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# Combination of GNSS Measurements

Revision as of 11:26, 23 February 2012 by Carlos.Lopez (talk | contribs)

Fundamentals | |
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Title | Combination of GNSS Measurements |

Author(s) | J. Sanz Subirana, J.M. Juan Zornoza and M. Hernández-Pajares, Technical University of Catalonia, Spain. |

Level | Intermediate |

Year of Publication | 2011 |

Starting from the basic observables (described in GNSS Basic Observables) the following combinations can be defined (where [math]R_{_{Pi}}[/math] and [math]\Phi_{_{Li}}[/math], [math]i=1,2[/math], indicate measurements in the frequencies [math]f_1[/math] and [math]f_2[/math]):

**Ionosphere-free combination**: It removes the first order (up to 99.9%) ionospheric effect, which depends on the inverse square of the frequency ([math]\alpha_i \propto \frac{1}{f_i^2}[/math], see article Ionospheric Delay).

- [math] \Phi_{_{LC}}=\frac{f_1^2\;\Phi_{_{L1}}-f_2^2\;\Phi_{_{L2}}}{f_1^2-f_2^2}~~~~~;~~~~~ R_{_{PC}}=\frac{f_1^2\;R_{_{P1}}-f_2^2\;R_{_{P2}}}{f_1^2-f_2^2} \qquad \mbox{(1)} [/math]

Satellite clocks are defined relative to [math]R_{_{PC}}[/math] combination (see article Combining pairs of signals and clock definition).

**Geometry-free combination**: it cancels the geometric part of the measurement, leaving all the frequency-dependent effects (i.e., ionospheric refraction, instrumental delays, wind-up) besides multipath and measurement noise. It can be used to estimate the ionospheric electron content, to detect cycle-slips in the carrier phase, or also to estimate antenna rotations as well. Note the change of terms order in [math]\Phi_{_{LI}}[/math] and [math]R_{_{PI}}[/math].

- [math] \Phi_{_{LI}}=\Phi_{_{L1}}-\Phi_{_{L2}}~~~~~;~~~~~ R_{_{PI}}=R_{_{P2}}-R_{_{P1}} \qquad \mbox{(2)} [/math]

**Wide-laning combinations**: These combinations are used to create a signal with a significantly wide wavelength. This longer wavelength is useful for cycle-slips detection and ambiguity fixing. Other feature of this combination is the change of the sign in the ionospheric term, which is exploited by the Melbourne-Wübbena combination to remove the ionospheric refraction.

- [math] \Phi_{_{LW}}=\frac{f_1\;\Phi_{_{L1}}-f_2\;\Phi_{_{L2}}}{f_1-f_2}~~~;~~~ R_{_{PW}}=\frac{f_1\;R_{_{P1}}-f_2\;R_{_{P2}}}{f_1-f_2} \qquad \mbox{(3)} [/math]

**Narrow-laning combinations**: These combinations create signals with a narrow wavelength. The signal in this combination has a lower noise than each separated component. It is used to reduce the code noise in the Melbourne-Wübbena combination to estimate the wide-lane ambiguity.

- [math] \Phi_{_{LN}}=\frac{f_1\;\Phi_{_{L1}}+f_2\;\Phi_{_{L2}}}{f_1+f_2}~~~;~~~ R_{_{PN}}=\frac{f_1\;R_{_{P1}}+f_2\;R_{_{P2}}}{f_1+f_2} \qquad \mbox{(4)} [/math]

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