Big Aviation Data Mining forRobust, Ultra-Efficient Air Transportation
Technical Monitor:Sarah D’Souza, Systems Analysis Office, NASA Ames Research Center
MIT International Center for Air TransportationMIT International Center for Air Transportation
NASA LEARNPhase 1 Outbrief16 February 2016
Big Data for Aviation - 2MITLL 2/16/16
Team Members
KajalClaypoolDataArchitectures
RichDeLauraCo-PI
RichJordanAnalytics
TomReynoldsCo-PI
HamsaBalakrishnanAnalytics,Grad studentadvisor
JohnHansmanAnalytics,Grad studentadvisor
JacobAveryAnalytics,Grad student
EmilyClemonsAnalytics
YanGlinaAnalytics
AlexProschitskyDataArchitectures Mayara Conde
Rocha MurcaAnalytics,Grad student
KarthikGopalakrishnanAnalytics,Grad student
CalBrooksAnalytics,Grad student
NgaireUnderhillAnalytics
Big Data for Aviation - 3MITLL 2/16/16
• Air transportation system is very safe, but efficiency & robustness challenges remain
• Most inefficiencies caused by capacity & demand imbalances at range of spatial & temporal scales
Air Transportation SystemChallenges
NY arrivalsNY departures
PHL arrivalsPHL departures
BOS, DC ops
100
Millions of departures / % on timeBTS Annual
80
60
40
20
6
4
2
2006 2008 2010 2012 2014
Big Data for Aviation - 4MITLL 2/16/16
Demographics, economics
System Planning Air Traffic Control (ATC) OperationsStrategic
Weather forecast
Constraint, capacity forecast
Flight planning
Resources
National Airspace System (NAS)…in a single slide
Traffic management
Tactical
Tactical response & execution
Plans
Networks, capital, schedules
Resources, procedures
Airlines
FAA, Airports
Analytics
Data
Delays, cancellations
NAS structure, resources
Trajectories, resource use
-400 -300 -200 -100 0 100 200 300
-300
-200
-100
0
100
200
300
Big Data for Aviation - 5MITLL 2/16/16
Space, Time, Data, and Impacts
Planning / operational horizon
Scal
e / s
cope
of i
mpa
ct
Minutes Hours Days Years
Loca
lR
egio
nal
NA
SN
AS
+
Goal: Demonstrate Big Data analytic framework for aviation across spatial/temporal scalesGoal: Demonstrate Big Data analytic framework for aviation across spatial/temporal scales
Strategic ATC Operations
Tactical ATC Operations
Airline Network PlanningFAA System Planning
ScheduleEconomicDemographicClimatology, Etc.Delay
CancellationTraffic management planningNational / regional weatherRoute planning, Etc.
Flight trajectoriesLocal / regional
high resolution weather
Tactical decisionsEtc.
Sample data typesExploredExample future
Big Data for Aviation - 6MITLL 2/16/16
Data Description Spatial Extent Spatial Resolution Temporal Extent Temporal Resolution
Planning
Flight operations NAS-wide Airport pair(>300 BTS airports)
2000 - 2014 Annual
Strategic ATC Operations
Flight delays,cancellations
NAS-wide Airport pair(>300 BTS airports)
2008 - 2014 Annual, Seasonal, Daily, Hourly
Traffic Management Initiatives
NAS-wide N/A 2008 - 2014 Daily
Tactical ATC Operations
Flight trajectories Regional (NY, DFW, SFO metro)
~5 miles 2013 - 2015 1 minute
Weather radar mosaics
Regional (NY, DFW, SFO metro)
1 km 2013 - 2015 2.5 minute
Convective weather impacts
NY metro Individual route 2013 - 2015 5 minute
Terminal windimpacts
NY metro Individual terminal 2013 - 2015 hourly
Data Descriptions
BTS = Bureau of Transportation Statistics
Big Data for Aviation - 7MITLL 2/16/16
Anatomy of the Big Data Analysis Framework
‘Raw’ system data
Aggregate into compact mathematical representation
Derive descriptive
metrics
Identify patterns
of system behavior
Identify anomalies, interesting behaviors
Develop descriptive, predictive
models
Evaluate performance, identify best
practices
Size of data…
Size of insight…
Demand
Weather
Capacity
Analytics must be scalable, generalizable, and interpretableAnalytics must be scalable, generalizable, and interpretable
Enabled insights, applications, solutions
Big Data for Aviation - 8MITLL 2/16/16
• Motivation: Air transportation system challenges and Big Data opportunities
• Technical approach & Selected results:– Strategic ATC Operations– Tactical ATC Operations– Airline Network Planning
• Summary of innovations, Potential impacts and Next step recommendations
• Distribution / Dissemination & Acknowledgements
Outline
Big Data for Aviation - 9MITLL 2/16/16
Space, Time, Data, and Impacts
Planning / operational horizon
Scal
e / s
cope
of i
mpa
ct
Minutes Hours Days Years
Loca
lR
egio
nal
NA
SN
AS
+
Tactical ATC Operations
Airline Network PlanningFAA System Planning
DelayCancellationTraffic management planningNational / regional weatherRoute planning, Etc.
