The use of NOAA AVHRR data as a tool for operational agroclimatic assessment in Asia
NOAA satellite and sensor characteristics
The remotely sensed data used in the operational agro-climatic assessment process are acquired from the NOAA polar orbiting satellites. Selected characteristics of the NOAA 11 satellite and the Advanced very high resolution radiometer (AVHRR) sensor appear in table 1.
Table 1. Selected characteristics of the NOAA-11 satellite
and AVHRR sensor system
| Characteristic |
NOAA/AVHRR |
| Inclination of orbit |
98.922° |
| Height above surface | 851 km |
| Number of orbits/day | 14.1 |
| Times of coverage at equator | 01:41 descending
13:41 ascending |
| Orbital period | 102 min. |
| Latitudinal coverage | 90°N - 90°s |
| Cycle duration | 1 day |
| Ground coverage | 2842 km |
| Field of view (FOV) | ± 55.4° |
| Instantaneous field of view (IFOV) | 1.39 - 1.51 mrad |
| Ground resolution (nadir) | 1.1 km |
| Number of channels | One visible
One near infrared
One middle infrared
Two thermal infrared |
Source : Adapted from Kidwell, 1988
Date are routinely acquired from the satellite and sensor system at two spatial resolutions. The finest resolution data, known as High Resolution
Picture Transmission (HRPT) data, are transmitted to ground receiving stations in real-time. Selected data sets of the same spatial resolution area recorded aboard the satellite for alter playback to ground receiving stations.
Recorder limitations dictate that a maximum of ten minutes of this data may be recoded on any single orbit. These data are subsequently processed as Local Area Coverage (LAC) data. A reduced spatial resolution data set, known as Global area coverage GAC data are created by the processor aboard the satellite by sampling the full resolution data and recording it at GAC resolution. Full resolution data are sampled by averaging four of every five adjacent pixels along a scan line and processing every third line. LAC and HRPT data are normally 1.1 kilometer spatial resolution data at the satellite subpoint; GAC data are generally resolution data (Kidwell, 1988). A third spatial resolution AVHRR product, known as global vegetation Index (GVI) data, by selecting the last value in an array of four by four GAC pixels and assigning its radio metric value to the resulting GVI pixel.
Satellite data products
The spatial resolution of data for use in operational agro climatic assessments must be appropriate in terms of data availability, cost, project objectives, available hardware and software, and the timeliness expected of an operational system. All three types of NOAA satellite data GVI, GAC and LAC, were considered for assessment products. GVI resolution data represents the mot economical data source with least quantity of data to handle. For this reason, a three year data set of GVI data covering the Asian continent was created for use in the satellite crop monitoring workshop component for this project. Because of the agricultural conditions in South East Asia, however, mot countries wish to work with the highest spatial resolution data available from the NOAA satellite. Consequently, software for analysis the 1.1 km resolution HRPt.LAC data has also been made available.
Remotely sensed data products take two distinct forms :
- composited imagery which is interpreted for vegetative
condition; and
- the Normalized difference vegetation index (NDVI) which is analyzed for vegetative trend through time an for change from a reference period.
