It seems it is common to combine basic observations to create new observable, which are then used for PPP and other applications. Basic observations such as pseudorange and carrier-phase observations are real measurement from GNSS. These real observations are combined to create entirely new observable which is not direct, physical, and real. Amazingly, these new observable solves the real problem such as PPP (e.g. Ionosphere -free combination).
- What is the theory behind this?
- Any similar approach like this in other scientific field or any simple analogous explanation?
- You could direct me to resources such as videos, or literature.
Shanker KC Dai, the paper by Han, D. et al. will quench your curiosity, Please have a look.
Han, D.'s team proposed an ionospheric modeling algorithm that used both the pseudo-range measurements and double-difference carrier phase measurements and was able to greatly reduce the ionospheric errors. This algorithm, which was originally based on regional augmentation systems was able to improve the accuracy of the ionospheric mapping at the regional level.
Reference: Han, D.; Kim, D.; Song, J.; Kee, C. Improving the Accuracy of Regional Ionospheric Mapping with Double-Difference Carrier Phase Measurement. Remote Sens. 2019, 11, 1849. https://doi.org/10.3390/rs11161849
Without describing the things too complecated technically., here it is a plain explanation to help with understanding this combination of pseduranges and carrier phases. The GNSS carrier phase measurements come from the short wave lengths of microwaves (e.g., L1 (18cm), L2 (24cm) from GPS) that can be measured only for their last fractional portion without knowing the number of their integer cycles (leave as ambiguities) between the receiver and a specific satellite. However, the pseudoranges (e.g. L1 C/A from GPS) are measured at the accuracy of meters. We can easilly position our receiver at the accuracy of meters by using the L1 C/A and then move on resolving the number of the integers (called ambiguity resolution) for L1 and L2, which could deliver a precison of mm upon the meter level positioning.
Think how an EDM (Eletronic Distance Measurement) works uisng the carrier phases! Multiple waves are deployed to ensure the reuired range and also the required accuracy at the same time to avoid the ambibuities.
furthermore, one a satellite is locked for recording the phases, a counter starts, keep add "1" to the number in the counter whenever a whole cycle carrier wave is passed based on the time a wave length corresponds to. So, the unknow ambiguity of a satellite maintains constant (associated with the time instant when the lock starts) as long as this satellite is being locked without interruption. In case an interruption, a different ambiguity will have to be resolved in the estimation.
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