**All Files**

The header text lines begin with either a
colon (:) or a pound sign (#). The header contains metadata on product name,
creation time, units, source, missing data, end of record identifier, time range,
and cadence. The product filename includes the begin date/time (UTC) of each
product run in the format of yyyyMMddhhmm_filename.txt. Where yyyy = 4-digit
year, MM = 2-digit month, dd = 2-digit day, hh = 2-digit hour, and mm = 2-digit
minute. The product run interval is 15 minutes and is indicated by the

Note: The data files cover regions outside
of the CONUS where no real-time data is available. Values outside the CONUS
have larger uncertainty and should be treated with caution.

Note also that the product has not been operating in real-time through many
extreme storm events so the TEC estimates should again be treated with caution.

**Vertical and Slant TEC**

yyyyMMddhhmm_ustec.txt contains the vertical and slant TEC. The number in the first row, first column indicates the number of GPS stations used in the calculation of this file. The first row, from second column to the last column, contains the longitude locations. The first column, from the second row to the row just before the row starting with 999xx, contains the latitude locations. The latitude and longitude values should be divided by 10 to obtain the correct values. The latitude and longitude values are written without decimals to minimize the file size. Latitude and longitude are in degrees x 10. The latitude column and longitude row are the left and top borders for a matrix containing the vertical TEC values, which are in TECU (10^16 electrons/m^2) x 10. An example of this matrix is given below.

The first value in the table after the header text provides the number of stations used in this assimilation cycle. In the example, the number of GPS stations used in this assimilation cycle is 81, the top-left number in the file. In this example, the latitude ranges from 10.0 to16.0 degrees where all positive values are in the northern hemisphere, and the longitude ranges from -150.0 to -146.0 degrees where all negative values are in the western hemisphere. The vertical TEC is found in the first array before the first line beginning with 999xx. Therefore, the TEC value at 13.0 degrees latitude and -147.0 degrees longitude is 47.0 TECU, or the TEC at 15.0 degrees latitude and -150 degrees longitude is 46.7 TECU. In this particular array, all values are in the northern and western hemispheres.

# Vertical and Slant Path Total Electron Content #------------------------------------------------------------------- 81 -1500 -1490 -1480 -1470 -1460 100 479 478 478 478 479 110 482 481 480 480 480 120 480 478 477 477 477 130 473 471 469 470 471 140 468 466 463 464 464 150 467 465 462 461 460 160 471 468 465 463 460 99901 -1500 -1490 -1480 -1470 -1460 100 0 0 0 0 0 110 0 0 0 0 0 120 0 0 0 0 0 130 698 699 700 701 703 140 728 728 728 730 731 150 754 754 753 755 757 160 779 778 777 779 780 99921 -1500 -1490 -1480 -1470 -1460 100 1215 1192 1169 1145 1121 110 1188 1165 1142 1118 1093 120 1159 1136 1113 1089 1065 130 1128 1106 1083 1060 1036 140 1090 1068 1046 1024 1001 150 1047 1027 1007 985 964 160 1002 983 963 943 922

The slant TEC arrays follow the first array containing the vertical TEC. These arrays begin with a number in the form 999xx, where the xx is the space vehicle number (SVN). Again, the slant TEC values are bordered by the latitude as the first column and longitude as the first row, matching the vertical TEC spacing. The value within the array are the TEC along the line of site from the latitude and longitude position indicated and the satellite with the SVN number xx. In the example above, two GPS satellites are viewed, 01 and 22, indicated by the values, 99901 and 99922. The TEC, from SVN 01 and the position 16.0 degrees latitude and -14.6 degrees longitude, is 78.0 TECU, or the TEC, from SVN 22 and the position 10.0 degree latitude and -15.0 degrees longitude, is 121.5 TECU. If a 0 is indicated, then the satellite is not in view. For example, the above array indicates that satellite 01 is only in view between 13.0 and 16.0 degrees latitude, and satellite 22 is in view for all locations.

**TEC Uncertainty**

yyyyMMddhhmm_ERR.txt contains the expected error in the vertical TEC. These errors are obtained from the Kalman filter’s estimate of the state error. The number in the first row, first column indicates the number of GPS stations used in the calculation of this file. The first row, from second column to the last column, contains the longitude locations. The first column, from the second row to the last row, contains the latitude locations. The latitude and longitude values should be divided by 10 to obtain the correct values. The latitude and longitude values are written without decimals to minimize the file size. Latitude and longitude are in degrees x 10. The latitude column and longitude row are the left and top borders for a matrix containing the expected vertical TEC values, which are in TECU (10^16 electrons/m^2) x 10. An example of this matrix is given below.

