GPS

1
GPS Big Five contribution to Users Needs
Showing Dependence of User Measures of
Effectiveness (MOE) on GPS System Design

• Prof. Brad ParkinsonDraft Developed for IRT -
  March 2008
• Thanks to Col. Dave Madden and Aerospace for
  help, Particularly Tom Powell and Paul Massatt

2
The IRT Big 5 - Essential GPS CharacteristicsA
Bridge between Users MOE and GPS System Design

1. Assured (Geometric) Availability of GPS signals
2. Resistance to (Deliberate or Unintentional)
   Interference
3. Accuracy of Users 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
Background

• IRTs Big 5 are critical GPS System
  Characteristics, but
• Relationship to users Measures of Effectiveness
  (MOE) not well understood and defined
• Impacts on GPS System Design not clear
• Any clarification builds on
• Explicit CONOPs (with MOE)
• Statement of boundary conditions and constraints
• Attempts to consider all possible missions not
  feasible nor desirable
• Instead focus on the challenging, important
  missions that push the envelope of GPS
  capabilities (the Envelope Missions)

Explicitly show decision makers the MOE
relationship to GPS Design trades
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Outline

• Define Envelope Uses (or missions)
• Show Civil/Military Examples - Relate to MOE
• Analyze one (Mission A Precision Weapon
  Delivery)
• Summarize the MOE trades for the Study of Mission
  A
• Identify three other Envelope Missions Studies
• Conclusions
• Recommendations

5
Envelope GPS Missions and Uses

• Many Missions/Uses have little impact on GPS
  system design (because capability included when
  other users satisfied)
• Operational Envelope Includes all senarios where
  GPS provides needed PT to the user.
• Envelope Missions Defined as those that have
  significant impact on the GPS system design
  Pushing the Envelope, Examples follow.

Focusing on EMs reveals the essential,
User-oriented trades between System Design and
the users Measures of Effectiveness
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Envelope Mission/Use Examples(Analyzed here will
be Mission A. Next steps B, C, and D.)

• Military
• Use of Small Diameter Bomb in region where
  ground target locator has impaired visibility
  (e.g. mountainous terrain or urban street) (In
  Mission A)
• Delivering weapons close to friendly troops, or
  close to sensitive dont hit locations (In
  Mission A)
• Operating with impunity in the vicinity of
  high-power (or multiple, distributed) Enemy
  Jammers (In Mission A)
• Operating in mined land or restrictive sea
  areas

• Civilian
• Precision Civil Aircraft Approach and Landing
  (Up to Cat III) demanding 10-9 integrity (mission
  B)
• First Responder PNT in Urban Area(Mission C)
• Precision Survey using GPS carrier Phase
• Use of GPS ADS-B mandated for future ATC System
  improving separation distances (Mission D)
• Resistance to inadvertent GPS interference or
  deliberate sabotage
• Obscuration in Open Pit Mining

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Performance EnvelopeConceptual Examples
Envelope Missions
Current GPS Capabilities(30 Sats)
Needs for SDB (Target Designation in
VisibilityImpaired Region)
Potential GPS Enhancements
Current GPS Specification(e.g. 213 Sats)
Cat III Aircraft Landing(Integrity Time to
Alarmor Availability)
Potential GPSAugmentations
A/J for the Ground User
Needs for Truck Navigation
The Envelope
Potential UE Enhancements
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Obscured Visibility Problem
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Relationships
1. Availability of Geometry
3. Positioning Accuracy
2. Resistance to Interference
4. Bounded Inaccuracy
5. Integrity
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Relationships
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Decision Makers Leverage on Future Capability
Pivotal GPS Design trades Pivotal GPS Design trades
Trade Why important to Users
T-I. Number of Satellites Constrains useable operating region, particularly for visibility impaired user. The Geometric Satellite Configuration is defined by of satellites (optimal constellation) and the local visibility constraints
T-II. Satellite Ranging Error Combined with the Geometric Satellite Configuration, determines the users Positioning Accuracy and bounds on inaccuracy. Principally a function of Satellite Block.
T-III. Jamming Resistance Allocations/ Pronav Insures ability to operate in jammed environment. Combines at least 4 system aspects Signal Structure/frequencies and user processing Degree of integration with Inertial (or zero velocity) User Antenna and local shielding against jamming Satellite Broadcast Power
T-IV. Integrity - Time to Alarm GPS System Problems Insures against wild position fixes, particularly important when Safety of Life is concerned (aircraft landing or possible fratricide situations for the warfighter).
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Decision Makers Trade I.How many GPS Satellites
are the Spec

