Transcript SV Selection and High-Risk SV Avoidance

Using Outage History to Exclude
High-Risk Satellites from GBAS
Corrections
Sam Pullen and Per Enge
Stanford University
[email protected]
ION GNSS 2011
Portland, Oregon
Session C5-8
23 September 2011
A Look Backward…
• Satellite selection in GPS user receivers first became
an issue in the early 1990’s
– GPS satellite constellation quickly expanded to the 24satellite standard.
– 4- and 6-channel receivers could not track all satellites in
view and needed to select the optimal subset for positioning.
• Use of almanac and minimum(PDOP) algorithms
became standard techniques.
– Research on computationally-efficient methods continues.
• Availability of 12-channel (or more) receivers has
made this “problem” a memory.
23 September 2011
Excluding High-Risk Satellites from GBAS
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A Look Forward…
• The combined use of multiple GNSS constellations
(GPS, GLONASS, Galileo, Compass, and augmentations) for positioning provides many more satellites.
– Are all of these satellites useful for positioning?
– Beyond a certain point, don’t additional satellites add more
integrity risk than they are worth?
• Channel-limited scenario: as in the past, hardware
limitations prevent all satellites from being used.
– Even though modern receivers can track plenty of satellites,
GBAS VDB can only broadcast so many SV corrections.
• Channel-unlimited scenario:
– No receiver or correction limits, but performance
improvement is desired (or needed)
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Excluding High-Risk Satellites from GBAS
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GPS Satellite Fault Probabilities
• From GPS SPS Signal Standard (4th Ed, Sept. 2008):
– No more than three (3) GPS service failures per year (across
GPS constellation) for a max. constellation of 32 satellites.
– Service failure: SV failure leading to SPS user range error >
4.42 URA without timely OCS warning or alert
• Assuming 3 failures/year over 32-SV constellation:
3 events year
1
8766 hours year 32 satellites
 1.07  105 events SV hour
events SV 150 sec
-4 approach
7
class.
1.–
07 GBAS
 105 assumes 10 events/SV/hour
 4.46per
 10fault
events
SV approach
hour
3600 sec hour
• Is it sufficient to treat all satellites as having the same
failure probability?
• “GPS Satellites are operated to failure.”
– Col. Gaylord Green, USAF (Ret.), former GPS JPO director
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Excluding High-Risk Satellites from GBAS
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Today’s GPS Satellite Constellation
(as of 20 September 2011)
SVN
order of
23
launch 32
24
24
23 September 2011
SV age (years)
SVN
Sorted inPRN
23
2422
26
3920
35
3418
36
33
16
40
43
3814
46
5112
44
4110
54
56
8
45
47
59 6
60
61 4
53
52 2
58
55
0
57
0
48
50
62
63
PRN
Block
32
24
26
09
30
04
06
03
10
13
08
11
20
28
14
18
16
21
22
19
23
02
17
31
12
15
29
07
05
25
01
IIA
IIA
IIA
IIA
IIA
IIA
IIA
IIA
IIA
IIR
IIA
IIR
IIR
IIR
IIR
IIR
IIR
IIR
IIR
IIR
IIR
IIR
IIR-M
IIR-M
IIR-M
IIR-M
5IIR-M
IIR-M
IIR-M
SV
IIF
IIF
Block
IIA
IIA
Launch Launch J- Orbit Slot (as
Age as of
Year
Day
of 9/20/11) 9/20/11 (years)
1990
330
1991
185
1992
188
1993
177
1993
242
1993
299
1994
069
1996
088
1996
198
fit
1997Linear204
1997
310
1999
280
2000
131
2000
197
2000
314
2001
030
2003
029
2003
090
2003
355
2004
080
2004
175
2004
311
2005
269
2006
268
2006
321
2007
290
2007
354
10
15
2008
075
2009
076
index
(rank from
2010
148
2011
197
E5
D5
F5
A1
B5
D4
C6
C2
E6
F3
A3
D2
E1
B3
F1
E4
SV ages
B1-A
D3
E2
C3
F4
D1
C4
A2
B4
F2-A
20C1
25
A6
newestE3to oldest)
B2
D2-A
20.81
20.21
19.20
18.23
18.05
17.90
17.53
15.48
15.18
14.16
13.87
11.95
11.36
11.18
10.86
10.64
8.64
8.47
7.75
7.50
7.24
6.87
5.98
4.98
4.84
3.92
3.75
30
3.51
2.51
1.31
0.18
Launch Launch J- Orbit Slot (as
Year
Day
of 9/20/11)
1990
1991
330
185
Excluding High-Risk Satellites from GBAS
E5
D5
“Reborn” satellites –
decomm., recomm.,
and still healthy
Fairly
Age
as of
uniform
age
distribution
9/20/11
(years)
at present,
20.81
but not
always
20.21 true
in past
Sources: GPS
NANU’s and
Status Messages.