In the example, the number of GPS stations used in this assimilation cycle is 78, the top-left number in the file. The latitude ranges from 21.0 to 25.5 degrees longitude where all positive values are in the northern hemisphere. The longitude ranges from -160.0 to -144.0 degrees, where all negative values are in the western hemisphere. Therefore, the estimated error in the TEC value at 22.5 degrees latitude and -156.0 degrees longitude is 4.5 TECU, or the estimated error in the TEC at 25.5 degrees latitude and -144.0 degrees longitude is 5.5 TECU. In this particular array, all values are in the northern and western hemispheres.

# Vertical and Slant Path Total Electron Content #------------------------------------------------------------------- 78 -1600 -1560 -1520 -1480 -1440 210 43 42 41 40 39 225 47 45 44 43 38 240 52 51 49 46 42 255 65 63 58 57 55

**Recent Trend**

yyyyMMddhhmm_DIF.txt contains the deviation of the vertical TEC from the average of the most recent 10 previous days where a US-TEC solution was obtained. US-TEC tries to obtain the most recent 10 days. If data is not available, US-TEC searches for a set of 10 days within the most recent 20 days. If 10 days of US-TEC output are not available within the last 20 days, then this file is not displayed.

The number in the first row, first column indicates the average number of GPS stations over the previous 10 days used in the calculation of this file. The first row, from second column to the last column, contains the longitude locations. The first column, from the second row to the last row, contains the latitude locations. The latitude and longitude values should be divided by 10 to obtain the correct values. The latitude and longitude values are written without decimals to minimize the file size. Latitude and longitude are in degrees x 10. The latitude column and longitude row are the left and top borders for a matrix containing the deviation in TEC from the previous 10-day average, which are in TECU (10^16 electrons/m^2) x 10. An example of this matrix is given below.

In the example, the number of average GPS stations over the 10 days used in the calculation of the file is 83, the top-left number in the file. The latitude ranges from 25.0 to 31.0 degrees longitude where all positive values are in the northern hemisphere. The longitude ranges from -160.0 to -144.0 degrees, where all negative values are in the western hemisphere. Therefore, the deviation from the 10-day average in the TEC value at 26.5 degrees latitude and -156.0 degrees longitude is 1.2 TECU, or the estimated error in the TEC at 31.0 degrees latitude and -144.0 degrees longitude is -0.5 TECU. In this particular array, all values are in the northern and western hemispheres.

# Vertical and Slant Path Total Electron Content #------------------------------------------------------------------- 83 -1600 -1560 -1520 -1480 -1440 250 11 9 10 9 7 265 9 12 4 2 2 280 8 3 0 -2 0 295 5 10 -3 -5 -2 310 11 -2 -5 -7 -5

**Stations Used**

yyyyMMddhhmm_stations.txt contains a list of the station used in the US-TEC calculation for this given time frame. The station can be identified through the web site, http://www.ngs.noaa.gov/CORS/sort_sites.html.

Data from each station listed was used in the calculations for the time-corresponding USTEC maps and data files.

**Daily Empirical Ortho-Normal Functions (EOFs)**

yyyyMMdd_EOF.txt contains the empirical orthonormal functions (EOFs). The 1st row of data in yyyyMMdd_EOF.txt lists 4 numbers describing the elements for the array immediately below this row. The 4 numbers in this 1st row, in order from left to right, are:

Na =number of altitude ticks (one tick for each row)

Ne = number of EOFs (one EOF for each column, 1st EOF in column 1,

2nd EOF in column 2, etc), (also, number of columns = Ne).

h = starting altitude from the center of the earth (km) in first row

Dh = spacing (km) of altitude ticks

Array size in yyyyMMdd_EOF.txt = Na X Ne

Examples: if Na=10, Ne=3, h=6550.0, Dh=25.0

The array size in yyyyMMdd_EOF.txt would be 10 x 3

The element in row 1, column 1: 6550 km alt., 1st EOF

The element in row 2, column 1: 6575 km alt., 1st EOF

The element in row 1, column 2: 6550 km alt., 2nd EOF

The element in row 6, column 2: 6675 km alt., 2nd EOF

The element in row 10, column 3: 6775 km alt., 3rd EOF

End of Examples.

