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Frequently asked questions
==========================

Clouds in the data
~~~~~~~~~~~~~~~~~~

Q: Is it necessary to provide only measurement periods with cloud free conditions and 
homogeneous atmosphere or is this part of ELDA or the pre-processing of the data? 
For example in cases with scattered low cumulus clouds.

A: At moment you should provide cloud free data because the automatic cloud screening 
is not yet implemented in the SCC. We are working on this issue and hopefully the new 
module will be implemented at the end of ACTRIS project . 


High/low range channels
~~~~~~~~~~~~~~~~~~~~~~~
Q: What is the definition of low range, ultra near range and high range channels? 
Are there any threshold values?

A: There are no threshold values defined for the different range types. This information 
is used only in the gluing procedures just to identify which channel should be taken 
as low range and which one as far range. If no gluing is applied by the SCC the range 
id flags are not taken into account.

Variable depolarization factor
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Q: Due to changes in the settings of our lidar system, the depolarization factor can 
change during the measurement. Is it necessary to provide one SCC-file for each settings 
or is it possible to set the Depolarization_Factor variable time dependent? 
   
A: Unfortunately it is not possible to have time dependence on Depolarization_Factor variable. 
So if this parameter changes you should provide different SCC files for each setting.

Extra netcdf parameters
~~~~~~~~~~~~~~~~~~~~~~~
Q: Is it possible for documentation purposes to put own parameters in the SCC-NetCDF file? 
For example who created the file.... Are there any reasons against this? 

A: Technically as long as you use not standard SCC variables for your own parameters there are
no problems for the SCC. It will just ignore these not standard variables.
 

Netcdf version
~~~~~~~~~~~~~~ 
Q: Which NetCDF version is to use? NetCDF3, NetCDF3 Classic, NetCDF4, NetCDF4 Classic? 

A: The NetCDF libraries 4.1.3 are used in all the SCC modules. So all the NetCDF formats you 
have indicated should be compatible with the SCC (we have tested NetCDF3 and NetCDF4).

Lidar ratio
~~~~~~~~~~~
Q: What are the values for the lidar ratio used in the SCC_DB? 

A: The values of (fixed) lidar ratio used by the SCC in the elastic retrieval can be set by 
the user using the SCC web interface. In particular you can define a lidar ratio value for 
each elastic backscatter product: in the product page there is the section "Elastic Backscatter 
options" in which there is the field "Fixed lr". In case you want to use a lidar ratio profile 
you should set LR_Input accordingly and provide an external LR profile NetCDF file 
(see documentation on SCC file format).

Calculation of Raman and elastic backscatter
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Q: In cases of measurements where Raman channels are available, the SCC will calculate the Raman 
backscatter profile. If I want to retrieve Klett-retrievals for this channel, too (e.g 532nm) 
is it sufficient to set the value in LR_input(channels) to 1 or 0 plus a LR-profile to get 
both retrievals? 

A: No. In general, for each lidar configuration you can define a set of optical products to be calculated 
for that configuration using the SCC web interface. So suppose you have a system with 532nm and 607nm 
channels. In this case you have 2 options:

1. Raman backscatter and extinction in the e532 file and Raman backscatter (full resolution) in the b532 
   file. In this case you should associate to the configuration a product of type "lidar ratio and  will 
   produce the e532 file and a product of type "Raman backscatter" which will produce the b532 file
2. Raman backscatter and extinction in the e532 file and elastic backscatter in the b532 file. 
   In this case you should associate to the configuration a product of type "lidar ratio and extinction"
   which will produce the e532 file and a product of type "elastic backscatter" which will produce the b532 file

Note: you cannot calculate a b532 file containing the Raman and elastic backscatters at the same time. 
The reason is that the 2 products will produce an output file with the same name (according to the EARLINET rules). 
Moreover in general, it makes no sense to calculate the elastic backscatter when you can calculate the Raman 
one which usually is better.

Filename conventions
~~~~~~~~~~~~~~~~~~~~
A: What are the conventions for the filenames for the various files that need to be uploaded?

Q: The following definitions apply:   

SCC raw lidar data file
^^^^^^^^^^^^^^^^^^^^^^^

This file should be named as measID.nc. For example if your measurement had a measurement ID of 20130101cc00 the corresponding NetCDF file should be named 20130101cc00.nc 

Sounding file
^^^^^^^^^^^^^

The file should be named as rs_measID.nc. Considering the above example the sounding file should be named rs_20130101cc00.nc

In this case you should also set the global attribute Sounding_File_Name in the raw lidar data file as::

   Sounding_File_Name=rs_20130101cc00.nc

Lidar ratio file
^^^^^^^^^^^^^^^^
The file should be named as lr_measID.nc. Considering the above example the sounding file should be named lr_20130101cc00.nc

In this case you should also set the global attribute LR_File_Name in the raw lidar data file as::

   LR_File_Name=lr_20130101cc00.nc

Overlap file
^^^^^^^^^^^^
The file should be named as ov_measID.nc. Considering the above example the sounding file should be named ov_20130101cc00.nc

In this case you should also set the global attribute Overlap_File_Name in the raw lidar data file as::

   Overlap_File_Name=ov_20130101cc00.nc


Photocounting values should be integers
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Q: In one of my measurements I get an error concerning the photoncounting values::

   Pre processing (177): Found no integer values in photoncounting signal

The Raw_Lidar_Data variable in the NetCDF-file is defined as double. So is it necessary for Photoncounting signals to only provide integer values?

A: Two important considerations:

1. The Raw_Lidar_Data array should contain your *real* raw data. 
   This means that *no corrections/operations* should be made on your signals 
   before filling the Raw_Lidar_Data array. This is particularly important because *all* 
   the operations and corrections should be applied by the SCC and not by the user before 
   the submission. In this way we can keep track of all the operations made on the signals 
   (for QA purposes) and moreover we are sure that all the corrections are applied in a 
   correct order  (this is particularly important for non linear operations, think for example to the dead time correction).

2. The analog signals should be expressed in mV and the photoncounting signals in raw counts

So if your photoncounting values are not integers they are not expressed in raw counts (which of course should be integers). 
So the point here is not how to convert them in integers but to submit the right quantity in the right units.

So please check carefully your converter and be sure to really submit raw counts for photoncounting channels and raw mV for the analog ones.


Preprocessing failed but no Exit code is provided
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Q: The preprocessing of one of my measurements failed (I get a status -127). However, when I check the Exit codes 
to see the description of the problem, I get an empty value (-). What does this mean?

A: This means that the preprocessor crushed unexpectedly! Sorry for that! Report the problem in the forum and it will be fixed 
soon.