HRPT (High Resolution Picture Transmission) AVHRR data is received directly from the NOAA-17, NOAA-16, and NOAA-15 satellites at Landcare Research's Lincoln site. All of these NOAA satellites carry an AVHRR/3 instrument. Compared to the AVHRR instrument on NOAA-14 and earlier satellites, this instrument has six bands, but only 5 are active at any one time. The additional band, 3a, is centered at 1.6 microns and during daylight hours (only in the case of NOAA-17 data) is used in place of the traditional 3.7 micron band 3 (now called 3b), which is used during the night and by the other NOAA satellites.
NOAA operate their polar orbiting satellites in either a "morning" or an "afternoon" orbit. We receive the two highest passes from the primary afternoon satellite (NOAA-16). One of these two passes is in the afternoon (2-4pm NZST), and other is at night time (1-3am NZST). Between October and March, when the sun angles are higher, we also collect the second highest afternoon pass but do not immediately generate a quick look from it. This pass may or may not be archived, depending on its coverage over the New Zealand land mass. We also currently receive and archive the highest morning (9-11am NZST) pass from the primary morning satellite (NOAA-17), and the highest early-morning (6-8am NZST) and early-evening (7-9pm NZST) passes from NOAA-15.
Each pass received is archived to tape in the original HRPT format and processed to a JPEG quick look for inclusion on the web page. The processing and presentation used for these quick look JPEGs depends on whether the image is calculated to have been acquired during daylight or at night. In either case the image should be available about 20 minutes from the end of the satellite pass (remember image times are shown as NZST not NZDT).
Click map to see latest NOAA satellite imageFor the NOAA-17 (and older NOAA-16 daylight images in the archive), a combination of AVHRR bands 1,2 and 3a are displayed as blue, green and red respectively. All three of these visible and near infrared channels are corrected for solar irradiance and calibrated to apparent top of atmosphere reflectance. After re-sampling to the New Zealand Metric Map Grid, a histogram equalisation enhancement is applied and a graphical coastline overlay added before the JPEG image is generated.
The effect of these steps is for vegetation to show up as reddish-brown through to dark green depending on type and condition. Forest is usually dark green, while actively growing grassland is bright green but can be reddish-brown in dry condition. The near-infrared band 3a gives improved separation between clouds and snow which appears blue.
For the NOAA-16 and NOAA-15 daylight images (and older NOAA-14 daytime images in the archive), a combination of AVHRR bands 1,2 and 4 are displayed as blue, green and red respectively. The visible and near infrared channels (1 and 2) are corrected as for NOAA-17. The 11 micron thermal band (4) is calibrated to brightness temperature and inverted so that black is warm and white is cold.
With this band combination, vegetation shows up as green and dense cold (high) clouds as white. Thin cirrus cloud, which is semi-transparent at visible wavelengths, appears red, while warmer low clouds will have a blue tint. Depending on ground temperatures, snow may also have a bluish tint. If the satellite orbit is close to dawn, the visible and near-infrared components (blue & green) may be lost towards the edge of the image giving a red appearance to the sea. If the sun angle gets below 2 degrees elevation, our algorithm sets those two components to zero leaving only the thermal (red) band.
For the NOAA-16 night time images, and those NOAA-15 images that are received outside daylight hours, AVHRR thermal bands 3b, 4 and 5 are displayed as blue, green and red respectively. All three bands are calibrated to brightness temperature and inverted so that black is warm and white is cold. After re-sampling to the New Zealand Metric Map Grid, a histogram equalisation enhancement is applied and a graphical coastline overlay added before the JPG image is generated.
The effect is a fairly grey thermal image, although different emissivities of the various cloud and ground surface types can show up as colour tints. The 3.7 micron (band 3b), displayed as blue, is sensitive to both emitted and reflected radiation. Images close to dawn or dusk may therefore be affected by sunlight which, as the IR bands are inverted, reduces the blue content causing a yellow tint, especially for higher cloud.