Transcript Slide 1

Mitigation of the Effects Early Skywaves
Ben Peterson, Peterson Integrated Geopositioning
&
Per Enge, Stanford University
Funded by Federal Aviation Administration, Mitch Narins, Program Manager
International Loran Association, November 5, 2003
Outline
• Review the environment & 1986 MOPS (TSO
C60b)
– 29 & 29 Oct 2003 sample data
• Shift tracking points for phase & ECD earlier
– Analysis of noise and bias in phase & ECD
measurements vs tracking point & pulse rise time
• Receivers w/ causal & non-Causal (block processing) filtering
– Change in time differences with shifted tracking
points
• Transmitting pulses with carrier frequencies of
96, 100, and 104 kHz
• Augmentation (monitors & warnings via LDC)
From Peter Morris (curves for 1 mmho/m)
Skywave Delay
45
microseconds
40
35
ECD bias @
M * 5 usec
PCD
Summer Day
30
TOA bias @
(N+1/2) * 5
usec
25
20
450
500
550
600
650
700
Skywave/Groundwave Ratio
750
800
850
(@ 1 mmho/m)
30
dB
20
PCD
Summer Day
10
0
-10
450
500
550
600
650
700
Distance in NM
750
800
850
690 NM
441 NM
591 NM
Simultaneous loss of WAAS
vertical guidance & Loran
horizontal is not operationally
significant.
• Ionosphere never gets bad enough that
WAAS HPL > 556 m
• Loran exists to address other
vulnerabilities (jamming, spoofing,
interference, etc)
Skywaves Specifications from TSO-C60b
30
Skywave/Groundwave (dB)
25
20
15
10
5
0
30
35
40
45
Delay (usec)
50
55
60
Auroral Zone 60deg
Basic Problem Statement
• Pulse design and 1986 MOPS address everyday
skywaves
• Issue is abnormally early skywaves caused by
solar event
• We can easily detect existence of skywaves,
tough part is to distinguish skywave with 25 us
delay from one with 35 us, 23 us from 33 us, etc.
in a user receiver
– With < 1e-7 integrity &
– With < 1e-4 to 1-e3 false alarms
• This can be done in a monitor receiver
Skywave/Groundwave Ratio for PCD Events
20
1 mmho/m
3 mmho/m
10 mmho/m
5 mho/m
15
10
5
dB
0
-5
-10
-15
-20
-25
450
500
550
600
650
700
Distance in NM
750
800
850
Effect on spectrum of faster rise time (tail w/65 usec time constant)
1
Envelope
0.8
0.6
0.4
0.2
0
0
20
40
60
80
100
usec
120
140
160
180
200
% power 90-110 kHz
99.5
99
Current requirement
98.5
98
45
50
55
Rise time in usec
60
65
Standard deviation of ECD measured via envelope ratio & for SNR = 33dB & rise time of 65 usec
0.9
10 usec separation
15 usec separation
20 usec separation
Standard deviation of ECD in usec
0.85
0.8
0.75
0.7
0.65
0.6
0.55
0.5
0.45
15
20
25
30
Time of second point - usec
35
40
Standard deviation of ECD measured via envelope ratio & for SNR = 33dB & rise time of 50 usec
0.9
10 usec separation
15 usec separation
20 usec separation
Standard deviation of ECD in usec
0.85
0.8
0.75
0.7
0.65
0.6
0.55
0.5
0.45
15
20
25
30
Time of second point - usec
35
40
No RF filtering. Curves will still apply when we filter, we just need to correct for the group delay &
the NEBW re the 30 kHz used here. Entire pulse phase noise will not change with NEBW.
