NOAA-17 status update

I got an very interesting email today. Maybe someone is interested in to read this…

received image (pristine) from the 20:46 UTC pass

I wanted to take the time to explain to you all what is going one with NOAA-17’s AVHRR and what you all are seeing in the images.
First a little lesson about the AVHRR scanner assembly. The scanner assembly consists of an 80-pole hysteresis synchronous motor that uses a set of duplex bearings. The scanner operates at 360 rpm and excess the jitter requirement which calls for less than 17us of jitter from scan line to scan line. “Usually”. What happens when the instrument ages is that the duplex bearings build up contaminants which comes from the wear and tear of its inner components. As this debris build up inside the bearings, a build up in resistants takes place and the current in the motor begins to rise. This excess force translates in an increase of heat in the scan motor and the loss of synchronicity that is seen in the image as jitter and worst case rephrasing by the MIRP (Manipulated Information Rate Processor).
Over the past three months the current in the AVHRR has steadily increased along with the sync deltas, which now has put the scan motor out of spec. NOAA engineers in a way to identify the problem decided to disabled rephasing in MIRP. With rephasing disabled, NOAA engineers are using measures whether by running an internal diagnostic programs in the MIRP or a series of scripts which are run within a stored command table to help identify how seriously unstable the scan motor really is. The images that we all see with the barcode patterns and the ramping up and down slope is representative of this type of testing being done and is not the scan motor stalling. These images are then ingested at NOAA where they are analyzed and interoperated to come up with a fix if possible to the AVHRR.
Now if you take an APT and a HRPT image from the same pass, the results will be very different. This is because the HRPT is a 1km resolution image and contains all the data received from the AVHRR while the APT image is 4km resolution image and is an average of three scan lines for every one scanned from the AVHRR. Also the MIRP corrects for any linier distortions in the APT image not the HRPT image due to the curvature of the earth and the way the AVHRR scans it. Because of these distinct differences in the two images, the bar-coding and ramping may appear in one image and not in the other.
So where does this leave us now? Well in the next two weeks the NOAA engineers will be talking with the instrument manufactures and reviewing the data to try and resolve or at best lessen the problem in the instrument. There are no guaranties that this will be resolved and in fact it is a real possibility that as the motor current continue to rise that the motor will fail permanently. In satellites past such as NOAA-12 and NOAA-16 this problem resolved itself over time but in the case of NOAA-11 the scan motor failed shortly after this event.
I will continue to keep you all informed as to any changes that take place in the status of the AVHRR or any announcements that comes from NOAA.
Thank you,
Steven Ross
Weatherscience.net