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

Background

RTK-GPS under Open Sky RTK-GPS on Downtown Street

Fixed Solution Float Solution by

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

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

Cycle Slip in Urban Area

Sky Plot of Satellites : Cycle Slips

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

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%)

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?

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?

Conventional RTK-GPS

GPS Receiver

RTK-GPS

bb φρ ,rr φρ ,

rtkr̂Rover Position

GPS Receiver

ρφ

: Pseudorange: Carrier Phase

Rover BaseStation

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

INS/GPS Navigation

ii aω , rr ρρ ,

),,,,( agiii bbrvψ δδδ

insinsins rvψ ˆ,ˆ,ˆ

INSNavigation

INS/GPSIntegrated

EKFiii rvψ ~,~,~

IMU GPSReceiver

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

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

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

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)

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

Snapshots

MEMS-IMU

GPS-702Antenna

Reference Trajectory

100m5m

BaseStation

2008-09-055:42:00-

5:49:30 GPST7min 30s

BaselineLength:

0.0-0.9 km

5m

1cm

Simulated Cycle Slips

81

388

718

(1)A FewSlips

(2)Moderate

Slips

(3)Extreme

Slips

# of slips

(1) A Few Cycle Slips

Without INS With INS

1m

Position Error

1m

Position Error

Fixed Solution Float Solution

(2) Moderate Cycle Slips

Without INS With INS

1m

Position Error

1m

Position Error

Fixed Solution Float Solution

(3) Extreme Cycle Slips

Without INS With INS

1m

Position Error

1m

Position Error

Fixed Solution Float Solution

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

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?