GPS “Big Five” contribution to Users Needs AN UPDATE

October 08Interim Report Big 5 and MOEs

1

GPS “Big Five” contribution to Users Needs AN UPDATE

Prof. Brad Parkinson

Draft Developed for IRT – August 2008Thanks to Col. Dave Madden and Aerospace for help, Particularly Tom Powell

and Paul Massatt

Also FAA with Sam Pullen and Todd Walter

Showing Dependence of User Measures of Effectiveness (MOE) on GPS System Design & Design Decisions


2

The IRT “Big 5” – Essential GPS PNt Characteristics

A Bridge between User’s MOE and GPS System Design

1. Assured (Geometric) Availability of GPS signals

2. Resistance to (Deliberate or Unintentional) Interference

3. Accuracy of User’s GPS Position (After satisfying #1 and #2)

4. Bounded inaccuracy –Limiting potential for very large

errors (Fratricide or Collateral Damage)

5. Integrity – Identifying and eliminating the non-normal

GPS system or local errors (e.g. extreme user multipath or runaway

clocks).


3

Performance EnvelopeConceptual Examples

Current GPS Capabilities

(30+ Sats)

Current GPS Specification

(e.g. 21+3 Sats)

Needs for SDB

(Target Designation in Visibility

Impaired Region)Cat III

Aircraft Landing

(Integrity – Time to Alarmor Availability)

Potential GPS Enhancements

Potential GPSAugmentationsThe

“Envelope”

“Envelope”

Missions

FAA ATCModernization

ADS-B


4

Envelope Examples of Uses(Summarize A, B, and D)

Military Uses M1. Use of Small Diameter Bomb

in region where ground target locator has impaired visibility (e.g. mountainous terrain or urban street) (In Mission A)

M2. Delivering weapons close to friendly troops, or close to sensitive “don’t hit” locations (In Mission A)

M3. Operating with impunity in the vicinity of high-power (or multiple, distributed) Enemy Jammers (In Mission A)

M4. Operating in mined land or restrictive sea areas

Civilian Uses C1. Precision Aircraft Approach and

Landing (Up to Cat III) demanding 10-9 integrity (Mission B – includes a military mission)

C2. First Responder PNT in Urban Area (Mission C)

C3. Precision Survey using GPS carrier Phase

C4. Use of GPS ADS-B mandated for future ATC System – improving separation distances (Mission D)

C5. Resistance to inadvertent GPS interference or deliberate sabotage (see military #3)

C6. Obscuration in Open Pit Mining


5

Mission Trade AnalysisMission A. Air Dropped Bomb against Ground located

target

Want to show effect of GPS Decision Maker’s Trades

onMeasures of Effectiveness

Note: this is illustrative of the technique and approach

It does not incorporate actual weapons system’s data

Sensitive results are presented in Relative Terms

UPDATE


6

Afghanistan in this Analysis

• Observer is assumed to be part way up Mountain (Red Dot)

• Slope assumed at 45 to 60 degrees (could be steeper)

• Target Building is on other side of Valley


7

Constraints and AssumptionsWithin current Availability In Red, the next step

possibilities – also analyzed • Terrain – Valley in Afghanistan

mountains, – Observer on side of 45 (or 60)

degree slope Obscuration ~40%

• Observer Laser Sight: – Gyrocompass North- – Azimuth - 3 mils, – Elevation 3 Mils– Range 3 Meters

• Observer GPS – 2.6 meter multipath-limited

receiver (1 meter multipath narrow tracking correlator)

– 0.75 meter receiver noise• Target

– 1 km away

• GPS Constellation– 18, 21, 24, 27, 30, 33, 36

considered with 1,2, or 3 satellites randomly out

– URE: Block II 0..57m, Block III 0.25m

• Bomb/Weapon– Same Constellations

considered– 3.5m Guidance error

Guidance Error 1.0m– GPS 0.8m noise, negl.

multipath URE as above– Vertical at impact

• Jamming interference– Assume a hostile 10W noise

Jammer


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Buildings on a Mountain RoadTarget is Largest Building

Numbers in Boxes are the number of Hits

Road


9

Observer on Slope of 45 Degrees


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99.9% Circle -Only 1 in 1000

exceeds

50% Circle Half in, Half out.

