This is a reminder about a known, long-standing problem with the HRIT Image Navigation Header
metadata LOFF,LFAC,COFF,CFAC, and the work-around.
Do not use those metadata in the Image Navigation Header to perform geolocation on GOES imagery
broadcast by HRIT/EMWIN. Instead, use the Ancillary Text Header metadata, which allow users
to compute image geolocation according to the GOES PUG (for example, see section 4 of the
GOES PUG Vol 5: Level2+ Products, on the OSPO site (
https://www.ospo.noaa.gov/resources/documents/goes-r.html))
-- History --
The HRIT Ancillary Text Header was introduced into the NOAA HRIT files in 2020 by OSPO, after user
complaints about geolocation errors and a re-examination of the HRIT INR calculation by OSPO developers.
The use of the HRIT Image Navigation Header metadata was then deprecated, but that was long
enough ago that new users (including me!) are not aware of the history.
The problem was rediscovered by Jamie Vital (
https://github.com/JVital2013), who contacted
us about inconsistencies in the [L,C][OFF,FAC] metadata between ABI bands.
I want to head-off any more wasted effort by the user community in developing code
using those metadata. Use the Ancillary Text Header for geolocation.
The team at OSPO is re-examining the issue with the aim to
avoid confusion in the future. We will touch upon it at the next User Group meeting (in planning).
My thanks (and apologies) to Jamie Vital and to Chris Wright for highlighting this problem
and bringing me up to speed on the history.
Yours,
-Ian