GGSPS Data

How to access data

How to read data

GGSPS or ROLSS ?

• GGSPS– Level 1.5 NANRG and ARG filtered radiances– Level 2 ARG fluxes and unfiltered radiances

from RGP (40 day delay)

• ROLSS– Level 2 ARG, BARG, SHI fluxes and

unfiltered radiances (< 40 days)

Access to GGSPS Datahttp://ggsps.rl.ac.uk/

Registration: Controlled by Imperial.

Access to GGSPS Data“Catalogue”: Catalogue searches only

several search criteria

“Products”: Catalogue searches and retrieval of products

Access to GGSPS Data

Downloading Products

• Three ways to download products:– click on link to an individual file– click on “Download” button

• maximum limit of 600 files

– “Access the most recent data products” (from the “Products” page) … tarred and zipped products in 6 hour bundles.

Structure of Products

• L1.5 NANRG and ARG Products– NANRG (6 individual unrectified images)

• up to 6 radiance datasets (256 x 282)

• up to 6 longitude and 6 latitude datasets

– ARG (2 time averaged rectified images)• 2 radiance, 2 standard dev, 2 valid. flag datasets

• 1 longitude, 1 latitude, 3 viewing angle datasets

• HDF5 Format (Hierarchical Data Format)

HDF5 Files

• Three entities to know about:– Groups: rather like directories, define a

hierarchy within which the data is stored– Datasets: tend to hold one or two dimensional

arrays of data. Has an associated “dataspace”.– Attributes: tend to hold individual values,

sometimes one dimensional arrays, of metadata. Groups and datasets can both have attributes.

NANRG

Groups Datasets Attributes

N A N R G

Q u a ntisa tio n Fa c to r

U n it

L a titu d e (o r E le va tio n)

L o n g itu d e (o r A z im u th )

S h ort W a ve Im ag e 1 S h ort W a ve Im ag e 2

G e o lo ca tion

Num ber of Colum ns in Short W ave Im age 1

...

Q u a ntisa tio n Fa c to r

U n it

S h ort W a ve R ad ia nce Im ag e 1

S h ort W a ve R ad ia nce Im ag e 2

T o ta l R ad ian ce Im ag e 1

R a d io m e try

/

Groups Datasets Attributes

ARGA R G

Q u a ntisa tio n Fa c to r

U n it

L a titude

L o ng itu de

R e la tive a z im u th a n g le

S o la r ze n ith an g le

V ie w in g ze n ith an g le

N x

N y

L ap

...

R e c tif ie d G rid

G e o lo ca tion

Q u a ntisa tio n Fa c to r

U n it

S h o rt W a ve F ilte re d R a d ia n ce

S hort W ave F iltered R ad iance S tand ard D eviatio n

S hort W ave F iltered R adiance V alid ity F la g

T o ta l F ilte red R ad ia n ce

T ota l F iltered R adian ce S tandard D eviatio n

T ota l F iltered R ad iance V alid ity F la g

R a d io m e try

/

Datasets

• Usually signed short integers

• Written out row by row (north-> south); each row west-> east

• use quantisation factor attribute to convert data to correct units:

Converted value = encoded value * quantisation factor

How to read products• HDF5 tools (h5dump, h5ls, H5View)

– useful for seeing what’s in an unknown file

• IDL– IDL v5.6+ (or v5.5 & plug-in) : HDF5– <= v5.5 : HDF4 only. Use h5toh4 to generate

HDF4 GERB products.– bugs in h5_browser/h5_parse

• HDF APIs– write your own Fortran/C/C++/Java/… code

Example (IDL HDF5 interface)

FILE_ID = H5F_OPEN(FileName) ; open file

; open root and radiometry groups

root_group_id = H5G_OPEN(FILE_ID,ROOT_GROUP)radiometry_group_id = H5G_OPEN(root_group_id,RADIOMETRY_GROUP)

; open and read the dataset

SW1_ID = H5D_OPEN(radiometry_group_id,SW_1)SW1Data = H5D_READ(SW1_ID)

; open and read the quantisation factor attribute

ATT_ID = H5A_OPEN_NAME(SW1_ID,QUANTISATION_FACTOR)SW1_QF_Data = H5A_READ(ATT_ID)

H5A_CLOSE, ATT_ID ; close attribute and datasetH5D_CLOSE, SW1_ID

H5G_CLOSE, radiometry_group_id ; close groupsH5G_CLOSE, root_group_id H5F_CLOSE, FILE_ID ; close file

Further Help• Example Code and Simple Tools on GGSPS

Web Site (see “Products” page)

• GGSPS User Guide will appear shortly

• RMIB User Guide available (more info on how to read HDF files)

• NCSA HDF Web Site– http://hdf.ncsa.uiuc.edu/– libraries, tools, tutorial, user guide, reference manual