Usually called CEP – a poor measure of

effectiveness

95 % Circle Should approximate Target

size, (for first round effectiveness)

Sometimes called “2d”bldgldg


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12


13


14

Observer on Slope of 60 Degrees


15


16


17


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Selected Civil “Envelope” Missions

• Precision Approach and Landing (Mission “B”)– Representative US Airports

– Desire Availability of >99.5% (99.9% ?)

• Advanced Air Traffic Control System (Mission “D”)– GPS Based

– Uses Automatic Dependent Surveillance Beacon (ADSB)

– Integrity Guaranteed - Issue is Geographic Coverage for 99.5% availability


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Constraints and Assumptions for Mission B – CAT III Precision Landing

• Terrain – Civil Airports and Military Airfields

• Aircraft guided down to 200’ HAT CAT I Decision Height solely by GPS Local Area Augmentation System (LAAS) fielded at airport/airfield where landing takes place

– Vertical guidance is limiting factor

• From 200’ to 100’ HAT, aircraft guided by LAAS with airborne inertial system as backup

• Below 100’ HAT (above runway threshold), aircraft primarily guided by radar altimeter

• GPS Constellation– 21, 24, 27, 30, 33, 36

considered with 1,2, or 3 satellites randomly out (cycle through all outage permutations)

– URE: dictated by LAAS ground and airborne error models

• RF interference– When present, assume

unintentional ground-based RF interference sufficient to make satellites below 10, 15 deg. elevation (TBC) unusable


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Four Measures of Effectiveness (MOEs) for Mission “B” – Cat III Landing

• MOE 1: Long-term probability that CAT III operation is available (without RF

interference)

Trade I – No. of GPS Satellites in Constellation

• MOE 2: Longest interval that CAT III operation is unavailable (without RF

interference)

Trade I – No. of GPS Satellites in Constellation

• MOE 3 : Loss-of-continuity probability when RF interference is suddenly

introduced

Trade II - Techniques to reduce RF interference vulnerability

• MOE 4: Availability probability when RF interference persists

Trades I and II


2124 23 22 21

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

Number of Healthy SV's

Ava

ilabi

lity

Results for 12 Airports

Max. Outage Duration (min)

27

67

142

284

Note Min. Avail. on

Plot

99.9 % Availability Threshold

Availability Results for IRT “Baseline” 24-SV Constellation – 1,2, or 3 GPS outages (Slide 1 of 2)


22MIA ATL MEM IAD JFK MS P ORD DFW S LC LAX S EA ANC

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

Airport Loca tion

Ava

ilab

ility

Availability Results for IRT “Baseline” 24-SV Constellation (Slide 2)

0

268

19

6

27

0 0

9

0 3 0 0

19

284

272

Max. Outage Duration

(min)

284

244276 272

228

236

264

248

164

110

116

102

106

8294 98

96

142

88

86

80

49

43

35

65

6745

51 46

50

51

33

43

3 SV Out (4-min updates)



0 SV Out (15-sec

updates)


2330 29 28 27

0.98

0.982

0.984

0.986

0.988

0.99

0.992

0.994

0.996

0.998

1

Numbe r of He a lthy SV's

Ava

ilab

ility

Availability Results for IRT 30-SV Constellation

Max. Outage Duration (min)

0 26

56

136

Note Min. Avail. on

Plot


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Comparison of CAT III Availability for All Six IRT Constellations (21 – 36 SV’s)

0 SVs Out 1 SV Out 2 SVs Out 3 SVs Out10

-6

10-5

10-4

10-3

10-2

10-1

100

Number of SV’s Unhealthy

Un

-ava

ilab

ilit

y

IRT 21-SV

IRT 24-SV

IRT 27-SV

Desired Availability

99.9%

IRT 33-SV

IRT 36-SV

IRT 30-SV


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• To compare to IRT constellations, a recent GPS

constellation almanac (Week 465, 25 July 2008) was downloaded and simulated.

