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Ship DetectionFrom Processing to Instrument
Characterisation
…and back !
Guillaume HAJDUCH BOOST [email protected], rue Claude Chappe, 29280 Plouzané
Context
SHIPDETECTION
Kerguelen
Context
• SARTool© is a modularsoftware including:– Oil detection– Wind inversion– Wave inversion
– Ship detection
SARTool
Context• Definition of new operational services requires to
assess/monitor the performances of the full ship detection processing chain wrt:
– SAR sensor/processor characteristics
– Local meteo/oceanic conditions
– Characteristics of ships to be detected
Product Cal/Valactivities
Ship detection reports vs meteo-oceanicconditions
Ship detection reports vs ground truth (AIS…)
• Context• Overview of Ship Detection Strategy• Results of Performance Assessment• Conclusion & Perspectives
Overview of Ship Detection Strategy
Preprocessing (Land) Masking Pre-screening Discrimination
SAR
Product
ShipDetection
Report
Calibration Focus on area of interest
First detection of
targetscandidates
(TFAC)
Rejection of False
Alarms
Overview of Ship Detection Strategy• Prescreening (CFAR
Approach)– Detection of bright points
within the SAR image– Requires a statistical
characterization of nominal intensity of sea clutter
• Hypothesis on probability law for homogeneous sea clutter intensity
– Apply an adaptive threshold with respect to a predefined False Alarm Rate
• Within the validity of local approximation of sea clutter characteristics (local discontinuities, instrument artifacts…)
False AlarmRate
To be checked and monitoredwrt SAR sensor, mode, processor, …
Overview of Ship Detection Strategy• Discrimination of False Alarms
– Related to instrument artefacts
Nadir ambiguities (RS1)
Nadir Observedarea
Ambiguousdistance
Adressed by geometric considerations
PRFnchtNadir //2 +=
Overview of Ship Detection Strategy• Discrimination of False Alarms
– Related to instrument artefacts
Nadir ambiguities (RS1)
For ScanSAR modes like
• ENVISAT ASAR Wide Swath
• Radarsat ScanSAR Narrow/Wide
The PRF is changing from one subswath to another
• must take into account PRF variation
• must estimate location of subswathborders and overlapping
PRFnchtNadir //2 +=
Overview of Ship Detection Strategy• Discrimination of False Alarms
– Related to instrument artefacts
Target
Ambiguities
Azimuth ambiguities
Observedarea
Ambiguousarea
Ambiguousdistance
Observedarea
Ambiguousarea
Overview of Ship Detection Strategy• Discrimination of False Alarms
– Related to instrument artefactsRD vV
=
Target
Ambiguities
Azimuth ambiguities
Adressed by geometric/radiometricconsiderations
.2ambv k PRFλ=
2ambR kD PRFV
λ=
Detection of main echo by radiometric considerations.
Main echo must be a candidate ship
Overview of Ship Detection Strategy• Discrimination of False Alarms
– Related to instrument artefactsInterferences ?
Overview of Ship Detection Strategy• Discrimination of False Alarms
– Related to erroneous hypothesis on sea clutterNot homogeneous
Inhomogeneous sea clutter or badprobability density law
Can be addressed by a trainedclassifier using a basic featurevector including radiometricmeasurements on targetcandidate
Adressed by radiometric considerations
Overview of Ship Detection Strategy• Discrimination of False Alarms
– Short life time events
Either:
• spike
• azimuth ambiguities
Azimuth ambiguities off Cherbourg harbour
Overview of Ship Detection Strategy• Discrimination of False Alarms
– Short life time events
[R67] [R67]
• Discrimination by analysis of sublooks of SLC products
• Short life time events vanishfrom one look to another, but are stacked in M-Look image
• Suppressed by studying cross-correlation of sublooks
• Allows to reject azimuthambiguities near shore whennative reflector is not a targetcandidate
LOOK 1 LOOK 2
LOOK 3 M-LOOKS
• Context• Overview of Ship Detection Strategy• Results of Performance Assessment• Conclusion & Perspectives
Performance assessment I• Analysis of detection reports
SAR products
SARTool
Shipdetectionreports
Statisticalanalysis
Performance assessment I• Histogram of target candidates wrt incidence angle
Incidence Angle
Num
bero
f can
dida
tes
•Performed on ASA/WSM products
•From 16 to 45 degrees
High occurrence of missing data atproduct border
Performance assessment I• 2D Histogram of targets wrt incidence angle and RCS
Incidence Angle
Rad
ar C
ross
Sec
tion
PRIOR TO DISCRIMINATION
Sensor NES0
Mean SeaBackscattering
NES0 from instrument calibration
Performance assessment I
• Effectiveness of false alarm discrimination process
• Requires accuratelyradiometricallycalibrated products
Incidence Angle
Rad
ar C
ross
Sec
tion
AFTER DISCRIMINATION
Adressed by radiometric considerations
No data processedin this area
Performance assessment I• Amendment of TSS off Portugal
Performance assessment II• Detection versus meteo/oceanic conditions
SAR products
SARTool
Shipdetectionreports
Statisticalanalysis
Database of meteo/oceanic
conditions
Performance assessment II• Histogram of target candidates wrt wind speed
Wind speed
Num
bero
f can
dida
tes
Computed localyfrom SAR product
using SARTool Wind
Local increase of computed windspeeddue to target sidelobes
SARTool
Database of meteo/oceanic conditions
Performance assessment III• Detection versus ground truth
SAR products
Database of meteo/oceanic
conditionsSARTool
Shipdetectionreports
Statisticalanalysis
AIS reports
Performance assessment III• Collecting information through a collaborative network of
AIS receiver from 2007/10 to now– After rough suppression of erroneous reports
• Information on 2500 ships where collected• Information on 700000 positions where collected (not all collocated with SAR
products)
Dev Interface to SARTool Interface to Google
Performance assessment III• Automatic analysis of AIS reports is under development
– Analysis of ship parameters– Co-localisation AIS / SAR– AIS/SAR Pairing
• Objectives– Variability of RCS– … wrt to ship length– … wrt to incidence angle– … wrt to wind speed (perf.)
Length
Wid
th
• Context• Overview of Ship Detection Strategy• Results of Performance Assessment• Conclusion & Perspectives
Conclusion• From performance assessment to instrument
characterization:– Illustration of impact of NES0 of the sensor in terms of
ship detection• And Back !
– NES0 of the sensor must be taken into account in order to reject a high number of false alarms
– False alarm rejection requires radiometricallycalibrated SAR products & the ability to invert wind information locally
Perspectives• Estimation of ship detection performances by using AIS
data base– To be continued
• Fit of ship RCS from length– Check / enhanced thumb rule
• Characterization of instrumental artefacts for new sensors / missions– TerraSAR X, CSK, ALOS
• Characterization of processor artefacts– …
QUESTIONS ?SARTool
