ION GNSS 2017 - Analysis of Carrier Phase Residual Using Geometry-Ionosphere-Free Combination of Triple-Frequency GPS Observations

Motivation:

TEC and Triple-Frequency

Background:

Linear Estimation of GNSS Parameters

Main Topic:

Geometry-Ionosphere-Free Combination

Analysis and Results:

GIFC From Real GPS Data

Motivation:

TEC and Triple-Frequency

Background:

Linear Estimation of GNSS Parameters

Main Topic:

Geometry-Ionosphere-Free Combination

Analysis and Results:

GIFC From Real GPS Data

Carrier Phase and TEC

\Phi_i = r + c\Delta t + T + I_i + \lambda_i N_i + H_i + S_i + \epsilon_i

\Phi_i = r + c\Delta t + T + I_i + \lambda_i N_i + H_i + S_i + \epsilon_i

HARDWARE BIASES

IONOSPHERE RANGE ERROR

CARRIER AMBIGUITY

SYSTEMATIC ERRORS / MULTIPATH

FREQUENCY INDEPENDENT EFFECTS

STOCHASTIC ERRORS

frequency $f_i$

I_i \approx -\frac{\kappa}{f_i^2} \text{TEC}

I_i \approx -\frac{\kappa}{f_i^2} \text{TEC}

plasma / free electrons

Multi-Frequency GNSS

GPS Block IIF / Block III
Galileo (E1, E5a/b)
GLONASS-K2

Signal	Frequency (GHz)
L1CA	1.57542
L2C	1.2276
L5	1.17645

triple-frequency GPS

12 satellites since 2016

Estimation of $G$ and $\text{TEC}$ Using Dual-Frequency GNSS

\text{TEC} = \displaystyle -\frac{\Phi_1 - \Phi_2}{\kappa \left(\frac{1}{f_1^2} - \frac{1}{f_2^2}\right)}

\text{TEC} = \displaystyle -\frac{\Phi_1 - \Phi_2}{\kappa \left(\frac{1}{f_1^2} - \frac{1}{f_2^2}\right)}

G = \displaystyle \frac{f_1^2 \Phi_1 - f_2^2 \Phi_2}{f_1^2 - f_2^2}

G = \displaystyle \frac{f_1^2 \Phi_1 - f_2^2 \Phi_2}{f_1^2 - f_2^2}

ionosphere-free combination

geometry-free combination

\Phi_i = G + I_i + S_i + \epsilon_i

\Phi_i = G + I_i + S_i + \epsilon_i

"geometry" term

\Phi_i = r + c\Delta t + T + I_i + \lambda_i N_i + H_i + S_i + \epsilon_i

\Phi_i = r + c\Delta t + T + I_i + \lambda_i N_i + H_i + S_i + \epsilon_i

we can solve bias terms

Examples of Two Dual-Frequency TEC Estimates

Poker Flat, Alaska, 2016-01-02

Can we characterize / find the source of these discrepancies?

Motivation:

TEC and Triple-Frequency

Background:

Linear Estimation of GNSS Parameters

Main Topic:

Geometry-Ionosphere-Free Combination

Analysis and Results:

GIFC From Real GPS Data

model parameters

\mathbf{\Phi} = \left[\Phi_1, \cdots, \Phi_m\right]^T

\mathbf{\Phi} = \left[\Phi_1, \cdots, \Phi_m\right]^T

\mathbf{m} = \left[G, \text{TEC}, S_1, \cdots, S_m \right]^T

\mathbf{m} = \left[G, \text{TEC}, S_1, \cdots, S_m \right]^T

\mathbf{\Phi} = \mathbf{A} \mathbf{m} + \mathbf{\epsilon}

\mathbf{\Phi} = \mathbf{A} \mathbf{m} + \mathbf{\epsilon}

Linear Inverse Problem

observations

\Phi_i = G + I_i + S_i + \epsilon_i

\Phi_i = G + I_i + S_i + \epsilon_i

\mathbf{\hat{m}} \approx \mathbf{A^*} \mathbf{\Phi}

\mathbf{\hat{m}} \approx \mathbf{A^*} \mathbf{\Phi}

estimate:

\mathbf{A^*} =\begin{bmatrix} \mathbf{C}_G \\ \mathbf{C}_\text{TEC} \\ \mathbf{C}_{S_1} \\ \vdots \\ \mathbf{C}_{S_m} \end{bmatrix}