Standard deviation of phase for SNR = 20dB (after averaging)
0.2
Rise time: 65 usec
Rise time: 50 usec
Entire Pulse
Standard deviation of phase in usec
0.18
0.16
0.14
0.12
0.1
0.08
0.06
0.04
20
25
30
35
40
45
50
Time of tracking point - usec
55
60
65
8th order Butterworth, 28 kHz -3 dB bandwidth (NEBW = 28.9 kHz)
Frequency Response: 8th order Butterworth
10
28 kHz -3dB bandwidth
50
Group delay - usec
Gain -dB
0
-10
-20
-30
-40
60
80
100
kHz
120
40
30
20
10
140
0
60
80
100
kHz
120
140
1
Original pulse
Filtered pulse
Envelope delayed 30 usec
Amplitude
0.5
0
-0.5
-1
0
10
20
30
40
Time - usec
50
60
70
80
8th order Butterworth, 28 kHz -3 dB bandwidth, group delay = 30 usec
TOA Bias due to early skywave,skywave/groundwave 0dB, )
0.06
Tracking point 50 usec
Tracking point 55 usec
Tracking point 60 usec
0.04
0.02
usec
0
-0.02
-0.04
-0.06
-0.08
-0.1
20
25
30
35
Skywave delay
40
45
50
8th order Butterworth, 28 kHz -3 dB bandwidth, group delay = 30 usec
(Vertical axis ECD bias in usec, x axis skywave delay in usec)
Sep = 10, Start = 37.5
4
Sep = 10, Start = 42.5
5
Sep = 10, Start = 47.5
10
2
10
5
0
5
0
0
-2
20
30
40
50
Sep = 15, Start = 37.5
4
Sep = 10, Start = 52.5
15
-5
20
30
40
50
Sep = 15, Start = 42.5
10
0
-5
20
30
40
50
Sep = 15, Start = 47.5
10
-5
20
30
40
50
Sep = 15, Start = 52.5
20
2
5
5
10
0
0
0
0
-2
20
30
40
50
Sep = 20, Start = 37.5
5
-5
20
30
40
50
Sep = 20, Start = 42.5
10
10
5
0
-10
20
30
40
50
Sep = 20, Start = 52.5
20
10
5
0
-5
20
-5
20
30
40
50
Sep = 20, Start = 47.5
15
30
40
50
-5
20
0
0
30
40
50
-5
20
30
40
50
-10
20
30
40
50
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter
Frequency Response
Group delay of analog: 64 kHz -3dB bandwidth
0
6
Group delay - usec
-5
Gain -dB
-10
-15
Analog
Overall
-20
-25
-30
60
80
100
kHz
120
5
4
3
2
1
60
140
80
100
kHz
120
140
1
Original pulse
Filtered pulse
Envelope delayed 5 usec
Amplitude
0.5
0
-0.5
-1
0
10
20
30
40
Time - usec
50
60
70
80
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter
TOA Bias due to early skywave,skywave/groundwave 0dB, )
0.06
Tracking point 25 usec
Tracking point 30 usec
Tracking point 35 usec
0.04
0.02
usec
0
-0.02
-0.04
-0.06
-0.08
-0.1
20
25
30
35
Skywave delay
40
45
50
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter ECD Bias
Sep = 10, Start = 12.5
1.5
Sep = 10, Start = 17.5
6
4
1
Sep = 10, Start = 22.5
Sep = 10, Start = 27.5
10
20
5
10
0
0
2
0.5
0
0
20
40
60
Sep = 15, Start = 12.5
3
2
-2
20
40
60
Sep = 15, Start = 17.5
10
5
1
0
0
-1
20
40
60
Sep = 20, Start = 12.5
4
2
-5
20
40
60
Sep = 20, Start = 17.5
10
5
-5
-10
20
40
60
20
30
40
50
Sep = 15, Start = 22.5
Sep = 15, Start = 27.5
15
30
10
20
5
10
0
0
-5
-10
20
40
60
20
30
40
50
Sep = 20, Start = 22.5
Sep = 20, Start = 27.5
20
30
20
10
10
0
-2
20
0
40
60
-5
20
0
40
60
-10
20
0
40
60
-10
20
30
40
50
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter, 50 usec rise time
Frequency Response
Group delay of analog: 64 kHz -3dB bandwidth
6
Group delay - usec
0
Gain -dB
-5
Analog
Overall
-10
-15
-20
70
80
90
100
110
kHz
120
130
5
4
3
2
1
70
140
80
90
100
110
kHz
120
130
140
1
Amplitude
0.5
0
Original pulse
Filtered pulse
Envelope delayed 5 usec
-0.5
-1
0
10
20
30
40
Time - usec
50
60
70
80
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter, 50 usec rise time
TOA Bias due to early skywave,skywave/groundwave 0dB, )
0.06
Tracking point 25 usec
Tracking point 30 usec
Tracking point 35 usec
0.04
0.02
usec
0
-0.02
-0.04
-0.06
-0.08
-0.1
20
25
30
Skywave delay
35
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter, 50 usec rise time, ECD Bias
(Green Locus LRS IIID data)
Sep = 10, Start = 12.5
Sep = 10, Start = 17.5
Sep = 10, Start = 22.5
4
4
10
2
2
5
0
0
0
-2
20
25
30
Sep = 15, Start = 12.5
35
4
2
0
-2
20
25
