Transcript Practical Implementation Considerations in the Detection of GPS Satellite Signal Failure A.

Practical Implementation Considerations in the
Detection of GPS Satellite Signal Failure
A. J. Van Dierendonck, AJ Systems
Dennis Akos, Sam Pullen, Eric Phelts and Per Enge
Stanford University
27 June 2000
IAIN/ION Meeting
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Topics
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Paper Objective
Satellite Failure Threat Models
Airborne Receiver Design Constraints
Detection Metric Definitions/Implementation
Detection Metric Performance Requirements
Detection Metric Performance
Some Simulation Results
Noise/Interference Considerations
Scan Peak Requirements
Summary & Conclusions
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IAIN/ION Meeting
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Paper Objectives
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Provide summary and update of SQM
requirements
Present practical implementation of SQM
detection metrics
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Reason why detection is not as complex as it may
seem
Provide an update on metric performance
Justification for changes to airborne receiver
requirements
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Satellite Failure Thread Models
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Undistorted Correlation Function
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Distorted Correlation Function
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Satellite Failure Threat Models
Threat A
Lead/Lag
-0.12 chips    0.12 chips
Threat B
Ringing
4 MHz  fd  17 MHz
0.8    8.8
Threat C
Combination
-0.12 chips    0.12 chips
4 MHz  fd  17 MHz
0.8    8.8
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Threat Model A
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Threat Model B
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Threat Model C
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Airborne Receiver Design Constraints
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Airborne E-L Discriminator Constraint
Space
Region
3 dB
Precorrelation
bandwidth (MHz)
Average
Correlator
Spacing
(Chips)
Instantaneous
Correlator
Spacing
(Chips)
Differential
Group Delay
(nsec)
1
2 < BW  7
0.045 - 1.1
0.04 - 1.2
 600
2
7 < BW  16
0.045 - 0.21
0.04 - 0.235
 150
3
16 < BW  20
0.045 - 0.12
0.04 - 0.15
 150
4
20 < BW  24
0.08 – 0.12
0.07 – 0.13
 150
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Airborne  Discriminator Constraint
Space
Region
3 dB
Precorrelation
bandwidth
(MHz)
Average
Correlator
Spacing
(Chips)
Instantaneous
Correlator
Spacing
(Chips)
Differential
Group Delay
(nsec)
1
2 < BW  7
0.045 – 0.6
0.04 – 0.65
 600
2
7 < BW  14
0.045 - 0.24
0.04 - 0.26
 150
3
14 < BW  16
0.07 - 0.24
0.06 - 0.26
 150
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SQM Detection Metric
Definitions/Implementation
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Don’t require pseudoranges
Don’t need a “picket fence”
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Close-in Correlation Function Values
P0.1
E0.025
E0.075
L0.025
(E - L)0.1
E0.1
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L0.075
L0.1
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SQM2b Test Metrics
ESFD1,k , j 
ESFD 2,k , j 
2 I P0.2,k , j

I E  L0.1,k , j
2 I P0.1,k , j
I E0.075,k , j  I L0.075,k , j
2 I P0.2,k , j
I E0.075,k , j
S P1,k , j 
S P 2,k 
I E  L0.2,k , j
I P0.2,k , j
I L0.075,k
I P0.2,k

  E0.2,0.1, j  0.2,k , j  0.1,k , j
I E  L0.1,k , j
2 I P0.1,k , j
  E0.15,0.1, j  0.15,k , j  0.1,k , j
  SP1, j
  SP 2, j
• Can use more ratios
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Detection Thresholds, Mean Values,
MDEs and MDRs
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Real World Thresholds, Mean Values
D, test   D ,test  TD
TD  K ffd D, test
R, test   R ,test  TR
TR  K ffd  R , test
Used for Simulations (Include missed
detection probability)
MDE  ( K ffd  Kmd )D,test
MDR  ( K ffd  Kmd )R,test
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Fault-Free Probability Densities
0.5
Fault-Free
 test
0.45
Negative Failure
Positive Failure
0.4
Probability Density
0.35
0.3
0.25
Kmd
0.2
Kffd
0.15
0.1
0.05
0
-16.7
-8.35
0
8.35
16.7
Hypothetical Test Value Normalized with  test
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Metric Statistics
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Based upon Stanford and NovAtel Roof Tests
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Two environmental extremes
Multipath dominates noise and interference
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Difference Metric Performance
Comparisons
0.45
0.4
0.15 - 0.1, NovAtel 502
0.2 - 0.1, NovAtel 502
Delta Sigma Value - meters
0.35
0.2 - 0.1 Stanford
0.15 - 0.1 Stanford
0.3
0.25
0.2
0.15
0.1
0.05
0
0
10
20
30
40
50
60
70
80
90
Elevation Angle
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Ratio Metric Performance
Comparisons
0.0035
Ratio +0.1, NovAtel 502
Ratio -0.1, NovAtel 502
Stanford Ratio -0.1
Stanford Ratio +0.1
0.003
Ratio Sigma Value
0.0025
0.002
0.0015
0.001
0.0005
0
0
10
20
30
40
50
60
70
80
90
Elevation Angle
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Some Simulation Results
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Justify simple metric definition
Justify change in airborne receiver design
constraints
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Additional constraints on  discriminator
receivers
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Smaller constraint space
Reduction of constraints on E – L discriminator
receivers
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Larger constraint space
No “Scan Peak” requirement
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 Discriminator Error Performance –
SQM3
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 Discriminator Error Performance –
SQM2b
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Threat Model C E-L Discriminator
Protection Results
8.0
0.09
Max
Spacing
Maximum Test Value
0.08
6.0
0.07
5.0
0.06
4.0
0.05
3.0
0.04
2.0
0.03
1.0
0.02
0.0
0
500
1000
1500
2000
Chip Spacing - chips, Bandwidth - GHz
BW GHz
7.0
0.01
2500
Test Number (With Multiple Zero Crossings)
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Summary & Conclusions
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SQM doesn’t have to be exceeding complex
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Performance is dominated by multipath
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Use correlation function amplitude measurements
instead of PRs
Performance varies widely with environment
Performance simulations are useful to define
airborne receiver design constraints
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 constraint space
E – L scan peak requirement
DoD receiver accommodation
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IAIN/ION Meeting
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