Local and Global IASI data intercomparison

Statistics reported for each station overpass (represented by a row in the table) pertain to the differences between local and global brightness temperatures (BTs), timestamps and field of view (FOV) coordinates for the selection of channels listed in the adjacent table.

Brightness temperature difference (BTD) plots present the differences between the local and global BTs (screened for invalid BTDs). Navigational difference plots depict the regional navigated fields of view, superimposed upon the global (screened for invalid coordinates).

The plots for each overpass are available by clicking on the corresponding "Overpass Time" entry in the table. Alternatively, view daily plots for each station (from the last week), here.

See also IASI monitoring statistics derived by Météo-France for the site in Lannion, which are available here. This includes L1C mean spectral differences in brightness temperature, maps of cloud fraction differences from local overpasses and statistics as well as cloud type retrievals from AVHRR.

Explanation of table statistics and data displayed in plots.

Click here for the differences between data displayed in the plots and used to calculate the statistics.

  • The Brightness Temperature Difference (BTD) plots display all valid BTDs, where BTDs are labelled invalid if the associated FOVs have invalid coordinates, spurious radiances or BTDs larger than 20K.
  • The navigational difference plots contain all navigational differences between local and global FOVs with valid coordinates. However, the local-global navigational difference mean is calculated after discarding any entirely spurious scanlines (in which all FOVs have spurious radiances or BTDs larger than 20K).
Header

Click here for information on how the different statistical measures are defined and calculated.

Overpass Time No. of spurious first FOV timestamps No. of common scanlines Mean local-global timestamp difference Percentage of valid Brightness Temperature Differences (BTDs) No. of spurious radiances Mean/maximum/minimum/standard deviation local-global BT difference Mean local-global navigational difference No. of spurious coordinates
Local Overpass Start time. A spurious local or global timestamp is not half-way between its adjacent scanlines’ first FOV timestamps. It could be:
  • A timestamp with corrupt or missing data.
 Having matched the local and global scanlines, any local or global timestamp with missing data is masked. The mean local-global timestamp difference across all FOVs is then calculated. Local-global radiance differences are converted to equivalent Brightness Temperature Differences (BTDs) using the Planck function, with 280K as the equivalent reference temperature.
BTDs are labelled ‘invalid’ if:
  • The BTD is larger than 20K.
  • The corresponding FOV local or global coordinates are invalid (see No. of spurious coordinates).
  • The corresponding FOV local or global radiances are spurious (see No. of spurious radiances).
The percentage of valid BTDs is calculated as a fraction of the number of potentially valid BTDs (all matched up FOVs).
  • A local or global FOV radiance is treated as spurious if the data is actually missing (the value has been set to a placeholder, -99999).
  • It is also treated as spurious if it has been flagged by GQisFlagQual as erroneous.
Local-global radiance differences are converted to equivalent BT differences using the Planck function, with 280K as the equivalent reference temperature.

The mean BTD is calculated from the BTDs between all FOVs in the matched local and global scanlines, after discarding BTDs labelled ‘invalid’ (see Percentage of valid BTDs). The other BTD statistics are also calculated in this way. 		The mean navigational difference is calculated from the navigational differences between all FOVs in the matched local and global scanlines (with valid coordinates).

This calculation is performed after discarding any scanlines where all FOVs have spurious radiances. 		The coordinates of a FOV are deemed spurious/invalid only if the data has been corrupted (the values are not physically possible).

Click here to see the thresholds used to generate warnings for overpasses.

Mean local-global timestamp difference 2s
Mean local-global navigational difference 15000m
Mean local-global BT difference 2K
Percentage valid BTDs in overpass interval 50%

How to use the table

Sort fields by clicking on the desired column heading; filter using the "search" bar.
Plots associated with each overpass can be viewed by clicking on the "Overpass Time" entries; clicking on each plot will enlarge the image.
Above the plots, there is a link to a text table of BT and BTD statistics for ALL channels for each overpass.
Overpasses which have not met the local-global data consistency thresholds have the relevant statistic highlighted in red.
Use the dropdown menu on the far right to selectively view these overpasses.


View overpasses with warnings:
File Name Satellite Station Day Overpass Time Channel Number Channel Index in MetDB Channel Wavelength No. of Local scanlines No. of Common scanlines Mean Timestamp Difference/s Mean Navigational Difference/m Percentage of valid BTDs Mean BTD Min BTD Max BTD STD of BTD No. of spurious local timestamps No. of spurious global timestamps No. of spurious local coordinates No. of spurious global coordinates No. of common spurious coordinates
File Name Satellite Station Day