Standard Point Positioning (SPP)
Nowadays, Global Navigation Satellite Systems (GNSS) have become the principal method for autonomous positioning. Millions of users utilize a GNSS device every second for obtaining the position of their vehicles, ships, drones, etc. Standard GNSS receivers usually exploit only code observations, i.e. representing absolute (pseudo-)ranges to visible satellites, and broadcast ephemerides describing satellites orbits and onboard clocks. Pseudorange measurements have limited accuracy and suffer from various biases (e.g. receiver/transmitter hardware delays, multipath effects), and thus meter-level positioning is possible only depending particularly on the quality of observations, environmental conditions, and satellite geometry.
Precise Point Positioning (PPP)
Expensive devices provide carrier-phase measurements in addition to pseudoranges. The carrier-phase observation is a measure of the range between a satellite and receiver expressed in units of cycles of the carrier frequency. Such observation can be made with a sub-millimeter accuracy in units of length, however, it represents only differential information of the range between a receiver and each satellite. It is expressed in the number of whole cycles, while the actual absolute range cannot be measured. Hence, an additional parameter called the initial carrier-phase ambiguity has to be introduced in a state vector. Carrier-phase observations are also contaminated by a number of biases. Some of them can be introduced as a priori known using external precise models or products, e.g. satellite precise orbits and clock offsets. Some others must be estimated simultaneously, e.g. receiver clock correction. External products are estimated on the basis of processing data from reference networks of global/regional stations, e.g. such as provided by the International GNSS service (IGS). Anyway, the receiver position can be estimated with an accuracy from centimeters to decimeters if utilizing carrier-phase measurements too. Achieved accuracy then depends on various factors including the availability of observations (e.g. dual-frequency, multi-constellation, satellite visibility, carrier-phase interruptions, quality of pseudoranges), quality and consistency of precise products and models, processing window and strategy (kinematic or static) and, last but not least, on resolving initial phase ambiguities as integer or float values. An epoch-by-epoch kinematic positioning supported with the traditional PPP can achieve a decimeter precision after an initial convergence period of twenty to sixty minutes.
Precise Point Positioning with ambiguity resolution (PPP AR)
An integer nature property of initial carrier-phase ambiguity is lost as it absorbs additional phase receiver and transmitter hardware biases, and these can not be estimated along with the ambiguities due to their one-to-one correlation. Fortunately, when applying a particular re-parametrization, the phase biases can be estimated again on the basis of processing observations from global/regional continuously operating reference stations. Consequently, when resolving initial ambiguities as integer values, the solution is improved in terms of the accuracy, the stability as well as the length of the convergence period. A centimeter-level kinematic receiver position can be achieved then within ten to twenty minutes.
Precise Point Positioning with fast ambiguity resolution (PPP RTK)
(centimeter-level accuracy in a few minutes)
If precise information about local atmospheric conditions (the ionosphere and the troposphere) are known and introduced into the GNSS processing, initial carrier-phase ambiguities can be resolved within a few minutes. Such local tropospheric and ionospheric products can be estimated on the basis of processing observations from local continuously operating reference stations.