Veripos Standard+ helps mitigate the effects of ionospheric activity which can have a detrimental effect on the positioning performance of standard DGPS. Using dual frequency (L1/L2) methods, permits the Veripos Standard+ service to remove these degrading effects by calculating and removing the ionospheric delays in each satellite signal thus producing DGPS positions free from ionospheric effects; this position type is usually referred to as "Iono-Free".
Description
Veripos Standard+ extends the validity of DGPS during periods of heightened ionospheric activity. The ionosphere contains electrons and ions that affect the propagation of the GPS signals through it, to a greater or lesser extent depending on the density of these particles; a higher density equates to a more active ionosphere. Increased 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 the auroral 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.
Increased ionospheric activity can introduce large biases into standard single frequency DGPS because of the failure in the differential process to cancel the effects of the ionospheric delay between the reference station and user end. Standard DGPS systems utilise the Klobuchar ionospheric correction model transmitted in the GPS navigation message. In areas of increased ionospheric activity this model does not reflect the true behaviour of the ionosphere due to the relatively slow update of its parameters as well as it's coarse resolution.
Veripos Standard+ removes the need to use an ionospheric model but instead uses dual frequency GPS receivers to calculate the true ionospheric delays at both the reference station sites and user end to derive an “iono-free” DGPS solution.
Scintillation
Scintillation is another symptom of ionospheric activity. It causes rapid fluctuations in the phase and amplitude of the L-band satellite signal as it passes through small-scale irregularities in the ionosphere. The effects of scintillation appear in different localised regions of the sky and thus only affect certain satellites at a time.
Veripos Standard+ service utilises state-of-the-art dual-frequency GPS receivers that help mitigate the effects of scintillation through better tracking of the GPS satellites. Differential corrections for Veripos services transmitted via L-band communications satellites can also be affected by scintillation. However, the use of high power communications satellite ensures that good signal levels are available to ensure that continuous tracking of the Veripos service can be maintained during periods of scintillation. While these steps help mitigate the effects of scintillation, under extreme conditions it may not be physically possible to track the signals.
Correction Message Format
The correction information for each Standard+ station is broadcast approximately every minute using the RTCM Type 15 message format. As the ionosphere is changing slowly over a period of minutes then a slower transmission rate for the Standard+ correction messages is used.
By combining the RTCM Type 1 and Type 3 messages from the Veripos Standard service and the Type 15 from the Veripos Standard+ service with the ionospheric delays calculated at the user end, an “iono-free” DGPS position can be produced.
