Effects of the scanning geometry

The reflectivity and brightness temperature values depend on several parameters, such as the solar and satellite viewing directions. The direction of the sun depends on date, time and geographical location. The satellite viewing direction depends on the scanning geometry and the technical design of the instrument. The AVHRR instrument is a so-called 'across-track scanner' in which the satellite viewing zenith angle varies considerably across the swath: from -55.37 to +55.37 degrees (Fig. 1). This broad angle interval has a considerable effect on the image.

Figure 1: Across-track scanner

How does the satellite viewing direction affect the measurement?

  • A longer path through the atmosphere (e.g. at the edge of the swath compared to the middle) causes increased absorption.
  • A semi-transparent cloud appears thicker at the edge of the swath because the radiation reaching the satellite took a longer path through it (see Fig. 2).
  • Both surface and cloud reflectivity depend on the solar and viewing angles.
  • IR3.74 emissivity also depends on the viewing angles.

Fig. 2 shows the same semi-transparent clouds in two successive Day Microphysics RGB images taken at different satellite zenith angles. The cirrus clouds near Lake Ladoga appear thinner in the left image (where they are closer to the middle of the swath) than in the right image (where they are closer to the edge of the swath).

Figure 2: AVHRR Day Microphysics RGB images on 15 May 2016, taken by METOP-B at 09:04 UTC (left) and METOP-A at 09:52 UTC (right)

For opaque clouds and the surface, the angle dependency of the measured signal is

  • rather strong in the solar channels,
  • while in the IR10.8 and IR12.0 channels it is weak. However, it is stronger in the IR3.74 channel.

Due to the broad satellite zenith angle interval and the anisotropy of the reflectivity, the brightness and shades of daytime AVHRR RGBs' colors may change across the swath. The color shift is stronger in Day Microphysics RGBs but less eye-catching in Natural Color and Cloud RGBs.

Fig. 3 shows two successive images in which the brightness of cloud-free land changes considerably - see Germany, for example. The brightness of the clouds also changes; see for example the water cloud over south Norway.

Figure 3: AVHRR Natural Colour RGB images on 2 October 2013, at 08:25 and 10:04 UTC

Fig. 4 shows two Day Microphysics RGB images where the brightness increases towards the edge. The green component increases more strongly than the red component, such as in the ice cloud over Romania, which turns from magenta towards orange.

Figure 4: AVHRR Day Microphysics RGB images on 2 October 2013 at 08:25 (left) and 09:14 (right) UTC

Night Microphysics RGBs are less sensitive to the satellite zenith angle. The green component is an exception, as the emissivity of the IR3.74 channel is more angle-dependent.

Fig. 5 shows two successive Night Microphysics RGB images. The water cloud over the Atlantic is greener in the right-hand image where it is close to the edge, whereas in the left-hand image it is closer to the middle of the swath.

Figure 5: AVHRR Night Microphysics RGB on 14 May 2016 at 21:19 (left) and 20:31 (right) UTC

The angular dependence of the green component is even more pronounced in the noisy green dots on top of the red-brown high-level ice clouds in the three successive Night Microphysics RGB images in Fig. 6.

Figure 6: AVHRR Night Microphysics RGB on 12 May 2016 at 19:31 (top), 20:19 (middle) and 21:13 (bottom) UTC

Note that the effect of the reflection/emission anisotropy is not always as strong as in Figs. 4 and 5.