All Terrastar Users: High levels of un-forecasted ionospheric activity

There is currently un-forecasted Ionospheric activity present at G3 (strong) levels, today, the 23^rd of April 2023. The ionosphere is expected to remain active for several hours before decreasing. The area of impact is predicted to be poleward of 50 degrees Geomagnetic Latitude.

Terrastar information with regard to heightened ionospheric activity in equatorial and polar regions remains as below.

   Increased ionospheric activity is correlated with the following factors:

• Sunspot Activity – increased ionospheric activity linked with the 11 year solar cycle.
• Solar and Magnetic storms – cause an increase in the ionospheric activity;

• Geographic Location – highest activity along geomagnetic equator and in auroral (polar) regions;
• Seasonal Variations – increased activity at the Vernal and Autumnal Equinoxes;
• Diurnal (Daily) Variations – maximum effects normally experienced one hour after local sunset until midnight.

       There are typically two different effects that are experienced:

1)    Increased ionospheric activity can introduce large errors/biases (up to 15 metres) into single frequency DGNSS (both GPS and GLONASS) because of the failure in the differential process to cancel the effects of the ionospheric delay between the reference station and user end.

2)    The second effect is scintillation which is caused by small scale irregularities in the ionosphere caused by the solar activity. Scintillation occurs mainly in the evening along the geomagnetic equator. It has the effect of causing fluctuations in the amplitude and phase of the carrier phase signal introducing noise or causing loss of lock to the satellite. This results in a reduced number of usable GNSS satellites and occasionally a reduction in the L-Band communications link strength causing intermittent reception of the augmentation data.

Scintillation affects are normally seen in a period of 6 hours after sundown and are not predictable.

Although there is no way to eliminate the effects of scintillation, Terrastar recommend the following to help mitigate the impact of scintillation:

• Use of GLONASS satellites to increase the number of observations available to the position solution (will help in the majority of situations but not all);
• Use of Dual Beam L-band (two independent downlink satellites) for DGNSS corrections to ensure that corrections can still be received if lock to one satellite is lost.

Although these steps may mitigate the effects of scintillation, in some extreme circumstances all GNSS and L-band signals may be lost, resulting in total loss of GNSS positioning. As a result of this, Terrastar recommends the following:

• The impact of increased ionospheric activity and the potential to lose both stable satellite and L-band communications is taken into consideration during risk assessments when planning critical offshore activities;
• Selection and use of non-GNSS positioning reference systems to mitigate the potential for loss or instability of GNSS position;
• Increased vigilance of DP Operators especially after sunset for scintillation effects.