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GNSS-based Flight Inspection Systems. International Flight Inspection Symposium Oklahoma City, OK USA June 2008. Euiho Kim Selex-si-us Todd Walter, David Powell Stanford University. Introduction. - PowerPoint PPT Presentation
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1 of 2404/22/23
GNSS-based Flight Inspection
Systems
International Flight Inspection International Flight Inspection SymposiumSymposium
Oklahoma City, OK USA June 2008Oklahoma City, OK USA June 2008
Euiho KimSelex-si-us
Todd Walter, David Powell Stanford University
2 of 2404/22/23
Introduction• Stanford Univ. has developed Global
Navigation Satellite System (GNSS)-based flight inspection systems:– WAAS-aided FIS: described at 14th IFIS at Toulouse – WAAS-based FIS– Standalone (unaugmented) GPS-based FIS
• The primary objective of this work was to develop a FI truth system with– High efficiency: onboard self-contained FIS– Low cost: no INS (WAAS-based FIS, standalone
GPS-based FIS)
3 of 2404/22/23
WAAS(SBAS)-based FIS• Sensors
– WAAS receiver (single frequency), Radar altimeter (RA), TVPS
• Positioning strategy− Reference position @ runway threshold from the RA & TVPS + a
relative positioning algorithm, called “T-D PRP”, with GPS/WAAS
• Characteristics – On-board self contained system– Does not use an INS– Does not need to fly over the entire runway– Near real-time positioning (WAAS optimized)– A receiver must be able to output raw GPS/WAAS measurements
• Reference – “WAAS-based Flight Inspection System”, Journal of Aircraft, , Vol.
45, No 2, AIAA
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EstimatedTrue
Trajectory
T-D PRP
RA,TVPS
Ref. Position at threshold
Raw GPS/WAAS• carrier phase meas.• pseudo-range• ephemeris• WAAS messages
WAASPosition
Fast-ClockCorrection
Filtering (MV)
Satellite Exclusion Tests
Y/N
Y/NSat 1
Y/NSat 2
…Ref. Position Validation
Y
Y
WAAS-based FIS Architecture
∑
Approach
RunwayThreshold
Approach finishes
Relativeposition
5 of 2404/22/23
Time-Difference Precise Relative Positioning (T-D PRP)
• Ranging source− Time-difference of carrier phase measurements− Fine resolution (~cm)− Immune to any bias-like GPS ranging error sources
• Utilizes WAAS correction messages if available − Does not use WAAS ionospheric delay corrections
• Reduce ionospheric effects− Ionospheric delay gradient is estimated and removed using the
combination of code and carrier (near real time)− The estimation process is further checked by exclusion tests
• Improvements− Fine resolution relative position (~cm)− Not vulnerable to abnormal ionospheric effects
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Satellite Exclusion Tests
GPS/WAASSatellite Health
Designation
Cycle-Slip
Residual Test on Estimated Ionospheric
Gradient
SatelliteOutage
• Satellites and carrier phase measurements must pass the following checks
7 of 2404/22/23
Reference Position Validation
T-D PRP
True
WAAS
Bias
Off-Set
Range wrt. Threshold
VerticalPosition
RadarAltimeter
• Test statistics for the reference position validation using WAAS position in vertical
• WAAS 95% accuracy (WAAS PAN Oct ~ Dec, 2006)
1.11 m in horizontal
1.34 m in vertical
• RA and TVPS 95% Accuracy
RA: 15cm
TVPS: 15cm
thr(WAAS T-D PRP) (RA T-D PRP )| |
| WAAS vertical bias Error (RA) |VT AVG
<Vertical position characteristics for WAAS, RA, T-D PRP>
8 of 2404/22/23-10000
-50000
5000
-1
0
1
2
x 104
0
200
400
600
800
1000
EAST (m)NORTH (m)
UP
(m
)
Flight Test Data• Flight test during Oct 30~31, 2006 in collaboration with
FAA flight inspection crews• 23 approaches used
–No radar altimeter or TVPS data was available, unfortunately• Truth source is RTK DGPS
9 of 2404/22/23
WAAS-based FIS Test Results• Assumes a perfect reference position
– Radar altimeter and TVPS were not available– A reference position was obtained from RTK DGPS
• Graphs show only the T-D PRP algorithm errors– (Total error = ref. position bias (RA, TVPS) + T-D PRP error)
<Horizontal> <Vertical>
0 2000 4000 6000 8000 10000 12000-3
-2
-1
0
1
2
3
Range wrt. Threshold (m)
Cro
ss-T
rack
Erro
r(m) CAT I
Error Limit
CAT II&III Error Limit
CAT II&III Error Limit
CAT I Error Limit
10 of 2404/22/23
0 2000 4000 6000 8000 10000 12000-3
-2
-1
0
1
2
3
Range wrt. Threshold (m)
Cro
ss-T
rack
Erro
r(m)
WAAS-based FIS Accuracy (95%) at Critical Region
• Error analysis− T-D PRP error grows over time− Total errors at critical region
= Ref. position bias + T-D PRP error @ CR
• T-D PRP error statistics @ CR
• Therefore, with:– RA accuracy (95%) : 15 cm– TVPS accuracy (95%) : 15 cm– T-D PRP Accuracy @ CR
• Total Error @ CR is:– Up : 18 cm < 30 cm (req.)– Cross-track : 17 cm < 60 cm (req.)