5
Unscheduled GPS Satellite Outages
Since 1999
A textbook example of a “bathtub” failure curve!
20
18
25
Increasing trend
due to increasing
age of older SV’s
16
“What happened to
Social Security?”
178 total
unscheduled
outages
20
22 22
20
53 above
“expected life”
No. of Occurrences
SV Age (years)
14
12
10
8
6
15
15
14 14
10
8
OCS
software
switchover
2
0
2000
2002
2004
2006
2008
2010
2012
5
“Prime of
life”
4 4
7
“Old Age
Champs”
10
9
6
4
“50 is not
the new
30”
“Infant
mortality”
7
4
3
2
3
2
2
18
20
0
0
2
4
6
8
10
12
14
16
Outage Date
Range of expected
SV lifetimes
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Excluding High-Risk Satellites from GBAS
SV Age (years)
6
Simplified Histogram of
Unscheduled Outages
60
Beyond expected lifetime,
outage rate is further
elevated.
• 178 outages over 156 mo, or 113,958 hrs
No. of Occurrences
50
40
30
• Total outage probability  1.16 × 10-3 per
hour
Outages of SV
above expected
lifetime grouped
together
• Dividing over 24 satellites gives  6.51 ×
10-5/SV/hr (below GBAS integrity fault
allocation per cause)
• Fraction of these that represent “service
failures” or potential GBAS threats is
From 10 to 13
unknown but likely small
years, outage rate
is significantly
elevated.
20
Within the first 10 years of life,
outage rates are low enough  no
need to differentiate.
10
0
1
2
3
4
5
6
7
8
9
10
11
12
13 14+
SV Age (years)
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Number of Outages by Individual SV
14
12
Number of Outages
10
8
6
4
2
0
15
20
25
30
35
40
45
50
55
60
SVN Index
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Individual Satellite Outages:
Age and Duration
20
18
SV Age and Duration (years)
16
SV age at end
of outage
14
Outage duration
12
SV age at start
of outage
10
8
6
4
Record
begins in
1999
2
0
15
20
25
30
35
40
45
50
55
60
SVN Number
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Zoom in on History of SVN 25
18
12 unsched.
outages from
6/11/99 to 12/17/09
( 9.22 × 104 hrs) 16
10/10/09 (~ 250 hrs)
6/26/09 (~ 130 hrs)
8/26/08 (~ 482 hrs)
4/22/07 (~ 85 hrs)
SV Age (years)
14
5/18/06 (~ 1000 hrs)
12/23/05 (~ 8.5 hrs); then
12/25/05 (~ 721 hrs)
3/22/06 (~ 144 hrs)
2/24/05 (~ 1 hr)
8/10/04 (< 1 hr)
Beyond
expected
lifetime
12
5 outages within
6 months!
10
10/02/00 (~ 35 hrs)
8
6/11/99 (< 1 hr)
6
25
SVN Number
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Example SV-Exclusion Heuristics
• Examine GPS VDOP as multiple SVs are removed
based on example heuristics (exclusion rules)
• Assume heuristics are “hard” rules
– “Channel-unlimited” scenario: SVs that fail tests are not
used even if spare channels remain
• Option 1: remove all satellites > 13 years old
– Currently, don’t use SVNs 26 – 40 & 43 (PRNs 3, 4, 6, 8, 9, 10,
13, 26, 30)
– 5 of these 9 SVs are in primary orbit slots
• Option 2: remove all satellites > 10 years old & with
one or more unscheduled outages in past 2 years
– Currently, don’t use SVNs 26, 35, 38, 40, 51 (PRNs 8, 10, 20,
26, 30)  2 of these 5 SVs in primary orbit slots
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Satellite Geometry Sensitivity (1):
Number of Usable Satellites in View
Palo Alto, CA (37.4o N latitude)
5o mask angle
Number of Usable Satellites in View
14
Use All SVs
Fairbanks, AK (64.8o N latitude)
5o mask angle
14
12
12
10
10
8
8
6
6
4
4
2
0
Elim. SVs >
13 yrs old
Elim. SVs > 10
yo w/outages
0
5
10
15
20
Time from midnight local time (hours)
on 9/20/11
23 September 2011
2
0
0
5
10
15
20
Time from midnight local time (hours)
on 9/20/11
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Satellite Geometry Sensitivity (2):
Vertical DOP (VDOP)
Palo Alto, CA (37.4o N latitude)
5o mask angle
10
10
Peak  45
9
Peak  37
Peak  16
9
8
8
Elim. SVs > 10
yo w/outages
7
VDOP
Fairbanks, AK (64.8o N latitude)
5o mask angle
Elim. SVs >
13 yrs old
6
7
6
5
5
4
4
3
3
2
2
1
1
Use All SVs
0
0
5
10
15
20
Time from midnight local time (hrs)
on 9/20/11
23 September 2011
0
0
5
10
15
20
Time from midnight local time (hours)
on 9/20/11
Excluding High-Risk Satellites from GBAS
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Heuristic Implementation in GBAS
• Proposed SV exclusion rules are simple but require
knowledge of SV age and outage history.
