PPP Standards

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

Precise point positioning (PPP) stands out as an optimal approach for providing global augmentation services using current and coming GNSS constellations. PPP requires fewer reference stations globally distributed rather than classic differential approaches (e.g. RTK), also one set of precise orbit and clock data is valid for all users everywhere, and the solution is largely unaffected by individual reference-station failures. There are always many reference stations observing the same satellite because the precise orbits and clocks are calculated from a global network of reference stations. As a result, PPP gives a highly redundant and robust position solution: mm precision level.

PPP Standards

Precise Point Positioning (PPP) is a global precise positioning service, since it requires the availability of precise reference satellite orbit and clock products in real-time using a network of GNSS reference stations distributed worldwide.

There are not "PPP Standards" as understood in SBAS Systems, but there are some conventions, models and formats commonly used. To achieve the highest accuracy and consistency (mm-cm level positioning), users must implement the GNSS-specific conventions and models adopted by the IGS.

Developers of GPS software are generally well aware of corrections they must apply to pseudorange or carrier-phase observations to eliminate effects such as special and general relativity, Sagnac delay, satellite clock offsets, atmospheric delays, etc. All these effects are quite large, exceeding several meters, and must be considered even for pseudorange positioning at the meter precision level. When attempting to combine satellite positions and clocks precise to a few centimeters with ionospheric-free carrier phase observations (with millimeter resolution), it is important to account for some effects that may not have been considered in pseudorange or precise differential phase processing modes.

The following sections summarize the adjustment models and procedures usually used and look at additional correction terms that are significant for carrier phase point positioning. The corrections models have been grouped under Satellite Effects, Site Displacements Effects and Compatibility and IGS conventions. A number of the corrections listed below require the Moon or the Sun positions which can be obtained from readily available planetary ephemeredes files, or more conveniently from simple formulas since a relative precision of about 1/1000 is sufficient for corrections at the mm precision level. Note that for centimeter level differential positioning and baselines of less than 100 km, the correction terms discussed below can be safely neglected.

PPP Adjustment Models and Procedures

The adjustment model is a weighted least-squared estimation. This adjustment can be done in a single step, so called batch adjustment (with iterations), or alternatively within a sequential adjustment/filter (with or without iterations) that can be adapted to varying user dynamics. The disadvantage of a batch adjustment is that it may become too large even for modern and powerful computers, in particular for un-differenced observations involving a large network of stations. However, no back-substitution or back smoothing is necessary in this case, which makes batch adjustment attractive in particular for double difference approaches. Filter implementations, (for GPS positioning, equivalent to sequential adjustments with steps coinciding with observation epochs), are usually much more efficient and of smaller size than the batch adjustment implementations, at least, as far as the position solutions with un-differenced observations are concerned. This is so, since parameters that appear only at a particular observation epoch, such as station/satellite clock and even zpdw parameters, can be preeliminated. However, filter (sequential adjustment) implementations then require backward smoothing (back substitutions) for the parameters that are not retained from epoch to epoch, (e.g. the clock parameters). Furthermore, filter/sequential approaches can also model variations in the states of the parameters between observation epochs with appropriate stochastic processes that also update parameter variances from epoch to epoch. For example, the PPP observation model and adjustment involve four types of parameters: station position (x, y, z), receiver clock (dT), troposphere zenith path delay (zpdw) and noninteger carrier-phase ambiguities (N). The station position may be constant or change over time depending on the user dynamics. These dynamics could vary from tens of meters per second in the case of a land vehicle to a few kilometers per second for a Low Earth Orbiter (LEO). The receiver clock will drift according to the quality of its oscillator, e.g. about 0.1 ns/sec (equivalent to several cm/sec) in the case of an internal quartz clock with frequency stability of about 10-10. Comparatively, the tropospheric zenith path delay (zpd) will vary in time by a relatively small amount, in the order of a few cm/hour. Finally, the carrier-phase ambiguities (N) will remain constant as long as the satellite is not being reoriented (e.g., during an eclipsing period, see the phase wind-up correction) and the carrier phases are free of cycle-slips, a condition that requires close monitoring. Note that only for double differenced data observed from at least two stations, all clocks dT’s, including the receiver clock corrections are practically eliminated by the double differencing.

IGS Formats

IGS has adopted and developed a number of standard formats, which for convenience are listed below. Note that some formats, like RINEX, SP3 and SINEX undergo regular revisions to accommodate receiver/satellite upgrades, or multi-technique solutions, respectively.

Real-Time PPP

No standard for Real-Time PPP has yet been defined. Nonetheless, a standarisation effort is being carried by the Radio Technical Comission for Maritime Services(RTCM) Special committee 104.