riding out the rough spots: scintillation-robust gnss carrier tracking dr. todd e. humphreys...
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Riding out the Rough Spots:Scintillation-Robust GNSS Carrier Tracking
Dr. Todd E. HumphreysRadionavigation LaboratoryUniversity of Texas at Austin
UT Radionavigation Laboratory
UT Radionavigation Lab Research Agenda GNSS Spoofing
Characterize spoofing signatures Develop receiver-autonomous defenses Develop augmentation-based defenses
(GPS + eLORAN + Iridium + …)
GPS Jamming Develop augmentation-based defenses Locate jamming sources by combining
data from a network of receivers
Indoor Navigation Pioneer collaborative navigation Develop augmentation-based indoor nav
techniques (GPS + eLORAN + Iridium + …)
Natural GNSS Interference Improve tracking loop robustness to
scintillation
UT Radionavigation Laboratory
Ionospheric Diagnosis via Arrays of GPS Receivers
Ionospheric Monitoring (sparse array) Ionospheric Tomography (dense array)
Incident plane wave
Disturbedionosphere
Diffractedwavefront
Linear array ofGRID receivers
Nominal magneticfield direction
CASESConnected Autonomous Space Environment Sensors
Cornell University, UT Austin, ASTRA LLC
AFOSR STTR Proposal, 2008
CASESConnected Autonomous Space Environment Sensors
Cornell University, UT Austin, ASTRA LLC
AFOSR STTR Proposal, 2008
UT Radionavigation Laboratory
Carrier Tracking GoalsReceiver noise and scintillation-induced
phase errors Cycle slips (phase unlock)
Total loss of carrier lock (frequency unlock)
Analyze scintillation effects on GPS receivers; isolate cause of phase unlock
Model scintillation well enough to generate realistic synthetic scintillation
Synthesize scintillation to test tracking loop strategies
Design phase tracking loops for operation in scintillation
Strategy
Long-term Goals Eliminate frequency unlock Minimize cycle slips and
generally reduce phase errors
UT Radionavigation Laboratory
Carrier Tracking Goals
Analyze scintillation effects on GPS receivers; isolate cause of phase unlock
Model scintillation well enough to generate realistic synthetic scintillation
Synthesize scintillation to test tracking loop strategies
Design phase tracking loops for operation in scintillation
Long-term Goals Eliminate frequency unlock Minimize cycle slips and
generally reduce phase errorsStrategy
UT Radionavigation Laboratory
Model: Distill Scintillation Down to Essential Characteristics for Carrier Tracking
Standard statistical
analysis techniques
Standard statistical
analysis techniquesDPSK bit error
prediction with Rice and 2nd-order
Butterworth models
DPSK bit error prediction with Rice
and 2nd-order Butterworth models
UT Radionavigation Laboratory
Hardware-in-the-loop Scintillation Robustness Evaluation
Scintillation Simulator Simulated time history
GNSS Signal Simulator
GNSS Receiver Phase difference time history
UT Radionavigation Laboratory
Design: Scintillation-hardened Tracking Loops
• Straightforward approach: navigation data bit prediction
• Incorporate the observed second-order dynamics into a Kalman filter whose state includes the complex components of z(t)
• Combine this with a Bayesian multiple-model filter that spawns a new tracking loop whenever a data bit is uncertain. Prune loops at parity check.
GOAL: Ts > 240 seconds for {S4 = 0.8, 0 = 0.8 sec., C/N0 = 43 dB-Hz}
(a factor of 10 longer than current best)
GOAL: Ts > 240 seconds for {S4 = 0.8, 0 = 0.8 sec., C/N0 = 43 dB-Hz}
(a factor of 10 longer than current best)
UT Radionavigation Laboratory
The GPS Assimilator
The GPS Assimilator modernizes and makes existing GPS equipment resistant to jamming, spoofing, and scintillation without requiring
hardware or software changes to the equipment
The GPS Assimilator modernizes and makes existing GPS equipment resistant to jamming, spoofing, and scintillation without requiring
hardware or software changes to the equipment
A Backward-Compatible Way to Harden Existing UE Against Scintillation
UT Radionavigation Laboratory
All digital signal processing implemented in C++ on a high-end DSP Marginal computational demands:
Tracking: ~1.2% of DSP per channel Simulation: ~4% of DSP per channel
Full capability: 12 L1 C/A & 10 L2C tracking channels 8 L1 C/A simulation channels 1 Hz navigation solution Acquisition in background
GPS Assimilator Prototype
UT Radionavigation Laboratory
Summary
Models of scintillation effects on phase tracking loops must faithfully capture deep fades
The mean time between differentially-detected navigation bit errors is a good lumped
indicator of scintillation severity The triple accurately
predicts For carrier tracking, scintillation
modeling & simulation can be boiled down to two parameters: S4 & τ0
A hardware-in-the-loop scintillation testbed has been built and validated
Carrier tracking techniques inspired by the proposed model promises to extend
UT Radionavigation Laboratory
Acknowledgements
CASES sensor development funded by STTR grant through AFOSR via ASTRA LLC
Adaptation of CASES sensor for Antarctic deployment funded by ASTRA LLC
UT Radionavigation Laboratory
Amplitude Distribution:
Rice distribution applies
p(|z(t)|) can be summarized by the S4 index
Rice distribution applies
p(|z(t)|) can be summarized by the S4 index