Quantization Noise for GOES-R ABI Bands

Certain specifications are set for the GOES-R Advanced Baseline Imager (ABI). Two of those specifications are allowable instrument noise and the instrument maximum scene temperature for each band. As a result of those specs, other characteristics of the ABI data stream (GOES Re-Broadcast or GRB) can be determined. One of those characteristics is the bit-depth or number of bits used to represent the radiances measured by the ABI. This in turn determines the quantization of the measured radiances and the quantization “step” or the minimum change that can be described in the digitized scale. The desire is that this quantization step per count be much less than the actual radiance noise in order to not put an artificial limit on the radiance noise of the ABI. Calculations are made and results will be presented on the minimum number of bits needed to capture the desired range of temperatures as well as exceed the noise spec for each ABI band. Of course it may turn out that the maximum number of bits needed for any band will be used for all bands. For example, the current-GOES instruments have 10-bit for the Imager and 13-bit for the Sounder.

Among the ABI bands, the 3.9 ìm band is of primary interest because its greater sensitivity to warm temperatures and the desire to capture very hot scene temperatures for detection and characterization of hot spots (e.g., forest and range fires). Thus this shortwave infrared (IR) band has a specified instrument maximum scene temperature of 400 K, much greater than the specified temperatures for the other ABI bands. Due to the finer field-of-view size of the ABI (compared to the current GOES imager), this hotter saturation temperature is needed. However, of the IR bands, this band suffers the most from increasing noise (in temperature units) at low scene temperatures, as a result of the basic physics of the Plank equation for shorter wavelengths. Of course this same shortwave band is also used for cloud characterization on the cold temperature end. Temperature noise in this band is much greater at low temperatures than that for the other ABI bands and can be an undesirable feature of the increased instrument maximum scene temperature. In particular, this band seems to require a 15-bit scale to meet ABI specifications, but that would still result in a quantization noise per count step of 2.1 K @200 K. Thus, it appears that a higher-resolution scale, or a 16-bit scale in this case, is more desirable, resulting in a quantization noise per count step of 1 K @200 K.