Sensitivity of the GOES Imager Infrared Channels

Stan Kidder

Counts count

The GOES Imager transmits its observations back to Earth as 10-bit counts (integers between 0 and 1023). These counts can be converted to equivalent blackbody temperatures using calibration equations. Basically, the counts are linearly related to radiance, and radiance is nonlinearly related to blackbody temperature through the Planck function. The figure below shows the counts versus blackbody temperature for the four GOES Imager infrared channels.

The 6.7 µm channel (water vapor) saturates (reaches count 1023) by 20°C because it is designed to see upper levels in the atmosphere, where it is colder. The other channels (which are designed to see the surface or clouds) saturate at something greater than 60°C.

Sensitivity

Sensitivity can be defined as the slope of the curves above. The greater the change in counts for a 1°C change in equivalent blackbody temperature, the greater the sensitivity of the channel. The graph below shows this sensitivity.

Sensitivity increases with increasing temperature for all channels, but the shorter the wavelength, the faster the sensitivity increases. Comparison of the 3.9 µm channel with the 10.7 and 12 µm channels is interesting. At warm temperatures (above about room temperature), the 3.9 µm channel is more sensitive than the 10.7/12 channels and is, therefore, preferred for investigating temperature variations. This explains the ability of the 3.9 µm channel to detect fires. Below about -40°C, the sensitivity of the 3.9 µm channel approaches zero. Because the satellite can transmit only integral counts, the 3.9 µm channel becomes useless for investigation of temperature variations somewhere below -40°C.

Noise counts, too

Finally, each GOES Imager channel has a certain noise, which is approximately constant in radiance and, therefore, in counts. For the infrared channels, the noise is between 1 and 2 counts. (For the visible channel it is 8 counts.) That is, if one of the infrared Imager channels measures a count of 500, one can expect the real value to be between 498 and 502.

Another way to say this is that if an imager channel measures a blackbody temperature of T, the real blackbody temperature will lie between T minus NEDT and T plus NEDT, where NEDT is the noise-equivalent temperature difference, which is calculated from the noise equivalent radiance difference and the channel sensitivity. Since the relationship between counts and blackbody temperature is nonlinear, NEDT is a nonlinear function of T as shown in the figure below.

All channels have good noise characteristics (NEDT << 1°C) at warm temperatures. Below about -40°C, however, the noise in the 3.9 µm channel becomes large, too large for useful measurements.