Strategic ATC Operations
Big Data for Aviation - 10MITLL 2/16/16
NAS-Wide Operational NetworkAt a glance…
Airport ConnectionsLinks colored by delay
Norm
alized departure delay(airport pair)
BOS
ATL
SFO
LAX
SEA
MIA
DFW
ORD NYCWAS
PDX
Big Data for Aviation - 11MITLL 2/16/16
Strategic ATC Operations:Analyzing the NAS-Wide Network
Eigencentrality:Airport connectivity
Application:Network structure
Goal: Characterize and model NAS-wide network dynamics and performance
Approach: Apply novel adjacency matrix weightings and metrics to define NAS-wide states that characterize propagation of disruptions
Goal: Characterize and model NAS-wide network dynamics and performance
Approach: Apply novel adjacency matrix weightings and metrics to define NAS-wide states that characterize propagation of disruptions
Adjacency matrix
Demand-weighted adjacency matrix
Eigencentrality:Airport throughput
Application:Network capacity
Delay, cancellation weighted adjacency matrix
Hub, authority metrics:Asymmetrical propagation of delay, cancellation
Application:Propagation of weighting metric (delay, cancellation, etc.)
HUB:Sendsdelay
AUT:Receives delay
DYNAMIC
High (Low)
High (Low)
Inbound, outbound delay balanced
High Low Delaypropagator
Low High Delay reducerAirport Flight connectionKEY:
Big Data for Aviation - 12MITLL 2/16/16
Delay State Identification:Methodology
Flight delays, cancellations (2008-2014)
Aggregate (daily, hourly) weighted
connectivity matrices (delay, cancellation)
Calculate Hub, Authority scores for major airports
Cluster into propagation
patterns
…
Daily Delay / Cancellation States
Post-event performance evaluation
Hourly Delay / Cancellation States
Dynamic delay propagation for predictive modeling
…
Framework key:
Insights
Big Data for Aviation - 13MITLL 2/16/16
Delay Distribution by Daily Delay StateSelected (5 of 12) Persistent Delay States (2008-2014)
Total delay on airport pair linksN
ormalized to m
aximum
observed link delay
NAS-wide HIGH Delay (11.4%)NAS-wide LOW Delay (29.4%)
ATL-dominated HIGH Delay (6.7%)ORD-dominated HIGH Delay (11.9%)
Daily Delay States provide insights into the scale and propagation of delayDaily Delay States provide insights into the scale and propagation of delay
SFO-dominated HIGH Delay (12.2%)
Big Data for Aviation - 14MITLL 2/16/16
NAS-Wide Delays by Daily Delay State2008 - 2014
Total delay is similar (but propagation is not) in single-airport dominated statesTotal delay in NAS-wide states tends to the extremes
NAS-wide HIGHNAS-wide LOW
ORD HIGH
ATL HIGHSFO HIGH
Big Data for Aviation - 15MITLL 2/16/16
Hourly Delay StatesCapturing Dynamics of Delay Propagation
ATL HIGHINCREASING
ATL HIGHDECREASING
FRO
M S
TATE
TO STATE
• Hourly Delay States capture delay propagation structure, magnitude, and trends– Local delays build and spread– Propagation is widest as delays
peak and begin decrease
• Observed Hourly Delay State transition probabilities, and dwell times can be calculated
Big Data for Aviation - 16MITLL 2/16/16
Day Delay Cancelled
July 26, 2012
26808 hours
554
Avg: 2008-2014
13054 hours
295
Norm
alized departure delay(airport pair)
July 26, 2012
Big Data for Aviation - 17MITLL 2/16/16
Network Dynamics Case Study26 July, 2012
NY Ground Delay Program (GDP) to reduce demand as thunderstorms impact local operations
NY GDP continues & delays persist and propagate as weather dissipates and major traffic corridors clear
