A rather technical question:

In the MODIS/Aqua .L2_LAC files that I receive through the ODPS subscription service (see e.g. ftp://ocdist1.sci.gsfc.nasa.gov/subscriptions/109/) it seems that the HDF global attribute 'Orbit Number' is set to zero. I want to use this number to automatically merge the 5-minute MODIS files when I have multiple scenes from one orbit over my area of interest (North Sea).

Can you help me to find out (automatically) what is the orbit number of a given MODIS L2_LAC scene?

Best regards and thanks in advance,

Reinold Pasterkamp
Institute for Environmental Studies,
Amsterdam, The Netherlands
www.vu.nl/ivm

Reinold,

The attribute in the L2_LAC file is a place-holder.  We need to add the code to transfer the orbit number from the L1B file.  Until that is done, there is no simple way to determine the orbit number automatically, unless you want to run two-line elements through an orbit propogator. The only other option is to pull it from the meta-data of the L1A or L1B file, if you have that. e.g.:

% ncdump -h A2003289175500.L1A_LAC | grep -w1 ORBITNUMBER
    "\n",
    "      OBJECT                 = ORBITNUMBER\n",
    "        CLASS                = \"1\"\n",
--
    "        VALUE                = 7726\n",
    "      END_OBJECT             = ORBITNUMBER\n",
    "\n",

If you don't have the L1A or L1B already, perhaps you can determine which granules go together by proximity in time.  We will put it on the list to incorporate the orbit number into the L2_LAC attributes.  It should be in the next code update, perhaps in a few weeks, but that won't have any impact to past products until we reprocess again.

BTW: The use of orbit number as a discriminator can be precarious for Aqua, as the orbit number changes on the ascending node crossing, which for Aqua is on the daylight side of the orbit. I realize this is not a problem for your high latitude situation, but I mention it here for others who may try to use it in equatorial regions.