PPP Systems

Fundamentals
Title	PPP Systems
Edited by	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 Kinematics (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).

• gLAB Software: ESA/UPC GNSS-Lab Tool suite (gLAB) is an interactive software package for GNSS data processing and analysis, including Precise Positioning.

• 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).

All the aforementioned PPP SW require the availability of precise reference satellite orbit and clock products, normally computed using a network of GNSS reference stations distributed worldwide. The International GNSS Service (IGS) uses its global network to compute orbit and clock products used by several of the PPP providers. In fact, some of the central processing facilities that collaborate with IGS in the computation of these products are the same as the ones listed above; JPL, ESOC, NRCan, CODE (Bern University).

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 back.

• 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 with GPS data. 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: provides post-processed position estimates from GPS observation files submitted by the user in RINEX format. Precise position estimates are referred to the CSRS standard North American Datum of 1983 (NAD83) as well as the International Terrestrial Reference Frame (ITRF). Single station position estimates are computed for users operating in static or kinematic modes using precise GPS orbits and clocks from IGS global data centers.

In order to obtain a simple and efficient means of comparing solutions provided by each PPP service, the Precise Point Positioning Software Centre has been created under the auspices of the Geomatics for Informed Decisions Network of Centres of Excellence. It allows for:

• An easy comparison of PPP solutions provided by different online PPP services.
• An increased reliability for the user by giving access to independent PPP solutions.
• An insight at the performance of different implementation strategies.
• A means of validation for potential PPP software developers.
• An opportunity to share information on PPP and identify its strengths and limitations.

More information on RINEX files,

Real-Time PPP

Precise Point Positioning (PPP) can be obtained in real-time if precise reference satellite orbit and clock products are available in real-time. There are some projects in development phase, both for real-time products and for real-time PPP. It should be noted that no standard for Real-Time PPP has been defined yet, but standarisation effort is being carried by the Radio Technical Comission for Maritime Services (RTCM) Special committee 104.

Among the real-time projects:

• Real-Time IGS: The main goals of the IGS Real-Time Pilot Project include:
  • Manage and maintain a global IGS real-time GNSS tracking network.
  • Generate combined real-time IGS analysis products.
  • Develop standards and formats for real-time data collection and distribution.
  • Develop standards and formats for the generation and distribution of real-time analysis products.

• Global Differential GPS: JPL's Real Time GIPSY (RTG) software provides with products in real-time, and also real-time PPP positioning.

• RETICLE : DLR via RETICLE also offers real-time data streams via the dedicated NTRIP-caster.

Regional PPP

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.

To overcome this problem, the PPP service must be able to compute their own orbit and clock products using a regional network of stations (e.g. magicGNSS). 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. It has been observed that PPP solutions based on regional networks, even for small regions, are as accurate as PPP solutions using global networks. This opens new ways for providing precise positioning services at regional basis.