PPP, “Precise Point Positionning”?
In the world of GNSS, “PPP” is the acronym for Precise Point Positioning. Developed in the 2000s, it defines a method of GNSS positioning that consists of correcting precisely the orbits and clocks of satellites. PPP makes it possible to achieve centimetric precision under optimal real-time conditions.
What are the characteristics of this positioning method? In which cases can we recommend this technology? What are its limitations and alternatives?
The principle of Precise Point Positioning (PPP)
PPP technology uses raw data retrieved from a global terrestrial network of reference stations to calculate highly accurate orbit-synchro corrections.
These corrections are transmitted to a receiver or software to improve the accuracy of positioning or time measurement. The PPP service has a coverage that can be global, but it is less accurate than NRTK .
PPP consists in calculating global corrections at the raw GNSS signal level, without using the principle of the differential effect with another nearby station.
This method requires a very thorough knowledge of all the parameters that disturb the measurements (clocks, orbits, atitude of each satellite, propagation in the atmosphere, earth tides, etc.). Thus, a whole series of corrections is made available, especially :
- clock corrections,
- orbit corrections,
- The electronic biases of the satellites on code and phase (the latter being necessary to fix ambiguities).
In addition, there are real-time PPP public services that can be used for research and testing purposes, made available via the IGS website : http://www.igs.org/rts
PPP: Centimetric precision with global coverage
Finally, PPP technology alone makes it possible to correct very precisely all errors related to the GNSS system by providing a global service. It then becomes possible to position oneself centimetrically in PPP with a single receiver in real time.
The real advantage of PPP is that it is operational worldwide. Indeed, it is not necessary to work close to a reference station, allowing coverage of the oceans for example.
This is the case of TERIA, which in 2019 entered into a partnership with France Brevet to exploit some of the CNES (Centre National des Études Spatiales) patents to set up a PPP-Global service. Combined with the know-how of its experts and its network, this algorithm will make it possible to extend the coverage of its services. TERIA will then offer complementary solutions to its customers and partners wishing to work beyond the TERIA network coverage area through a PPP service alone.
Thanks to the different TERIAsat service levels, TERIA offers locally a PPP-RTK service and globally a PPP-only service. (already available since 2018)
To this end, work is being finalised so that new algorithms can be integrated into the receiver market.
Limitations of the PPP and recommended uses.
However, PPP technology has a major disadvantage when used alone. Currently, the atmospheric correction models used only converge to an accurate estimate after 20 to 30 minutes. This time corresponds to the initialization time that the user will have to wait before making measurements if he wants a centimetric positioning.
Despite centimetric accuracy and global coverage, this significant convergence time reduces its uses.
Nowadays, 20 to 30 minutes of convergence time creates a waiting time that is far too long in application areas that require real time with a high level of responsiveness.
It should also be noted that this convergence time is reproduced in the event of signal loss (masks, vegetation cover, tunnels, etc.). Under certain conditions it could be reduced, however it will remain significantly longer than the convergence times in NRTK or PPP-RTK where it is a few seconds for example.
This precise positioning technology is therefore only used in very specific cases. This is particularly the case in geographical areas where it is not economically or technically possible to provide an RTK or NRTK service:
However, some R&D developments indicate that the convergence time will gradually decrease. Indeed, the improvement in computing power of electronic chips capable of tracking more frequencies and the use of several constellations will in the future reduce this convergence time.
Alternatives and evolutions of the "Precise Point Positionning".
The PPP has therefore an important limitation, but this can be overcome with the combination of other technologies.
TERIA has reached this point by capitalising on the best of both the PPP and RTK techniques to offer an optimal result.
The PPP-RTK association consists of transmitting PPP corrections and broadcasting corrections in parallel via tropospheric and ionospheric error modelling. These very accurate corrections come from a dense network of GNSS stations (NRTK). This is the concept of SSR (http://www.geopp.com/pdf/ion2005_fw.pdf )
De ce fait, il est possible de travailler avec une précision centimétrique sur l’ensemble de la zone de couverture des modèles atmosphériques, tout en gardant un temps de convergence équivalant au NRTK (Network Real Time Kinematic).
Compared to NRTK technology alone, this combination has several advantages:
The PPP-RTK is currently not yet standardised in format, only the PPP part has its own RTCM messages.
TERIA has responded to this problem by creating the TERIAsat service based on a proprietary format. This solution builds on existing RTCM messages and provides ionosphere and troposphere corrections via proprietary messages.
Current receivers are not compatible with RTCM messages for PPP. All commercial PPP solutions (such as Omnistar, Starfire, Atlas, Etc.) work with proprietary formats.
In addition, it can be configured in any hardware on the market through the new TERIAsat Terminal.