1. Calculation Steps

1. A global-navigation-satellite-system (GNSS) receiver measures the apparent transmitting time, [math]\displaystyle{ \displaystyle \tilde{t}_i }[/math], or "phase", of GNSS signals emitted from four or more GNSS satellites ([math]\displaystyle{ \displaystyle i \;=\; 1,\, 2,\, 3,\, 4,\, ..,\, n }[/math] ), simultaneously.^[1]
2. GNSS satellites broadcast the messages of satellites' ephemeris, [math]\displaystyle{ \displaystyle \boldsymbol{r}_i (t) }[/math], and intrinsic clock bias (i.e., clock advance), [math]\displaystyle{ \displaystyle\delta t_{\text{clock,sv},i} (t) }[/math]^{[clarification needed]} as the functions of (atomic) standard time, e.g., GPST.^[2]
3. The transmitting time of GNSS satellite signals, [math]\displaystyle{ \displaystyle t_i }[/math], is thus derived from the non-closed-form equations [math]\displaystyle{ \displaystyle \tilde{t}_i \;=\; t_i \,+\, \delta t_{\text{clock},i} (t_i) }[/math] and [math]\displaystyle{ \displaystyle \delta t_{\text{clock},i} (t_i) \;=\; \delta t_{\text{clock,sv},i} (t_i) \,+\, \delta t_{\text{orbit-relativ},\, i} (\boldsymbol{r}_i,\, \dot{\boldsymbol{r}}_i) }[/math], where [math]\displaystyle{ \displaystyle \delta t_{\text{orbit-relativ},i} (\boldsymbol{r}_i,\, \dot{\boldsymbol{r}}_i) }[/math] is the relativistic clock bias, periodically risen from the satellite's orbital eccentricity and Earth's gravity field.^[2] The satellite's position and velocity are determined by [math]\displaystyle{ \displaystyle t_i }[/math] as follows: [math]\displaystyle{ \displaystyle \boldsymbol{r}_i \;=\; \boldsymbol{r}_i (t_i) }[/math] and [math]\displaystyle{ \displaystyle \dot{\boldsymbol{r}}_i \;=\; \dot{\boldsymbol{r}}_i (t_i) }[/math].
4. In the field of GNSS, "geometric range", [math]\displaystyle{ \displaystyle r(\boldsymbol{r}_A,\, \boldsymbol{r}_B) }[/math], is defined as straight range, or 3-dimensional distance,^[3] from [math]\displaystyle{ \displaystyle\boldsymbol{r}_A }[/math] to [math]\displaystyle{ \displaystyle\boldsymbol{r}_B }[/math] in inertial frame (e.g., Earth-centered inertial (ECI) one), not in rotating frame.^[2]
5. The receiver's position, [math]\displaystyle{ \displaystyle \boldsymbol{r}_{\text{rec}} }[/math], and reception time, [math]\displaystyle{ \displaystyle t_{\text{rec}} }[/math], satisfy the light-cone equation of [math]\displaystyle{ \displaystyle r(\boldsymbol{r}_i,\, \boldsymbol{r}_{\text{rec}}) / c \,+\, (t_i - t_{\text{rec}}) \;=\; 0 }[/math] in inertial frame, where [math]\displaystyle{ \displaystyle c }[/math] is the speed of light. The signal time of flight from satellite to receiver is [math]\displaystyle{ \displaystyle -(t_i \,-\, t_{\text{rec}}) }[/math].
6. The above is extended to the satellite-navigation positioning equation, [math]\displaystyle{ \displaystyle r(\boldsymbol{r}_i,\, \boldsymbol{r}_{\text{rec}}) / c \,+\, (t_i \,-\, t_{\text{rec}}) \,+\, \delta t_{\text{atmos},i} \,-\, \delta t_{\text{meas-err},i} \;=\; 0 }[/math], where [math]\displaystyle{ \displaystyle \delta t_{\text{atmos},i} }[/math] is atmospheric delay (= ionospheric delay + tropospheric delay) along signal path and [math]\displaystyle{ \displaystyle \delta t_{\text{meas-err},i} }[/math] is the measurement error.
7. The Gauss–Newton method can be used to solve the nonlinear least-squares problem for the solution: [math]\displaystyle{ \displaystyle (\hat{\boldsymbol{r}}_{\text{rec}},\, \hat{t}_{\text{rec}}) \;=\; \arg \min \phi ( \boldsymbol{r}_{\text{rec}},\, t_{\text{rec}} ) }[/math], where [math]\displaystyle{ \displaystyle \phi ( \boldsymbol{r}_{\text{rec}},\, t_{\text{rec}} ) \;=\; \sum_{i=1}^n ( \delta t_{\text{meas-err},i} / \sigma_{\delta t_{\text{meas-err},i} } )^2 }[/math]. Note that [math]\displaystyle{ \displaystyle \delta t_{\text{meas-err},i} }[/math] should be regarded as a function of [math]\displaystyle{ \displaystyle \boldsymbol{r}_{\text{rec}} }[/math] and [math]\displaystyle{ \displaystyle t_{\text{rec}} }[/math].
8. The posterior distribution of [math]\displaystyle{ \displaystyle \boldsymbol{r}_{\text{rec}} }[/math] and [math]\displaystyle{ \displaystyle t_{\text{rec}} }[/math] is proportional to [math]\displaystyle{ \displaystyle \exp ( -\frac{1}{2} \phi ( \boldsymbol{r}_{\text{rec}},\, t_{\text{rec}} ) ) }[/math], whose mode is [math]\displaystyle{ \displaystyle (\hat{\boldsymbol{r}}_{\text{rec}},\, \hat{t}_{\text{rec}}) }[/math]. Their inference is formalized as maximum a posteriori estimation.
9. The posterior distribution of [math]\displaystyle{ \displaystyle \boldsymbol{r}_{\text{rec}} }[/math] is proportional to [math]\displaystyle{ \displaystyle \int_{-\infty}^\infty \exp ( -\frac{1}{2} \phi ( \boldsymbol{r}_{\text{rec}},\, t_{\text{rec}} ) ) \, d t_{\text{rec}} }[/math].

