PPP Systems

Fundamentals
Title	PPP Systems
Author(s)	GMV
Level	Basic
Year of Publication	2011

Precise point positioning (PPP) stands out as an optimal approach for providing centimeter-level error positioning using current and coming GNSS constellations. The Precise Point Positioning (PPP) processes measurements from a single user receiver, using detailed physical models and corrections, and precise GNSS orbit and clock products computed beforehand. PPP differs from other precise-positioning approaches like Real Time Kinematic (RTK) in that no reference stations are needed in the vicinity of the user. Another advantage is that since the GNSS orbit and clock products are by nature global, the PPP solutions are also global. However, it should be noted that it is possible to set up a regional PPP service using a regional network of stations.

Several software products implementing a PPP processing strategy have been developed recently by government agencies, universities, industries and individuals. There are also some online PPP services available. They are summarized in this article.

PPP Software

The precise point positioning (PPP) algorithms using un-differenced carrier phase observations have been added to sophisticated processing software:

• GIPSY-OASIS, or GIPSY: the GNSS-Inferred Positioning System and Orbit Analysis Simulation Software package, developed by the Jet Propulsion Laboratory (JPL), and maintained by the Orbiter and Radio Metric Systems (ORMS) group.

• NRCan PPP: Global GPS post-processing service, developed by Natural Resources Canada.

• magicGNSS: GNSS Orbit Determination and Precise Positioning software developed by GMV, Spain.

• Bernese Software: GPS/GLONASS post processing package developed by Astronomical and the Physical Institutes of the University of Bern, Switzerland.

• GAPS : The University of New Brunswick (UNB) developed the GPS Analysis and Positioning Software (GAPS).

• NAvigation Package for Earth Observation Satellites(Napeos): software system for GNSS data processing and Multi-satellite high precision orbit determination, developed and maintained by the European Space Operations Centre (ESOC) of the European Space Agency (ESA).

Among these institutions, some of them offer online processing as it is explained in next section.

PPP online Services

There are free online PPP services. When submitting RINEX observation files on each website, the data will be processed by those services and then the PPP solution is obtained and sent it.

• GAPS: available as an online post-processing engine via their webpage. Static as well as kinematic processing is possible. They accept an observation file in the RINEX 2.10 or 2.11 formats. IGS orbits and clocks necessary for processing the observations are automatically retrieved from one of the IGS global data centers.

• magicPPP: processing static and kinematic GPS and GLONASS real-time data in RINEX format. Only dual-frequency PPP is supported. Real-time GPS and GLONASS orbits and clocks needed by PPP are generated internally (magicODTS). Rapid and final GPS orbits and clocks from IGS are also used, if available.

• Automatic Precise Positioning Service (APPS): using GIPSY software, APPS supports input in RINEX 2.10, RINEX 2.11 files and GIPSY TDP files to process static and kinematic GPS real-time data.

• [ NRCan PPP]:

Regional PPP

Blog magicGNSS: Precise Point Positioning (PPP) is normally understood as a global positioning service, since precise satellite orbit and clock products used as input in PPP are best calculated using a network of GNSS reference stations distributed worldwide. However orbit and clock corrections for PPP can also be provided over a world region in a similar way to WAAS or EGNOS corrections. It is even possible to provide a national PPP service over a single country, using national GNSS stations exclusively.

Satellite orbits and clocks calculated using regional stations do not have of course a high overall accuracy, but they do have a higher accuracy over the service area and, most importantly, orbit and clock errors largely cancel out over the service area. For a static or kinematic PPP user located inside the region, positioning accuracy using global or regional products is very similar.

In magicGNSS version 3.1 it is possible to evaluate the performance of regional PPP in a very simple way. Just process an ODTS scenario to calculate orbits and clocks covering the time period of your interest. On the Stations tab, select stations over your region only. Click on the station icons to select/deselect, you can use core stations and/or your own stations.

In regional ODTS it is better to deselect “Refine Station Coordinates” in Settings. Core stations have already precise coordinates, and for your own stations you are supposed to have processed them beforehand in PPP using the “Update My Station Coordinates” option. Try also to use a longer ODTS scenario duration (3 to 4 days), in order to maximize satellite visibility. It is also important to select a good reference clock in ODTS. The reference clock time should be close enough to UTC (or GPS Time), with an offset of the order of the microsecond or less. You can see the offset of a station clock by processing it in PPP.

Coordinates: A PPP service is normally a global service, considering that the orbit and clock products are themselves global. This is true as long as the tracking stations used for the computation of the products are distributed worldwide. In this case, there is good visibility of the satellites along all their orbits, and the accuracy of the orbit and clock estimations does not depend on the location. This may pose some limitations as there are mainly two options:

1. To deploy a global network of stations, this may be complex and expensive to operate for a regional service provider

2. To relay on an external orbit and clock provider, this may limit accuracy, real time capabilities and multisystem approaches

GMV has overcome this problem as magicGNSS is able to compute their own orbit and clock products using a regional network of stations. In this case, the accuracy of the orbits and clocks is slightly degraded but this degradation occurs mainly outside the area where the stations are deployed. Inside this region, the combination of orbit and clock products is such that the positioning performances are good. Indeed, it is possible to achieve positioning performances at the same level as with a global network. This opens very interesting possibilities to regions already operating networks of GNSS receivers (e.g. for RTK), as they can deploy a PPP service using their own resources. Such service could complement RTK for areas far away from any of the base stations requiring much less stations than a classical RTK approach for the same level of precision.