Masks and multipaths

In wooded areas or bordered by buildings, some satellites may be obscured and therefore unusable by the equipment on the ground to establish a correct position. It is also possible that these signals come from the reflections of a body of water, glass or metal surfaces, mountains, trees or constructions creating multipaths which alters the precision of the positioning.

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Masks and multipaths

GPS / GNSS Positioning Accuracy Limits

Several natural errors affect the satellite/receiver distance measurement:

        • The electronics bias of the satellite
        • The effect of orbit error.
        • The Effect of Satellite Clock Error.
        • The satellite antenna phase center position error.
        • Ionospheric elongation.
        • Tropospheric elongation.
        • Possible multi-paths.
        • The position error of the phase center of the antenna of the mobile receiver.
        • The Effect of Receiver Clock Error.
        • Receiver Electronics Bias.

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.
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Errors affecting positioning accuracy by GPS / GNSS methods are related to:

Origin of these errors :

Source d’erreur Influence absolue
 Orbites Entre 2 et 50 mètres
 Ionosphère Entre 50cm et plus de 100 mètres
 Troposphère Entre 1 et 50 cm
To determine the position, the GPS receiver calculates the distance at which the satellites are at the same time from the data of their ephemerides and based on its internal clock, knowing precisely the trajectory that the satellites follow, the receiver must theoretically use the same time as these satellites. But due to the desynchronization of these clocks and disturbances in the propagation of the signals, this calculation loses precision (we speak of pseudo-distances).
        • the signal is slowed down during its passage through the atmosphere (ionosphere and troposphere) in a variable manner.
        • the signal can be reflected by elements on the ground (metal surfaces, glass, buildings, etc.) before being picked up by the receiver.
        • the signal may be blocked and not reach the receiver (masks), in cities, due to trees, bridges, tunnels etc.
        • the geometry of the satellites used (PDOP),
        • errors affecting the distance measurement between the various satellites seen and the user’s receiver.

To work properly :

Care must be taken to :
        • have a minimum of 5 satellites well distributed
        • the correct distribution of satellites in the sky
        • either in sufficiently clear environments,
        • either with a sufficiently large number of satellites,
        • so that the user’s mobile receiver can choose the 5 best usable satellites among the 24 to 26 satellites traditionally observable in France with a mask of 10° above the horizon.

Different satellites

Until today, we used the generic term GPS for Global Positioning System to describe the various geopositioning equipment. However, this term is actually the name of a set of 24 American satellites placed in orbit at an altitude of 20,000 km. This constellation of satellites, which was the first in history, therefore left its name to geopositioning technology. These satellites revolve around the earth and are therefore in constant motion. To establish a position, a minimum of 5 satellites is required. Other satellite constellations have thus been commissioned by other countries: Glonass for Russia, Beidou for China and more recently Galileo for Europe have also been commissioned, all of which are available in France. All of these constellations are grouped together under the single name of GNSS for Global Navigation Satellite System. These satellites revolve around the earth and are therefore in constant motion. To establish a position, a minimum of 5 satellites is required.

Dilution of GDOP, HDOP accuracy…

The HDOP, a question of geometry

The positioning accuracy of a satellite navigation system (GPS) is affected by mathematical errors related to the geometry of observable satellites either because of their position and their distribution above the user.

The idea of ​​geometric DOP (GDOP) is to indicate how measurement errors will affect the final state estimate.

DOP can be expressed in a number of distinct measurements :
  • HDOP – horizontal dilution of precision
  • VDOP – vertical dilution of accuracy
  • PDOP – position dilution (3D) of accuracy
  • TDOP – temporal dilution of precision
  • GDOP – geometric dilution of precision

Benchmarks

These values ​​derive mathematically from the positions of the usable satellites. The signal receivers allow the display of these positions (skyplot) as well as the DOP values.
Valeur DDP Classement La description
<1 Idéal Highest possible level of confidence to be used for applications requiring the highest possible accuracy at all times.
1-2 Excellent At this level of confidence, position measurements are considered accurate enough to meet all but the most sensitive applications..
2-5 Bien Represents a level that marks the appropriate minimum for making accurate decisions. Position measurements could be used to make reliable navigation suggestions to the user.
5-10 Modérer Position measurements could be used for calculations, but the quality of the correction could still be improved. A clearer view of the sky is recommended.
10-20 Équitable Represents a low confidence level. Position measurements should be ignored or used only to indicate a very rough estimate of the current location.
>20 Pauvre At this level, measurements are inaccurate out to 300 meters with an accurate 6 meter device (50 DOP × 6 meters) and should be discarded.

Sky map

The sky map allows you to get a good idea of ​​the distribution of the satellites above you and therefore potentially of the DOP problems that you could encounter. Example of observation of the sky of the GPS and Glonass constellations with a cut-off angle of 45° compared to the sky map in Full GNSS with a cut-off angle of 10°.

The Orpheon network compatible with Galileo and BeiDou

Orphéon becomes the first 100% Full GNSS network in France

Since September 7, 2017, all of the reference stations, the infrastructure and the correction calculation software allow us to deliver NRTK Full GNSS corrections including

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GPS + GLONASS + Galiléo + BeiDou.

The most complete range of services, everything is done so that you can take advantage of the performance of the Orphéon network and work in the best conditions. Real-time corrections, post-processing, smartphone application.

100% Galileo and BeiDou correction services in addition to GPS and GLONASS:

  • more satellites (+22 Galileo)
  • faster fixing,
  • more measurement stability,
  • more availability,
  • better positioning
  • fewer multipaths

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