Slant Total Electron Content (STEC)

The ionosphere have many ions and free electrons. When the sun radiates through this layer of atmosphere, the particles will be separated into electrons and ions. These free electrons affect the GPS satellite signals and cause varying propagation delays. The electron density is higher during daytime than nighttime, in general. And it is higher during equniox seasons (Mar,Apr,Sept, Oct) than other seasons. Slant total electron content (STEC)  is the amount of electron density  along a slant path between a satellite and a receiver, expressed in TECU (1 TECU = 1016 electrons/m2).

Fig.1. Slant TEC (STEC) from a satellite to a receiver.

Slant TEC can be computed from the integral path of the Ne  is Electron density (electrons/m3), i.e.,

where  S is the distance along the propagation path (m).

In generally, the STEC values at night time are smaller than those at day time. Fig.2 shows an example of the STEC as calculated from the GPS satellite data, each color represents the computed STEC from the pseudorange and carrier phase from each satellite.

       Fig.2. Examples of STEC curves which are calculated by using the GPS data.

The ionosphere can cause the delay to the GPS signal called Ionospheric delay (I) expressed in meters. It  can be computed from


where f is the frequency (Hz).


Vertical Total Electron Content (VTEC)

VTEC is the total number of electron density  in a vertical path, also expressed  in TECU (1 TECU = 1016 electrons/m2)



 –  Zenith angle (degree).

 –  Elevation angle of the satellite (degree).

RE – Earth’s radius is equal 6378.137 km.

–  Height of the ionospheric layer, assume that height at 350 km.

IPP –  Ionospheric pierce point.

The STEC (blue) and the VTEC(red) are calculated from all PRNs on the same date are shown in Fig.3.

Fig.3. Examples of STEC and VTEC.


  1. P. Misra and P. Enge, GLOBAL POSITIONING SYSTEM Signals, Measurements, and Performance. United States of America, 2012.
  2. Norsuzila et al., American J. of Engineering and Applied Sciences., pp. 223-229, 2008.