9AM EDT 1PM EDT
Big Data for Aviation - 18MITLL 2/16/16
Network Dynamics Case Study26 July, 2012
Delays rapidly increase storms bisect the NAS (but coastal corridor remains clear)
5PM EDT
Delay growth and propagation appear to be driven by weather-related airspace constraints and control decisions with long time
constants
Delay State dwell times, transition probabilities provide
insight into NAS system response times
Big Data for Aviation - 19MITLL 2/16/16
Strategic ATC Operations:Next Steps
Delay Propagation ModelingMarkov Jump Linear System
Vector of airport delays at time t
Delay state at time t
Delay-state dependent system matrixDerived from network delay matrix
Probability of transition from delay state i to state j
Delay statesDwell times
Observed transition probabilities
Forecast, observed weather
Traffic management decisions
Delay / demand prediction modeling
Control strategy assessment
Big Data for Aviation - 20MITLL 2/16/16
Airline Network PlanningFAA System Planning
Space, Time, Data, and Impacts
Planning / operational horizon
Scal
e / s
cope
of i
mpa
ct
Minutes Hours Days Years
Loca
lR
egio
nal
NA
SN
AS
+
Strategic ATC OperationsFlight trajectoriesLocal / regional
high resolution weather
Tactical decisionsEtc.
Tactical ATC Operations
Big Data for Aviation - 21MITLL 2/16/16
Tactical ATC OperationsNY Metro Focus
Fair weather operationsNY Metro Arrival Trajectories
Convective weather operations
Goal: Develop a generalizable method to characterize tactical use of terminal and transition airspace to guide airspace design and support operational best practices
Approach: Identify patterns of arrival / departure resource use through trajectory analysis and link them to constraints and outcomes
Key:LGAEWRJFK
‘arrival (departure) resource’ = routinely used arrival (departure) path
Big Data for Aviation - 22MITLL 2/16/16
Tactical ATC Operations:Methodology
Observed trajectories
Resource Identification
Cluster trajectories using
DBSCAN
Resource Use
Assign trajectories to resources using
Random Forest &identify non-
conforming trajectories
Operational Patterns
Cluster Resource Use Vectors to
identify patterns of hourly use
-400 -300 -200 -100 0 100 200 300
-200
-100
0
100
200
300
1
2
345
67
8910
11
12
13
14
15
16
1718
19
20
21 22
23
13 day training set
57 day weather impact dataset1000 day pattern dataset (2013-2015)
Daily Resource Use MatricesPost-event analysis of operational dynamics
Hourly Resource Use VectorsReal time operational dynamics
Hourly Resource Use PatternsPredictive modeling
Framework key:
Insights
Big Data for Aviation - 23MITLL 2/16/16
85 ° W 80° W 75° W 70° W 65° W
35° N
40° N
45° N
NU
MB
ER
OF
FLIG
HTS
200
400
600
800
1000
1200
1400
1600
• Cluster algorithm parameterization involves tradeoffs between compactness, separability, and dissimilarity of clusters
• Resulting clusters captured ~92% of all trajectories
Resource Identification
85 ° W 80° W 75° W 70° W 65° W
35° N
40° N
45° N
‘Emergence’ of EWR Arrival Resources
13 days of arrivals… …23 clusters… …23 cluster centroids = Arrival Resources
Big Data for Aviation - 24MITLL 2/16/16
• Random Forest trajectory classification assigns individual trajectories to resources and identifies non-conforming trajectories
• Non-conforming trajectories take many forms– Dynamically alter flow structure– Workload consequences for Air Traffic Control?