30
Sep = 20, Start = 12.5
35
-2
20
25
30
Sep = 15, Start = 17.5
35
-5
20
6
15
4
10
2
5
0
0
-2
20
25
30
Sep = 20, Start = 17.5
35
-5
20
4
10
20
2
5
10
0
0
0
-2
20
25
30
35
-5
20
25
30
35
-10
20
25
30
Sep = 15, Start = 22.5
35
25
30
Sep = 20, Start = 22.5
35
25
30
35
Field strength predictions of BALOR code for Seneca-Little Rock path
Difference in field strength
90 kHz - 100 kHz
100 kHz - 110 kHz
1.2
1
Undoes the effect
of differentiation
dB
0.8
0.6
0.4
0.2
0
0
100
200
300
400
500
600
Distance km
700
800
900
1000
In near far field, zeros crossings are < 5 usec apart
Range = 6 km, Delta ECD = -0.01 usec
600
400
200
0
-200
Original
Propagated
Env shftd dECD
-400
-600
0
5
10
15
20
25
usec
30
35
40
45
50
0.3
0.4
0.5
Zero Crossings re 10 usec multiples (-2.5 usec)
50
0
-50
-0.5
10
20
30
40
50
60
-0.4
-0.3
-0.2
-0.1
0
usec
0.1
0.2
After propagation has cancelled frequency response of differentiation
Range = 758 km, Delta ECD = -0.85 usec
600
400
200
0
-200
Original
Propagated
Env shftd dECD
-400
-600
0
5
10
15
20
25
usec
30
35
40
45
50
0.3
0.4
0.5
Zero Crossings re 10 usec multiples (-2.5 usec)
50
0
-50
-0.5
10
20
30
40
50
60
-0.4
-0.3
-0.2
-0.1
0
usec
0.1
0.2
At long differences, zeros crossings are > 5 usec apart
Range = 1011 km, Delta ECD = -1.4 usec
600
400
200
0
-200
Original
Propagated
Env shftd dECD
-400
-600
0
5
10
15
20
25
usec
30
35
40
45
50
0.3
0.4
0.5
Zero Crossings re 10 usec multiples (-2.5 usec)
50
0
-50
-0.5
10
20
30
40
50
60
-0.4
-0.3
-0.2
-0.1
0
usec
0.1
0.2
•Frequency modulation
•Transmit mixture of 96, 100, & 110 kHz pulses in known
pattern
•SSX controller switches L or C but not during pulse as in IFM
•Measure ECD & TOA for each frequency & compare
•Early skywave will cause different biases at the different
frequencies
•Issues:
•Spectrum
•Effect on legacy receivers
•Ability to detect early skywaves
Rise Time = 72 usec
1
0.5
0
-0.5
-1
0
20
40
60
usec
80
100
120
0
96 kHz
100 kHz
104 kHz
Sum
-10
dB
-20
-30
-40
-50
80
85
90
95
100
kHz
105
110
115
120
Rise time = 65 usec
Percent of power 90-110 kHz
99.2
99
98.8
98.6
2 ea.@
96 &
104 kHz
98.4
98.2
98
97.8
0
2
4
6
8
10
12
14
Number of 100 kHz pulses in PCI of 18 total
16
18
16
18
Rise time = 72 usec
100
99.8
Percent of power 90-110 kHz
Lengthening
pulse permits
balanced
distribution
among
frequencies &
still meeting
spectrum
requirement
99.4
5 ea.@
96 &
104 kHz
99.6
99.4
99.2
99
98.8
98.6
98.4
0
2
4
6
8
10
12
14
Number of 100 kHz pulses in PCI of 18 total
Lengthening pulse makes leading edge of average pulse the same to legacy receivers
Rise time = 72 usec
0.6
96 kHz
100 kHz
104 kHz
Sum
Envelope w/65 usec rise time shifted 1.2 usec
0.5
0.4
0.3
0.2
0.1
0
0
5
10
15
20
25
30
TOA Bias due to early skywave, (skywave/groundwave 0dB)
0.06
D = approx. 10% of bias
0.04
96 kHz
100 kHz
104 kHz
0.02
-0.02
-0.04
-0.06
-0.08
-0.1
20
25
30
ECD Bias due to early skywave, (skywave/groundwave 0dB)
35
2
96 kHz
100 kHz
104 kHz
1
0
usec
usec
0
-1
-2
-3
-4
-5
20
25
30
Skywave delay
35
Horizontal: Candidate detection statistic, Vertical: Bias to detect
0
27
0.04
26
100 kHz TOA Bias in usec
28
0.02
29
25
30
35
34
31 3233
20
-0.02
24
-0.04
21
-0.06
23
0.02
27
28 26
29
25 35
34
20 30 31
33
32
0
-0.02
24
-0.04
21
23
-0.06
22
-0.08
-0.1
22
-0.05
0
0.05
104 kHz - 96 kHz TOA Difference in usec
0.1
-0.08
-2
2
24
25
1
2326
27
3233
34
35
31
28
30
29
22
0
-1
-2
21
-3
-4
-5
-0.1
-1
0
1
2
104 kHz - 96 kHz ECD Difference in usec
3
2
100 kHz ECD Bias in usec
100 kHz ECD Bias in usec
100 kHz TOA Bias in usec
0.04
20
-0.05
0
0.05
104 kHz - 96 kHz TOA Difference in usec
0.1
25
1
0
26
27
28
24
23
29
35
34
33
32
30 31
22
-1
-2
21
-3
-4
-5
-2
20
-1
0
1
2
104 kHz - 96 kHz ECD Difference in usec
3
Assumptions
• Time constant (after Doppler has been measured and
