using outage history to exclude high-risk satellites from gbas corrections ion gnss 2011portland,...

Using Outage History to Exclude High-Risk Satellites from GBAS

Corrections

ION GNSS 2011 Portland, Oregon

Session C5-8 23 September 2011

Sam Pullen and Per Enge

Stanford University

[email protected]

23 September 2011 Excluding High-Risk Satellites from GBAS 2

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 24-satellite 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.

3

• The combined use of multiple GNSS constellations (GPS, GLONASS, Galileo, Compass, and augmenta-tions) 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)

A Look Forward…

23 September 2011 Excluding High-Risk Satellites from GBAS

• 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:

– GBAS assumes 10-4 events/SV/hour per fault class.

• 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

approachSVevents1046.4hoursec3600

approachsec150

hour

SVevents1007.1

hourSVevents1007.1satellites32

1

yearhours8766

yearevents3

75

5

4

GPS Satellite Fault Probabilities


Today’s GPS Satellite Constellation (as of 20 September 2011)

23 September 2011 5

SVN PRN BlockLaunch

YearLaunch J-

DayOrbit Slot (as

of 9/20/11)Age as of

9/20/11 (years)

23 32 IIA 1990 330 E5 20.8124 24 IIA 1991 185 D5 20.2126 26 IIA 1992 188 F5 19.2039 09 IIA 1993 177 A1 18.2335 30 IIA 1993 242 B5 18.0534 04 IIA 1993 299 D4 17.9036 06 IIA 1994 069 C6 17.5333 03 IIA 1996 088 C2 15.4840 10 IIA 1996 198 E6 15.1843 13 IIR 1997 204 F3 14.1638 08 IIA 1997 310 A3 13.8746 11 IIR 1999 280 D2 11.9551 20 IIR 2000 131 E1 11.3644 28 IIR 2000 197 B3 11.1841 14 IIR 2000 314 F1 10.8654 18 IIR 2001 030 E4 10.6456 16 IIR 2003 029 B1-A 8.6445 21 IIR 2003 090 D3 8.4747 22 IIR 2003 355 E2 7.7559 19 IIR 2004 080 C3 7.5060 23 IIR 2004 175 F4 7.2461 02 IIR 2004 311 D1 6.8753 17 IIR-M 2005 269 C4 5.9852 31 IIR-M 2006 268 A2 4.9858 12 IIR-M 2006 321 B4 4.8455 15 IIR-M 2007 290 F2-A 3.9257 29 IIR-M 2007 354 C1 3.7548 07 IIR-M 2008 075 A6 3.5150 05 IIR-M 2009 076 E3 2.5162 25 IIF 2010 148 B2 1.3163 01 IIF 2011 197 D2-A 0.18

Fairly uniform age distribution at present,

but not always true

in past

Sorted in order of launch

“Reborn” satellites – decomm., recomm.,

and still healthy

Sources: GPS NANU’s and

Status Messages.

Excluding High-Risk Satellites from GBAS

SVN PRN BlockLaunch

YearLaunch J-

DayOrbit Slot (as

of 9/20/11)Age as of

9/20/11 (years)

23 32 IIA 1990 330 E5 20.8124 24 IIA 1991 185 D5 20.21

0

2

4

6

8

10

12

14

16

18

20

22

0 5 10 15 20 25 30

SV index (rank from newest to oldest)

SV

age

(ye

ars

)

SV ages

Linear fit

Unscheduled GPS Satellite Outages Since 1999

23 September 2011 6

0 2 4 6 8 10 12 14 16 18 200

5

10

15

20

25

SV Age (years)

No

. o

f O

ccu

rre

nce

s

2000 2002 2004 2006 2008 2010 20120

2

4

6

8

10

12

14

16

18

20

Outage Date

SV

Ag

e (

yea

rs)

Increasing trend due to increasing age of older SV’s

A textbook example of a “bathtub” failure curve!

“Infant mortality”

“Prime of life”

“50 is not the new

30”

“What happened to Social Security?”