Resource Assignment and Non-conformance: JFK Arrivals
Illustrations of non-conformanceTrajectories assigned to Arrival Resources
(all conforming)
September 9, 2013February 11, 2013October 8, 2014
Non-conforming trajectoriesArrival resources
NYCairports NYC
airportsNYC
airports
Big Data for Aviation - 25MITLL 2/16/16
D
2013
0131
2013
0211
2013
0227
2013
0421
2013
0522
2013
0523
2013
0524
2013
0624
2013
0701
2013
0717
2013
0718
2013
0809
2013
0828
2013
0901
2013
0911
2013
0912
2013
1127
2013
1214
2013
1217
2014
0328
2014
0429
2014
0714
2014
0820
2015
0623
2015
0714
2013
0108
2013
0118
2013
0121
2013
0214
2013
0225
2013
0305
2013
0314
2013
0330
2013
0415
2013
0430
2013
0501
2013
0503
2013
0531
2013
0619
2013
0621
2013
0726
2013
0816
2013
0909
2013
0918
2013
1021
2013
1027
2013
1029
2013
1104
2013
1113
2013
1218
2013
1220
2014
0416
2014
0722
2014
0919
2014
1008
2015
0603
Per
cent
age
of n
on-c
onfo
rmin
g tra
ject
orie
s
0
10
20
30
40
50
60JFKEWRLGA
Non-conformance and Weather
• Trajectories assigned for dataset of 56 days including weather impacted (convection or adverse winds / ceiling / visibility) and fair weather days
• Significant increase in non-conforming trajectories during weather impacted days
Weather impacted
days
Fair weather
days
Mean 13.8% 4.1%
Standard Deviation 6.9% 2.3%
Big Data for Aviation - 26MITLL 2/16/16
NY Metro Operational DynamicsA Tale of Two Days… (EWR Arrivals)
Resource Use Matrix Full day summary
October 8, 2014:Fair weather
July 14, 2015: Convective impacts
Arr
ival
reso
urce
ID
Non-conforming
Non-conforming
Period of convective impacts
Big Data for Aviation - 27MITLL 2/16/16
Hourly Resource Use Patterns (RUP)
RUP 1: Departure
RUP 2: JFK, EWR Arrival
JFK
AR
R
EWR
AR
R
LGA
ARR
JFK
DE
P
EWR
DE
P
LGA
DE
P
NC
NU
MB
ER
OF
FLIG
HTS
0
5
10
15
20
25
30
35
arrivals departures NC
JFK
AR
R
EWR
AR
R
LGA
ARR
JFK
DE
P
EWR
DE
P
LGA
DE
P
NC
NU
MB
ER
OF
FLIG
HTS
0
5
10
15
20
25
30
35
EWR Arrivals EWR DeparturesA
verage Hourly N
umber of Flights
Average H
ourly Num
ber of Flights
Average H
ourly Num
ber of Flights
Average H
ourly Num
ber of Flights
Big Data for Aviation - 28MITLL 2/16/16
Hourly Resource Use Patterns (RUP)
RUP 3: Arrival / Low Throughput
RUP 4: Very Low Throughput
EWR Arrivals EWR Departures
JFK
AR
R
EWR
AR
R
LGA
ARR
JFK
DE
P
EWR
DE
P
LGA
DE
P
NC
NU
MB
ER
OF
FLIG
HTS
0
5
10
15
20
25
30
35
JFK
AR
R
EWR
AR
R
LGA
ARR
JFK
DE
P
EWR
DE
P
LGA
DE
P
NC
NU
MB
ER
OF
FLIG
HTS
0
5
10
15
20
25
30
35
Average H
ourly Num
ber of Flights
Average H
ourly Num
ber of Flights
Average H
ourly Num
ber of Flights
Average H
ourly Num
ber of Flights
Big Data for Aviation - 29MITLL 2/16/16
EWR Arrivals
Hourly Resource Use Patterns (RUP)
RUP 5: High Demand / High Throughput
RUP 6: High Demand / High Non-conformance
EWR Departures
JFK
AR
R
EWR
AR
R
LGA
ARR
JFK
DE
P
EWR
DE
P
LGA
DE
P
NC
NU
MB
ER
OF
FLIG
HTS
0
5
10
15
20
25
30
35
JFK
AR
R
EWR
AR
R
LGA
ARR
JFK
DE
P
EWR
DE
P
LGA
DE
P
NC
NU
MB
ER
OF
FLIG
HTS
0
5
10
15
20
25
30
35
EWR ArrivalsA
verage Hourly N
umber of Flights
Average H
ourly Num
ber of Flights
Average H
ourly Num
ber of Flights
Average H
ourly Num
ber of Flights
Big Data for Aviation - 30MITLL 2/16/16
Occurrence of Resource Use PatternsBy Hour
High demand, high throughputDepartureArrival / Low throughputJFK / EWR arrivalVery low throughputHigh demand / High non-conformance
Clear Weather(427 days)
Obs
erve
d R
UP
Prob
abili
ty
High non-conforming (High Throughput) RUP observed more (less) frequently on
days with measurable convection / rain impacts
Convection / Rain(523 days)
Big Data for Aviation - 31MITLL 2/16/16
Tactical ATC Operations:Next Steps
Resource Use Matrices
Weather impact / constraint
Clustering to identify days with similar constraints,
resource use
Constraint-normalized performance assessment
Case day identification / scenario generation
Daily Aggregations
Hourly Aggregations
Resource Use Patterns
Correlation of Resource Use Patterns with
constraints, demand
Constrained capacity modeling and prediction
for decision support
Development of best practices
Weather impact / constraint
Big Data for Aviation - 32MITLL 2/16/16
Space, Time, Data, and Impacts
Planning / operational horizon
Scal
e / s
cope
of i
mpa
ct
Minutes Hours Days Years
Loca
lR
egio
nal
NA
SN
AS
+
Strategic ATC Operations
Tactical ATC Operations
ScheduleEconomicDemographicClimatology, Etc.