• Current Requirement 24 satellites (21 plus
  three active spares)
• Currently on orbit are 31 or 32 - not in optimal
  orbits
• Much improved availability over requirement due
  to geometry
• Users now expect this performance
• Paired Orbits not optimal for 30 (Some used as
  standbys)
• DOD Not committed to this current level of
  service
• Many studies have suggested the knee in the
  curve for user availability is 30 to 36
• Critical users those with impaired sky
  visibility
• Potential 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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Decision Makers Trade II. User Range Error
Note the decision is the Satellite
GenerationDual Frequency does not include User
Multipath and receiver Noise
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Decision Makers Trade III. Procuring or
IncorporatingProven Jamming Resistance Methods

• Smorgasbord of techniques to increase resistance
  to interference
• The goal of a Hostile jammer is to deny use over
  an area
• The payoff of Resistance to that jamming (A/J) is
  the reduction in effective area

Techniques Available dB Improvement(?J/So) Reduction in Jammer effective area up to
Signal Structure/frequencies and user processing 3 to 7 80.0
Degree of integration with Inertial (or zero velocity) To 15 96.8
User Antenna and local shielding against jamming To 20 To 99.0
Satellite Broadcast Power(IIIA increase of 8 dB 6.3 times Block II) 8 TO 84.2
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Connecting EMs/MOEs and GPS Design Trades

• Select Four Envelope Uses or Missions and Show
  the GPS Trades
• Major National/Warfighter Importance
• Span critical Design Trades to Illustrate Process
• Most other Important Missions are subtended

Missions (or uses) Selected for Current or Future
Analysis Are Mission A. Ground designated
Bombing in region where ground target locator has
impaired visibility (e.g. mountainous terrain or
urban street), and friendly troops or dont hit
areas are nearby, and there is a threat of enemy
jamming Mission B. Precision Civil Aircraft
Approach and Landing (Up to Cat III) Mission C.
First Responder pinpointing urban
locations Mission D. Use of GPS based ADS-B
mandated for future ATC System improving
separation distances
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Mission Trade AnalysisMission A. Air Dropped
Bomb against Ground located target

• Want to show effect of GPS Decision Makers
  TradesonMeasures of Effectiveness

Note this is illustrative of the technique and
approach It does not incorporate actual weapons
systems data Sensitive results are presented in
Relative Terms
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A Representative User Need Statement for Mission
AGround designated Bombing in region where
ground target locator has impaired visibility
(e.g. mountainous terrain or urban street)Note
that this includes 4 significant Measures of
Effectiveness (MOE)

• Mission Destroy a ground-located target with an
  airdropped, GPS-guided bomb (MOE 1 of
  Sorties/weapons to deliver bomb within distance,
  R - 95)
• Where Mountainous Terrain Will include
  representative masking angles producing sky
  obscuration for ground observer (Constraint)
• When All-weather, 24/7 (Constraint)
• Enemy countermeasures Ground observer to operate
  within X km of 10 W GPS noise jammer (MOE 2 Min
  Operating Range to enemy Jammer)
• Collateral Damage Probability of hitting
  friendly troops Z meters away less than Y due
  to GPS (MOE 3 P (miss distance gt Z m)
• Integrity -GPS System Malfunction Notify User of
  GPS System/single satellite malfunctions (out of
  normal Specification) within S seconds (MOE 4
  is Time to Alarm TTA S seconds)