• This information can be obtained from GPS NANUs
but is not included in the broadcast satellite signals.
• For GBAS ground stations that can only observe
broadcast signals, less-informative means to update
age and outage information would have to be used.
– Can update satellite ages, PRN assignments, and outage
statistics when new or re-commissioned satellites are
manually added to the list of “usable” SVs.
– Otherwise, sub-optimal tracking of outages observable to
each ground station would be needed.
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Satellite-Geometry-based Heuristics
• In principle, a real-time model that trades risk of
using SV against geometry benefit gained from that
SV would be superior to fixed (use/don’t use) rules.
• The multiple-hypothesis (MH) protection level
approach used by ARAIM makes this possible.
– Resulting protection levels incorporate both positioning
geometry and risk from each hypothesized fault
– However, like existing protection levels, MH is only as good
as the failure assumptions and probabilities that go into it.
• Fixed heuristics are less sensitive to modeling errors
but are likely to sacrifice performance to gain
conservatism.
• Experimentation with MH approach to follow…
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Summary
• GPS outage data since 1999 shows that satellite
failure probabilities are not evenly distributed.
– Older satellites fail much more often than younger ones.
– Satellites that begin experiencing outages are much more
likely to continue having outages.
• At present, the satellite integrity failure rate assumed
by GBAS appears to cover all GPS satellites.
• To provide more margin, or if circumstances change,
heuristics derived from observed outage rates can be
used to remove satellites with excessive risk.
• Future GBAS using multiple constellations should
allow marginal satellites to be excluded with
negligible performance penalty.
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Backup Slides
• Backup slides follow…
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A Preliminary Analysis Approach
• “Back to the Future…”
– Model GPS-only scenario with today’s constellation of
several satellite “blocks” (IIA, IIR, IIR-M, IIF).
– Consider the “channel-limited” case with 6- and 8-channel
receivers (as in mid-1990’s).
• Examine satellite failure probabilities and effects as a
function of satellite age and recent failure history.
• Explore simplified heuristics for inclusion or
exclusion of individual satellites.
• Examine potential system impacts.
– Integrity could be threatened by including satellites with
unacceptably high prior failure probabilities.
– Availability and continuity potentially affected by exclusion of
too many “marginal” satellites.
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Distribution of Current SV Ages
22
20
18
16
SV age (years)
Linear fit
14
12
10
SV ages
8
6
4
2
0
0
5
10
15
20
25
30
SV index (rank from newest to oldest)
23 September 2011
Satellite Selection for Modernized GNSS/GBAS
19
Failure Probability Estimates from
Outage Data
• 178 unscheduled outages recorded from Jan. 1999 –
Aug. 2011 (156 months, or 113,958 hours)
– Relatively few of these represent “service failures” or
potential GBAS threats, but actual fraction is unknown.
– Total outage probability  1.16 × 10-3 per hour
– Dividing over 24 satellites gives  6.51 × 10-5 per SV per hour
(GBAS integrity failure assumption per cause exceeds this).
• Satellites > 10 years old cause 117 of these outages
(~ 66% of total) and are greatly over-represented.
– SVs between 10 + d and 13 years old cause 46 outages (~
36% of total).
 If SV ages were evenly distributed between 0 and 20 years, this
would imply 2.36 × average failure rate.
– 53 outages (~ 30% of total) from SVs beyond 13 years old
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Significance of SV Outage History
• Unscheduled satellite outages are rare, but they are
not “memoryless” (assumed by Poisson approx.)
– SVs with recent unscheduled outages are more likely to have
future unscheduled outages.
• This factor is correlated with SV age but has
independent value in estimating future SV failure risk
and should also be used in SV selection heuristics.
– “Trackable” by reviewing NANUs, but NANUs represent
external information not broadcast by satellites.
– For GBAS, easier to track observed SV outages (“unhealthy”
flags), but not all can be observed from one location, and
observed flags would include scheduled outages.
– Manual updates needed (e.g., when new SVs are approved) if
NANUs are not used.
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