Airline Network PlanningFAA System Planning
Big Data for Aviation - 33MITLL 2/16/16
Air Carrier Competition:Methodology
Extract all city pairs
Identify top 40 routesCalculate # of flights, # of airlines on each
2000 - 2014
Number of Flights on route (x )
Num
ber o
f City
Pa
irs
Annual Route Use, Competition Networks
Inputs to Strategic Operations analysesBasis for predictive models to guide capital investment
Define use, competition
network structures
Framework key:
Insights
Big Data for Aviation - 34MITLL 2/16/16
Top 40 RoutesBy number of operations
2006 2007 2008
2009 2010 2011
2012 2013 2014
2
3
1
x104
Annual num
ber of departures
Big Data for Aviation - 35MITLL 2/16/16
Competition on Top 40 RoutesNumber of airline operators
2006 2007 2008
2009 2010 2011
2012 2013 2014
Num
ber of flight operators
Big Data for Aviation - 36MITLL 2/16/16
Air Carrier Competition:Next Steps
Characterize operational and competitive network
structure as weighted connectivity matrices
Effect of structure on annual NAS performance
measured by delay, cancellation
Correlation to observed frequency of Delay, Cancellation States
Network operations
Market competition
Big Data for Aviation - 37MITLL 2/16/16
• Motivation: Air transportation system challenges and Big Data opportunities
• Technical approach & Selected results:– Strategic ATC Operations– Tactical ATC Operations– Airline Network Planning
• Summary of innovations, Potential impacts and Next step recommendations
• Distribution / Dissemination & Acknowledgements
Outline
Big Data for Aviation - 38MITLL 2/16/16
• Developed Big Data analysis framework using novel metrics & analytics to provide new insight across a range of fundamental scales in air transport:
• Insights provide foundation for performance evaluation and predictive models
Phase 1 Innovation Summary
Aggregate Metrics Patterns Insights
Tact
ical
ATC
Ope
ratio
nsSt
rate
gic
ATC
Ope
ratio
nsA
irlin
e/FA
APl
anni
ng
• Airport-pair delay and cancellation weighted directional connectivity matrices
• NAS network hub and authority scores at range of temporal scales
• Assessed over multi-years
• Identification of small number of key NAS-wide delay and cancellation states
• System-wide delay and cancellation dynamics across operating conditions
• Terminal area trajectory clustering under range of operating conditions
• Assignment of trajectories to standard resources
• Determination of non-conforming flights
• Identification of small number of key resource use patterns
• Resource use pattern dynamics across airport locations and operating conditions
• Airline network definitions across decades
• Top route and competition evolutions over decades
• Identification of dominant scheduled routes
• Competition dynamics
• Network structural evolution over time
• Initial correlations of network structure with external influences
Big Data for Aviation - 39MITLL 2/16/16
Phase 1 Innovation & Impact Summary => Phase 2 Recommendations
DataLayer
AnalyticsLayer
ApplicationLayer
Impact &Tech
TransferLayer
Phase 1 Phase 2
• Flight trajectories• Flight delay
• Weather• Cancellations• Schedules
• Traffic Management Initiatives• Emerging data types (FAA SWIM, other?)• Database structure & technology
Tactical ATC
Operations Analysis
Strategic ATC
Operations Analysis
Airline/ FAA
Planning Analysis
• Diagnostic system characterization• Baseline, anomaly, scenario
identification
• Predictive modeling• Control action analysis• Tool building (visualization & analysis)
• NASA: technical interchange meetings• Other: Publications
• NASA: tools for integration into existing programs
• FAA / Industry: performance analysis Tech
tran
sfer
opp
ortu
nitie
s in
form
rese
arch
nee
ds
