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. pysatMadrigal currently supports Vertical TEC (VTEC), Line-of-Site (LoS) Slant TEC, and ground receiver data handling through pysatMadrigal.instruments.gnss_tec.

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.

import datetime as dt
import matplotlib as mpl
import matplotlib.pyplot as plt
import numpy as np
import pysat
import pysatMadrigal as pysat_mad

vtec = pysat.Instrument(inst_module=pysat_mad.instruments.gnss_tec,
                        tag='vtec')
ftime = dt.datetime(2013, 1, 1)

if not ftime in vtec.files.files.index:
    vtec.download(start=ftime, user='firstname+lastname', password='myname@email.address')

vtec.load(date=ftime)
print(vtec.variables)

The result of the above command should be ['time', 'gdlat', 'glon', 'dtec', 'gdalt', 'tec'], where 'tec' is the VTEC and 'dtec' is the error in the VTEC. To plot the data on a grid where each value takes up the desired grid size, we need to extend the latitude and longitude coordinates, as the data is specified at the lower edge of each coordinate bin.

coords = {}
for ckey in ['gdlat', 'glon']:
    coords[ckey] = list(vtec[ckey].values)
    coords[ckey].append(vtec.meta[ckey, vtec.meta.labels.max_val])
    coords[ckey] = np.array(coords[ckey])

Now, create a figure using pcolormesh to create a regular grid with VTEC value indicated by color.

fig = plt.figure()
ax = fig.add_subplot(111)

itime = 0
vmin = vtec.meta['tec', vtec.meta.labels.min_val]
vmax = np.ceil(vtec['tec'][itime].max().values / 10.0) * 10.0
con = ax.pcolormesh(coords['glon'], coords['gdlat'],
                    vtec['tec'].values[itime], vmin=vmin, vmax=vmax)

ax.set_facecolor('0.9')
ax.xaxis.set_major_locator(mpl.ticker.MultipleLocator(60))
ax.yaxis.set_major_locator(mpl.ticker.MultipleLocator(30))
ax.set_xlabel('Geographic Longitude ($^\circ$)')
ax.set_ylabel('Geodedic Latitude ($^\circ$)')
plt.colorbar(mappable=con, ax=ax, location='right', fraction=.1, pad=.01,
             label='VTEC ({:s})'.format(vtec.meta['tec',
                                                  vtec.meta.labels.units]))
fig.suptitle('{:s} {:s} at {:s}'.format(
    vtec.platform.upper(), vtec.tag.upper(),
    vtec.index[itime].strftime('%d %b %Y %H:%M:%S UT'), fontsize='medium'))

plt.show()

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.

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='myname@email.address')

# 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')

When loading data by a specific time or receiver, it may be desirable to determine what times and receivers are available. The functions pysatMadrigal.instruments.methods.gnss.get_los_receiver_sites() and pysatMadrigal.instruments.methods.gnss.get_los_times() can be used on a list of filenames to determine what loading options are available.