mitigation of the effects early skywaves ben peterson, peterson integrated geopositioning & per...
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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)
450 500 550 600 650 700 750 800 85020
25
30
35
40
45Skywave Delay
mic
rose
cond
s
PCDSummer Day
450 500 550 600 650 700 750 800 850-10
0
10
20
30Skywave/Groundwave Ratio
dB
Distance in NM
PCDSummer Day
ECD bias @ M * 5 usec
TOA bias @ (N+1/2) * 5
usec
(@ 1 mmho/m)
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
0
5
10
15
20
25
30
30 35 40 45 50 55 60
Delay (usec)
Skyw
ave/
Gro
undw
ave
(dB
)
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
450 500 550 600 650 700 750 800 850
-25
-20
-15
-10
-5
0
5
10
15
20Skywave/Groundwave Ratio for PCD Events
dB
Distance in NM
1 mmho/m3 mmho/m10 mmho/m5 mho/m
0 20 40 60 80 100 120 140 160 180 2000
0.2
0.4
0.6
0.8
1E
nvel
ope
usec
Effect on spectrum of faster rise time (tail w/65 usec time constant)
45 50 55 60 6598
98.5
99
99.5
Rise time in usec
% p
ower
90-
110
kHz
Current requirement
15 20 25 30 35 400.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
Sta
ndar
d de
viat
ion
of E
CD
in u
sec
Time of second point - usec
Standard deviation of ECD measured via envelope ratio & for SNR = 33dB & rise time of 65 usec
10 usec separation15 usec separation20 usec separation
15 20 25 30 35 400.45
0.5
0.55
0.6
0.65
0.7
0.75
0.8
0.85
0.9
Sta
ndar
d de
viat
ion
of E
CD
in u
sec
Time of second point - usec
Standard deviation of ECD measured via envelope ratio & for SNR = 33dB & rise time of 50 usec
10 usec separation15 usec separation20 usec separation
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.
20 25 30 35 40 45 50 55 60 650.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2S
tand
ard
devi
atio
n of
pha
se in
use
c
Time of tracking point - usec
Standard deviation of phase for SNR = 20dB (after averaging)
Rise time: 65 usecRise time: 50 usecEntire Pulse
8th order Butterworth, 28 kHz -3 dB bandwidth (NEBW = 28.9 kHz)
60 80 100 120 140-40
-30
-20
-10
0
10Frequency Response: 8th order Butterworth
kHz
Gai
n -d
B
60 80 100 120 1400
10
20
30
40
50
kHz
Gro
up d
elay
- u
sec
28 kHz -3dB bandwidth
0 10 20 30 40 50 60 70 80-1
-0.5
0
0.5
1
Am
plitu
de
Time - usec
Original pulseFiltered pulseEnvelope delayed 30 usec
8th order Butterworth, 28 kHz -3 dB bandwidth, group delay = 30 usec
20 25 30 35 40 45 50-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06TOA Bias due to early skywave,skywave/groundwave 0dB, )
Skywave delay
usec
Tracking point 50 usecTracking point 55 usecTracking point 60 usec
8th order Butterworth, 28 kHz -3 dB bandwidth, group delay = 30 usec (Vertical axis ECD bias in usec, x axis skywave delay in usec)
20 30 40 50-2
0
2
4Sep = 10, Start = 37.5
20 30 40 50-5
0
5Sep = 10, Start = 42.5
20 30 40 50-5
0
5
10Sep = 10, Start = 47.5
20 30 40 50-5
0
5
10
15Sep = 10, Start = 52.5
20 30 40 50-2
0
2
4Sep = 15, Start = 37.5
20 30 40 50-5
0
5
10Sep = 15, Start = 42.5
20 30 40 50-5
0
5
10Sep = 15, Start = 47.5
20 30 40 50-10
0
10
20Sep = 15, Start = 52.5
20 30 40 50-5
0
5Sep = 20, Start = 37.5
20 30 40 50-5
0
5
10Sep = 20, Start = 42.5
20 30 40 50-5
0
5
10
15Sep = 20, Start = 47.5
20 30 40 50-10
0
10
20Sep = 20, Start = 52.5
60 80 100 120 140-30