Up (m) Cross-Track (m)
Mean -0.04 0.00
Std 0.05 0.04
RMS 0.06 0.04
CR
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Standalone GPS-based FIS• Sensors
– GPS receiver (single frequency), Radar altimeter, TVPS
• Positioning strategy− Reference position @ runway threshold (RA, TVPS) + relative
positioning using the T-D PRP algorithm with standalone GPS
• Characteristics– On-board self contained system– Does not use INS– Does not need to fly over the entire runway– Near real time positioning– Operational worldwide – A receiver must be able to output raw GPS measurements
• Reference– “Standalone GPS-based Flight Inspection System”, ENC-GNSS 2007,
Geneva, Switzerland
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Standalone GPS-based FIS Architecture
EstimatedTrue
Trajectory
T-D PRP
RA,TVPS
Ref. Position at threshold
GPS measurements• carrier• pseudo-range• ephemeris
Satellite Exclusion Tests
Y/N
Y/NSat 1
Y/NSat 2
…FIS-RAIM
Y
∑
Approach
RunwayThreshold
Approach finishes
13 of 2404/22/23
Integrity Issues• The WAAS-based FI system derived its
integrity from the underlying WAAS integrity messages.
• GPS-based FI system does not have that feature; therefore,– An autonomous procedure was developed to
meet that need– Uses redundant satellite measurements to
establish integrity of the FI procedure
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Receiver Autonomous Integrity Monitoring (RAIM)
• Basic RAIM methods to detect satellite failures− Range comparison method [Lee, 1986]− Least-squares-residual method [Parkinson and Axelrad, 1988]− Parity method [Sturza, 1989]− Maximum separation of solutions [Brown and McBurney, 1987]
Sat 1 Sat 5
Sat 2 Sat 4Sat 3
unhealthy
nominalfault
<Illustration of separated solutions under 1 satellite failure>
d
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Flight Inspection System RAIM (1/3) (FIS-RAIM)
• Current baseline RAIM scheme – Not suited for high-accuracy
application [Ober,06]– Detects large satellite failures
• Minor satellite failures – Slow ramp clock error (a few cm/s)– Low amplitude satellite clock
dithering (a few meters)
• FIS-RAIM– Designed to detect the violation of
CAT I error limit
0 2000 4000 6000 8000 10000 12000-6
-4
-2
0
2
4
6
Range wrt. Threshold (m)
UP
Erro
r(m)
ramp clock error (4cm/s)
CAT I Error Limit
CAT II & IIIError Limit
0 2000 4000 6000 8000 10000 12000-6
-4
-2
0
2
4
6
Range wrt. Threshold (m)
UP
Erro
r (m
)
sinusoidal clock dithering (1m amp.)
CAT I Error Limit
CAT II & IIIError Limit
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0 2000 4000 6000 8000 10000 120000
1
2
3
4
Range wrt. Threshold (m)
Max
imum
Sep
arat
ion
of S
olut
ions
(m)
0 2000 4000 6000 8000 10000 120000
2
4
6
8
10
Range wrt. Threshold (m)
Max
imum
Sep
arat
ion
of S
olut
ions
(m)
• FIS-RAIM
– Uses maximum separation of solutions of T-D PRP in vertical
– Uses full measurements during approach
– Uses 2 test statistics • A slope of observed
maximum separated solutions using linear regression (ramp clock error)
• RSS for the residual test on the regression (clock dithering)
sat. clock ramp error(4cm/s)
sat. clock dithering (1m amp.)
