NOAA-16/15 HIRS Scan Line Fix and NOAA-16 AVHRR

NOAA-16/15 HIRS:
The following software update to the AIP preprocessor will be implemented on
May 31, 2001 to correct the calibration for the first scan line of the HIRS
passes. This update has been tested internally and no differences in the
data were detected when compared to the operational level 1Bs. This should
be a no impact change to the user community. Below is a more detailed
explanation of why the update is needed.
If you need to look at test data, please use only the AIP (HIRS, AMSU-A, and
AMSU-B) level 1B and 1B* data found on the CEMSCS (P1B and P1S) and the same
data from the anonymous ftp site ( under subdirectory
pub/test1b/NOAA15 and NOAA16). The AVHRR data contains preliminary
corrections for the PRT conversion coefficients and visible calibration
coefficients. Please contact me, if you have questions or concerns.

NOAA-16 AVHRR PRTs and VISIBLE Calibration Coefficients:
Problems with the PRT conversion coefficients was mentioned in the notice
outlining our plans over the next four months. Since then, we have
discovered that the visible coefficients for NOAA-16 must be updated. We
are conducting preliminary tests of the AVHRR level 1B and 1B* data
containing these updates. The test data are on the above sites, however,
this data has not been verified by the instrument scientist and should not
be considered final. Once internal testing is complete and the instrument
scientist has completed the data verification, a notice will be sent out to
the user community. We believe the changes will have a minimal impact and a
week of testing will be sufficient for all users. The forthcoming notice
will contain detailed information on the changes that the user should expect
to see. Please look at the data as soon as possible after you receive the
notification and notify us of your findings and your needs for testing.
Our tentative plan is to allow a week of testing after the notice has been
Thank you.
Emily Harrod

Details on the correct calibration for first scan line of HIRS pass:
Sometime since the NOAA-16 AIP processing became operational, it
was noticed that the intercept in the first scan in the HIRS NOAA-15/16
1b*/1b was incorrect. The problem caused the temperatures in the first
scan line to be wrong and appear as a distinct anomaly in the images
generated from the HIRS data. Complicating the problem was the
erroneous setting of a time sequence error flag. Analysis revealed that
the the time sequence flag started being set when clock corrections were
turned on to correct an along track error.
It was discovered that the flag was being inadvertently set because the
initial expected time used for comparing successive scans was not a
corrected time. This error was corrected and rather than concentrate on
a time sequence problem, personnel began examining the HIRS calibration.
After much examination of the entire calibration process, an anomaly was
discovered in the part of the secondary mirror interpolation scheme
which uses a polynomial algorithm to convert counts to secondary mirror
temperatures. The interpolation algorithm assumes that three
temperatures exist; a previous, a current and a subsequent mirror
temperature. At the
beginning of an orbit there are usually missing scans and very rarely a
complete superswath. Consequently one or two of the counts were missing
and were assigned a value of zero. The algorithm that converts from
counts to temperature was bypassing code based on the count being a
missing value (not zero). The algorithm was executed in some cases
where some of the counts were 0 causing erroneous secondary mirror

* Emily D. Harrod
* Product Systems Branch
* Pre-Product Processing Group
* E/SP13, Federal Building #4, room 0318
* 5200 Auth Road
* Suitland, Maryland 20746-4304
* voice 301-457-5247
* fax 301-457-5199
* email