Ionosphere Scintillation-Induced
Phase Transitions (Cycle Slips)
in Triple-Frequency GPS Measurements
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Brian Breitsch, Jade Morton, Dongyang Xu
24 January 2020
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ION ITM/PTTI 2020
are ionosphere-induced phase transitions?
What
Why
How
Outline
are they so difficult to deal with?
can we mitigate their impact?
are ionosphere-induced phase transitions?
What
Why
How
Outline
are they so difficult to deal with?
can we mitigate their impact?
Ascension Island - 2013-10-05 - G24
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(typically equatorial) ionosphere plasma bubbles / structures
signal diffraction
Ionosphere Scintillation
[Xu et al 2017]
Signal Model
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IONOSPHERE EFFECTS
BIASES
NONDISPERSIVE EFFECTS
phase model
ionosphere effect
REFRACTIVE
DIFFRACTIVE
WARNING: \(I_\text{diffr}\) may contain phase transitions...
...but why?
Ionosphere Diffraction Simulations
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instantiate realistic phase screen structure \(\bar{\phi}\) from stochastic model
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propagate using parabolic wave equation methods
generate complex field \(\Psi(t)\) at the receiving antenna
Simulated Phase Transitions
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Most of common variation in \(\phi_k(t)\) is due to ionosphere refraction, which is approximately captured by the model phase screens \(\bar{\phi}(t)\).
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[Rino et al 2017]
Origin of Phase Transitions
Assume the phase screen structure approximates refractive phase, i.e. \(I_\text{refr} \propto \bar{\phi}\).
Then \(\tilde{\phi}(t) = \text{unwrap}\left(\angle \tilde{\Psi}(t) \right)\) is the diffractive phase and \(I_\text{diffr} \propto \tilde{\phi}\)
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histograms of \(\tilde{\Psi}\)
We can decompose \(\Psi(t)\) as \(\Psi(t) = \tilde{\Psi}(t) \exp(i\bar{\phi}(t))\).
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Phase Transitions
not instantaneous!
phase rate-of-change as cycle slip indicator would produce many false positives!
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singularity at origin causes phase bifurcation
Phase Transitions as Cycle Slips
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[phase transitions] occur gradually over many samples...[they are] distinct from cycle slips, which are abrupt phase changes of a cycle (or more)
- Carrano et al 2013
Phase transitions and cycle slips due to other factors are often indistinguishable
- Me (Brian) 2020
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"[The] increased measurement noise associated with an active ionosphere makes correcting cycle slips an ongoing challenge which requires further investigation."
- Banville 2010
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"...precise navigation is possible under strong scintillation conditions as long as the problem with the cycle slips could be properly addressed."
- Juan et al 2018
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are ionosphere-induced phase transitions?
What
Why
How
Outline
are they so difficult to deal with?
can we mitigate their impact?
Cycle Slip Detection
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Cycle Slip Detection
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Cycle Slip Detection
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Cycle Slip Detection
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Cycle Slip Detection
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Cycle Slip Detection
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Things that make cycle slip detection hard
Effects of ionosphere diffraction
large phase noise
unknown trends / dynamics
frequent / consecutive slips
Geometry-Free Cycle Slip Detection
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Geometry-Free combination not viable during diffraction
are ionosphere-induced phase transitions?
What
Why
How
Outline
are they so difficult to deal with?
can we mitigate their impact?
Mitigation Strategies
- multi-frequency observations
- triple-frequency [de Lacy 2012, etc.]
- quadruple-frequency [Juan et al 2019]
- geodetic detrending [Juan et al 2017]
- cascaded detection [Qile Zhao 2014]
- first remove easy-to-detect slips
- allows detection of weaker signatures
- expanding window of observations [Tao Li 2018]
What remains is to carfully apply these techniques to the diffraction-induced cycle slip problem.
Linear Combinations of Phase
Ionosphere-Free
Geometry-Free
Geometry-Ionosphere-Free
not so useful during diffraction
need to know your clocks!
need triple-frequency
Linear Combinations of Phase
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Septentrio PolaRxS - Hong Kong - 2013-10-05 - G24
Slip Detectability
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(1, 1, 1)
(0, 1, 1)
(1, 0, 0)
(1, 1, -)
(1, -, 1)
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cycle-slip combinations
Geometry-Ionosphere-Free Combination
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L5
L2
L1
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Key Points
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GNSS phase measurements are plagued by diffraction-induced cycle slips (and phase transitions) during strong ionosphere scintillation
Diffraction-induced cycle slip detection is challenging
- methods that only use GF combination are not viable
- many consecutive slips with large noise residuals
A combination of strategies will be necessary to detect cycle slips during diffraction
- triple-frequency offers unique advantages
- cycle slip combinations (1, 1, 1), (0, 1, 1), and (1, 0, 0) are especially challenging
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References
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Xu, Dongyang, and Yu Morton. "A semi-open loop GNSS carrier tracking algorithm for monitoring strong equatorial scintillation." IEEE Transactions on Aerospace and Electronic Systems 54.2 (2017): 722-738.
Rino, Charles, et al. "A New GNSS Scintillation Model." Proc. ION GNSS. 2017.
de Lacy, Maria Clara, Mirko Reguzzoni, and Fernando Sansò. "Real-time cycle slip detection in triple-frequency GNSS." GPS solutions 16.3 (2012): 353-362.
Wang, Yaoding, et al. "Real-Time Quadruple-Frequency Cycle Slip Detection and Repair Algorithm Based on the Four Chosen Linear Combinations." IEEE Access 7 (2019): 154697-154710.
Juan, José Miguel, et al. "Feasibility of precise navigation in high and low latitude regions under scintillation conditions." Journal of Space Weather and Space Climate 8 (2018): A05.
Zhao, Qile, et al. "Real-time detection and repair of cycle slips in triple-frequency GNSS measurements." GPS Solutions 19.3 (2015): 381-391.
Li, Tao, and Stavros Melachroinos. "An enhanced cycle slip repair algorithm for real-time multi-GNSS, multi-frequency data processing." GPS Solutions 23.1 (2019): 1.
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Cycle Slip Detection
Attempted removal in real scintillation data (Hong Kong, Septentrio ground antenna)
phase transitions
Phase Behavior in Real Data
L5
L2
L1
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Phase Behavior in Real Data
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L5
L2
L1
Phase Behavior in Real Data
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L5
L2
L1
Phase Transitions
Linear Combinations
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Ionosphere Scintillation
L5
L2
L1
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ionosphere phase screen
2013, Ascension Island, PRN 24
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fading of 15 to 20 dB or more in signal amplitude
Deep Fading
canonical fades: associated with half-cycle phase changes
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L5
L2
L1
Simulating Scintillation
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shows validation of occurrence of full-cycle transitions during canonical fades
simulation
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Ionosphere Scintillation-Induced Phase Transitions in Triple-Frequency GNSS
By Brian Breitsch
Ionosphere Scintillation-Induced Phase Transitions in Triple-Frequency GNSS
ION ITM meeting 2020
- 456