\mathbf{A^*} =\begin{bmatrix} \mathbf{C}_G \\ \mathbf{C}_\text{TEC} \\ \mathbf{C}_{S_1} \\ \vdots \\ \mathbf{C}_{S_m} \end{bmatrix}

geometry estimator

TEC estimator

systematic-error estimators

Linear Coefficient Constraints

\sum_i c_i = 0

\sum_i c_i = 0

\sum_i c_i = 1

\sum_i c_i = 1

geometry-free

geometry-estimator

\sum_i -\frac{\kappa}{f_i^2}c_i = 1

\sum_i -\frac{\kappa}{f_i^2}c_i = 1

\sum_i \frac{c_i}{f_i^2} = 0

\sum_i \frac{c_i}{f_i^2} = 0

TEC-estimator

ionosphere-free

\Phi_i = G + I_i + S_i + \epsilon_i

\Phi_i = G + I_i + S_i + \epsilon_i

solutions at intersection of constraint hyperplanes

|G| , |I_i| \gg |S_i|

|G| , |I_i| \gg |S_i|

a-priori information:

Estimators

$\text{TEC}$ Estimator

TEC-estimator + geometry-free constraints

$G$ Estimator

geometry-estimator + ionosphere-free constraints

$\text{L1,L2,L5}$

$\text{L1,L5}$

$\text{L1,L2}$

choose min-norm

Triple-Frequency GPS Estimator Coefficients

\begin{matrix} \text{Estimate} & c_1 & c_2 & c_3 & \sum_i c_i^2 \\[.5em] G_\text{L1,L2,L5} & 2.327 & -0.360 & -0.967 & 2.546 \\ G_\text{L1,L5} & 2.261 & 0 & -1.261 & 2.588 \\ G_\text{L1,L2} & 2.546 & -1.546 & 0 & 2.978 \\ G_\text{L2,L5} & 0 & 12.255 & -11.255 & 16.639 \end{matrix}

\begin{matrix} \text{Estimate} & c_1 & c_2 & c_3 & \sum_i c_i^2 \\[.5em] G_\text{L1,L2,L5} & 2.327 & -0.360 & -0.967 & 2.546 \\ G_\text{L1,L5} & 2.261 & 0 & -1.261 & 2.588 \\ G_\text{L1,L2} & 2.546 & -1.546 & 0 & 2.978 \\ G_\text{L2,L5} & 0 & 12.255 & -11.255 & 16.639 \end{matrix}

\begin{matrix} \text{Estimate} & c_1 & c_2 & c_3 & \sum_i c_i^2 \\[.5em] \text{TEC}_\text{L1,L2,L5} & 8.294 & -2.883 & -5.411 & 10.314 & \\ \text{TEC}_\text{L1,L5} & 7.762 & 0 & -7.762 & 10.977 & \\ \text{TEC}_\text{L1,L2} & 9.518 & -9.518 & 0 & 13.460 & \\ \text{TEC}_\text{L2,L5} & 0 & 42.080 & -42.080 & 59.510 & \end{matrix}

\begin{matrix} \text{Estimate} & c_1 & c_2 & c_3 & \sum_i c_i^2 \\[.5em] \text{TEC}_\text{L1,L2,L5} & 8.294 & -2.883 & -5.411 & 10.314 & \\ \text{TEC}_\text{L1,L5} & 7.762 & 0 & -7.762 & 10.977 & \\ \text{TEC}_\text{L1,L2} & 9.518 & -9.518 & 0 & 13.460 & \\ \text{TEC}_\text{L2,L5} & 0 & 42.080 & -42.080 & 59.510 & \end{matrix}

$G$ Estimator

$\text{TEC}$ Estimator

Triple-Frequency TEC Estimate Examples

Motivation:

TEC and Triple-Frequency

Background:

Linear Estimation of GNSS Parameters

Main Topic:

Geometry-Ionosphere-Free Combination

Analysis and Results:

GIFC From Real GPS Data

Estimating Systematic Errors

apply both geometry-free and ionosphere-free constraints

For triple-frequency GNSS:

\begin{aligned} c_1 & = \ \ x \frac{\frac{1}{f_3^2} - \frac{1}{f_2^2}}{\frac{1}{f_2^2} - \frac{1}{f_1^2}} \\[1em] c_2 & = - x \frac{\frac{1}{f_3^2} - \frac{1}{f_1^2}}{\frac{1}{f_2^2} - \frac{1}{f_1^2}} \\[1em] c_3 & = \ \ x \end{aligned}