2. The Solution Illustrated

• 

  Essentially, the solution, [math]\displaystyle{ \scriptstyle (\hat{\boldsymbol{r}}_{\text{rec}},\, \hat{t}_{\text{rec}}) }[/math], is the intersection of light cones. https://handwiki.org/wiki/index.php?curid=2077021

3. The GPS Case

• For Global Positioning System (GPS),^[2] the non-closed-form equations in step 3 result in

[math]\displaystyle{ \scriptstyle \begin{cases} \scriptstyle \Delta t_i (t_i,\, E_i) \;\triangleq\; t_i \,+\, \delta t_{\text{clock},i} (t_i,\, E_i) \,-\, \tilde{t}_i \;=\; 0, \\ \scriptstyle \Delta M_i (t_i,\, E_i) \;\triangleq\; M_i (t_i) \,-\, (E_i \,-\, e_i \sin E_i) \;=\; 0, \end{cases} }[/math]

in which [math]\displaystyle{ \scriptstyle E_i }[/math] is the orbital eccentric anomaly of satellite [math]\displaystyle{ i }[/math], [math]\displaystyle{ \scriptstyle M_i }[/math] is the mean anomaly, [math]\displaystyle{ \scriptstyle e_i }[/math] is the eccentricity, and [math]\displaystyle{ \scriptstyle \delta t_{\text{clock},i} (t_i,\, E_i) \;=\; \delta t_{\text{clock,sv},i} (t_i) \,+\, \delta t_{\text{orbit-relativ},i} (E_i) }[/math].

• The above can be solved by using the bivariate Newton–Raphson method on [math]\displaystyle{ \scriptstyle t_i }[/math] and [math]\displaystyle{ \scriptstyle E_i }[/math]. Two times of iteration will be necessary and sufficient in most cases. Its iterative update will be described by using the approximated inverse of Jacobian matrix as follows:

[math]\displaystyle{ \scriptstyle \begin{pmatrix} t_i \\ E_i \\ \end{pmatrix} \leftarrow \begin{pmatrix} t_i \\ E_i \\ \end{pmatrix} - \begin{pmatrix} 1 && 0 \\ \frac{\dot{M}_i (t_i)}{1 - e_i \cos E_i} && -\frac{1}{1 - e_i \cos E_i} \\ \end{pmatrix} \begin{pmatrix} \Delta t_i \\ \Delta M_i \\ \end{pmatrix} }[/math]

• Tropospheric delay should not be ignored, while the Global Positioning System (GPS) specification^[2] doesn't provide its detailed description.

4. The GLONASS Case

• The GLONASS ephemerides don't provide clock biases [math]\displaystyle{ \scriptstyle\delta t_{\text{clock,sv},i} (t) }[/math], but [math]\displaystyle{ \scriptstyle\delta t_{\text{clock},i} (t) }[/math].

The content is sourced from: https://handwiki.org/wiki/Engineering:GNSS_positioning_calculation