MODIS "Missing datas"

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jcfischer
Posts: 47
Joined: Wed Aug 23, 2017 12:23 pm America/New_York

MODIS "Missing datas"

by jcfischer » Thu Feb 15, 2018 12:10 am America/New_York

Dear all,
I am working with MODIS Aqua satellite imagery for the full period of operation of this sensor. My AOI consists of more than 500 000 square kilometers in the central Indian Ocean. The very high revisit frequency of this sensor is one of its great advantages.
I realized that for the full period starting with the 4th July 2002 until the end of 2017 there is no data provided for my research area for the the following dates (year-month-day): 2002-07-30, 2002-07-31, 2002-08-01, 2002-08-02, 2002-08-03, 2002-08-04, 2002-08-05, 2002-08-06, 2005-10-18, 2007-12-02, 2008-04-11 and 2009-02-03. Does anybody know something about potential reasons for this?
Thanks!

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gfireman
Posts: 59
Joined: Thu Jan 07, 2010 2:59 pm America/New_York

MODIS "Missing datas"

by gfireman » Thu Feb 15, 2018 1:17 pm America/New_York

Every satellite mission has occasional data outages.  Here is one site that tracks them for the MODIS and VIIRS instruments:
https://modaps.modaps.eosdis.nasa.gov/services/production/outages.html
That page notes, for instance, that MODIS Aqua was in safe mode from July 29, 2002 - August 08, 2002.

fredoceansips
Posts: 7
Joined: Fri Nov 09, 2007 1:39 pm America/New_York

MODIS "Missing datas"

by fredoceansips » Thu Feb 15, 2018 1:18 pm America/New_York

All of these periods represent MODIS data outages due to various anomalies.  The contiguous period from 2002-07-30 through 2002-08-06 was an instrument safehold event that occurred early in the mission.  The other dates all represent either instrument anomalies, spacecraft anomalies or downlink failures.

Fred Patt

jcfischer
Posts: 47
Joined: Wed Aug 23, 2017 12:23 pm America/New_York

MODIS "Missing datas"

by jcfischer » Wed Feb 21, 2018 12:24 am America/New_York

Thanks for this link! Good to know what happened in these respective time periods.
Greetings, JC

obdaac_forum_user
Posts: 86
Joined: Wed Jan 27, 2021 1:52 pm America/New_York

MODIS "Missing datas"

by obdaac_forum_user » Thu Feb 22, 2018 7:20 pm America/New_York

Hi.  I retrieved the file T2000091000000.L2_LAC_OC.nc MODIS TERRA Level-2 LAC Ocean Color data file and tried to access the Variables in the NetCDF file using IDL.  After opening the file, I then I queried the file using the IDL function,
IDL> Result = NCDF_INQUIRE(Cdfid)
IDL> help, result
** Structure <f7d928>, 4 tags, length=16, data length=16, refs=1:
   NDIMS           LONG                 5
   NVARS           LONG                 0
   NGATTS          LONG                51
   RECDIM          LONG                -1
The above result seems to indicate that there are no named variables in the file.  In the past, I would get the data using an id of "chlor_a" for a variable, for instance.  It looks like there is data in the file, but how do I access the data using IDL library functions or procedures?
Thank you for your help.

gnwiii
Posts: 611
Joined: Fri Jan 29, 2021 5:51 pm America/New_York
Answers: 1

MODIS "Missing datas"

by gnwiii » Fri Feb 23, 2018 7:21 am America/New_York

You should start a new topic since this has nothing to do with the missing files due to satellite issues.   This topic should probably be under non-SeaDAS Packages. You don't mention the IDL version or platform.  Working with the NetCDF4-CF format level2 files means you have to deal with groups.  It is also helpful to know that
NetCDF4 files are also HDF5 files:

IDL> H5_LIST, "T2000091000000.L2_LAC_OC.nc"
% Loaded DLM: HDF5.
file        T2000091000000.L2_LAC_OC.nc             
group       /geophysical_data                       
[...]
dataset     /geophysical_data/chlor_a                H5T_FLOAT [1354, 2030]
[...]


NASA's IDL Library has READL2NC.PRO:

;+NAME/ONE LINE DESCRIPTION OF ROUTINE:
;    READL2NC reads a geophysical dataset from an l2gen netcdf4 file, applies scaling
;
;  NAME:
;    readl2nc
;
;  PURPOSE:                                  
;    Simplifies the reading of standard k2gen netcdf4 file. Auto
;    applies slope/intercept scaling. Can read multiple band-specific
;    SDS products into a single array.
;
;  CALLING SEQUENCE:
;    data = readl2nc(filename,prodname)
;
;  INPUT:
;    filename - L2 filename string.
;    prodname - L2 product name string. If this ends in '_', all
;               products of name prodname_nnn (where nnn is wavelength)
;               will be returned.
;
;  OUTPUT:
;    data - 1, 2, or 3-dimensional array containing the requested product.
;
;  SUBROUTINES CALLED:
;
;  WRITTEN BY:
;    B. Franz, October 2014.
;

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