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Concept
Precise Point Positioning
Summary
Precise Point Positioning (PPP) is a global navigation satellite system (GNSS) positioning method that calculates very precise positions, with errors as small as a few centimeters under good conditions. PPP is a combination of several relatively sophisticated GNSS position refinement techniques that can be used with near-consumer-grade hardware to yield near-survey-grade results. PPP uses a single GNSS receiver, unlike standard RTK methods, which use a temporarily fixed base receiver in the field as well as a relatively nearby mobile receiver. PPP methods overlap somewhat with DGNSS positioning methods, which use permanent reference stations to quantify systemic errors.
PPP relies on two general sources of information: direct observables and ephemerides.
Direct observables are data that the GPS receiver can measure on its own. One direct observable for PPP is carrier phase, i.e., not only the timing message encoded in the GNSS signal, but also whether the wave of that signal is going "up" or "down" at a given moment. Loosely speaking, phase can be thought of as the digits after the decimal point in the number of waves between a given GNSS satellite and the receiver. By itself, phase measurement cannot yield even an approximate position, but once other methods have narrowed down the position estimate to within a diameter corresponding to a single wavelength (roughly 20 cm), phase information can refine the estimate.
Another important direct observable is the differential delay between GNSS signals of different frequencies. This is useful because a major source of position error is variability in how GNSS signals are slowed in the ionosphere, which is affected relatively unpredictably by space weather. The ionosphere is dispersive, meaning that signals of different frequency are slowed by different amounts. By measuring the difference in the delays between signals of different frequencies, the receiver software (or later post-processing) can model and remove the delay at any frequency.
Official source
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