GPS / GNSS Positioning Accuracy Limits
The large distance traveled by the signal between satellites and GPS / GNSS receivers leads to a number of phenomena that influence the accuracy of GPS positioning.
The reception conditions at the level of the receivers also generate other phenomena. It is therefore the conjunction of all these factors that contributes to degrading precision.
Of all these sources of error, atmospheric refraction, responsible for ionospheric and tropospheric elongation, and orbital errors are the most problematic, as:
- they cannot be finely modeled beforehand (unlike electronic biases or antenna phase center variations),
- they cannot completely cancel each other out by multiple differentiations.
there are therefore “natural” factors that limit the accuracy of GPS / GNSS receivers. In order of their influence on the loss of precision related to refraction in the ionosphere.
This refraction is not constant and suffers the consequences of daily solar activity. It is therefore interesting to understand and monitor this activity and its influence on the operation of our instruments.
Intense solar activity
On October 2, 2013, all satellite networks (GNSS, telecommunications, etc.) around the world suffered temporary disruptions due to intense solar activity.
The Earth can be likened to a magnetic dipole whose field protects the Earth’s atmosphere from energetic particles by deflecting them from their trajectory. Fluctuations in the number, energy or speed of solar wind particles can cause a variation in the Earth’s magnetic field (orientation and amplitude) and therefore disturb this magnetic shield system; we talk about geomagnetic disturbance
The consequences caused by solar phenomena such as eruptions or geomagnetic storms are numerous and varied: accelerated corrosion of pipelines, breakdowns of satellites or the electrical network… Phenomena of solar origin also cause ionospheric disturbances which lead to a degradation of the accuracy of GNSS applications. At present, recent GPS positioning techniques based on the sending of differential corrections by a reference station (such as Real-Time Kinematic or RTK) make it possible, in general, to obtain a precision of the of a few centimeters in real time. However, the ionosphere is the main limitation to the accuracy of these positioning methods.
Significant solar activity every 11 yearsSolar activity follows an eleven-year cycle, with 2015 being a maximum of this cycle. This year 2020 is therefore synonymous with the end of a cycle of reduction in ionospheric disturbances for our applications. We must expect in the future to have to deal with increasing solar activity, hence the importance of having a dense network of permanent GNSS stations to properly model this error. As you can see in the graph below, the cycle of solar activity is reaching its minimum and the forecast for this activity is now on the rise:
During a day, ionospheric activity related to solar activity peaks around 12:00 p.m. solar time. Thus, as shown in the graph below, this ionospheric activity was very high on Wednesday October 2 at noon with I95 index values beyond the limit of 8 indicating very high activity for the targeted applications and therefore impacting all communications satellites during their passage in the ionosphere then rich in charged particles:
Disturbances related to geomagnetic storms
As you can see on the website SpaceWeatherLive.com who observes the Solar Weather, this phenomenon is global with an ionospheric index K (quantification of geomagnetic disturbances). This is an average of the K indices relating to 13 stations located between 44 and 60◦ latitude; it is therefore an overall index.
The K index is a local index which characterizes the variation of the magnetic field at the station considered in relation to a calm reference day; these measurements are carried out using magnetometers. The Kp index scale includes 10 rungs:
Les échelons de l’Indice KpWhen the Kp index is greater than 5, it corresponds to a geomagnetic storm:
Moreover, in a completely de-correlated manner, the distribution of GPS satellites is not optimal in this same time slot with a GDOP exceeding the limit set at 3 as shown below:As a reminder, the DOP corresponds to a coefficient of attenuation of the precision linked to the good distribution of the satellites in the sky (calculation of the position of the mobile by spatial multilateration). The GDOP (General Dilution of Precision), ideally equal to 1, should not exceed 2 or 3 for precision positioning applications. Beyond the notion of DOP, a minimum of 5 visible satellites are also required beyond the fact that they are well distributed, which can prove to be complicated at this time of day, this particular configuration linked to the very nature of GPS orbits advancing by 4 minutes per day:
Finally, as a reminder, GLONASS serving as an augmentation to GPS for the majority of GNSS mobiles, a minimum of well-distributed GPS satellites (at least 4) are required to be able to work in GPS+GLONASS.
Predict the state of the constellations on your sites
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