“Old Age Champs”

Range of expected SV lifetimes

8

6

4 4

2

4

7 7

910

20

22 22

14 1415

3 322

53 above “expected life”

178 total unscheduled

outages


OCS software

switchover

7

Simplified Histogram of Unscheduled Outages

23 September 2011

1 2 3 4 5 6 7 8 9 10 11 12 13 14+0

10

20

30

40

50

60

SV Age (years)

No.

of

Occ

urre

nces

Outages of SV above expected lifetime grouped

together

Within the first 10 years of life, outage rates are low enough no

need to differentiate.

From 10 to 13 years, outage rate

is significantly elevated.

Beyond expected lifetime, outage rate is further

elevated.• 178 outages over 156 mo, or 113,958 hrs

• Total outage probability 1.16 × 10-3 per hour

• 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 unknown but likely small


8

Number of Outages by Individual SV

23 September 2011

15 20 25 30 35 40 45 50 55 60

SVN Index

Num

ber

of

Ou

tag

es

2

4

6

8

10

12

14

0


9

Individual Satellite Outages:Age and Duration

23 September 2011

15 20 25 30 35 40 45 50 55 60

2

0

4

6

8

10

12

14

16

18

20

SVN Number

SV

Age

and

Dur

atio

n (y

ears

)

SV age at start of outage

SV age at end of outage

Outage duration


Record begins in

1999

10

Zoom in on History of SVN 25

23 September 2011

256

8

10

12

14

16

18

SVN Number

SV

Age

(ye

ars

)12 unsched.

outages from 6/11/99 to 12/17/09 ( 9.22 × 104 hrs)

6/11/99 (< 1 hr)

10/02/00 (~ 35 hrs)

8/10/04 (< 1 hr)

2/24/05 (~ 1 hr)12/23/05 (~ 8.5 hrs); then

12/25/05 (~ 721 hrs)

3/22/06 (~ 144 hrs)Beyond

expected lifetime

5/18/06 (~ 1000 hrs)

4/22/07 (~ 85 hrs)

8/26/08 (~ 482 hrs)

10/10/09 (~ 250 hrs)6/26/09 (~ 130 hrs)

5 outages within 6 months!


23 September 2011 11

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 slotsExcluding High-Risk Satellites from GBAS


Satellite Geometry Sensitivity (1): Number of Usable Satellites in View


0

2

4

6

8

10

12

14

0 5 10 15 20

Time from midnight local time (hours)

Num

ber

of

Usa

ble

Sat

ellit

es in

Vie

w

0 5 10 15 200

2

4

6

8

10

12

14


Palo Alto, CA (37.4o N latitude)5o mask angle

Fairbanks, AK (64.8o N latitude)5o mask angle

on 9/20/11 on 9/20/11

Elim. SVs > 13 yrs old

Use All SVs

Elim. SVs > 10 yo w/outages


Satellite Geometry Sensitivity (2): Vertical DOP (VDOP)


Palo Alto, CA (37.4o N latitude)5o mask angle

Fairbanks, AK (64.8o N latitude)5o mask angle

on 9/20/11 on 9/20/11

0 5 10 15 200

1

2

3

4

5

6

7

8

9

10

Time from midnight local time (hrs)

VD

OP

Peak 45

0 5 10 15 200

1

2

3

4

5

6

7

8

9

10


Peak 37Peak 16

Elim. SVs > 10 yo w/outages

Use All SVs

Elim. SVs > 13 yrs old

• 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.

14

Heuristic Implementation in GBAS


• 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…

15

Satellite-Geometry-based Heuristics


• 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.

16

Summary


• Backup slides follow…

17

Backup Slides


18

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.


23 September 2011 Satellite Selection for Modernized GNSS/GBAS 19

0

2

4

6

8

10

12

14

16

18

20

22

0 5 10 15 20 25 30

SV index (rank from newest to oldest)

SV

age

(ye

ars

)

SV ages

Linear fit

Distribution of Current SV Ages

• 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 + 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

20

Failure Probability Estimates from Outage Data



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.


using outage history to exclude high-risk satellites from gbas corrections ion gnss 2011portland,...

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