August 19, 2016 – The Space Weather Prediction Center (SWPC) has deployed an updated and expanded version of its US Total Electron Content (US-TEC) model. The model now uses more GPS stations, has increased coverage to all of North America, and has added scale information that indicates position error.
The Total Electron Content (TEC) is the total number of electrons present along a path between a radio transmitter and receiver. Radio waves are affected by the presence of electrons, and the more electrons are in the path of the radio wave, the more the radio signal is affected. For ground to satellite communication and satellite navigation, TEC is a good parameter to monitor for possible space weather impacts.
The TEC in the ionosphere is modified by changing solar extreme ultraviolet radiation, geomagnetic storms, and the atmospheric waves that propagate up from the lower atmosphere. The TEC therefore depends on local time, latitude, longitude, season, geomagnetic conditions, solar cycle and activity, and tropospheric conditions.
The propagation of radio waves is affected by the ionosphere. The velocity of radio waves changes when the signal passes through the electrons in the ionosphere. The total delay suffered by a radio wave propagating through the ionosphere depends both on the frequency of the radio wave and the TEC between the transmitter and the receiver. At some frequencies the radio waves pass through the ionosphere. At other frequencies, the waves are reflected by the ionosphere.
The change in the path and velocity of radio waves in the ionosphere has a big impact on the accuracy of satellite navigation systems such as GPS/GNSS. Neglecting changes in the ionosphere TEC can introduce tens of meters of error in the position calculations.
Two web pages are supported by the new model: North American Total Electron Content and US Region Total Electron Content. Both are designed for single and dual frequency GPS applications and provide a near real-time assessment of the Total Electron Content (TEC) which is often used as a proxy for GPS position error. The TEC maps can be used to estimate the GPS signal delay due to the ionospheric electron content between a receiver and a GPS satellite. This delay can be translated into GPS positioning error.
Both pages are currently hosted on experimental pages for customer evaluation before being promoted formally to operations. Promotion to operations is planned for September 26. At the same time the current US-TEC model will be decommissioned.
Comments on the updated model can be provided on SWPC’s feedback page.
The Space Weather Prediction Center is located in Boulder, Colorado. SWPC provides a wide range of alerts, warnings, watches and forecasts related to space weather.