“…The equations of carrier phase ( L ) and code observations (pseudorange P ) of dual‐frequency GPS are expressed as [ Jin et al ., , ] where subscript k stands for the frequency ( k = 1, 2), superscripts i and j represent the satellite and ground‐based GPS receiver, respectively, ρ 0 is the true distance between the GPS receiver and satellite, d ion and d trop are the ionospheric and tropospheric delays, c is the speed of light in vacuum space, τ is the satellite or receiver clock offset, b is the phase delay of satellite and receiver instrument bias, dq is the code delay of satellite and receiver instrument bias, λ is the carrier wavelength, φ is the total carrier phase between the satellite and receiver, N is the ambiguity of the carrier phase, and ε is other residuals. Ignoring the high‐order ionospheric effect in the Appleton‐Hartree equation, the ionospheric total electron content (TEC) along the signal path could be derived from the dual‐frequency GPS carrier phase and pseudorange measurements as [ Jin et al ., , ] …”