cycle slip detection and fixing by mems imu/gps
TRANSCRIPT
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Cycle Slip Detection and Fixing byMEMS IMU/GPS Integration for MobileEnvironment RTK-GPS
Tomoji TAKASU, Akio YASUDATokyo University of Marine Science and Technology
ION GNSS 2008, Savannah, USA, September 16-19, 2008
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Background
RTK-GPS under Open Sky RTK-GPS on Downtown Street
Fixed Solution Float Solution by
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Mobile RTK-GPS in Urban Area
• A lot of issues to be resolved:– Multipath (code, carrier-phase)– Signal outage/data gap– False tracking in receiver– Communication link interruption– ...– Cycle slip
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Cycle Slip
• Discontinuity in the carrier-phasemeasurement caused by loss-of-lock of signaltracking in a GPS receiver
• Integer cycle (or +half cycle) jump appears incarrier-phase ambiguity
• Most slips are caused by signal obstructionsdue to surrounding obstacles, like buildings,trees, bridges, poles, cars,...
• Far more slips in the mobile condition
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Cycle Slip in Urban Area
Sky Plot of Satellites : Cycle Slips
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Typical Cycle Slip Pattern
ReceiverDetects
Loss-of-LockReacquisition
(<1s)
Half-Cycle AmbiguityResolution(0-12s) *
Carrier-Phase
Cycle Slip(Half-Cycle
Slip)
Time
Signal Outage/Data Gap
* Depend on Receiver
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Statistics of Cycle Slips
• Most data gaps areshorter than 10 s.
• With short data gaps,cycle slips could befixed aided by INS?
• If slips fixed,availability of solutioncould be muchimproved.
0 10 200
100
200
300
400
500
Span of Data Gap (s)
Num
ber
of S
ampl
es
EL>15°
T<1s: 724(68.2%)T<3s: 869(81.8%)T<5s: 932(87.8%)
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Cycle Slip Detection
• Various ways:– Receiver loss-of-lock indicator or lock-time– Detect jump of L1-L2 Geometry-Free LC– Phase prediction with delta-range– Innovation test in navigation filter ...
• Combination of multiple methods to ensurethe reliability
• It’s effective aided by INS?
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Cycle Slip Fixing
• Most receivers have no cycle slip fixing. Ifdetecting a slip, just reinitialize and restartthe ambiguity estimation.
• Someone employ “slip-free” solutions, likeinstantaneous AR. But its performance greatlydepends upon the quality of code observables.May not be practical under ill multipathenvironment.
• It’s hard, but feasible aided by INS?
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Conventional RTK-GPS
GPS Receiver
RTK-GPS
bb φρ ,rr φρ ,
rtkr̂Rover Position
GPS Receiver
ρφ
: Pseudorange: Carrier Phase
Rover BaseStation
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INS Aided RTK-GPS
INS/RTK-GPSIntegration
bb φρ ,rr φρ ,
insrtk−r̂Rover Position
GPS ReceiverIMU
INS/GPSNavigation
insins vQv ˆ,ˆ
GPS Receiver
rr ρρ ,ii aω ,
ρ
ω
: Delta Range
: Angular Ratea : Acceleration
ρ : Pseudorange
Rover BaseStation
φ : Carrier Phase
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INS/GPS Navigation
ii aω , rr ρρ ,
),,,,( agiii bbrvψ δδδ
insinsins rvψ ˆ,ˆ,ˆ
INSNavigation
INS/GPSIntegrated
EKFiii rvψ ~,~,~
IMU GPSReceiver
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GPSReceiver
INS/RTK-GPS Integration
INS/GPSNavigation
insins vQv ˆ,ˆ
States Prediction
Measurement Update
Ambiguity Resolution
Cycle Slip Detection
2/))(()()(
2/))(ˆˆ()(ˆ)(ˆ
11,ˆ,ˆ1,,
11,,1
−−−
−−−
−+++=−
−+++=−
kkkkkrkr
kkkinskinskk
tt
tt
insins vv QQPP
vvrr
bb φρ ,,, rr φρ
GPSReceiver
insrtk−r̂
RTK-GPSFilter
Rover Position
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Cycle Slip Detection/Fixing
• Cycle slip detection– Innovation test in RTK-GPS Filter– Test threshold is determined by position
covariance with INS velocity covariance• Cycle slip fixing
– Slip amounts are estimated as float valueswith predicted position aided by INS
– Estimated slips are resolved into integervalues in usual RTK-GPS ILS solver process
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Experiments
Cycle Slip Simulation
Evaluation
Proposed Algorithm
Raw GPSData (4Hz)
Raw IMUData (50Hz)
SimulatedGPS Data
ReferenceTrajectory
INS-AidedRTK-GPSSolutions
Under Open Sky
Baseline Analysis
Drive Rover with GPS/IMU
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Configuration
NovAtelOEMV-3(L1/L2)
USB
Hub
Laptop PC(Data Logger)
NovAtelGPS-702
Splitter
MEMS-IMU
u-blox AEK-4T x 3 for Reference Attitude
SPI/USBAdapter
Base Station
NovAtelOEMV-3(L1/L2)
Rover (Car)
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MEMS-IMU
• Analog Devices ADIS16354– Tri-axis Gyros+Accelerometers– 23 x 23 x 23 mm– Range: ±300°/s, ±1.7g
– Embedded ADC/Filter, SPI I/F– Factory calibrated sensitivity, alignment
with temperature sensors– Price: $700/sample
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Snapshots
MEMS-IMU
GPS-702Antenna
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Reference Trajectory
100m5m
BaseStation
2008-09-055:42:00-
5:49:30 GPST7min 30s
BaselineLength:
0.0-0.9 km
5m
1cm
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Simulated Cycle Slips
81
388
718
(1)A FewSlips
(2)Moderate
Slips
(3)Extreme
Slips
# of slips
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(1) A Few Cycle Slips
Without INS With INS
1m
Position Error
1m
Position Error
Fixed Solution Float Solution
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(2) Moderate Cycle Slips
Without INS With INS
1m
Position Error
1m
Position Error
Fixed Solution Float Solution
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(3) Extreme Cycle Slips
Without INS With INS
1m
Position Error
1m
Position Error
Fixed Solution Float Solution
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Solution Availability/Accuracy
CaseWithout INS With INS
92.9% 93.0%
58.8% 64.5%
28.4% 48.4%
0.3 0.4 0.5 0.3 0.4 0.6
74.2 125.6 57.5 7.7 13.4 12.9
346.6 573.4 306.4 16.2 24.3 26.3
Availability of Fixed Sol. / RMS Error E-W,N-S,U-D (cm)
(1)A FewSlips(2)
ModerateSlips(3)
ExtremeSlips
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Summary and Conclusions
• A simple integration scheme of GPS/INSNavigation to RTK-GPS filter is proposed
• Cycle slip detection and fixing aided by INS isfeasible and effective especially on thecondition with extreme cycle slips
• Needs more experiments in the real situationswith ill multipath environment
• Direct integration of INS measurements toRTK-GPS filter improves the performance?