**Coefficients**

yyyyMMddhhmm_COE.txt contains the Coefficients for the empirical orthonormal functions (EOFs) in yyyyMMdd_EOF.txt. The 1st row of data in yyyyMMddhhmm_COE.txt lists 6 numbers describing the elements for the array immediately below this row. The 6 numbers in this 1st row, in order from left to right, are:

Lat.Ticks = number of latitude ticks (one tick for each row)

Lon.Ticks = number of longitude ticks (one tick for each column)

StartLat = value (degrees) of the first latitude tick in the upper left corner

StartLon = value (degrees) of the first longitude tick in the upper left corner

LatSpacing = spacing (degrees) of the latitude ticks

LonSpacing = spacing (degrees) of the longitude ticks

Array size in yyyyMMddhhmm_COE.txt = (Ne * N) x M

where Ne = number of EOFs (Ne found in yyyyMMdd_EOF.txt). The 1st NxM array is the factor for the 1st EOF (column 1 of yyyyMMdd_EOF.txt), the 2nd NxM array is the factor for the 2nd EOF (column 2 of yyyyMMdd_EOF.txt), etc.

Examples: if N=3, M=5, No=40.0, Mo=-90.0, Ns=10.0, Ms=2.5, columns in EOF file = 3

The array size in yyyyMMddhhmm_COE.txt would be (3*3) x 5 = 9 x 5

The element in row 1, column 1: 40 deg. lat., -90 deg. long., factor for 1st EOF

The element in row 2, column 1: 50 deg. lat., -90 deg. long., factor for 1st EOF

The element in row 1, column 2: 40 deg. lat., -87.5 deg. long., factor for 1st EOF

The element in row 3, column 5: 60 deg. lat., -80 deg. long., factor for 1st EOF

The element in row 4, column 1: 40 deg. lat., -90 deg. long., factor for 2nd EOF

The element in row 5, column 3: 50 deg. lat., -85 deg. long., factor for 2nd EOF

The element in row 7, column 1: 40 deg. lat., -90 deg. long., factor for 3rd EOF

The element in row 9, column 4: 60 deg. lat., -82.5 deg. long., factor for 3rd EOF

**Using Daily Empirical Ortho-Normal Functions (EOFs) and Coefficients**

**Electron Density**

To solve for the Electron Density (Ne) at a given latitude (lat), longitude (long), and altitude (alt):

EOF denotes values from yyyyMMdd_EOF.txt.

COE denotes values from yyyyMMddhhmm_COE.txt.

Ne(lat,long,alt)=COE(lat,long,EOF(column1))*EOF(column1,alt) +COE(lat,long,EOF(column2))*EOF(column2,alt)+... ...+COE(lat,long,EOF(columnN))*EOF(columnN,alt)

where N is the number of EOFs, and Ne(lat.long,alt) is the electron density for an individual bin at (lat,long,alt) in units of 10^{11} electrons/m^{3} .

Ne calculation example using the above examples: (3 EOFs):

To find the Ne at 60 deg. lat., -80 deg. long., and 6675 km alt:

Ne(60,-80,6675)=COE(row3,column5)*EOF(row6,column1) +COE(row6,column5)*EOF(row6,column2) +COE(row9,column5)*EOF(row6,column3)*

**Vertical and Slant Total Electron Content**

To calculate the "Vertical" Total Electron Content (TEC) at a given latitude and longitude, or the line-of-sight Electron Content along a given "Slant" direction :

EOF denotes values from yyyyMMdd_EOF.txt.

COE denotes values from yyyyMMddhhmm_COE.txt.

For "vertical" TEC, first evaluate Ne as outlined above at a given latitude (lat), longitude (long), and altitude (alt):

Ne(lat,long,alt)=COE(lat,long,EOF(column1))*EOF(column1,alt)

+COE(lat,long,EOF(column2))*EOF(column2,alt)+...

...+COE(lat,long,EOF(columnN))*EOF(columnN,alt)

where N is the number of EOFs and Ne(lat,long,alt) is the electron density for an individual bin at (lat,long,alt) in units of 10^{11} electrons/m^{3} .

With a bin size of 25 km, the electron content in each bin is given by: Ne(lat.long,alt) * 2.5x104 electrons/m^{2}

Secondly, by summing over all altitudes bins at a given latitude and longitude provides the total vertical TEC in units of electrons/m^{2}. This number will correspond to the vertical TEC in yyyyMMddhhmm_ustec.txt, which is in TEC units (TECU), where 1 TECU = 10^{16} electrons/m^{2}.

For "slant" TEC:

To obtain the slant TEC in yyyyMMddhhmm_ustec.txt, the geometry of the bin and raypath between satellite and receiver must be considered. The TEC for the slant, slantTEC, will increase proportionally to the distance traveled through the bin as compared to the vertical TEC value and distance, i.e. slantTEC(lat,long,alt) = TEC(lat,long,alt) *dh/Dh, where dh = distance traveled by slant raypath through a bin, Dh thick, centered at (lat,long,alt).

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