Simple differentially-corrected pseudorange factor for precise positioning.

Simple differentially-corrected pseudorange factor for precise positioning.This factor implements the model prescribed by chapter 5.8.2 from [1], where a reference GNSS receiver with known position provides differential pseudorange corrections for a "user" receiver to eliminate common-mode atmospheric errors. The idea being that spatially local receivers experience the same atmospheric errors since their signal paths pass through the same regions of Earth's atmosphere. Therefore, this factor accepts an additional "differential correction" variable from a reference receiver to cancel-out local-area biases from the user's pseudoranges.

Note that this factor is designed for code-phase measurements.

see the `DifferentialPseudorangeExample.ipynb` notebook for a demonstration of this factor on CORS datasets.

REFERENCES: [1] P. Misra et. al., "Global Positioning Systems: Signals, Measurements, and Performance", Second Edition, 2012.