-25
-20
-15
-10
-5
0Frequency Response
kHz
Gai
n -d
B
AnalogOverall
60 80 100 120 1401
2
3
4
5
6
kHz
Gro
up d
elay
- u
sec
Group delay of analog: 64 kHz -3dB bandwidth
0 10 20 30 40 50 60 70 80-1
-0.5
0
0.5
1
Am
plitu
de
Time - usec
Original pulseFiltered pulseEnvelope delayed 5 usec
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter
20 25 30 35 40 45 50-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06TOA Bias due to early skywave,skywave/groundwave 0dB, )
Skywave delay
usec
Tracking point 25 usecTracking point 30 usecTracking point 35 usec
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter ECD Bias
20 40 600
0.5
1
1.5Sep = 10, Start = 12.5
20 40 60-2
0
2
4
6Sep = 10, Start = 17.5
20 40 60-5
0
5
10Sep = 10, Start = 22.5
20 30 40 50-10
0
10
20Sep = 10, Start = 27.5
20 40 60-1
0
1
2
3Sep = 15, Start = 12.5
20 40 60-5
0
5
10Sep = 15, Start = 17.5
20 40 60-5
0
5
10
15Sep = 15, Start = 22.5
20 30 40 50-10
0
10
20
30Sep = 15, Start = 27.5
20 40 60-2
0
2
4Sep = 20, Start = 12.5
20 40 60-5
0
5
10Sep = 20, Start = 17.5
20 40 60-10
0
10
20Sep = 20, Start = 22.5
20 30 40 50-10
0
10
20
30Sep = 20, Start = 27.5
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter, 50 usec rise time
70 80 90 100 110 120 130 140-20
-15
-10
-5
0Frequency Response
kHz
Gai
n -d
B AnalogOverall
70 80 90 100 110 120 130 1401
2
3
4
5
6
kHz
Gro
up d
elay
- u
sec
Group delay of analog: 64 kHz -3dB bandwidth
0 10 20 30 40 50 60 70 80-1
-0.5
0
0.5
1
Am
plitu
de
Time - usec
Original pulseFiltered pulseEnvelope delayed 5 usec
Cascade of 2nd order analog filter, 64 kHz BW w/non-causal digital filter, 50 usec rise time
20 25 30 35-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06TOA Bias due to early skywave,skywave/groundwave 0dB, )
Skywave delay
usec
Tracking point 25 usecTracking point 30 usecTracking point 35 usec
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)
20 25 30 35-2
0
2
4Sep = 10, Start = 12.5
20 25 30 35-2
0
2
4Sep = 10, Start = 17.5
20 25 30 35-5
0
5
10Sep = 10, Start = 22.5
20 25 30 35-2
0
2
4Sep = 15, Start = 12.5
20 25 30 35-2
0
2
4
6Sep = 15, Start = 17.5
20 25 30 35-5
0
5
10
15Sep = 15, Start = 22.5
20 25 30 35-2
0
2
4Sep = 20, Start = 12.5
20 25 30 35-5
0
5
10Sep = 20, Start = 17.5
20 25 30 35-10
0
10
20Sep = 20, Start = 22.5
0 100 200 300 400 500 600 700 800 900 10000
0.2
0.4
0.6
0.8
1
1.2
Distance km
dBDifference in field strength
90 kHz - 100 kHz100 kHz - 110 kHz
Field strength predictions of BALOR code for Seneca-Little Rock path
Undoes the effect of differentiation
0 5 10 15 20 25 30 35 40 45 50-600
-400
-200
0
200
400
600Range = 6 km, Delta ECD = -0.01 usec
usec
OriginalPropagatedEnv shftd dECD
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-50
0
50
usec
Zero Crossings re 10 usec multiples (-2.5 usec)
102030405060
In near far field, zeros crossings are < 5 usec apart
After propagation has cancelled frequency response of differentiation
0 5 10 15 20 25 30 35 40 45 50-600
-400
-200
0
200
400
600Range = 758 km, Delta ECD = -0.85 usec
usec
OriginalPropagatedEnv shftd dECD
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-50
0
50
usec
Zero Crossings re 10 usec multiples (-2.5 usec)
102030405060
0 5 10 15 20 25 30 35 40 45 50-600
-400
-200
0
200
400
600Range = 1011 km, Delta ECD = -1.4 usec