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Constraints and AssumptionsWithin current
Availability In Red, the next step possibilities
also analyzed

• Terrain Typical Mountain Valley,
• Observer on side of 45 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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Reminder The Weapon Delivery Double-Right
Triangle

• Total Error (TE) due to Three Components
• Weapon Location (WLE)
• Weapon Guidance (WGE)
• Target Location (TLE)
• For independent errors, square of result equals
  sum of squares.
• Can be visually depicted with two right triangles
• All three can be affected by GPS Accuracy

Target Location Error
TOTAL Error
Weapon Error
WGE
WLE
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Summary The Measures of Effectiveness (MOEs)
for Mission/Use A

• MOE 1 Number of Sorties/weapons to deliver bomb
  within R meters with 95 probability. Trades I
  and II No. of GPS Sats and Ranging Errors
• MOE 2 Closest Operating Range to enemy Jammer
  Trades III - Techniques to reduce jamming
  effectiveness
• MOE 3 P (miss distance gt Z m) Also Trades I
  and II No. of GPS Sats and Ranging Errors
• MOE 4 is Time to Alarm TTA -Seconds) This is
  currently about 45 minutes for DOD, the trades to
  improve this have been studied, but will not be
  discussed here

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Mountain Valley for Analyses

• Observer is assumed to be part way up Mountain
  (Red Dot)
• Slope assumed at 45 to 60 degrees (could be
  steeper)
• Target is on other side of Valley

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MOE 1 Assured Weapon Accuracy

• Use 95th percentile as more meaningful than CEP
  which is the 50th percentile (only half the time)
• Two Decision Trades
• Number of GPS Satellites
• Satellite Generation (Ranging Accuracy)
• Major Environmental Variable Slope of
  observers mountain side
• Nominally assumed to be 45o also looked at 60o

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Results 95th Percentile Radius Achievable with
N Sorties normalized to 30 SVs 1 random SV
inoperable in constellation Block II Ranging
ErrorMountainous Terrain Observer on 45o slope
Number Sorties
  1 2 3
18 67.57 33.78 16.89
21 3.38 1.69 1.18
24 2.03 0.97 0.66
27 1.53 0.70 0.55
30 1.00 0.64 0.51
33 0.88 0.60 0.49
36 0.84 0.57 0.47
24 Sats take twice the sorties for assured
delivery at Radius R compared to 30 Sats
Constellation Size
24 Sats take 50 more sorties for assured
delivery at 2/3rds R compared to 30 Sats
meters
24
Tradeoff I Number of Satellites (Block II
Ranging Accuracy)MOE of Sorties for 95
Accuracy AssuranceMountainous Terrain, 45o Slope
Number of Satellites
25
Tradeoff I and II Number of Satellites Block
IIIA Ranging AccuracyMOE of Sorties for 95
Accuracy Assurance Mountainous Terrain, 45o Slope
Number of Satellites
Normalized to 30 Block II Spacecraft
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Trade II. Comparing MOE 1 for Block II and Block
III with improved Target Locator
Number of Satellites
30 Satellites seems to be the natural knee
27
MOE 2 Closest Operating Range to enemy Jammer

• Baseline is a dual-frequency handheld with no
  extra enhancements to minimize interference
• Improvements generally cascade and are additive
  in their dB effects
• Suggested improvement trades are all well
  demonstrated and clearly feasible
• Implementing requires direction and budget
• Capabilities of Jammers is not 1/R2 for ground
  users there is greater ground attenuation and
  interference
• The calculations should be viewed as conservative
  the effective range of jammers may fall of as
  1/R4, giving substantially less effectiveness at
  longer ranges

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Trade III. Effect of Decisions onImproving
Dual-Frequency Handheld- Anti-JamMOE 2
Effective Range of 10W Jammer
Case Description J/S Improve-ment (dB)
A Upward Beam Antenna 15
B Ultra-tight Coupling 12
C GPS III 8
ALL 35
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Decision Makers Trade III. Applying the Possible
Mitigation Techniques to a Handheld GPS set
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MOE 3 Probability (miss distance gt Z m)

