality control on these data. NOAA Polar-orbiting Operational Environmental Satellites obtain global imagery daily. These data are transmitted to the Command and Data Acquisition (CDA) stations. The CDA stations relay the data to the National Environmental Satellite, Data and Information Service (NESDIS), 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 contain only one out of three original AVHRR lines. 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. 4.0 DATA FORMAT AND FILE NAMING 4.1 File Naming conventions NSS.HRPT.NM.D06080.S0433.E0442.B1942626.WI.L6592061 where: HRPT is the data type NM is the satellite (M) D06080 is the year (06) and Julian day (080) S0433 is the scan start time (UTC) E0442 is the scan end time (UTC) 4.2 Data Format These data are in Level 1-b format. Complete information on the NOAA POES AVHRR format is available from the NOAA KLM User's Guide: http://www2.ncdc.noaa.gov/docs/klm/ AVHRR Level 1b data are present as a collection of data sets. Each data set contains data of one type for a discrete time period. Thus, for AVHRR, there are separate HRPT, LAC, and GAC data sets. Time periods are arbitrary subsets of orbits, and may cross orbits (i.e., may contain data along a portion of an orbital track that includes the ascending node, the reference point for counting orbits). Generally, GAC data sets are available for corresponding time periods and usually have a three to five minute overlap between consecutive data sets. 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. 5.0 DATA REMARKS 6.0 REFERENCES None.