usec
OriginalPropagatedEnv shftd dECD
-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5-50
0
50
usec
Zero Crossings re 10 usec multiples (-2.5 usec)
102030405060
At long differences, zeros crossings are > 5 usec apart
•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
0 20 40 60 80 100 120-1
-0.5
0
0.5
1
usec
80 85 90 95 100 105 110 115 120-50
-40
-30
-20
-10
0
kHz
dB
96 kHz100 kHz104 kHzSum
Rise Time = 72 usec
0 2 4 6 8 10 12 14 16 1898.4
98.6
98.8
99
99.2
99.4
99.6
99.8
100
Number of 100 kHz pulses in PCI of 18 total
Per
cent
of
pow
er 9
0-11
0 kH
zRise time = 72 usec
0 2 4 6 8 10 12 14 16 1897.8
98
98.2
98.4
98.6
98.8
99
99.2
99.4
Number of 100 kHz pulses in PCI of 18 totalP
erce
nt o
f po
wer
90-
110
kHz
Rise time = 65 usec
Lengthening pulse permits balanced distribution among frequencies & still meeting spectrum requirement
2 ea.@ 96 &
104 kHz
5 ea.@ 96 &
104 kHz
0 5 10 15 20 25 30
0
0.1
0.2
0.3
0.4
0.5
0.6
Rise time = 72 usec
96 kHz100 kHz104 kHzSumEnvelope w/65 usec rise time shifted 1.2 usec
Lengthening pulse makes leading edge of average pulse the same to legacy receivers
20 25 30 35-5
-4
-3
-2
-1
0
1
2ECD Bias due to early skywave, (skywave/groundwave 0dB)
Skywave delay
usec
96 kHz100 kHz104 kHz
20 25 30 35-0.1
-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
0.06TOA Bias due to early skywave, (skywave/groundwave 0dB)
usec
96 kHz100 kHz104 kHz
= approx. 10% of bias
-0.1 -0.05 0 0.05 0.1-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
20
21
22
23
24
25
2627
28
29
3031 32333435
104 kHz - 96 kHz TOA Difference in usec
100
kHz
TO
A B
ias
in u
sec
-2 -1 0 1 2 3-0.08
-0.06
-0.04
-0.02
0
0.02
0.04
20
21
22
23
24
25
262728
29
303132333435
104 kHz - 96 kHz ECD Difference in usec
100
kHz
TO
A B
ias
in u
sec
-0.1 -0.05 0 0.05 0.1-5
-4
-3
-2
-1
0
1
2
20
21
22
23
242526
2728293031 32333435
104 kHz - 96 kHz TOA Difference in usec
100
kHz
EC
D B
ias
in u
sec
-2 -1 0 1 2 3-5
-4
-3
-2
-1
0
1
2
20
21
22
23
242526
2728 29 30 3132333435
104 kHz - 96 kHz ECD Difference in usec
100
kHz
EC
D B
ias
in u
sec
Horizontal: Candidate detection statistic, Vertical: Bias to detect
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 = 1.126 usec /(N x SNR) = 0.159 usec for average of 500 @ -10dB SNR = 0.225 usec for difference between 2 frequencies = 0.089 usec for average of 1600 @ -10dB SNR
• ECD = 30.4 usec/(N x SNR) = 4.29 usec for average of 500 @ -10dB SNR = 6.07 usec for difference between 2 frequencies= 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 = 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 = 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 = 3.3 x 0.225 usec = 0.74 usec, corresponding ECD bias is 26 usec
79 Paths of < 900 nm using only LORSTA’sWith addition of Dunbar Forest, dense enough to see PCD.
-130 -120 -110 -100 -90 -80 -70 -60 -50
25
30
35
40
45
50
55
+ Caribou
+ Nantucket
+ Cape Race
+ Fox Harbor+ Williams L
+ Shoal Cove
+ George
+ Port Hardy
+ Malone + Grangevlle
+ Raymondvll + Jupiter
+ Carolina B
+ Havre + Baudette
+ Boise City
+ Gillette
+ Dana + Fallon + Middletown
+ Searchlght
+ Las Cruces
+ Seneca
+ Comfort CvDunbar Forest??
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:
-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.