\begin{aligned} c_1 & = \ \ x \frac{\frac{1}{f_3^2} - \frac{1}{f_2^2}}{\frac{1}{f_2^2} - \frac{1}{f_1^2}} \\[1em] c_2 & = - x \frac{\frac{1}{f_3^2} - \frac{1}{f_1^2}}{\frac{1}{f_2^2} - \frac{1}{f_1^2}} \\[1em] c_3 & = \ \ x \end{aligned}

system is linear subspace

there is "only one estimate" of systematic errors

note this requires $m \ge 3$

\Phi_i = G + I_i + S_i + \epsilon_i

\Phi_i = G + I_i + S_i + \epsilon_i

Geometry-Ionosphere-Free Combination

\mathbf{C}_{\text{TEC}_{1,2,3}}

\mathbf{C}_{\text{TEC}_{1,2,3}}

\mathbf{C}_\text{GIFC}

\mathbf{C}_\text{GIFC}

information about systematic and stochastic errors present in GNSS carrier phase observables

\begin{aligned} \mathbf{C}_{\text{GIFC}_\text{L1,L2,L5}} & = \mathbf{C}_{\text{TEC}_{\text{L1,L5}}} - \mathbf{C}_{\text{TEC}_{\text{L1,L2}}} \\[1em] & = \left[-1.756, \ 9.520, \ -7.764\right] \end{aligned}

\begin{aligned} \mathbf{C}_{\text{GIFC}_\text{L1,L2,L5}} & = \mathbf{C}_{\text{TEC}_{\text{L1,L5}}} - \mathbf{C}_{\text{TEC}_{\text{L1,L2}}} \\[1em] & = \left[-1.756, \ 9.520, \ -7.764\right] \end{aligned}

We (arbitrarily) choose:

\Phi_i = G + I_i + S_i + \epsilon_i

\Phi_i = G + I_i + S_i + \epsilon_i

\mathbf{C}_{G_{1,2,3}}

\mathbf{C}_{G_{1,2,3}}

$\text{GIFC} = G_{1,3} - G_{1,2}$

$\text{GIFC} = \text{TEC}_{1,3} - \text{TEC}_{1,2}$

Motivation:

TEC and Triple-Frequency

Background:

Linear Estimation of GNSS Parameters

Main Topic:

Geometry-Ionosphere-Free Combination

Analysis and Results:

GIFC From Real GPS Data

Experiment Data

Sense Lab high-rate GNSS data collection network

Alaska, Hong Kong, Peru
2013 - 2016
1 Hz GPS L1/L2/L5 measurements
Septentrio PolarXs

GIFC Examples

PRN 24, Peru

PRN 24, Hong Kong

PRN 25, Peru

PRN 01, Alaska

2013-01-01

2014-05-15

2013-01-01

2014-06-29

GIFC

Sat. Elevation

GIFC Calendar

PRN 24, Hong Kong

PRN 25, Hong Kong

PRN 24, Alaska

PRN 25, Alaska

GIFC Calendar

PRN 25, Alaska

large, long-term GIFC trend likely due mostly to satellite thermal oscillations
studied by (Montebruck et. al. 2012) and (Li et. al. 2013)

$\theta$

GIFC Histogram

Peru Example

estimate trend $\Rightarrow$ reduce major component of TEC and G estimate errors

(all satellites, all data)

Conclusions

GIFC is an indicator of the presence of systematic / stochastic errors

dominant GIFC component is periodic
- long, arc-wide trend due to satellite thermal variations
- errors on order of up to 0.5 TECu

need to start estimating variable IFB corrections to accommodate TEC / geometry estimates

Next Steps

use to analyze other systematic dispersive errors such as multipath and strong scintillation

References

O. Montenbruck, U. Hugentobler, R. Dach, P. Steigenberger, and A. Hauschild, “Apparent clock variations of the Block IIF-1 (SVN62) GPS satellite,” GPS Solutions, vol. 16, no. 3, pp. 303–313, 2012.

H. Li, X. Zhou, and B. Wu, “Fast estimation and analysis of the inter-frequency clock bias for Block IIF satellites,” GPS Solutions, vol. 17, no. 3, pp. 347–355, 2013.

B. Breitsch, "Optimal Linear Combinations of GNSS Phase Observables to Improve and Assess TEC Estimation Precision," Masters Thesis, Colorado State University

Acknowledgements

This research was supported by the Air Force Research Laboratory and NASA.

GIFC Spectrum

PRN 01, Peru

PRN 24, Peru

PRN 25, Peru

Hz

Normalized Power Spectrum

Analysis of Carrier Phase Residual Variations

Using the Geometry-Ionosphere-Free Combination