• Results for two cases shown on the following slide:

– Optimistic – use all 31 satellites listed in almanac (24 “primary” 7 “spare” orbit slots)

– Realistic: remove 5 satellites in “spare” orbit slots that are older than 15 years of age

» Retain use of 2 satellites in “primary” orbit slots that exceed 15 years of age

» 26 satellites are used (24 “primary” 2 “spare” orbit slots)

Simulations with Current GPS Constellation


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Comparison of CAT III Availability for IRT and Current Constellations

0 SVs Out 1 SV Out 2 SVs Out 3 SVs Out10

-6

10-5

10-4

10-3

10-2

10-1

100

Number of SV's Unhealthy

Un-

avai

labi

lity

IRT 36-SV

IRT 30-SV

IRT 21-SV

IRT 24-SV

IRT 27-SV

Current/Optimistic (31-SV)

Desired Availability

99.9%IRT 33-SV

Current/Realistic (26-SV)

Current/Optimistic (31-SV)


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• More availability results to follow…

• Results now available for all SV constellations for no-RFI case

• Now experimenting with best ways to plot these results

Status of CAT III Analysis


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Mission D – GPS-Based ADS-B Support of Air Traffic Control

Many aircraft in flight• Each equipped with

GPS/SPS and/or WAAS• Each equipped with ADS-B

transponder to share GPS-based “PVT” information

Airport C

ATC Tower

ATC Tower

Airport B

FAA ARTCC

Airport A

ATC Tower

ADS-B PVT

ADS-B PVT

ADS-B PVT

ADS-B PVT


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Perfect Constellation: Comparison of GIC (WAAS) and RAIM Integrity Techniques

(Table with Numerical Values)

Satellite Constellation

Architecture 24 27 30

WAAS Integrity 100% 100% 100%

RRAIM (300-sec coasting)

76.1% 99.6% 100%

ARAIM 44.7% 94.1% 100%

Fraction of Airspace (inside ± 70 deg. Latitude) with ≥99.5% availability of support

for Precision Approach to 200’ Height Above Terrain (Like CAT I)


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Realistic Constellation Comparison of GIC (WAAS) and Self-Integrity (RAIM) Techniques

(Table with Numerical Values)

Satellite Constellation

Architecture 24 minus significant

SV

27 minus significant

SV

30 minus significant

SV

WAAS Integrity 86.6% 97.8% 100%RRAIM (300-sec

coasting) 28.0% 52.3% 93.9%

Absolute RAIM 7.8% 30.6% 90.5%

Fraction of Airspace (inside ± 70 deg. Latitude) with ≥99.5% availability of support

for Precision Approach to 200’ Height Above Terrain (Like CAT I)


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Summary and Path Forward

• Evaluation of civil missions/uses B and D (CAT III precision landing and ADS-B support of ATC) will be conducted using common simulation approach

– CAT III application is more clear-cut (based on use of already-defined single-frequency LAAS)

– ADS-B application has more options and trades

• The simulation needed to evaluate Mission B has been built and run for IRT constellations and for two variations of recent GPS Week 465 broadcast almanac


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Decision # I.The Number of GPS Satellites

• Current “Requirement” – 24 (21 plus three active spares)

• On orbit are 31but not optimal– Much improved geometric availability - Users now expect this

performance– Paired Orbits – not optimal for 30 (ready for Failure)

• Many studies have suggested the “knee in the curve” for user availability is 30 to 36– Critical users – those with impaired sky visibility or extreme integrity req.

• A key to increasing commitment to 30 + X is on-orbit cost of Satellites– Major driver Additional Payloads (reduce size, weight, power and complexity)

– Cost savings opportunity - dual launch

• Decision: A National commitment to increased number of SVs– Civil users could have significantly improved availability

– Military Users more effective in impaired situations


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Conclusions• The concept of “Envelope” missions places

focus on those missions that really drive GPS system design and illuminate trades for the decision makers

• We have shown a Process :– relates GPS System Design Trades to Measures of

Effectiveness (MOE)– Closely related to the “Big 5 GPS Characteristics” but

adds the advantage of quantification• MOEs are very mission specific

– relate to particular use and/or users• Additional “Envelope” missions are suggested as

worthy of further MOE analysis


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PL Fundamental Issues – Operations

• Most impaired users are in “harms way”– Placing PLs in the Afghan Mountains not plausible

• One PL usually only benefits a narrow geographic area

• Support for PL requires monitoring• GPS receivers must be specially configured to

handle PL signal– Near-Far problem

• Airborne PLs suffer degraded accuracy, and complex support architecture


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Comment on MOE 1:The Accuracy Payoff

• Reducing error by 3 improves PK by up to 9

• CNN wars dictate reduced collateral damage – the stray bomb is important

• Improve 1st round effectiveness = less US attrition.

• Sorties to destroy = ~ 1/ PR

Issue: Need both TLE and WLE accuracy

Documents

GPS “Big Five” contribution to Users Needs AN UPDATE