DOC: updated the TEC example

Updated the TEC example to express available data and show how to use the selective LoS loading.

docs/examples/ex_gnss_tec.rst

@@ -1,18 +1,21 @@
 .. _ex-gnss-tec:
 
-Plot GNSS TEC
-=============
+GNSS TEC
+========
 
 The Global Navigation Satellte System (GNSS) Total Electron Content (TEC) is one
 of the most valuable ionospheric data sets, given its long and continuous
 operational duration and expansive coverage.  :py:mod:`pysatMadrigal` currently
-only supports Vertical TEC (VTEC) data handling through
-:py:mod:`pysatMadrigal.instruments.gnss_tec`.
+supports Vertical TEC (VTEC), Line-of-Site (LoS) Slant TEC, and ground receiver
+data handling through :py:mod:`pysatMadrigal.instruments.gnss_tec`.
 
-The VTEC measurements are median filtered to fill 1 degree latitude by 1
-degree longitude bins.  This can be represented with spatially representative
-coverage as shown in the example below.  Start by obtaining the desired data
-and loading it.
+Plot VTEC Maps
+--------------
+
+The Madrigal VTEC maps are made up of median filtered data placed in 1 degree
+latitude by 1 degree longitude bins.  This can be represented with spatially
+representative coverage as shown in the example below.  Start by obtaining the
+desired data and loading it.
 
 .. code::
 
@@ -81,3 +84,39 @@ a regular grid with VTEC value indicated by color.
     :width: 800px
     :align: center
     :alt: Mapped median Vertical Total Electron Content over the globe.
+
+
+Load LoS TEC
+------------
+
+The data used to create the Madrigal VTEC maps is available in the LoS TEC
+files.  These files contain both the Vertical and Slant TEC from the available
+GNSS satellite networks.  These files are large (several GB) and may not
+successfully download through the MadrigalWeb interface.  In such instances, it
+may be simpler to download the desired data directly from the Open Madrigal
+website.
+
+Once downloaded, the data is best loaded in subsets.  Current load options
+include loading data by a unique receiver name ('site'), a unique satellite ID
+('prn', 'sat', 'sat_id'), a time ('time', 'unix', 'ut1_unix', or 'ut2_unix'),
+an orientation ('azm', 'elm'), or a location ('gdlatr', 'gdlat', 'gdlonr',
+'glon').  If specifying a time, orientation, or location a range may also be
+specified, as illustrated in the following example.
+
+.. code::
+
+   tec = pysat.Instrument(inst_module=pysat_mad.instruments.gnss_tec,
+                          tag='los')
+   ftime = dt.datetime(2013, 1, 1)
+
+   # If this fails, access data at:
+   # http://cedar.openmadrigal.org/single?isGlobal=on&categories=17&instruments=8000&years=2013&months=1&days=1
+   # Then save the data with a '.hdf5' extension in the directory found by:
+   # print(tec.files.data_path)
+   if not ftime in tec.files.files.index:
+       tec.download(start=ftime, user='firstname+lastname', password='[email protected]')
+
+   # Load only the GPS data from -40 to -20 degrees longitude
+   tec.load(date=ftime, los_method='glon', los_value=-30, los_range=10,
+            gnss_network='gps')
+