4.2 Microphonic Noise Detection

LWR images are preprocessed in order to detect the presence of periodic noise interference (``microphonic noise''). In the SWP camera, this interference often covers all of the image and its amplitude is generally low (often only 1-3 DN) compared to random background noise (Northover, 1980), making detection difficult. Microphonic noise is also present in LWP images and is similar in appearance and amplitude to SWP microphonic noise. In the LWR, however, the microphonic noise has different characteristics, being localized chiefly in a small number of image lines, well-modeled by an exponentially dampened sinusoid, and with a peak amplitude typically in excess of 20 DN (Northover, 1980; Panek and Schiffer, 1981). The LWR microphonic noise is descriptively referred to as a ``ping'' because of its sudden onset and rapid decay. Unless an extended heater warm-up prior to image read is used as a ping avoidance technique, a given LWR image has about an 85% probability of suffering a ping, generally in the lower one-third of the image (Holm and Panek, 1982). If the heater warmup procedure is used, the ping is typically displaced to the top of the image where it does not impact the useful data.

Utilizing techniques developed by K. Northover (1980), the microphonics screening done by NEWSIPS for all LWR images is based on the characteristics of the image data in the last 32 samples of each image line. This area is outside of the target region, and pixel values are zero except for noise. Image lines affected by the microphonics are initially identified by thresholding the variance of the last 32 samples; the interference amplitude is then estimated on the basis of the power spectrum of the sampled data, with successive image lines processed in pairs. Pixels in image lines with estimated noise amplitudes in excess of the chosen threshold (corresponding to a peak-to-peak noise amplitude of approximately 10 DN) are flagged in the data quality ($\nu$) flag image with a value of -16. This condition, which is documented by a notation in the image processing history portion of the label (Chapter 12) and noted in the core data item ABNMICRO, is subsequently used by the spectral extraction routine (Chapters 9 and  10 ) to flag extracted fluxes derived from lines affected by the microphonic interference.