removed) = 20 sec
• GRI = 9990, 1800 pulses (1600 unmodulated) in 20 sec
– 500 @ 96 kHz, 500 @ 104 kHz, 600 unmodulated @ 100 kHz, &
200 modulated @ 100 kHz
• SNR = -10dB
– SNR = +17 dB after average of 500
– SNR = +22 dB after average of 1600
• Phase s = 1.126 usec /(N x SNR)
s = 0.159 usec for average of 500 @ -10dB SNR
s = 0.225 usec for difference between 2 frequencies
s = 0.089 usec for average of 1600 @ -10dB SNR
• ECD s = 30.4 usec/(N x SNR)
s = 4.29 usec for average of 500 @ -10dB SNR
s = 6.07 usec for difference between 2 frequencies
s = 2.41 usec for average of 1600 @ -10dB SNR
Candidate test statistics for TOA bias
• Delta TOA to detect TOA bias
– Can’t use 104 re 96 kHz because delta goes to 0 at bias max
(22.5 & 27 usec)
– For max of 104 re 100 kHz and 96 re 100 kHz
• Delta TOA = 0.1 x TOA bias
• For probability of false alarm = 1e-3, 3.3 x delta TOA s = 3.3 x 0.225
usec = 0.74 usec, corresponding bias is 7.4 usec or 2,200 meters
• Delta ECD to detect TOA bias
– Delta ECD = 40 x TOA bias
– For probability of false alarm = 1e-3, 3.3 x delta ECD s = 3.3 x
6.07 usec = 20 usec, corresponding bias is 0.5 usec or 150
meters
– At 0 dB SNR, this bias becomes 0.17 usec or 50 meters
Candidate test statistics for ECD bias
• Delta ECD to detect ECD bias
– Can’t use because all deltas go to 0 at bias
max (24.5 usec)
• Delta TOA (104 re 96) to detect ECD bias
– Delta TOA = 0.028 x ECD bias
– For probability of false alarm = 1e-3, 3.3 x
delta TOA s = 3.3 x 0.225 usec = 0.74 usec,
corresponding ECD bias is 26 usec
79 Paths of < 900 nm using only LORSTA’s
With addition of Dunbar Forest, dense enough to see PCD.
55
+ Shoal Cove
+ Fox Harbor
+ Williams L
+ Port Hardy
50
Dunbar Forest??
+ Havre
+ Comfort Cv
+ Baudette
+ George
+ Caribou
45
+ Cape Race
+ Gillette
+ Seneca
+ Nantucket
40
+ Dana
+ Fallon
+ Middletown
+ Boise City
+ Searchlght
35
+ Carolina B
+ Las Cruces
+ Malone
+ Grangevlle
30
+ Raymondvll
+ Jupiter
25
-130
-120
-110
-100
-90
-80
-70
-60
-50
Conclusions
• Problem is much worse at high geomagnetic latitudes
(Alaska) but occasionally exists in large portion of northern
CONUS
– Problem for NELS, Russia, but probably not FERNS
• Problem can be detected with existing monitor
infrastructure
– Monitoring at LORSTA’s (plus Dunbar Forest) only is OK
• Receiver issues
– Faster rise time is possible and helps
– Non causal digital vice causal analog or digital filtering helps
– Moving tracking point earlier helps, phase and ECD measurement
noise get worse but are reasonable
– Moving tracking point affects TDs, issue for maritime, less for
aviation
– Frequency modulation is elegant but preliminary analysis indicates
poor performance at low SNR
• Simultaneous loss of WAAS vertical guidance & Loran
horizontal is not operationally significant.
Options (in order of cost)
A. New receiver MOPS
Probably not enough
B. Real time warnings via data channel (&
new MOPS)
– Hopefully enough but need to study impact on
availability
C. Faster rise time (& new MOPS)
D. Frequency modulation (& new MOPS)
A, B & C or A, B & D
Options (in order of cost)
A. New receiver MOPS
Probably not enough
B. Real time warnings via data channel (&
new MOPS)
– Hopefully enough but need to study impact on
availability
C. Faster rise time (& new MOPS)
D. Frequency modulation (& new MOPS)
A, B & C or A, B & D
Acknowledgements, etc.
Funded by Federal Aviation Administration
– Mitch Narins – Program Manager
For additional info:
[email protected]
-Note- The views expressed herein are those of the authors
and are not to be construed as official or reflecting the views
of the U. S. Federal Aviation Administration, or the U.S.
Departments of Transportation and Homeland Security.