• Also Called Bounded Inaccuracy (The normally
  occurring Wild Points limited by Good Geometry
  and bounds on ranging error)
• Estimation of effects similar to MOE 1
• More strongly affected by weak satellite
  geometry due to local obscuration
• Also affected by satellite ranging error
• Particularly important for weapon delivery
• Senisitive non-targets (Mosques, Embassies)
• Working close to friendly troops
• GPS in weapon or used by ground spotter can be
  critical

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MOE 3 Bounds on inaccuracy (Probability of
Large Error) - 1 GPS Satellite not
operatingMountainous Terrain, 45o slope, Block
II Ranging error
24 Satellites have gt15 chance of exceeding 100 m
error
30 Satellites will not exceed 40 meter error more
than 1 of cases
32
MOE 3 Bounds on inaccuracy (Probability of
Large Error) - 2 GPS Satellites not
operatingMountainous Terrain, 45o slope, Block
II Ranging error
24 Satellites have gt25 chance of exceeding 100 m
error
30 Satellites will not exceed 70 meter error more
than 3 of cases
33
MOE 3 Bounds on inaccuracy (Probability of
Large Error) - 1 GPS Satellites not
operatingMountains of Afghanistan, 45o slope,
Block III Ranging error
24 Satellites have gt10 chance of exceeding 100 m
error
30 Satellites will not exceed 20 meter error more
than 1 of cases
34
MOE 3 Bounds on inaccuracy (Probability of
Large Error) - 1 GPS Satellites not
operatingMountainous Terrain, 60o slope, Block
III Ranging error- More restricted user case-
similar to urban streets
24 Satellites have gt40 chance of exceeding 100 m
error
30 Satellites will not exceed 40 meter error more
than 10 of cases
35
MOE 4 is Time to Alarm TTA S secondsNotify
User of GPS System/single satellite malfunctions
(out of normal Specification) within S seconds

• Sometimes Called Integrity Assurance
• Identifying and eliminating the non-normal GPS
  system errors that would cause Hazardous/Misleadin
  g information
• Examples include
• Satellite Runaway clock
• Extreme Local Multipath
• Evil Waveforms (distortion that leads to
  erroneous ranging measurement)
• Discontinuities in Ionosphere
• Methods for eliminating include
• Satellite self checking (FAA would have
  limitations)
• Direct external measurement and notification
  (WAAS)
• Notification through GPS message (may be a time
  to alarm issue)
• User Self Checking Using Satellite Redundancy
  (RAIM Receiver Autonomous Integrity Monitoring)
• Current Military Technique TTA about 45 minutes

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Comments

• RAIM is perhaps the simplest to implement, but
  requires sufficient satellites to have Geometric
  leverage
• Two Cases
• Alert that there is a problem
• Identification of the offending satellite and
  elimination from the users solution

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MOE Summary for Mission APrecision Bomb Delivery
with Ground Observer in Mountainous Terrain
24 Sats Blk II 30 Sats Blk. II 30 Sats Blk III Improved Locator
MOE 1 1 sortie 95 Accuracy 2.0 1.0 0.4
MOE 3 Bounds on Miss gt100m, gt15 of time gt40m,lt1 of time gt20M,lt1 of time
Current Available Improvements Available Improvements Available Improvements Available Improvements
Current Ant Tight Couple IIIA All
MOE 210WJammer Range (nm) 31.5 5.1 7.9 12.0 0.6
Current FAAs WAAS RAIM
MOE 4Time to Alarm 45 Min. 10 sec 3 sec?
MOE 1 results have been normalized
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Next Steps Repeat Analysis for other
Uses/Missions

• Other Envelope Missions/Uses
• Mission B. Precision Civil Aircraft Approach and
  Landing (Up to Cat III)
• Mission C. First Responder pinpointing urban
  locations
• Mission D. Use of GPS based ADS-B mandated for
  future ATC System improving separation distances

39
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