The ionosphere is a part of the earth’s atmosphere, which extends from approximately 50 – 1000 km in altitude. The sun's ultraviolet and x-ray emissions are primarily responsible for causing ionization in the ionosphere. When a signal from the GNSS satellite is travelling through the ionized layer in the ionosphere, refraction, or bending of the wave, occurs. The amount of refraction that occurs depends on three main factors: (1) the density of ionization of the layer, (2) the frequency of the radio wave, e.g. for GPS L1 it is 1575.42 MHz, and (3) the angle at which the wave enters the layer.
An important descriptive quantity in describing the effect of the ionosphere on the GNSS signal is the total electron content (or TEC). TEC is the total number of electrons present along a path between the satellite and the receiver on earth, with units of electrons per square meter, where 1016 electrons/m² = 1 TEC unit (TECU).
The relationship between TECU and the group delay of a GNSS signal is described in the first approximation by
where k is a constant equal to approximately 40.3 and f is the signal carrier frequency. For the GPS L1 frequency where fL1=1575.42 MHz one TECU corresponds to a delay of approximately 0.162 meters. TEC is strongly affected by solar activity.
Trimble is continuously computing a global ionospheric model, which is represented by a spherical harmonic expansion of vertical TEC units. This model is derived from a global GNSS tracking network, compiled into a map and updated every five minutes. The latest map is available via the following link http://www.trimbleionoinfo.com/Images.svc/TEC
The level of ionospheric activity is dependent on:
§ Solar activity; it is highest at solar maxima during an 11 year solar cycle; the next solar maximum will be expected to be reached during 2013
Ionospheric delay is frequency dependent, i.e. under normal conditions, dual-frequency (L1 and L2) code and carrier observations can be used to essentially remove ionospheric errors. The increased solar activity has the following negative impact on GNSS:
§ Moderate to severe tracking problems around equatorial anomaly and polar regions and occasional problems due to Travelling Ionospheric Disturbances at mid-latitudes
§ Degraded RTK and RTX initialization and positioning performance
§ Degraded VRS network performance
Normally the ionospheric effects decorrelates at a rate of 1 part per million (ppm) of the receiver separation (i.e. 1mm per kilometer). However, large gradients can occur in ionosphere following solar storms (> 30ppm). Under extreme conditions, ionosphere can become highly stratified (irregular distribution of charged particles) leading to GNSS signal scintillation.
Scintillation involves fluctuation in the phase and amplitude of GNSS signals. In extreme cases, scintillation can cause loss of signal tracking (i.e. cycle slips). It is important to note that the effects of scintillation are not removed by dual-frequency observations. Trimble has setup a global ionospheric scintillation sounding network, which detects scintillation effects and is able to give up to date warning information on scintillation effects in different parts of the world.
Typically scintillation occurs in equatorial regions after sunset for several hours. In polar regions, scintillation can occur at any time. Mid-latitude regions are sometimes affected by Travelling Ionospheric Disturbances (TIDs). A map showing the current ionospheric scintillation activity can be found here http://www.trimbleionoinfo.com/Images.svc/SCINTI
As an indicator for the strength of the ionospheric scintillations the noise level of carrier phase measurements of a global tracking network is analyzed. The normalized results of this analysis are mapped to this idealized ionosphere layer and represented by spherical harmonic expansion to generate the global scintillation map.
Trimble is also providing an ionospheric index for a given time and location on Earth. In the GNSS Planning Online product this index is shown with respect to time. The ionospheric index is the maximum of the TEC value and the scintillation index (both values normalized to 10, i.e. 10 is the highest value possible).