Flight Inspection System RAIM (2/3)(FIS-RAIM)
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0 2000 4000 6000 8000 10000 120000
1
2
3
4
5
6
7
Range wrt. Threshold (m)
Max
imum
Sep
arat
ion
of S
olut
ions
(m)
Threshold
• The thresholds are– Slope: 0.05/2 deg (determined for CAT I error limit)– RSS : (20cm)2*chi2inv(99.9%,n-2)
no satellite failures
Flight Inspection System RAIM (3/3)(FIS-RAIM)
0 2000 4000 6000 8000 10000 12000-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
Range wrt. Threshold (m)
Res
idua
ls (m
)
no satellite failures
18 of 2404/22/23
Standalone GPS-based FIS Test Results
• Assumes a perfect reference position– Radar altimeter and TVPS were not available – The reference position was determined from DGPS
• Graphs show only T-D PRP errors– (Total error = ref. position bias (RA, TVPS) + T-D PRP error)
<Vertical> <Horizontal>
0 2000 4000 6000 8000 10000 12000-3
-2
-1
0
1
2
3
Range wrt. Threshold (m)
Cro
ss-T
rack
Erro
r(m)
CAT I Error Limit
CAT II & III Error LimitCAT I
Error Limit
CAT II & IIIError Limit
19 of 2404/22/23
0 2000 4000 6000 8000 10000 12000-3
-2
-1
0
1
2
3
Range wrt. Threshold (m)C
ross
-Tra
ck E
rror
(m)
Standalone GPS-based FISAccuracy (95%) at Critical Region
(CR)• Error analysis
− T-D PRP error grows over time− Total errors at critical region
= Ref. position bias + T-D PRP error @ CR
• T-D PRP error statistics @ CR
• Therefore, with:• RA accuracy (95%) : 15 cm• TVPS accuracy (95%) : 15 cm• T-D PRP Accuracy @ CR
• Total Error @ CR is:– Up : 18 cm < 30 cm (req.)– Cross-track : 17 cm < 60 cm (req.)
Up (m) Cross-Track (m)
Mean -0.04 0.00
Std 0.05 0.04
RMS 0.06 0.04CR
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WAAS-based FIS vs. GPS-based FIS
0 2000 4000 6000 8000 10000 12000-3
-2
-1
0
1
2
3
Range wrt. Threshold (m)
Cro
ss-T
rack
Erro
r(m)
0 2000 4000 6000 8000 10000 12000-3
-2
-1
0
1
2
3
Range wrt. Threshold (m)
Cro
ss-T
rack
Err
or(m
)
WAAS-based FIS
WAAS-based FIS
GPS-based FIS
GPS-based FIS
• Almost identical accuracy− Strong temporal correlation
in satellite clock-ephemeris residual correction error of WAAS and GPS (usual)
− A FI approach occurs over a very short time
21 of 2404/22/23
Summary of WAAS-based FIS
• WAAS-based FIS Accuracy (95%)– Meets flight inspection system accuracy requirement for CAT
III ILS calibration • Up : 18 cm < 30 cm (req.) @ CR• Cross-track : 17 cm < 60 cm (req.) @CR
• Benefits– Firm integrity
• Satellite health from WAAS integrity messages • Protect against sharp and nonlinear ionospheric delay gradient• Reference position validation
– Low cost and high efficiency
• Limitation– Operational where WAAS (or any SBAS) is available
22 of 2404/22/23
Summary of Standalone GPS-based FIS
• Standalone GPS-based FIS accuracy (95%)– Meets flight inspection system accuracy requirements for
CAT III ILS calibration • Vertical : 18 cm < 30cm (req.) @ CR• Cross-track : 17 cm < 60cm (req.) @CR
• Advantages– Firm integrity
• Satellite health from FIS-RAIM• Protect against sharp and nonlinear ionospheric delay gradient
– Low cost and high efficiency for worldwide use
• Limitation– Not able to check the integrity of a reference position
23 of 2404/22/23
Accuracy Efficiency Cost-Effect. Worldwide Usefulness
Inertial-based AFIS
DGPS-based AFIS
WAAS(SBAS)-based FIS
Standalone GPS-based
FIS
Comparison with Current FIS’s
24 of 2404/22/23
Thank you !Special thanks to FAA AVN for the generous
support in this research