• Refinements across areas• Extensions where appropriate
Big Data for Aviation - 40MITLL 2/16/16
• Tactical Operations / 4D-TBO: end-to-end modeling of TBO-based traffic management (illustrated)
• Strategic, Tactical Operations / SMART-NAS Testbed: real-time analytics and visualization tools– Simulation modules– Review of archives to identify case studies and define scenarios
• All / Sherlock Data Warehouse: information models for analytic products
Current & Potential Future Connections to NASA Efforts
-600 -400 -200 0 200 400 600-500
-400
-300
-200
-100
0
100
200
300
400
500
LEARN Phase 1 / 2DFW departure resources
NASA ARCDFW-LGA trajectory prediction
LEARN Phase 1 / 2LGA arrival resources
85° W 80° W 75° W 70° W
35° N
40° N
45° N
4D-TBO = 4 Dimensional-Trajectory Based OperationsSMART-NAS = Shadow Mode Assessment Using Realistic Technologies
for the National Airspace System
Big Data for Aviation - 41MITLL 2/16/16
System Planning Air Traffic Control (ATC) OperationsStrategic
Ultimate Impact: Influencing Future National Airspace System Operations
TacticalAnalytics
-400 -300 -200 -100 0 100 200 300
-300
-200
-100
0
100
200
300
Performance-driven best practices(post-event analysis)
Operational decision support(real-time predictive models)
Structural inefficienciesCapital needs projection
Big Data for Aviation - 42MITLL 2/16/16
• Motivation: Air transportation system challenges and Big Data opportunities
• Technical approach & Selected results:– Strategic ATC Operations– Tactical ATC Operations– Airline Network Planning
• Summary of innovations, Potential impacts and Next step recommendations
• Distribution / Dissemination & Acknowledgements
Outline
Big Data for Aviation - 43MITLL 2/16/16
• Papers– “Multi-Scale Data Mining for Air Transportation System Diagnostics”, accepted to 16th AIAA
Aviation Technology, Integration, and Operations Conference, 13-17 June 2016, Washington DC.– “Clusters and Communities in Air Traffic Delay Networks”, accepted to 2016 IEEE American
Control Conference, 6-8 July 2016, Boston, MA.– “A Visual Analytic Platform for Air Traffic System Strategic and Tactical Operational Evaluation
and Control”, accepted to 2016 Integrated Communications Navigation and Surveillance (ICNS) Conference, 19-21 April 2016, Herndon, VA.
– “Airline Network & Competition Characterization using Big Data Approaches”, to be submitted to 35th Digital Aviation Systems Conference, 25-29 September 2016, Sacramento, CA.
• Presentations– “Big Aviation Data Mining for Robust, Ultra-Efficient Air Transportation”, Kick-off Meeting &
Overview for NASA ARC Aviation Systems Division researchers, NASA Ames Research Center, 4 April 2015.
– “Big Aviation Data Mining for Robust, Ultra-Efficient Air Transportation”, Status report & Technical Interchange Meeting for specific NASA ARC ASD programs, NASA Ames Research Center, 18-19 November 2015.
• Other– Numerous telcons with NASA researchers to discuss potential mutual value from
collaboration (including SMART-NAS, 4D-TBO, Sherlock data warehouse programs)
Distribution/Dissemination
Big Data for Aviation - 44MITLL 2/16/16
• Many thanks to the following:
– NARI for supporting the project and promoting collaboration
– Sarah D’Souza and Michael Bloem, NASA ARC for providing excellent technical oversight and helping connect us to relevant NASA researchers
– NASA ARC program researchers for their invaluable technical discussions, feedback on our approach and identification of relevant problem areas• 4D-TBO (Paul Lee, Heather Arneson, Tony Evans, …)• SMART-NAS (John Robinson, Kee Palopo, Gano Chatterji, …)• Sherlock data warehouse team (Michelle Eshow, Rich Keller, Ron
Reisman, …)• William Chan (Branch Chief)• Sandy Lozito (Division Chief)
Acknowledgments