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27 June 2000 IAIN/ION Meeting 1
Practical Implementation Considerations in the Detection of GPS Satellite Signal Failure
A. J. Van Dierendonck, AJ SystemsDennis Akos, Sam Pullen, Eric Phelts and Per
Enge Stanford University
27 June 2000 IAIN/ION Meeting 2
Topics 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
27 June 2000 IAIN/ION Meeting 3
Paper Objectives Provide summary and update of SQM
requirements Present practical implementation of
SQM detection metrics Reason why detection is not as complex as
it may seem Provide an update on metric
performance Justification for changes to airborne
receiver requirements
27 June 2000 IAIN/ION Meeting 4
Satellite Failure Thread Models
27 June 2000 IAIN/ION Meeting 5
Undistorted Correlation Function
27 June 2000 IAIN/ION Meeting 6
Distorted Correlation Function
27 June 2000 IAIN/ION Meeting 7
Satellite Failure Threat Models
Threat ALead/Lag
-0.12 chips 0.12 chips
Threat BRinging
4 MHz fd 17 MHz
0.8 8.8
Threat CCombination
-0.12 chips 0.12 chips4 MHz fd 17 MHz
0.8 8.8
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Threat Model A
27 June 2000 IAIN/ION Meeting 9
Threat Model B
27 June 2000 IAIN/ION Meeting 10
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)
AverageCorrelator
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
27 June 2000 IAIN/ION Meeting 13
Airborne Discriminator Constraint Space
Region 3 dB Precorrelation
bandwidth (MHz)
AverageCorrelator
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
Don’t require pseudoranges Don’t need a “picket fence”
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Close-in Correlation Function Values
P0.1
L0.075(E - L)0.1E0.075
E0.025
E0.1
L0.025
L0.1
27 June 2000 IAIN/ION Meeting 16
SQM2b Test Metrics0.2, , 0.1, ,
0.2,0.1,
0.2, , 0.1, ,
0.075, , 0.075, , 0.1, ,
0.15,0.1,
0.2, , 0.1, ,
0.075,
1, , 0.2, , 0.1, ,
2, , 0.15, , 0.1, ,
1, ,
2 2
2 2
k j k j
j
k j k j
k j k j k j
j
k j k j
E L E L
SFD k j E k j k jP P
E L E L
SFD k j E k j k jP P
E
P k j
I IE
I I
I I IE
I I
IS
,
1,
0.2, ,
0.075,
2,
0.2,
2,
k j
P j
k j
k
P j
k
SP
L
P k SP
I
IS
I
• Can use more ratios
27 June 2000 IAIN/ION Meeting 17
Detection Thresholds, Mean Values, MDEs and MDRs
Real World Thresholds, Mean Values
Used for Simulations (Include missed detection probability)
,D ffd D testT K
,R ffd R testT K
,MDE ( )ffd md D testK K
,MDR ( )ffd md R testK K
,, D test DD test T
,, R test RR test T
27 June 2000 IAIN/ION Meeting 18
Fault-Free Probability Densities
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
-16.7 -8.35 0 8.35 16.7
Hypothetical Test Value Normalized with test
Pro
bab
ility
De
nsi
ty
Fault-Free
Negative Failure
Positive Failure
Kffd
Kmd
test
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Metric Statistics
Based upon Stanford and NovAtel Roof Tests
Two environmental extremes Multipath dominates noise and interference
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Difference Metric Performance Comparisons
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0 10 20 30 40 50 60 70 80 90
Elevation Angle
De
lta
Sig
ma
Va
lue
- m
ete
rs
0.15 - 0.1, NovAtel 502
0.2 - 0.1, NovAtel 502
0.2 - 0.1 Stanford
0.15 - 0.1 Stanford
27 June 2000 IAIN/ION Meeting 21
Ratio Metric Performance Comparisons
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0 10 20 30 40 50 60 70 80 90
Elevation Angle
Ra
tio
Sig
ma
Va
lue
Ratio +0.1, NovAtel 502Ratio -0.1, NovAtel 502Stanford Ratio -0.1Stanford Ratio +0.1
27 June 2000 IAIN/ION Meeting 22
Some Simulation Results Justify simple metric definition Justify change in airborne receiver
design constraints Additional constraints on discriminator
receivers Smaller constraint space
Reduction of constraints on E – L discriminator receivers
Larger constraint space No “Scan Peak” requirement
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Discriminator Error Performance – SQM3
27 June 2000 IAIN/ION Meeting 24
Discriminator Error Performance – SQM2b
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Threat Model C E-L Discriminator Protection Results
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
0 500 1000 1500 2000 2500
Test Number (With Multiple Zero Crossings)
Ma
xim
um
Te
st
Va
lue
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
Ch
ip S
pa
cin
g -
ch
ips
, Ba
nd
wid
th -
GH
z
Max
Spacing
BW GHz
27 June 2000 IAIN/ION Meeting 26
Summary & Conclusions SQM doesn’t have to be exceeding
complex Use correlation function amplitude
measurements instead of PRs Performance is dominated by multipath
Performance varies widely with environment Performance simulations are useful to
define airborne receiver design constraints constraint space E – L scan peak requirement DoD receiver accommodation
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