VERIPOS GPS & GLONASS services: Standard2, Apex2 and Ultra2 are all extensions of the VERIPOS GPS only services: Standard, Apex and Ultra. These services are designed to increase the availability of GNSS observations during periods of local masking, ionospheric scintillation or general poor GPS geometry.
A minimum of 4 GPS satellites must be used for a DGPS Solution to work and 5 satellites for a PPP (Ultra or Apex) Solution. If the number of usable satellites drop below these values the positioning will stop. When working in areas where masking to the GNSS satellites is an issue, working alongside platforms or other high structures for example, GPS + Glonass solutions can continue to operate after a GPS Only solution has failed.
As can be observed in the below pictures if a platform masks 100° of the sky view to an elevation of 60° the number of GNSS satellites can be reduced significantly.
In the example below the GPS + Glonass Constellation with no obstruction shows 14 Satellites, the GPS Only constellation with the obstruction shows 4 Satellites and the GPS + Glonass constellation with the obstruction shows 4 Satellites:
|Full Constellation – No Obstruction||Obstruction curtain ≈100° Azimuth / 60° Elevation|
|GPS Only Constellation with Obstruction||GPS + Glonass Constellation with Obstruction|
Poor GPS Geometry
Example Nanu from Navcen regarding Poor GPS Geometry (5th March 2015):
Users in certain parts of the world may experience a very short period of high Dilution of Precision (DOP >6) in the vicinity of Western Sahara and Morocco at their border and within a large area offshore the United Kingdom and Ireland in the Atlantic from 052117Z - 052120Z Mar 2015 (three minutes), due to satellite maintenance (NANU 2015013, below). DOP depends only on the geometry of the four satellites used by the user equipment to determine its position and time and is a multiplier. Simply speaking, Signal-in-Space error (how much accuracy the signal lost during its travel from the satellite to your receiver) plus Receiver Error (inherent in the manufacture of the device) multiplied by the DOP equals Navigation Error (how far off you can expect the position to be within). Graph illustration of the predicted event can be seen at: http://navcen.uscg.gov/?pageName=degradation_05302015
Predictions are based on a "Best Four" analysis to illustrate the worst case scenario and are provided for situational awareness only.
Offshore UK and Ireland
GNSS coverage at high latitudes
When working at higher latitudes GPS only systems will observe all satellites at a low elevation and no satellite at higher elevations. As the Glonass system is designed for working at higher latitudes primarily over Russia its coverage at higher latitudes is superior to GPS.
GPS Only Satellite Track GPS + Glonass Satellite Track
Increased resilience to scintillation
Scintillation is a symptom of ionospheric activity which causes rapid fluctuations in the phase and amplitude of the satellite signal as it passes through small-scale irregularities in the ionosphere. Effects of scintillation appears in different localised regions of the sky and thus only affect certain satellites at a time. Two effects occur during periods of scintillation:
- Amplitude scintillations can lead to periods of reduced signal levels at the GNSS antenna which results in an increase in the measurement noise within the code and carrier tracking loops
- Phase scintillations increase the dynamic stress on the carrier tracking loops which results in additional phase measurement jitter.
Both effects result in an increase in pseudo-range measurement errors and under extreme conditions can lead to complete loss of signal lock
When Scintillation causes a loss of signal lock it reduces the number of usable satellites. It has been observed during Solar Cycle 24 that GPS only calculations can be affected severely enough by Scintillation to stop working. Whereas the GPS + Glonass increase the number of satellites available and the solutions can work through or at least be affected less.
Ultra solution using GPS Only during Scintillation:
Apex solution using GPS + Glonass during Scintillation: